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Electrical anomaly

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Just fitted a small 4-way fuse box in the back of the 110. Took a line off it to a small USB power socket I'm fitting - the kind that has a built-in volt meter. Despite having no fuse in the 4-way I'm getting 11.5V at the USB socket. Put the fuse in the holder on the 4-way and I get 12.7V. Metering any of the four outlet tabs on the 4-way shows voltage (11.5 or 12.8) fuse out/fuse in respectively, and the same with the meter at the USB socket.

I'm not any kind of expert but I assumed that a circuit minus a fuse should be dead - 0V?  Do I have a problem?


Edited by Jocklandjohn
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Feed comes from a main distribution board which has switched & unswitched feeds (so not directly connected to the battery). Its on the switched side so only active when ignition is on. Its a one-wire 4-way box (live connection only) and I;ve securely earthed it to the chassis at the rear. I've never seen a fuse box that passes current witout a fuse - what it does have though and which may be significant is some led's that illuminate should a fuse blow - would that circuit account for this?



Edited by Jocklandjohn
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It’s because without the fuse you’d normally have an open circuit, but in this case even without the fuse, the led is there allowing current to flow and you therefore have voltage on the output side of the fuse connection.

The led is in the feed but it’s normally short circuited by the fuse and so no current flows through it. With the fuse out or blown, the led can pass current and therefore illuminate.

You then see the lower voltage as the led drops a little when it conducts. 

Edit: should add that the usb socket is providing enough load to provide current through the led but I assume not enough to illuminate it. 

Edited by Peaklander
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