Large earth fault current mysterie

[avkester]

Dear All,

I am posing a question regarding the occurence of an earth fault trip (the G function on a 4 pole circuit breaker, set at 250 A).

The system is 3-phase WHY TN-S and the load uses all 3 phases without the neutral.

The earth protection trips on a Siemens 3WN6 and the reading shows the following currents: L1 = 260A, L2= 253A, L3 = 0A, N= 0A, PE =250A

The strange thing is the zero current on L3.

Since there is a faulty neutral-PE connection (apart from the one at the incoming supply point) in the system this may explain the excessive earth current experienced.

Could someone explain what happens to the neutral (and effectively in this case also the neutral since they are joined) when there is a phase loss on L3 in this situation (neutral not used by the load) ?

 

[electricuwe]

The breaker doesn't observe the current in the earth connection although it displays it. The current on PE is calculated be summing up all the other currents. If PE and N are connected at the source and somewhere else a fraction of the current flows in the PE-conductor (or, thats worse, in gounded parts of your installation) instead of the N-conductor. The breaker has detected this and tripped correctly.