8.4.4.5 TROUBLESHOOTING THE DIFFERENTIAL CURRENT TEST
Under what conditions will the differential current test fail by indicating a non-zero current?
A failure in the differential current test can occur if there is a Phase rotation at one of the stations or if the phase
relationship between stations shows a 180 degree inversion. If there is a phase rotation the value will be non-
zero and above 1 amp. Phase rotation occurs when the 3 phases are 120 degrees apart and in sequence, but
instead of having phases A, B, and C connected in the correct order, the unit is wired (B, C and A) or (C, A, and
B) or some other phase rotated combination.
Another condition which will fail the differential current test is when PHASE A on one relay is connected to a
different phase on one of the other relays.
If the relay displays a value much greater than 0 amps, then there is a problem that needs to be corrected be-
fore the relays may be put into service.
Verify the Addressing and communications between stations,
Verify that the phases are connected to the correct inputs,
Verify that the phase relationship between stations is correct,
Check for current on 3I0.
Be sure that there are no tapped loads in the system affecting the test.
NOTE
BRINGING A RELAY ON LINE WITHOUT CORRECTING A FAILED DIFFERENTIAL CURRENT
TEST WILL CAUSE THE RELAY TO MISOPERATE.
RFL 9300 RFL Electronics Inc.
October 20, 2004 8 - 30 (973) 334-3100
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