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Switchgear Riddle No.7 - Outgoing feeder protection inactivity
Many years ago when I worked in new power plant site for doing of auxiliary system commissioning, we fronted a wonder accident in low voltage switchgear. One of withdrawable outgoing of switchgear (200 A feeder) burned due to a sever fault. Related outgoing protection device was a 200A molded case circuit breaker (MCCB) which could not have suitable action against mentioned fault. The space of drawable compartment failed due to sever arcing current. That sever fault current interrupted via, 4000 A - 4 pole ACB, MCC incoming with a long delay time instead of related outgoing protection. Indeed the incoming circuit breaker had prevented to fault extension in switchgear but it could not prevent to outgoing failure. Later investigation and verification shown, 200 A MCCB which made by an effectual manufacturer had suitable protection setting and it must be tripped against huge fault current instantaneously.
How can you explain that outgoing feeder protection inactivity?
Author : Hamid - From: Iran- Firouzabad in Fars
 
#1
Tue, March 16th, 2010 - 11:20
Faults resulting from spark and thereafter arcing ,  in spite of very high heat and temperature , due to impedance of arcing can limit value of fault current . Probably the value of earth fault current has been below the setting of 200 Amp. MCCB . Instead , the back-up earth fault relay of incoming breaker has acted . Weakness of withdrawable pieces ( probably load stabs)  may cause the problem .  
Author : amir khashayar - From: iran
 
#2
Wed, March 17th, 2010 - 08:30
I am not sure , the problem has been arisen from load stabs ( outgoings) . It seems , it has been arisen from line stabs (incoming and on upstream side of 200 Ampere MCCB) .In this case MCCB is not bound to interrupt the fault current  on its own upstream side.  
Author : GLAVIZH - From: IRAN
 
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