The role and functions of earth leakage protection
This type of protection systems is defined by the NEC (National Electrical Code) to provide fire protection for electrical installations.
To provide protection against fire:
- The NEC defines the use of a very low sensitivity differential called GFP
- The IEC 60 364 standard uses the characteristics of the TT system combined with low or high sensitivity differential RCDs.
These protections use the same principle. Fault current measurement with:
- A sensor sensitive to earth fault or residual current (earth fault current)
- A measurement relay that compares the current to the setting threshold
- An actuator that sends a trip order to the breaking unit on the monitored circuit if the defined threshold is exceeded.
1. Visual and mechanical inspection
- Compare equipment nameplate data with drawings and specifications.
- Inspect components for damage and incorrect conductor polarity or routing:
- Check that the ground connection is made in front of the neutral disconnect link and on the line side of any ground fault sensor.
- Check that the neutral sensors are connected with correct polarity on the primary and secondary.
- Check that all phase conductors and neutral cross the sensor in the same direction for homopolar protection systems.
- Check that the ground conductors do not pass through the zero-sequence sensors.
- Check that the grounded conductor is securely grounded.
- Verify that the bolted electrical connections have high resistance by using one of the following methods:
- Use of low resistance ohmmeter according to Section 7.14.2.
- Check the tightness of the accessible bolted electrical connections with a torque wrench calibrated in accordance with the manufacturer’s published data or in Table 100.12.
- Perform thermographic readings in accordance with section 9.
- Check the correct operation of all the functions of the self-test panel.
- Check that the control transformer has sufficient capacity for the system.
- Set the pickup and timeout settings according to the settings defined in the owner’s specifications. Record the appropriate operations and test sequences required by National Electrical Code NFPA 70, section 230.95.
2. Electrical tests
- Perform resistance measurements through bolted connections with a low resistance ohmmeter, if applicable, according to Section 7.14.1.
- Measure the insulation resistance of the system between neutral and earth with the neutral disconnect link temporarily removed. Replace the neutral disconnect link after testing.
- Perform an insulation resistance test on all control wiring against the ground. The applied potential should be 500 volts dc for 300-volt cable and 1000 volts dc for 600-volt cable. The duration of the test should be one minute. For units with solid static components or control devices that cannot tolerate the applied voltage, follow the manufacturer’s recommendations.
- Perform the following sample tests using the primary injection:
- Verify that the relay is not operating at 90% of the pickup setting.
- Check that the pickup is less than 125% of setting or 1200 Amps, whichever is less.
- For summing type systems using phase and neutral current transformers, verify the correct polarities by applying current to each pair of phase-to-neutral current transformers.
This test also applies to molded case circuit breakers using an external neutral current transformer. </ p>
- The relay should operate when the current direction is the same with respect to the polarity marks in both current transformers.
- The relay should not operate when the current direction is opposite to the polarity marks in the two current transformers.
- Measure the relay delay at 150% or more of a start-up.
- Verify that the reduced control voltage trip capability is 55% for AC systems and 80% for DC systems.
- Check the blocking capacity of the locking systems zone.
3. Test values
- Compare bolted connection resistors to values of similar connections.
- Bolt tightening torques shall be in accordance with Table 100.12 unless otherwise specified by the manufacturer.
- The microhm or millivolt drop values should not exceed high levels of the normal range indicated in the data published by the manufacturer. If manufacturer data is not available, look for values that diverge more than 50% from the lower value.
- A neutral-to-earth insulation system must be a minimum of 1.0 megohm.
- Insulation resistance values for control wiring should be a minimum of 2.0 megohms.
- The timing of the relays should conform to the manufacturer’s specifications but should not exceed one second at 3000 amps.