Any Protection Relay used on Neutral (Ground Fault Protection of Double ended System):
The use of four poles or three poles CB will depend on system protection and system configuration.
Normally in 3 phase with neutral we just use 3 pole CB and Neutral is connected on common Neutral Link but if application of 3 pole will affect the operation of protective relay then we must use 4 pole CB.
System evaluation has to be required to decide whether three-pole circuit breakers plus neutral link can be used or four-pole breakers are required. If unrestricted ground fault protection is fitted to the transformer neutral, then the bus section circuit breaker should have 4-poles and preferably incomer circuit breakers should also have 4-poles because un cleared ground fault located at the load side of a feeder have two return paths. As shown in fig a ground fault on a feeder at the bus section “A” will have a current return path in both the incomers, thus tripping both Bus. The sensitivity of the unrestricted ground fault relay is reduced due to the split current paths.
For System Stability
In an unbalanced 3 phase system or a system with non-linear loads, the neutral gives the safety to the unbalanced loads in the system and therefore It must not be neglected. In perfectly balanced conditions the neutral functions as a safety conductor in the unforeseen short-circuit and fault conditions. Therefore by using 4-pole MCB will enhance the system stability.
4 Poles will be decided after knowing the Earthing Systems (TT, TN-S, TN-C, IT).
IT (with distributed neutral) System: The Neutral should be switched on & off with phases. Required MCB: TPN or 4P MCB.
IT (without distributed neutral) System: There is no neutral. Required MCB: TP MCB.
TN-S System: Required MCB: TP MCB because even when neutral is cut off system remains connected with Ground.
TN-C System: Required MCB: TPN or 4P only, because we cannot afford to cut neutral doing so will result in system loosing contact with Ground.
TN-C-S System: Neutral and Ground cable are separate. Required MCB: TP MCB Because Neutral and Ground cable are separate.
TT System: Ground is provided locally. Required MCB: TP MCB because ground is provided locally.