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Inline fusing 12v feed

john2k

Jun 13, 2012
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Jun 13, 2012
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If I have a 12v supply going from a car battery to the common pin of multiple relays and I fuse it with a single correctly rated blade fuse at the battery end, do I need to also put blade fuse at the point of each relay? A few relays will be further back in the cars trunk and few at the front.

Also, when using a supply to energise the relay on pin 86 of a relay, these supplies are fused by the car already for example the ignition feed is already fused from the fusebox, do I need to fuse the supply on pin 86 again as it enters the relay?

Thank you.
 

duke37

Jan 9, 2011
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I do not think that the relay coil would be likely to take excess current. In my experience fuse failure is because rodents strip the wire insulation and the bare wire then rubs against the chassis.
 

Bluejets

Oct 5, 2014
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If I have a 12v supply going from a car battery to the common pin of multiple relays and I fuse it with a single correctly rated blade fuse at the battery end, do I need to also put blade fuse at the point of each relay? A few relays will be further back in the cars trunk and few at the front.

In addition to what you have, I'd be inclined to fuse each relay as each circuit exits each relay(87) with a fuse for that circuit.
Having only the one line fuse at the source means it would have to be rated at the maximum of all circuits and would probably only blow when some dead short is present.
Even then, you loose the lot.
With your present arrangement it protects the line up to all the relays but with outgoing smaller fuses on each relay output, if that circuit goes down due to a fault , then chances are it will only blow that one particular fuse.

For the same reason, it could be argued the same applies on pin 86 control circuits, but as mentioned above, rarely would this be a problem as coils rarely short out, mainly the wiring chafing somewhere or other.
 
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