Ground Fault Systems are designed to limit fault damage to power equipment caused by an arcing or solid fault current from line to ground.
NEC 230-95 “Ground-fault protection of equipment shall be provided for solidly grounded WYE services of more than 150 volts to ground, but not exceeding 600 volts phase to phase for each service disconnect rated 1000 amperes or more”.
There are three types of ground fault protection for solidly grounded WYE systems; Residual, Zero Sequence and Source Ground. The residual is the most common, utilizing a circuit breaker and neutral sensor. Zero Sequence utilizes 1 sensor surrounding all line and neutral conductors. Source ground is the least common, it utilizes one sensor located at the main bonding jumper between neutral and ground.
NEC 230-95(c) Performance Testing “The ground-fault protection system shall be performance tested when first installed on-site. The test shall be conducted in accordance with instructions that shall be provided with the equipment. A written record of this test shall be made and shall be available to authority having jurisdiction”.
This testing is critical, since a failed system could cause a no trip when a fault occurs, or could cause nuisance tripping, or both. At minimum, the following inspections and tests should be performed:
1) Inspect grounding electrode conductor for proper sizing (NEC 250-94) and proper connection to the equipment ground bus.
2) Inspect main bonding jumper for proper sizing (NEC 250-79d) and proper connection in accordance with the manufacturer.
3) Test neutral insulation resistance to ground.
4) Check for proper location of sensors.
5) Test for proper sensor polarity by primary current injection.
6) Systems requiring control power, check control power transformer for proper sizing. Test system at 55% reduced voltage.
7) On multiple source ground fault systems, test for all possible ground fault paths by primary current injection.
8) Test ground fault pickup current at the prescribed setting using primary current injection.
9) Test ground fault trip time at the prescribed setting using primary current injection.
QPT’s Comprehensive Service Program will:
1) Check for design and installation errors.
2) Verify the integrity of the sensing unit.
3) Provide general application or minimum settings if no settings are available.
4) Test All pickup values or as needed by primary current injection.