This article looks at ground-fault protection in switchgear, what works and what doesn’t. We’ll look at some of the ways ground-fault protection may inadvertently become inoperable and what steps are needed to prevent this from happening. We will discuss some possible grounding schemes that are incompatible with ground fault sensing and may thereby disable your ground-fault protection equipment.
What does the electrical code require? The Canadian Electrical Code, Rule 14-102 requires that solidly grounded 3-phase, 4-wire services or feeders must have ground-fault protection when an electrical circuit is:
- 1000 amperes or higher at 600/347 volts; or
- 2000 amperes or higher at 120/208volts
Two of the more common ground fault sensing methods are zero-sequence and residually-connected current transformer arrangements for sensing ground faults. Both systems are prone to failure when combined with incorrect grounding.
Zero-sequence ground fault sensing involves a large current transformer surrounding all four of the phase and the neutral busbars in a switchboard. This device is designed to measure the vector sum of all of the currents flowing in the neutral and all phases, which under normal conditions should equal zero, providing a zero measurement.
But when a ground fault occurs, some of the current returning to its source of supply flows along a path outside the zero-sequence current transformer. In that case, the measured vector sum of all the phase and neutral currents is no longer zero. When the fault current reaches its preset ground fault trip level, the controlling circuit-breaker opens to safely isolate the fault.
A residually-connected ground-fault protection system operates in a similar fashion, except that a smaller current transformer surrounds each phase and the neutral busbar. The outputs of all four current transformers are totalized to provide a signal that trips the controlling circuit breaker to isolate ground faults at a pre-set level.
As mentioned above, either of these ground-fault protection methods will fail when combined with incorrect grounding. With either of these arrangements, it’s extremely important that the electrical system neutral be grounded only on the supply side of the zero-sequence or residually-connected current transformers. When grounds are connected on the load side, fault current can enter the neutral bus downstream from the current sensor(s), allowing fault current to return through them. The result? No available signal to trip the breaker, since vector sum of all the current measurements is now zero.
Inadvertent load-side grounding can happen in several different ways.
1. Failure to remove electrical equipment case bonding screws or bonding jumpers from panelboards or switches, thereby creating an accidental ground connection. Electrical equipment installed on the load side of the main service should be carefully checked to make sure that all equipment bonding has been removed. Even when there is no ground-fault protection, equipment bonding to the neutral violates Rule 10-204(1)(d), which prohibits all connections to ground on the load side of the service disconnecting means.
2. In come cases, Rule 10-208(b) does permit the electrical system in a building to be grounded when it is supplied from the main electrical service in a different building. Of course this automatically means that the electrical system is grounded on the load side of the service disconnect in the first building. Although the code does permit this grounding arrangement, this arrangement will not allow ground-fault protection located in the first building to operate correctly. As mentioned earlier, this will result in an unwanted grounding connection on the load side of the current sensing equipment. In this case, an obvious solution is to ground the electrical system only at the electrical service in the first building.
3. When emergency standby power is required in a building, there is usually a solid interconnection between the neutrals of the main electrical service and the standby generator. This presents no problems when generator windings are isolated from the case of the equipment. But this does cause problems when the generator frame and its neutral are interconnected, creating an unwanted grounding connection. Of course, this also violates Rule 10-204(1)(d), even without ground-fault protection In this case, 4-pole automatic transfer switching should be used to isolate the generator neutral when not in use. But when the case and the windings of the generator are not interconnected, standard 3-pole transfer equipment will do.
As indicated in previous articles, you should always consult with the electrical inspection authority in each Province or Territory as applicable for a more precise interpretation of any of the above.