Surge protection on a single phase of three phase system - why?

[eleclearner]

I was working (mostly learning) on a power system the other day. Single primary transformer winding 22kV and 2 secondary windings, one is WYE one in DELTA - for load balancing i think. Secondary WYE neutral is not earth. For the following you can assume i'm talking about the WYE side of the transformer secondary.

The secondary side is 610V used to supply DC motors (big ones like 1 mW). What i noticed is on the secondary line there was surge suppressors for each DC motor (on the AC side i.e. the transformer secondary). What i don't understand is the surge protectors were from 1 phase to earth. But always on the same phase???? Say phase A. Why does this work? I thought they should be on each phase. All the surge protectors are then in parallel.







As a further question...so one of the surge protectors failed and it was shorted to earth. So this basically means that the A phase was at earth potential. This didn't effect the plant operation although there was 3 1mW dc motors that blew while the plant was running unknowlingly with the A phase at earth potential. Could this have caused the brushed DC motors to blow? It's hard to picture for me because the neutral and the earth are not tied together at the transformer + the neutral is unused.

 

[waross]

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Bill
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"Why not the best?"
Jimmy Carter