Could someone help me with this.
In my building there is PDU (fed from UPS with solid ground neutral) having 125A TPN MCCB, which is followed by four 63A TP MCBs downstream. Each 63A TP MCB feeds one 8-way DB. Each 8-way DB has 32A TP Incomer, followed by 03 R-Y-B DP isolators of 20A each. Further, each DP isolator is followed by 16A SP MCBs.
One fine day, one particular 8-way DB, which feeds the various workstation, printers and LCD TVs in a floor, started experiencing erratic trips along with 125A upstream MCCB. On the first instance, the 8-way DB incomer tripped along with 125A MCCB. On second instance, Y-phase DP isolator tripped along with 125A MCCB. On the third instance R-phase DP isolator tripped along with the 125A MCCB.
Each subsequent trip occurred within the duration of 30-60 minutes. During the first and third instance, the 125A MCCB Ground Fault was set to OFF. On the second instance the ground fault was set to 20%.
Post restoration of circuit and after having switching ON all connected load on the PDU, the current measured on each line was 14, 9, 20. Using power clamp meter, I also found the net current at the output terminal of PDU to be between 1.2 to 1.5 Amps, and the total harmonic distortion (THD) in each line was between 40% - 60%.
Since last 10 days we did not have any trip after the initial incident.
We know there is need for coordination checks. Besides that what could have caused erratic tripping behavior. Could harmonics be the culprit or any spurious load or neutral.
In my building there is PDU (fed from UPS with solid ground neutral) having 125A TPN MCCB, which is followed by four 63A TP MCBs downstream. Each 63A TP MCB feeds one 8-way DB. Each 8-way DB has 32A TP Incomer, followed by 03 R-Y-B DP isolators of 20A each. Further, each DP isolator is followed by 16A SP MCBs.
One fine day, one particular 8-way DB, which feeds the various workstation, printers and LCD TVs in a floor, started experiencing erratic trips along with 125A upstream MCCB. On the first instance, the 8-way DB incomer tripped along with 125A MCCB. On second instance, Y-phase DP isolator tripped along with 125A MCCB. On the third instance R-phase DP isolator tripped along with the 125A MCCB.
Each subsequent trip occurred within the duration of 30-60 minutes. During the first and third instance, the 125A MCCB Ground Fault was set to OFF. On the second instance the ground fault was set to 20%.
Post restoration of circuit and after having switching ON all connected load on the PDU, the current measured on each line was 14, 9, 20. Using power clamp meter, I also found the net current at the output terminal of PDU to be between 1.2 to 1.5 Amps, and the total harmonic distortion (THD) in each line was between 40% - 60%.
Since last 10 days we did not have any trip after the initial incident.
We know there is need for coordination checks. Besides that what could have caused erratic tripping behavior. Could harmonics be the culprit or any spurious load or neutral.