Low-Voltage Switchgear and Circuit Breakers:Testing of Ground Fault Sensing and Relaying Equipment

Testing of Ground Fault Sensing and Relaying Equipment

Ground fault sensing and relaying equipment is covered by UL standard 1053. It classifies ground fault protection into class I and class II. Class I

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ground fault protection is intended to be used with disconnecting devices at high levels of fault current, whereas class II ground fault protection is used with disconnecting devices with limited interrupting current capacity. This testing application guidance is directed toward class I ground fault relaying.

In accordance with NEMA Publication PB2.2-2004 (Application Guide for Ground Fault Protective Device and Equipment), manufacturers are required to perform design and production tests. The design tests include calibration, temperature rise, overvoltage, overload, dielectric withstand, endurance, and the like. The production tests are conducted to determine if calibration settings are within performance limits, control circuits are working properly, and current sensors have correct turns ratio. The field testing of GFP relaying equipment is discussed in the following section.

Preparation for Fielding Training

• Review the electrical drawings for the power system, as well as the manufacturer equipment drawings, to ensure that ground fault equipment is installed as designed.

• With the power off, remove the disconnect link on the switchboard to isolate the neutral of the wiring system from both supply and ground. Measure the insulation resistance of the neutral to ground with the main disconnect open to ensure that no ground connections exist downstream of the GFP devices being checked. For a dual fed (double-ended) power system, remove all the disconnect links to isolate the neutral from both the supply and ground before measuring the insulation resistance.

• Visually inspect the wiring system to confirm there is an adequate grounding connection at the service equipment upstream of any ground fault sensor, and that the neutral connection is run from the supply transformer to the service equipment in accor- dance with the National Electric Code. Where dual power sources are involved, confirm that the main grounding connection at the service equipment is in accordance with manufacturer’s recommendations.

• Once these steps have been accomplished, return all neutral and ground connections to their normal intended operating condition.

Field Testing

• Field testing should be limited to only those tests that are necessary to determine that the installation is correct and the ground fault protection system is operational. Because of the many variables involved, field testing cannot be considered as an accurate check of the calibration of any sensing relay. Field test current sources can

introduce errors owing to nonsinusoidal wave shapes, power source regulation problems, and metering accuracy. In addition, timing measurements often include additional delay times owing to the use of auxiliary relays and timers. Field testing should be limited to a go/no-go type of testing, which confirms the serviceability of the system involved.

• Before field testing is initiated on any ground fault sensing and relaying equipment, the manufacturer’s installation and instruction literature should be reviewed and understood. The manufacturer’s field test recommendations should be followed. Although some manufacturer’s test setups may be difficult to perform in the field, the configuration is very important in order to obtain good results. If a particular device is self-powered, considerable current (100–700 A for a 4000 A device) may be required to activate it. This is especially true where multiple sensors are being used in a vector summation scheme. It should not be assumed that inducing current on a ground return or neutral sensor alone will be sufficient to activate the device and get accurate time–current characteristics. This practice can lead to finding a device defective when nothing is actually wrong with it. However, as noted below, there are systems that use external control power for which this practice is acceptable.

• Ground fault sensing and relaying equipment utilizing either ground return or vectorial summation sensing methods can be checked in the field by passing a measured test current directly through the sensing transformer or test windings. To confirm the proper func- tioning of the equipment while it is installed in the switchboard or panelboard. The following tests can be performed.

Simulated ground fault test using sensors without built-in test windings

• Turn off all power to the switchboard section or panelboard. Set the relay to its minimum current setting.

• Loop a test coil of wire having sufficient current-carrying capacity through the sensor window. Prefabricated multiturn test cables may be used for convenience.

• Provide control power only and close the disconnect associated with the GFP device being tested.

• Apply sufficient test current so that the ampere turns of the test wind- ing numerically equal or exceed 125% of the relay current setting. The relay should trip the disconnect. Immediately return the test current to zero.

• Turn off all power, remove the test winding, and restore all equipment to the operating condition.

• Reset the relay to the predetermined setting, reestablish control power, and turn on main power as needed.

Simulated ground fault test using sensors with integral (built-in) test windings A go/no-go test for the proper tripping of the GFP devices and the intercon- nections between the sensor, the relay, and the disconnect mechanism can be made by following manufacturer’s test instructions. The manufacturer usually provides for a test current >125% of the maximum current setting, so a test can be made anytime without disturbing the current settings. If there is any question concerning the ability of the GFP device to operate at its minimum setting and for low ground fault currents, a test as described in simulated ground fault tests using sensors without built-in test windings can be made immediately following installation. Periodic tests using the manufacturer’s test circuit should be adequate after installation.

Equipment with built-in test circuitry but without a built-in test winding Following installation, the GFP devices should be tested in accordance with “Simulated ground fault test using sensors without built-in test windings” to confirm that sensors and interconnections to the ground fault relay are functioning. Thereafter, the manufacturer’s test circuit can be used to check the operation of the GFP relay and the tripping circuitry.

Test buttons and indicators

Operate test buttons to check the functions described in the manufacturer’s instructions. Pilot lights and other indicators should signal ground-fault tripping or other functions as described in the manufacturer’s instructions.

Zone selective interlocking function

The manufacturer should be consulted for specific instructions when this test is to be performed in the field.

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