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DC power circuit protection

DC power circuit protection

DC power circuit protection

(OP)
We have a large 125VDC source that feeds control relays, indicating lights, power relays, etc. I notice on the control schematics that in some cases both the (+) and the (-) are protected by a fuse or circuit breaker, but in other cases, particularly on the negative side of a coil or an indicating lamp the (-) side is not fused.
And,
We're installing a new computer cabinet that has a new power supply converting to 24VDC that is fed from this 125VDC source, and I notice the engineer didn't fuse the  incoming negative wire, but did the positive. The positive side of the 24VDC is fused. The negative side of the 24VDC supply is connected to ground, so I'm sure that is why this side isn't fused. Ok.

This inconsistenty is confusing...can anyone clear this up?

 

RE: DC power circuit protection

Fusing both negative and positive often implies a system that is floating from ground. The grounded polarity is seldom fused in a grounded system. If that is the case, using a ground connection for one terminal of a floating supply is a challenging concept. Conversely, if the supply is grounded there are safety implications to fusing the grounded conductor. Many codes discourage the use of a ground connection for one conductor of a power circuit. Often it works fine but the law of unintended consequences is always waiting to bite your but.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: DC power circuit protection

(OP)
Waross,

On further examination, the 125VDC source is derived from 60 batteries in series together. I don't see where it's connected to ground on the negative side. Therefore, we have an ungrounded system. For consistency sake wouldn't it seem prudent to fuse both the positive and the negative to all loads?  

RE: DC power circuit protection

That is normal practice as I understand it. You may be stuck with a case of "What is, is!".

Bill
--------------------
"Why not the best?"
Jimmy Carter

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