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I wasn't sure whether to ask this here or on https://electronics.stackexchange.com/ but this seemed the more appropriate place, please migrate it if not.

I'm in the UK and I've got a wall mounted connection unit (see picture below) with a 5 amp fuse in that leads to a socket outside and a lamp post in the garden.
The outside socket is used for plugging in a catering van to charge the battery, but also to use the sockets inside and power the 12v lighting when it's parked on the drive.

Now the fuse blows sometimes if you plug two things into the sockets in the van, but today I found it had blown sometime between last night and this morning (it was fine last night). When I went to change the fuse it blew the new one straight away. I had to unplug the van and remove the bulb from the outside lamppost before I could put a fresh fuse in and not have it blow.
I can still charge the van (which is the main use for the socket outside) but it would be nice not to have to worry about it.

So my question is, can I put a 13 amp fuse in the unit instead of a 5 amp one? Will this cause any problems (like short circuiting the houses fuse box instead of merely blowing the fuse in the unit) or be dangerous?

wall mounted connection unit

  • Is the lamppost connected to the socket? – Chris M. Jun 1 '17 at 16:41
  • @ChrisM. It's on the same length of wire, but splits off before the socket. – adaliabooks Jun 1 '17 at 17:23
  • Why are you messing with the bulb on the outside lamppost? Even if you absolutely insist on continuing to use archaic incandescent lights, it still won't account for more than 1/4 amp out of 5 amps. And if you have gone LED, more like .05 amp if even that. It's just not a factor (unless it's shorted out) and can be for all practical purposes ignored. – Harper Jun 1 '17 at 18:33
  • @Harper Because it was the only other thing connected to the circuit. It is only about 40w (halogen I think) but I was thinking the wire out to it might be damaged or faulty in some way (it's got a light sensor and stuff on it which has been playing up) and causing surges. – adaliabooks Jun 1 '17 at 18:37
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In general, NO. Replacing a fuse with a higher-amp fuse is a very bad idea and can lead to fires.

Fuses (and circuit breakers) are rated such that they blow or trip before any part of the circuit gets to a dangerous current. You rate the fuse or breaker based on the lowest current rating of all the devices and wires in the circuit .

In your case, the manufacturer of the connection unit determined (somehow) that the "weakest link" in that circuit can only draw 5A before it exceeds its limits. Putting in a 13A fuse would thus allow that weakest link to draw almost 3 times what it may be designed to, which leads to increased heat and then fires.

There is a reason your fuse is tripping and you need to figure out what that reason is. Perhaps there is a problem with your lamp post, the bulb, one of the sockets, or the van charger. Fix the underlying problem, and you won't have any more blown fuses.

  • That's what I'd thought, but I wanted to check and make sure it wasn't as easy as putting a higher fuse in. The wiring outside is a bit of a bodged job and I imagine the issue is there somewhere (or just using too many high power things on it at once) but I'll see if I can get someone to look at it. – adaliabooks Jun 1 '17 at 17:26
  • @adaliabooks Indeed, it could be as simple as trying to plug in too much. A lot of circuits in older houses especially have trouble coping with all of the electricity-drawing devices that are common nowadays. The temptation is to simply put in a bigger fuse (or something similarly sized, like a penny), but that is how a lot of house fires start. – mmathis Jun 1 '17 at 17:32
  • The puzzling part is that we can plug in a few things and have them run no problem half a dozen times, then all of a sudden doing the same thing will blow the fuse. Last night we had a little camping fridge plugged in (to the 240v plug as it can do either) in the van which we have done before with no problems, but presumably that was part of the problem. The 5 amp fuse doesn't seem to be rated very highly so it might just take a surge or exposed wire or something to blow it.. – adaliabooks Jun 1 '17 at 18:08
  • @adaliabooks I'd suggest asking a new question to determine why your fuse might be tripping some times but not others – mmathis Jun 1 '17 at 18:24
  • I might do, but it's probably easier to get someone to come and look at it if it keeps doing it or gets worse. Thanks for your help, I'll mark your answer as correct. – adaliabooks Jun 1 '17 at 18:28
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The question is why was a 5A fuse put in there. There are a couple of possibilities.

One possibility is that the cable is too small for a 13A fuse, but if that is the case then the installation is a serious bodge.

Another is that the light itself specified a maximum fuse/breaker rating. Unfortunately I suspect the installation instructions for the light are long gone.

Another is that the installer simply didn't think it was a good idea to put the light on a circuit fused above 6A (6A is the normal breaker rating for lighting circuits).

Personally what I would do is inspect the cable, if it is of sufficient size I would increase the fuse to 13A but add a second fuse in the line to the light since I could not be confident what if anything the installation instructions for the light said about maximum fuse/breaker ratings.

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