Testing ground fault reduced voltage and blocking capability

[Sanjayo]

Blocking capability and reduced voltage tripping are two tests listed for ground fault relays in NETA-ATS, can someone explain how these two tests are performed?

[alexanderchaney]

Reduced voltage is accomplished with reduced control power voltage (we typically use 55%). This simulates what the ground fault control relay would have to function with if one of the legs feeding the control power transformer was grounded.
The blocking is, i believe, tested to ensure that the switch does not close back in on an active ground fault, or at least on an unacknowledged one.

[Sanjayo]

Thanks for the reply. Is this test done with a simple variable voltage source? For example, if I had 120V AC control power, at 55% my AC variable source would be set to 66V, then I would push current through CT and the relay should still trip?

Reduced voltage is accomplished with reduced control power voltage (we typically use 55%). This simulates what the ground fault control relay would have to function with if one of the legs feeding the control power transformer was grounded.

[alexanderchaney]

Yep. Exactly.

[SecondGen]

NETA references zone-interlocking for the blocking capability test. Unless a restraint signal is used, any ground fault relay with zone interlocking will trip instantaneously regardless of the delay setting.

The restraint signal is normally applied with a small jumper between specified terminals on the ground fault relay -- consult the relay manufacturers literature for this procedure. Perform a time test with and without this signal applied to ensure proper operation.

The blocking is, i believe, tested to ensure that the switch does not close back in on an active ground fault, or at least on an unacknowledged one.

[alexanderchaney]

ahh, thanks for the clarification.