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What Is The Separation Of A Three Relay Differential Control Relays?

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On October 18th 2017 one engineer and one electrician were continuing work on decommissioning two relays of a three relay differential protection relaying scheme in preparation for installation of a new relay to be used a replacement for the three. The protection scheme used the three differential relays for bus protection during different main and transfer bus utilization scenarios. Two weeks prior, October 4th, preliminary work began on removal of the relays from service. The goal of the day’s work on October 4th was to shift all differential protection to 87B63 while decommissioning 87B62 and 87B61 relays. To decommission these two relays the CT’s from PCB3652 and PCB3552 were shorted at the PCB’s in the field. Both PCB’s were open …show more content…

The electrician doing the removal examined the wiring from the switch and noted to the engineer that it was CT wiring. It was distinguished that it was one of the two CT circuits coming from 3652, one which was shorted and one that was not. The one that was not shorted was previously daisy chained from relay to relay in the previous protection scheme and work on that circuit was thought to be all removed two weeks prior when the engineer was on site for decommissioning. The cabinet in the field was examined and the cables were noted which CT was and what CT was not shorted. Upon returning to the control room the CT cable running to the test switch was determined incorrectly by the engineer to be the shorted CT and the A phase circuit was cut and opened by the electrician. Upon cutting of the wire the circuit was instantly noted to be live by the nature of the “buzzing” of the cut wire. North dispatch desk was contacted to see if PCB3652 could be opened so that the circuit may be examined safely while saving possible damage to the CT’s on PCB3652. When PCB3652 was opened the “buzzing” stopped further confirming the wrong circuit was opened. With the breaker open the cable was examined in the field and it was noted that where the cable came out of the conduit in the panel that the wire had burned the insulating jacket off. The circuit that was opened was a remaining leg of a daisy chain that supplied current for differential

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