low votlage ground fault testing
low votlage ground fault testing
(OP)
One of my clients recently made some modifications to the ground fault detection system on a low voltage switchboard. We have ground fault on the main breaker and feeder breakers, with zone selective interlocking. These are draw-out mounted breakers with internal residual ground fault sensing. There are cell position contacts and breaker auxillary contacts wired in the neutral CT circuits. The wiring of these was changed when the bus was re-configured.
I have recommended primary injection testing of the revised ground fault system. Due to the complexity of the system, tseting will take a number of hours. My client insists that the switchboard cannot be shut down for sufficient time to perform the tests. Does anyone have ideas on alternative methods to test the neutral CT circuits without shut down?
I have recommended primary injection testing of the revised ground fault system. Due to the complexity of the system, tseting will take a number of hours. My client insists that the switchboard cannot be shut down for sufficient time to perform the tests. Does anyone have ideas on alternative methods to test the neutral CT circuits without shut down?






RE: low votlage ground fault testing
If the client does not already have a philosophy of applying current-injection testing—associated with the larger issue of periodic power-circuit-breaker maintenance—it may be a hard sell now.
A “no outage” insistence may mean, 1) business is good and he doesn’t want to slow production, or 2) scheme testing is a waste of money and he’s sure it’ll be fine. It is likely the client has not experienced a significant unscheduled outage that had a serious impact on production reports. Adding external {neutral} CTs to power-circuit-breaker lineups is not my idea of ensuring flawless protective-scheme operation.
Unscheduled testing can be tedious if one of two scenarios prevail. Besides the pressure to resolve possible problems when the facility is dark, someone has to intelligently determinate what problem external to the switchgear and its communication scheme has surfaced? If nothing can be found, who decides to go ahead with the “burn-and-learn” approach? [In this case, you are the default expert {whipping boy} for almost any undesirable outcome.]
Do your best to make the client understand that the no-test decision is risky, and may cost at least an order of magnitude more if “debugging” is required in conjunction with an unscheduled outage. Too often managers have to learn from their poor choices, although it [realization of a hard lesson] may not happen the first time around.
RE: low votlage ground fault testing
The neutral CT's are part of the original ground fault detection system. More were added when the system was expanded. I think the design is sound. My client and I have little confidence in the installer however.
RE: low votlage ground fault testing
I would have preferred to do injection testing.
RE: low votlage ground fault testing
My concern is that there are breaker aux contacts and cell position switches involved in the neutral sensor circuits and ZSI circuits. The wiring is somewhat complex and has not been tested.
Would it be possible to do secondary injection at the neutral sensors while they are in the circuit and live? At present, there are no loads connected to the neutral bus. If this can be done without damage to the sensors or trip units, we could at least verify portions of the circuits.