View Full Version : Testing ground fault reduced voltage and blocking capability
Sanjayo
January 19, 2016, 02:37 PM
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
January 25, 2016, 11:02 AM
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
January 25, 2016, 11:22 AM
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
January 25, 2016, 11:24 AM
Yep. Exactly.
SecondGen
January 26, 2016, 10:21 AM
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
January 29, 2016, 02:46 PM
ahh, thanks for the clarification.
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