Prospective Short Circuit (PSC) and Prospective Fault Current (PFC) are both designed to calculate the maximum current that will flow within a fault loop path during the event of an electrical fault as required by regulations.
The Prospective Short Circuit Current is the maximum current that could flow between Line and Neutral conductors on a single phase supply or between Line conductors on a three phase supply. A PSC test calculates the current that will flow in the event of a short circuit fault between the live conductors. That is, Line and Neutral on a single phase installation or Line to Line/ Line to Neutral on a three phase installation. A PFC test calculates the maximum current that will flow in the event of an earth fault; i.e., Line to Earth.
The test result can be determined by calculation, ascertained by enquiry to the relevant electricity board, or measured using a Loop Tester. If you are using a Loop Tester, you would measure both PSC and PFC values and record the highest value. Due to the nature of different supply types, you would expect to find a PSC value higher than a PFC value on both TT and TN-S systems, however on a TNC-S system both the PFC and PSC value should be identical.
Why PSC and PFC are Done?
It is important that we conduct the tests to make sure that the protective devices installed within a circuit are rated at the correct breaking capacity. Within a domestic installation, it is common to find 6000A (6kA) rated MCB’s installed within a circuit. If a domestic premises is situated particularly close to a supply sub-station and the measured external impedance (Ze) of the property was 0.03 Ohms, Ohm’s law calculates that 7,666A may flow in the event of a fault on a 230V supply. This may cause concern if the switch gear is only rated at 6000A.
What is done During PFC and PSC Tests?
PSC is determined by the voltage and impedance of the supply system. It is of the order of a few thousand amperes for a standard domestic mains electrical installation, but may be as low as a few milliamperes in a separated extra-low voltage (SELV) system or as high as hundreds of thousands of amps in large industrial power systems.
Protective devices such as circuit breakers and fuses must be selected with an interrupting rating that exceeds the prospective short-circuit current, if they are to safely protect the circuit from a fault. When a large electric current is interrupted an arc forms, and if the breaking capacity of a fuse or circuit breaker is exceeded, it will not extinguish the arc. Current will continue, resulting in damage to equipment, fire or explosion.
PFC is conducted at the origin of the installation, such as at the main switch or at other switchgear connected directly to the tail from the electricity distributor’s metering equipment. Where a measurement is made at a point in the installation other than the origin, such as an item of switchgear fed by a distribution circuit, it would not be the maximum value for the installation.
Particular care should be exercised during the testing process, as fault conditions are most severe at the origin of an installation, where this test is performed. The earthing conductor, main protective bonding conductors and circuit protective conductors should all be connected as for normal operation during these tests, because the presence of these and any other parallel paths to earth may reduce the impedance of the earth fault loop and so increase the level of prospective fault current.
PSC will be higher than the PFC. Prospective fault current and short circuit current of a circuit is automatically calculated when making a loop impedance test. The calculation uses a nominal circuit voltage, not the actual circuit voltage.
How is PSC and PFC Performed?
The nominal supply voltage used in the calculation is automatically selected depending on the actual circuit voltage. The instrument uses the following voltage values:
Actual measured voltage
Nominal voltage
< 75 V
55 V
≥= 75 V and <150 V
110 V
≥= 150 V and <300 V
230 V
≥=300 V
400 V
The Prospective Short Circuit Current Test Sequence
Step 1: Prospective Fault Current tester or the PFC function of a multifunctional tester such as the Megger 1553 is selected, and we make sure that the supply is ON, but the Main Switch is in OFF position.
Step 2: The test leads are connected on the incoming side of the Main Switch, one test lead on Line and another on the Neutral terminals of the Main Switch.
Step 3: TEST switch is pressed and a note of the value (kA) is made.
For three phase installations each phase is tested separately and the measured reading (test between Line 1 and Neutral, then Line 2 and Neutral and last Line 3 and Neutral) is doubled.
Some test meters require that the third (usually green) lead to be connected on the Neutral during this test. Please refer to the test meter manufacturer’s instruction.
There are two methods for measuring the value of PSC, but these can only be used when the supply has already been connected. By then, the fuses and circuit breakers will already be installed.
The first method is to measure the impedance of the supply by determining its voltage regulation, that is, the amount by which the voltage falls with an increase in current. For example, consider an installation with a no-load terminal voltage of 240 V. If, when a current of 40 A flows, the voltage falls to 238 V, the volt drop will be due to the impedance of the supply.
A second measurement method is to use a loop impedance tester see connected to phase and neutral (instead of phase and earth) to measure supply impedance. This can then be used with the supply voltage as above to calculate PSC. Some manufacturers modify their earth-loop testers so that this connection is made by selecting ‘PSC’ with a switch. The instrument measures supply voltage, and calculates, then displays, PSC.
A possible difficulty in measuring PSC, and thus being able to use fuses or circuit breakers with a lower breaking capacity than that suggested by the Supply Company, is that the supply may be reinforced. More load may result in extra or different transformers and cables being installed, which may reduce supply impedance and increase PSC.
In terms of PFC, there is no such thing as acceptable PFC. It is what it is. When testing on 3-phase supplies the simplest and quickest way is to take the highest PFC reading off the single phase and double the value. Use Ohms Law to check. Fault current readings should be very high. Anyway its result must be lower than breaking capacity of the protective device. For example, a BS 1361 type 1 cartridge fuse has a rating of approximately 16.5KA. The result must be lower than this at the origin of installation.
Benefits of PSC and PFC tests
They give accurate results as its live testing.
The testing is simple and not much calculations are needed.
Increased safety for employees and third parties.
Reduced Insurance Premiums.
Asset Data management and tracking systems.
Minor repairs of equipment made onsite to minimise down time.
A ground fault is any short circuit that outcomes in an unintentional connection amid a ground and energized ungrounded phase conductor and ground. Ground faults are the most common type of fault on power distribution systems. They are due to the insulation failure or accidental grounding of an ungrounded phase conductor that causes the ungrounded phase conductor to meet ground. Unintentional grounding of a phase conductor can occur when a small animal enters a piece of equipment and contacts both an ungrounded phase conductor and the grounded enclosure. They monitor that the current going out any one phase is coming back on another phase or neutral. If current is going out on a phase but is coming back on the ground path a ground fault has occurred. All systems with ground fault protection include:
Current transformers to identify ground fault current(GFC)
A relay or logic box to determine tripping current value and time
An operating mechanism to trip the breaker or switch
Certain systems have a monitor panel indicating system status and a test panel for generating a ground fault signal to test the breaker.
Why Testing Ground Fault Systems is Important?
According to National Electrical Code (NEC) sections 230-95 and 517-17 performance testing is required. Around 16% of ground fault protection systems tested by NETA comprises of components which are not installed properly, damaged components or does not operate properly. Arcing ground faults can seriously damage distribution equipment, causing fires, which damage facilities and endanger personnel. They also cause extended downtime during system repair. Ground fault protection is the initial step of shield. Once installed, ground fault protection systems stand by until needed to protect services and feeders. However, if these systems malfunction when a ground fault occurs, the distribution system and facility will be as damaged as if no systems were installed.
Ground fault systems must be fitted correctly and maintained and tested periodically. An arcing ground fault of even small values spoil switchgear before the main service overcurrent protection gets time to operate. A 480V solidly grounded system possess enough voltage to support an arc between 1 phase and a ground, but not adequate current to cause big main breaker or fuse to eradicate the fault rapidly. The result of this is an arc that is like an electric weld, munching huge values of metal during the time the breaker or fuse functions. A correctly installed and functioning ground fault protection system will identify and eradicate the fault in milliseconds, speedy enough to regulate damage to satisfactory levels.
Few Points to Remember
Complete field acceptance testing as required by the NEC
Inspect neutral main bonding connection
Verify proper installation of sensor(s) and grounding connections
Inject current through the current sensor and verify pickup and timing characteristics of the relay
Test operation with control voltage supply decreased to 277V as an alternative of 480V
Verify functioning of exclusive features like zone interlocks
NEC Article 100 defines ground fault protection of equipment as, “A system envisioned to give safety of equipment from harming line to ground fault currents by functioning to bring about a detaching means to open whole ungrounded conductors of the faulted circuit. This protection is offered at current levels smaller than those needed to shield conductors from harm through the functioning of a supply circuit overcurrent device.” Ground-Fault Sensing and Relaying Equipment requires that manufacturers provide information sheets describing system testing instructions. As a minimum, UL requires the following performance testing for manufacturers’ test requirements:
Have a trained personal examine the ground fault protection system to safeguard that it was fixed properly according to manufacturer’s recommendations. – Verify that the location of sensors and the polarity of their connections are correct.
Detect system grounding points to ensure that no ground paths occur that would sidestep the sensors.
Test the ground fault protection system using either a simulated or actual controlled ground fault to determine that the system settings are correct and that the system is operating as intended.
Record the results of the performance testing on the manufacturer-provided test form.
In addition, NEMA Publication PB 2.2 requires the following for performance testing:
Manufacturer’s installation and instruction literature should be reviewed and understood prior to performance testing.
Performance testing should follow manufacturer’s recommendations.
Performance testing must be regulated to those tests that find out that the ground fault system has been fixed properly and is functional.
What is Done During Testing of Ground Fault System?
There are two test methods for evaluating ground fault protection systems by using simulated fault current or by high-current primary injection. These methods can be applied to ground-fault relay systems, but to test a system with integral ground-fault trip circuit breakers, only the high-current primary injection method can be utilized. If the high-current test does not create necessary tripping, verify control power at fuses, transformers and at relays. The ground fault relay systems can be tested by the simulated fault current testing method plus a detailed visual inspection, if it is suitable to the local inspection agencies. Or else, it will be essential to utilize the high current primary injection test method.
Simulated Fault Current Method
A simulated fault current is created by a coil round a window type sensor or with the help of a distinct test winding in the sensor.
A secondary current in the sensor is created, when the monitor panel sends a small current through the test winding, which the relay act in response to as if it were created by a primary current of thousand six hundred amperes.
In a similar method which can be utilized with any window type sensor providing a ground fault relay, number of twists of wire are covered round the sensor core, like twenty turns of #14 wire.
A current is passed through the wire to simulate the ground-fault current, which is approximately 125 percent of the pickup setting of the relay divided by the number of turns.
Testing with simulated fault current offers a way of explaining the functioning of the relay, sensor and shunt trip and the sufficiency of the control power supply.
GFP system must be checked to confirm. that neutral ground points are placed properly with regard to sensors, that sensor polarities are correct when several are connected in parallel, and that conductors which pass through a sensor window all run in the same direction.
The significance of adding simulated fault current testing with sufficient inspection is highlighted when 1 understands that the first 5 items on the Checklist are problems that cannot be identified by just simulated fault current testing only.
High-Current Primary Injection Method
The high-current injection test method might be used to test ground fault protection systems with integral ground fault trips on circuit breakers or ground fault relays.
It is an alternative to simulated fault current testing along with inspection in the case of relays.
Integral ground fault protection in circuit breakers can be system tested only with the help of high-current injection test method.
No. TAK-TS2, which is utilized along with AKR-SST/ECS trips can be used to determine the internal electronics of these breakers.
High-current testing of ground fault protection systems comprises of injecting full-scale current into the equipment phase and neutral conductors to replicate the ground fault current flow under various states.
The testing gear needed consists of a high-current supply sufficient enough to deliver up to thousands of amperes or more at 2.5V, or similar.
With the help of smaller ground fault current pickup settings on relays and breakers or switches, the current necessary to trip can be maintained to a minimum, such as 400 or 300 amperes or less.
If inspection experts needs tests at complete ground fault protection setting, a current supply sufficient enough to deliver thousand two hundred amperes or more may be necessary.
Connect the current supply and jumpers between the points indicated in the tables accompanying the diagrams.
Ground fault protection can be supplied for three wires and four wire gear supplied from a solidly grounded four wire supply.
This is necessary to offer a low-impedance ground fault current return path to the neutral to make sure functioning of the overcurrent device is correct.
How is Ground Fault Test Performed?
Visual and Mechanical Inspection
Compare nameplate data of equipment with specifications and drawings.
Inspect the components for damage and errors in polarity or conductor routing:
Verify that ground connection is made ahead of the neutral disconnect link and on the line side of any ground fault sensor.
Verify that the neutral sensors relate to correct polarity on both primary and secondary.
Verify that all phase conductors and the neutral pass through the sensor in the same direction for zero sequence systems.
Check that grounding conductor does not pass through the 0 sequence sensors.
Verify that the grounded conductor is solidly grounded.
Inspect bolted electrical connections for high resistance using one of the following methods:
Use of low-resistance ohmmeter in accordance with Section 7.14.2.
Check stiffness of reachable bolted electrical connections by calibrated torque wrench method according to manufacturer’s published data.
Perform thermographic survey.
Verify correct operation of all functions of the self-test panel.
Verify that the control power transformer has adequate capacity for the system.
Set time-delay and pickup settings according to the settings given in the owner’s specifications. As asked by NFPA, record suitable functional and test sequences.
Electrical Tests
Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable.
Measure the system neutral-to-ground insulation resistance with the neutral disconnect link temporarily removed. Replace neutral disconnect link after testing.
Execute insulation resistance test on whole control wiring in connection with ground. Employed potential should be 500 V dc for 300V rated cable and 1000V dc for 600V rated cable. Test duration shall be one minute. Follow manufacturer’s recommendation, for units that cannot tolerate the applied voltage or units with solid state components.
Perform the following pickup tests using primary injection:
Verify that the relay does not operate at 90 percent of the pickup setting.
Verify pickup is less than 125 percent of setting or 1200 amperes, whichever is smaller.
Verify right polarities by employing current to each phase neutral current transformer pair, for summation type systems, utilizing phase and neutral current transformers.
This test also applies to molded-case breakers utilizing an external neutral current transformer.
Relay must function when current direction is constant relative to polarity marks in the 2 current transformers.
Relay should not function when current course is contrasting to polarity marks in the 2 current transformers.
Quantify relay time-delay at 140% or greater of pickup.
Check decreased control voltage tripping ability is 81 percent for dc systems and 56 percent for ac systems.
Verify blocking capability of zone interlock systems.
Test Values
Match up bolted connection resistances to values of same connections.
Bolt-torque levels should be in accordance with Table 100.12 unless otherwise specified by manufacturer.
Millivolt drop values or Microhm should not cross the high-level limit of the normal range as stated in the manufacturer’s published data. Investigate any values which stray from similar connections by more than fifty percent of the lowest value, if manufacturer’s data is not accessible,
System neutral to ground insulation should be at least 1.0 megohm.
Insulation-resistance values for control wiring shall be a minimum of 2.0 megohms.
Relay timing should be matching with manufacturer’s specifications but must be slower than 1 second at three thousand amperes.
Electrical safety testing is essential to ensure safe operating standards for any product that uses electricity. Various governments and agencies have developed stringent requirements for electrical products that are sold world-wide. Several tests are conducted to check the safety of products. One among that is earth test.
Potentially the most dangerous appliances are Class I appliances (earthed appliances), example: microwave ovens/bench grinders and the like, but also in this category are extension leads. Class I appliances are designed to have a connection to the ground via an earth conductor. This may, or may not, be a suitable low resistance path for electric current to protect personnel and equipment. If this conductor is damaged anywhere then the consequences can be virulent.
Why is Earth Ground Testing Needed?
The measurement of ground resistance for an earth electrode system should be done when the electrode is first installed, and then at periodic intervals thereafter. This ensures that the resistance-to-ground does not increase over time. The International Electrical Testing Association specifies ground electrode testing every three years for a system in good condition with average up-time requirements.
Poor grounding not only increases the risk of equipment failure; it is dangerous too. Facilities need to have adequately grounded electrical systems so in the event of a lightning strike, or utility overvoltage, current will find a safe path to earth. Although the ground system when initially installed had low earth ground resistance values, the resistance of the grounding system can increase if the ground rods are eaten away by corrosive soils with high moisture content, high salt content, and high temperatures.
If our technician finds an increase in resistance of more than 20 percent, we will investigate the source of the problem and make the correction to the grounding system to lower the resistance.
Factors That Can Change Your Minimum Earth Resistance
A plant or other electrical facility can expand in size. Also, new plants continue to be built larger and larger. Such changes create different needs in the earth electrode. What was formerly a suitably low earth resistance can become an obsolete “standard.”
As facilities add more modern sensitive computer-controlled equipment, the problems of electrical noise is magnified. Noise that would not effect cruder, older equipment can cause daily problems with new equipment.
As more non-metallic pipes and conduits are installed underground, such installations become less and less dependable as effective, low-resistance ground connections.
In many locations, the water table is gradually falling. In a year or so, earth electrode systems that formerly were effective may end up in dry earth of high resistance.
These factors emphasize the importance of a continuous, periodic program of earth-resistance testing. It is not enough to check the earth resistance only at the time of installation.
Factors Influencing Requirements for a Good Grounding System
Limiting to definite values the voltage to earth of the entire electrical system. Use of a suitable grounding system can do this by maintaining some point in the circuit at earth potential. Such a grounding system provides these advantages:
Limits voltage to which the system-to-ground insulation is subjected, thereby more definitely fixing the insulation rating.
Limits the system-to-ground or system-to-frame voltage to values safe for personnel.
Provides a relatively stable system with a minimum of transient over-voltage.
Permits any system fault to ground to be quickly isolated.
Proper grounding of metallic enclosures and support structures that are part of the electrical system and may be contacted by personnel. Also to be included are portable electrically operated devices. Consider that only a small amount of electric current — as little as 01 A for one second — can be fatal! An even smaller amount can cause you to lose muscular control. These low currents can occur in your body at voltages as low as 100 V, if your skin is moist.
Protection against static electricity from friction. Along with this are the attendant hazards of shock, fire and explosion. Moving objects that may be inherent insulators – such as paper, textiles, conveyor belts or power belts and rubberised fabrics – can develop surprisingly high charges unless properly grounded.
Protection against direct lightning strokes. Elevated structures, such as stacks, the building proper, and water tanks may require lightning rods connected into the grounding system.
Protection against induced lightning voltages. This is particularly a factor if aerial power distribution and communications circuits are involved. Lightning arresters may be required in strategic locations throughout the plant.
Providing good grounds for electric process control and communication circuits. With the increased use of industrial control instruments, computers, and communications equipment, accessibility of low resistance ground connections in many plant locations — in office and production areas — must be considered.
Resistance to earth can vary with changes in climate and temperature. Such changes can be considerable. An earth electrode that was good (low-resistance) when installed may not stay that way; to be sure, you must check it periodically. We cannot tell you what your maximum earth resistance should be. For specific systems in definite locations, specifications are often set. Some call for 5 Ω maximum; others accept no more than 3 Ω. In certain cases, resistances as low as a small fraction of an ohm are required.
Nature of an Earth Electrode
Nature of an Earth Electrode Resistance to current through an earth electrode actually has three components:
Resistance of the electrode itself and connections to it.
Contact resistance between the electrode and the soil adjacent to it.
Resistance of the surrounding earth.
Electrode Resistance: Rods, pipes, masses of metal, structures, and other devices are commonly used for earth connections. These are usually of sufficient size or cross-section that their resistance is a negligible part of the total resistance.
Electrode-Earth Contact Resistance: This is much less than you might think. If the electrode is free from paint or grease, and the earth is packed firmly, contact resistance is negligible. Rust on an iron electrode has little or no effect but if an iron pipe has rusted through, the part below the break is not effective as a part of the earth electrode.
Resistance of Surrounding Earth: An electrode driven into earth of uniform resistivity radiates current in all directions. Think of the electrode as being surrounded by shells of earth, all of equal thickness. The earth shell nearest the electrode naturally has the smallest surface area and so offers the greatest resistance.
Principles Involved in Earth Resistance Testing
The resistance to earth of any system of electrodes theoretically can be calculated from formulas based upon the general resistance formula:
R = ρ LA
Where ρ is the resistivity of the earth in ohm-cm, L is the length of the conducting path, and A is the cross-sectional area of the path. All such formulas can be simplified a little by basing them on the assumption that the earth’s resistivity is uniform throughout the entire soil volume under consideration.
There are five basic test methods as noted below
Soil Resistivity Testing: Wenner four-pole equal method [19] has been considered in measuring the soil resistivity. The correct design of an earthing system is dependent upon detailed knowledge of the local ground resistivity. This is measured as a function of depth at a series of locations around the site, using an expanding four electrode Wenner array (BS EN 50522). The procedure is known as soil resistivity or earth resistance testing. Correct measurement is particularly important in areas of high resistivity ground, where electrical currents are not able to dissipate. In these conditions obtaining an earth can be problematic, and information on ground resistivity is required to much greater depths for the successful installation of an earthing system.
Fall-of-Potential Method: With a four-terminal tester, P1 and C1 terminals on the instrument are connected to the earth electrode under test. With a three-terminal instrument, connect X to the earth electrode. Although four terminals are necessary for resistivity measurements, the use of either three of four terminals is largely optional for testing the resistance of an installed electrode. The use of three terminals is more convenient because it requires one lead to be connected. The trade-off is that the resistance of this common lead is included in the measurement. Normally, this effect can be minimized by keeping the lead short, to accommodate simple test requirements. The small additional resistance thus introduced is negligible. When performing more complex tests or meeting stringent requirements, however, it may be better to use all four terminals by a lead from the P1 terminal to the test electrode (connecting it inside the lead from C1). This is a true four wire test configuration which eliminates all lead resistance from the measurement.
The added accuracy may prove significant when meeting very low resistance specifications or using test methods that necessitate an extra digit of measurement in order to meet the mathematical requirements. The decision is optional, based on the operator’s testing goals and the method used. The driven reference rod C should be placed as far from the earth electrode as practical; this distance may be limited by the length of extension wire available, or the geography of the surroundings. Leads should be separated and “snaked,” not run close and parallel to each other, to eliminate mutual inductance. Potential-reference rod P is then driven in at a number of points roughly on a straight line between the earth electrode and C. Resistance readings are logged for each of the points.
Dead Earth Method: When using a four-terminal instrument, P1 and C1 terminals connect to the earth electrode under test; P2 and C2 terminals connect to an all-metallic water-pipe system. With a three-terminal instrument, connect X to the earth electrode, P and C to the pipe system. If the water system is extensive (covering a large area), its resistance should only be a fraction of an ohm. You can then take the instrument reading as being the resistance of the electrode under test. The dead earth method is the simplest way to make an earth resistance test. With this method, resistance of two electrodes in series is measured — the driven rod and the water system. But there are three important limitations:
The waterpipe system must be extensive enough to have a negligible resistance.
The waterpipe system must be metallic throughout, without any insulating couplings or flanges.
The earth electrode under test must be far enough away from the water-pipe system to be outside its sphere of influence. In some locations, your earth electrode may be so close to the water-pipe system that you cannot separate the two by the required distance for measurement by the two-terminal method.
Under these circumstances, if conditions 1 and 2 above are met, you can connect to the water-pipe system and obtain a suitable earth electrode. As a precaution against any possible future changes in the resistance of the water-pipe system, however, you should also install an earth electrode.
Clamp-on Method: Fall-of-potential testing, and its modifications, is the only ground testing method that conforms to IEEE 81. It is extremely reliable, highly accurate and can be used to test any size ground system. Additionally, the operator has complete control of the test set-up and can check or proof his/her results by testing at different probe spacing. Unfortunately, the Fall of Potential method also comes with drawbacks:
It is extremely time consuming and labour intensive.
Individual ground electrodes must be disconnected from the system to be measured.
The clamp-on ground testing method, although it does not conform to IEEE 81, does provide the operator with the ability to make effective measurements under the right conditions. The clamp-on methodology is based on Ohm’s Law (R=V/I). A known voltage is applied to a complete circuit and the resulting current flow is measured. The resistance of the circuit can then be calculated. The clamp-on ground tester applies the signal and measures the current without a direct electrical connection. The clamp includes a transmit coil that applies the voltage and a receive coil that measures the current.
Selective Measurement Testing: Selective testing is very similar to the Fall-of-Potential testing, providing all the same measurements, but in a much safer and easier way. This is because with Selective testing, the earth electrode of interest does not need to be disconnected from its connection to the site! The technician does not have to endanger himself by disconnecting ground, nor endanger other personnel or electrical equipment inside a no grounded structure.
How to Improve Earth Resistance
When you find that your earth electrode resistance is not low enough, there are several ways you can improve it:
Lengthen the earth electrode in the earth.
Use multiple rods.
Treat the soil.
Effect of Rod Size: As you might suspect, driving a longer rod deeper into the earth, materially decreases its resistance. In general, doubling the rod length reduces resistance by about 40 percent.
Use of Multiple Rods: Two well-spaced rods driven into the earth provide parallel paths. They are, in effect, two resistances in parallel. The rule for two resistances in parallel does not apply exactly; that is, the resultant resistance is not one-half the individual rod resistances (assuming they are of the same size and depth).
Treatment of the Soil: Chemical treatment of soil is a good way to improve earth electrode resistance when you cannot drive deeper ground rods because of hard underlying rock, for example. It is beyond the scope of this manual to recommend the best treatment chemicals for all situations. You have to consider the possible corrosive effect on the electrode as well as EPA and local environmental regulations. Magnesium sulfate, copper sulfate, and ordinary rock salt are suitable non-corrosive materials. Magnesium sulfate is the least corrosive, but rock salt is cheaper and does the job if applied in a trench dug around the electrode. It should be noted that soluble sulfates attack concrete, and should be kept away from building foundations. Another popular approach is to backfill around the electrode with a specialized conductive concrete. A number of these products, like bentonite, are available on the market.
Effect of Temperature on Earth Resistivity
Not much information has been collected on the effects of temperature. Two facts lead to the logical conclusion that an increase in temperature will decrease resistivity:
Water present in soil mostly determines the resistivity
An increase in temperature markedly decreases the resistivity of water.
The resistivity continues to increase as temperatures go below freezing.
Grounding testers are indispensable troubleshooting tools to help you maintain uptime. It is recommended that all grounds and ground connections be checked at least annually as a part of your normal predictive maintenance plan. Should an increase in resistance of more than 20% be measured during these periodic checks, the technician should investigate the source of the problem and make the correction to lower the resistance by replacing or adding ground rods to the ground system.
Leakage current is the current that streams from either DC or AC circuit in an equipment to the ground or framework and can be from the output or input. If the equipment is not properly grounded, the current flows through other paths such as the human body. This mighty also occur if the ground is incompetent or is disrupted unintentionally or intentionally.
The leakage current in an equipment flows when an unintentional electrical connection occurs between the ground and an energized part or conductor. The ground may be the reference point of zero voltage, or the earth ground. Ideally, the current leaking from the power supply unit should flow through the ground connection and into the installations earth ground.
The inadequacies in the materials that build up the elements like the capacitors and semiconductors are the main cause of leakage current. These results in to small current leaking or flowing through the through the dielectric, in the case of a capacitor.
This measurement is done during the electrical safety test of a device. The currents flowing through the protective conductor or metallic parts of the earth are measured.
Why is Leakage Current Measurement Important?
Electrical system usually consists of a grounding technique that offers shield against a shock hazard if an insulation fault occurs. The grounding system comprises of a grounding rod that connects the instrument to the earth. If ever a disastrous failure of insulation between power line and conductive parts occur, the voltage will be pushed to ground. The current that is created because of this event will flow, causing a circuit breaker to open or a fuse to blow thus avoiding a shock hazard.
Clearly, a shock hazard prevails if the earth or ground connection is intruded, either accidentally or intentionally. The possibility for a shock might be larger than assumed if there is case of leakage currents. Even in the scenario of no insulation failure, intrusion of leakage currents streaming through the grounding rod still pose a threat of electric shock to somebody meeting the ungrounded system and ground at the same time.
This is a huge concern when it comes to the field of medical applications, where a patient might be the receiver of the electric shock. A shock can be even fatal if the patient is weak or unconscious, or if the current flows to internal organs. The two-layered insulation offered in non-grounded equipment ensures protection. The security in this scenario is made sure because both coats of insulation are not likely to collapse together. Nevertheless, the situations that leads to leakage currents still exists and must be considered.
Hence, how can you eradicate or reduce the outcomes of leakage current? Measure the leakage current and then recognize the cause. Purpose of the Test is to measure the amount of current that passes through a person when that person touches an electrical product.
What is Done During Leakage Current Measurement?
Meter particularly designed for determining leakage currents is utilised.
The current streaming through the ground rod is quantified by attaching the meter in series with the earthing connection.
The ground connection is unsealed and the current streaming to the neutral side of the power line is measured, for data processing equipment.
The meter may also be connected between the outputs of the power supply and ground.
Test conditions consists of exchanging the neutral connections and ac line and turning power switches on and off while monitoring the current.
The test is done once the system has warmed to typical functioning temperature.
The intention is to identify and measure the worst-case leakage current.
For very small leakage currents, the meter is substituted with a network comprising of either a resistor or a resistor and capacitor grouping.
The voltage drop throughout the network is then quantified using an ac voltmeter.
Double-insulated equipment or ungrounded is verified by attaching the meter amid any touchable conductive part and earth.
A copper foil of a specific dimension is placed on the housing, for a nonconductive housings, and the current flowing from it to ground is determined.
Type of Equipment
Maximum Leakage Current
Class I
0.75mA for hand held devices, 3.5mA for other devices
Class II
0.25mA
Class III
No hazardous voltages
How is Leakage Current Measurement Performed?
Direct Measurement
Direct measurement has precision and a meter especially designed for determining leakage currents is used. The current flowing in the ground conductor is measured by connecting the meter in series with the grounding connection of the device concerned.
Leakage current clamp meter is the most popular device used to measure leakage current. They are like the clamp meters utilized for finding load currents but gives considerably better results when quantifying currents less than 5mA. Generally, clamp meters wouldn’t register such small currents. After we position the jaws of a clamp meter around a conducting rod or wire, the current reading is taken, and the value depends upon the intensity of the alternating electromagnetic field around the conductor. The clamp meter will identify the magnetic field around conductors like a wire armor cable, single core cable, a water pipe etc. The paired neutral and phase conductors of a single-phase circuit, or all live conductors of a three-phase circuit.
Testing different kinds of conductors:
When testing the grouped live conductors of a circuit, the magnetic fields produced by the load currents cancel each other out. Any uneven current coming from the conductors to ground is measured with a leakage clamp meter and must have a reading less than 0.1 mA.
If you performed an insulation test on a circuit that was powered down, the result would be in the range of 50MΩ or further, because the insulation tester utilizes s a dc voltage for checking, which do not consider the capacitive effect.
If you measured the same circuit loaded with office equipment, the result would be significantly different due to the capacitance of the input filters on these devices.
When a lot of parts of equipment are functioning on a circuit, the result will be collective, that is, the leakage current will be greater and could well be in the range of milliamps. Adding new pieces of equipment to a circuit protected by a GFCI could trip the GFCI. And as the value of leakage current differs based on how the equipment is functioning, the GFCI may trip unintentionally.
When telecommunications equipment is present, the value of leakage indicated by a clamp meter may be considerably more than that resulting from insulation impedance at 60 Hz because, telecommunications system usually consists of filters that generate functional grounding currents and other gears that generates harmonics, etc.
Measurement of Leakage Current to Ground
When the load is switched on, the leakage current measured includes leakage in load equipment. If the leakage is adequately small with the load attached,
then circuit wiring leakage is even smaller. If circuit wiring leakage alone is required, disconnect the load.
If you test single-phase circuits by clamping the phase and neutral conductor, the obtained amount will be any current streaming to ground.
Test 3 phase circuits by fastening a clamp around all 3 phase conductors. If a neutral is present, it must be clamped along with the phase conductors and the measured amount will be any current flowing to ground.
Measuring leakage current through the ground conductor
To quantity the sum of leakage streaming to the proposed earth connection, position the clamp around the ground rod.
Measuring leakage current to ground via unintentional paths to ground.
Clamping neutral/phase/ground all together recognizes uneven current that means leakage at a passage or electrical panel via unintended pathways to ground.
If a connection to a water pipe or other electrical connections occur, similar inequality might happen.
Tracing the source of leakage current
This series of measurements identifies the overall leakage and the source. The first measurement can be made on the main conductor to the panel.
Measurements 2 to 5 are made consequently to find out circuits carrying the bigger amounts of leakage current.
Leakage Current Measurement in Medical Devices
The objective of the Leakage Current test is to verify that the electrical insulation used to protect the user from a Risk of Shock is suitable for the application. Leakage Current testing is used to verify that the product does not leak excessive current when contacted by the user. For medical equipment, the current flowing to ground is measured.
Excessive leakage current can cause the heart to go into ventricular fibrillation resulting in cardiac arrest which can lead to death.
Leakage current measurement levels depend on the amount of capacitance in the products’ solid insulating materials. Different types and number of layers of an electrical insulation results in varying amounts of inherent capacitance through the insulation. This capacitance causes low amounts of current to “leak” through the insulation.
Leakage current levels can be significantly elevated in products that are subject to EMI requirements (FCC, CE-EMC). These products must incorporate EMI filters on their incoming mains power to provide clean power to sensitive electronics while also protecting from radiating emissions back onto the power line. These filters incorporate capacitors to ground, these capacitors can cause high leakage current when operating normally. If the product is for professional use only, the standard may permit high leakage current with warning markings for the user to insure the product is reliably grounded (so the user isn’t subjected to the high leakage current). Otherwise, an isolation transformer must be added to power the product thereby isolating the product from ground – which will almost eliminate leakage current to ground.
Hipot Leakage Current Testers
The HIPOT test also called Dielectric Withstand Test is a routine test that is performed in electrical production industry. This is a high voltage test that stresses the insulation of an electrical product far 80 M.
If the insulation of product can withstand a much higher voltage for a given time, then it can withstand normal voltage for its whole life.
The basic function of HIPOT tester is to monitor excessive leakage current to ground.
Hipot tester applies a high voltage across the insulation of device that is tested. This is generally higher 1400 Volts to test a device that is planned to be operated on 220 Volts.
Terminals A and B are connected to supply voltage of 220 or 110, terminal C is grounded, return lead is floating as shown here.
The device to be tested should be separated electrically from ground.
One lead from winding is connected to HV out probe and return lead to motor body. This applies high voltage across winding and case.
If winding is short or weak at any point a current will stream to return lead and meter will display that current.
All HIPOT testers have an over-current trip to secure the tester itself. This is vital in case if device is completely shorted to its body and extreme current flows upon application of high voltage from HIPOT tester.
Benefits of Leakage Current Measurement
Advantages of leakage current measurement are:
The device under test is not placed into service, and its polarity need not be reversed
No stressing due to high switching current
Leakage current can be a sign of the inefficiency of insulation on conductors. It is achievable to trace the cause of leakage current with the help of a low current leakage current clamp to interpret orderly measurements as needed. If required, this allows you to re-allocate loads all around the installation in a better unbiased manner.
Published by Randy Barnett, Certified Energy Auditor and Trainer for NTT, Centennial, Colo. Email: randy@randybarnett.net , March 1st, 2012.
How to interpret the results of a power quality site survey
Analyzing electrical parameters associated with distributing electricity is viewed by many as complex engineering work. Yet, for engineers, electricians, and technicians troubleshooting equipment problems these days — and for contractors maintaining electrical systems they may have once installed — measuring power quality is becoming as much of a necessity as using the clamp ammeter to find out why the overloaded circuits keep tripping.
When any electrical system fails to meet its purpose, it is time to investigate the problem, find the cause, and initiate corrective action. The purpose of the electrical distribution system is to support proper operation of the loads. When a load does not operate properly, the quality of the electric power in the system should be suspected as one possible cause. Whether it’s used for troubleshooting purposes or to obtain baseline data, measuring/analyzing electrical system parameters is called power quality analysis.
The setup and use of power quality equipment — and obtaining and interpreting usable data — can be intimidating for those not familiar with the process. The key to success is knowing where and how to measure as well as how to interpret the results.
Organization and planning is key to success. Dedicating an equipment cart to hold analyzers, test equipment, drawings, manuals, notebook, digital camera, and safety equipment can help.
Measurement Tools
Several measurement tools are available for power quality measurement. Power quality analyzers are the most commonly used tools to observe real-time readings and also collect data for downloading to computers for analysis. While some are permanently installed in the distribution system, handheld analyzers are necessary for many applications, especially troubleshooting.
Handheld power quality analyzers are fairly lightweight (generally 4 lb to 5 lb) and will measure a variety of parameters. The most typical include voltage, amperage, frequency, dips (sags) and swells in voltage values, power factor, harmonic currents, and the resulting distortion and crest factor, power and energy, voltage and current unbalance, inrush current values, and light flicker. If an analyzer measures and records such basic parameters, you can address most power quality issues successfully.
Portable data loggers typically monitor many of the same parameters as the power quality analyzer; however, they are meant for long-term recording (days to several weeks). In addition, the data logger does not typically provide the real-time values on-screen that an analyzer can provide. Additional test equipment, such as scopemeters and recording digital multimeters, also find specific use applications.
The Process
Conducting a power quality survey begins with planning. Simply determine the purpose of the survey, and write it down in a notebook or binder that will be used throughout the process to organize and maintain data. Start with a good one-line diagram of the facility electrical distribution system. If one does not exist, then this is an excellent time to get one up to date.
If conducting a general power quality survey to obtain baseline data for future comparisons — or to help identify any immediate hidden electrical distribution problems that may exist — start monitoring as close as practical at the point of service. Beware, however, measuring near the service typically means large amounts of fault current available. Therefore, be careful when connecting the analyzer at a point in the distribution system downstream of the main breaker that limits incident energy levels to acceptable values. Because power quality problems can either come from the electric utility — or be generated within the facility — be sure to contact the utility in order to identify any possible issues on this side of the meter.
Inside the facility, continue to “drill down” into the distribution system following the one-line diagram. Obtain data at the source of each separately derived system. For example, take recordings at the first panelboard or switchboard after a 480V to 208Y/120V transformer. Be sure to mark up drawings, and take plenty of notes for future reference.
Digital cameras work well for quickly capturing nameplate data and later identifying exact connection locations. Note plant conditions and any equipment that was running. Print out digital pictures, and maintain all data for the survey in the notebook binder. These notes will become valuable when analyzing data and conducting further studies.
Follow manufacturer’s instructions for connecting and setting up the analyzer. Because of the amount of test equipment and supporting documentation that is needed, it is often best to have an equipment cart dedicated for power quality work. In addition to technical expertise, the underlying key to a successful survey is planning and organization. Three common mistakes when connecting power quality analyzers are:
Failure to observe current polarity. Make sure the arrow on current clamps points toward the load. If the arrow points in the wrong direction, a negative current value is obtained on the analyzer for that phase.
Not matching current/voltage probes. If analyzer input phase “A” is clipped onto phase “B,” it is obvious readings will be erroneous. Color code individual leads such that voltage and current leads for each phase are the same color, and connect carefully to prevent such errors.
Relying on battery power to complete a lengthy monitoring session. While fully charged analyzer batteries are meant to last hours, nothing is more frustrating than to find key power quality events were not recorded because the analyzer shut down. Be sure to keep the analyzer plugged into an AC source for recording parameters when you will be away from the equipment.
Analyzing the Data
Whether observing values real-time on the analyzer color screen or analyzing downloaded data on the laptop back in the shop, an understanding of power quality parameters and their characteristics must be understood. IEEE Power Quality Standards and NFPA 70B are excellent resources to help understand power quality terminology, issues, and corrective actions. To help with data analysis, each manufacturer provides software for its specific test equipment. Here is what to look for when analyzing data:
If experiencing overheating of neutrals, overheating of transformers or motors, nuisance tripping of circuit breakers, blown fuses, unusual audible noise in larger distribution equipment, or if distorted voltage sine waves are found, then suspect harmonics. The magnitude of the various harmonic frequencies and the amount of total harmonic distortion created by the harmonics are the critical factors to determine the severity and correction techniques for any harmonic problem. Measure harmonics at their source, (e.g., VFD, UPS), and expect them to lessen further upstream from the equipment. Sine wave distortion is a good indicator that you should analyze harmonics values (Figures 1, 2, and 3).
Fig. 1. While performing a power quality survey in a commercial office, distortion of the current sine wave on phase “C” at a panelboard indicated nonlinear loads and potential harmonic problems.
Transients are extremely short-duration voltage surges, sometimes incorrectly called “spikes.” The voltage levels achieved during a transient can cause equipment problems ranging from malfunction to destruction. If you’re experiencing unusual insulation failures, record data for extended periods at the equipment. The most severe transients are often caused by nearby lightning strikes. However, they can also be the result of switching of loads.
Voltage sags and swells are the most common type of power quality culprits. While definitions provide specific numbers for magnitude and duration of changes up or down in voltage values, the bottom line is changes of 10% or more in either direction from normal voltage can cause problems. These conditions only need to last from ½ cycle to 1 min. Too high a voltage (swell) can occur when large loads are dropped off the line. Sags, the decrease in voltage, are typically more bothersome and can cause contactors and relays to chatter or drop out completely. Equipment such as PLCs and variable-speed drives can malfunction, and computers may lock up. Observe voltage recordings for sags and swells, and try to relate these variations to changes in plant conditions or operations, (e.g., a chiller or other large load cycling off or on).
Fig. 2. Switching the analyzer to the harmonic function found indications of primarily 3rd and 5th harmonics (180 Hz) on phase “C.” These harmonics can distort the voltage sine wave causing mis-operation of equipment and increasing heat on the neutral conductor — and in motor and transformer windings.
Voltage unbalance between phases on a 3-phase motor can cause current values to reach six to 10 times the value of the voltage unbalance. Because current causes heat — and overheating is one of the leading causes of motor failure — distribution systems should be monitored for unbalance. Unbalance is often the result of single-phase loads cycling off and on, so monitor for unbalance at panelboards and switchboards throughout a typical plant cycle.
Fig. 3. The concern is that the harmonic currents may severely distort the voltage sine wave causing distribution system problems. A normal crest factor (CF) should read 1.41 (Vpeak ÷ Vrms). Here, phase “C” voltage crest factor is 1.47, slightly higher than normal. The crest factor for amperage on phase “C” is 2.09.
The key to success in power quality measurement and analysis can be attributed to success in three key areas. Set goals and plan the survey by reviewing one-line diagrams to determine points to monitor. Learn the functions and features of the test equipment and how to use it to capture the needed values. Finally, know what to look for while observing data whether in the field or after it is downloaded to the computer. Learning how to successfully measure electrical parameters associated with proper operation of equipment is obviously a key step in solving power quality issues.
Overcurrent breakers are triggering without a reason!
Energy bills are higher than expected!
Undesirable Asymmetry
The economic benefits of energy suppliers and their users are strongly dependent on reliability, safety and efficiency of the power system. One of the phenomena that is strongly related to the efficiency of the power system is an asymmetry.
Current and voltage asymmetry degrades the efficiency of the power system. It reduces the efficiency of generation, transmission and distribution of electricity. Ultimately, this results in an increase in the price of electricity. The electricity consumer will also incur additional costs due to a decrease in the efficiency of electrical equipment with the appearance of voltage asymmetry.
Figure 1. Asymmetry in power network causes higher costs.
About Asymmetry
There are three types of asymmetry states in the power grid. It is the current asymmetry, voltage asymmetry and the simultaneous occurrence of both current and voltage asymmetry.
With regard to three-phase systems, voltage asymmetry is defined as a state in which the effective values of the three phase voltages are not the same and/or the angles between them differ significantly from 120 ° as can be seen in figure 2b. Any three-phase system of voltage or current vectors can be decomposed into a sum consisting of three components: zero sequence, negative sequence and positive sequence component. Coefficients that are scaled to the positive sequence can be calculated and are used to describe the quantitatively phenomenon of voltage and current asymmetry.
Figure 2. Phasor graph in a) reflects perfect symmetry in 3 phase power system. All vectors corresponding to 3 phases have the same magnitude and there is 120° between two adjacent vectors. Phasor graph in b) reflects asymmetry in power system. The vectors have different length and the angular shifts are different than 120° between adjacent phases.
Sources of Asymmetry
Voltage asymmetry and current asymmetry are two different types of asymmetry in the power system. The source and nature of these asymmetries are different. Voltage unbalance results from the structural asymmetry of generators (variations in internal construction), transformers, and transmission and distribution lines. In addition, asymmetry can be caused by a voltage drop on the system impedance by asymmetrical currents. In turn, the main source of current unbalance is load imbalance, caused by a single phase load in the distribution system or a fault on the load side.
Voltage asymmetry can also cause asymmetry in the supply current. This is particularly evident in the current of induction motors supplied with asymmetric voltage. For example, a 1% asymmetry in the supply voltage can cause several times greater current unbalance in induction motors.
Negative Effect of Asymmetry
Asymmetry of the current, i.e. the occurrence of the asymmetrical component, causes the dissipation of energy in the elements of the power system in the form of heat. As a result, current asymmetry reduces performance when generating, transmitting and distributing electricity. Therefore, the distribution system cables and wires must be selected taking into account the level of asymmetry.
The negative voltage asymmetry component will contribute to the creation of a magnetic field with the opposite direction in induction motors. It’s like accelerating and braking at the same time. For example, the occurrence of voltage asymmetries of up to several percent is able to significantly increase the temperature of the motor winding and reduce the winding life by more than half. Therefore, the load on motors should be reduced accordingly to compensate heating losses resulting from asymmetry.
Figure 3. High asymmetry caused overheating windings of the engine and fire.
Asymmetry also has a negative effect on three-phase rectifiers and inverters. Voltage unbalance causes asymmetry of the supply current, which increases the temperature of the rectifier diodes and disrupts the operation of the safety devices. Other negative effects occur during transient states mainly caused by faults in the power system.
Transient current asymmetry occurs, for example, due to phase-to-phase faults. In this case, extreme levels of current asymmetry will occur, which will last only for a few seconds. However, this may lead to instability and system failure if the causes are not eliminated on time.
Methods of Mitigating Asymmetry
Adoption of appropriate standards regarding acceptable levels of current and voltage asymmetry and their control.
Applying regulations and standards to the equipment and transmission line.
Structural modifications of single-phase loads.
Single-phase voltage regulators.
Parameters and Devices for Measuring the Asymmetry
The asymmetry is related to the presence of positive sequence and zero sequence components and parameters u0 and u2 (equations shown in the table) are commonly used in measurements in power engineering:
u0=U0/U1 × 100% u2=U2/U1 × 100%
where: u0 – zero sequence asymmetry parameter, u2 – asymmetry of the opposite parameter, U0 – zero sequence component, U1 – positive sequence component, U2 – negative sequence component.
Commonly used devices for measuring power network parameters, including asymmetry, are power quality analyzers. Series of devices named PQM is the series of power quality analyzers by Sonel. All analyzers in the PQM series have the ability to measure asymmetry parameters.
A Few Simple Measures to Perform the Measurement and Diagnostics of Asymmetry:
Connect any PQM Sonel series analyzer according to the manufacturer’s instructions and set the voltage and/or current asymmetry measurement mode.
After the appropriate measurement time, download the data to the computer and use the Sonel Analysis program to plot the time graph of the course of the asymmetry parameters.
If the level of the asymmetry parameter exceeds the threshold of EN 50160 or other standards, take action to mitigate the adverse effects.
Figure 4. Electrician is configuring connected PQM for measurement of asymmetry.
Power Quality Blog 