problem with replacement fuse wire repeatedly blowing

dh

I have replaced the fuse wire (5A) in the fusebox three/four times now for the upstairs lights after it went a week ago. Once replaceed, all lights work for several hours (the same evening, say) but are no longer working by the following morning, when the fuse wire needs replacing again. Lights never actually stop working while they are on but just won't come on once switched off (usually the next morning). I would appreciate any thoughts/advice on this. The big mystery to me is why the circuit can work for several hours but not indefinitely.

DESL · Posted: Mon Apr 30, 2007 3:27 pm Post Subject:

You have an overload problem on the circuit in question.

Quote:

The big mystery to me is why the circuit can work for several hours but not indefinitely.

The time/current characteristics of a 5A semi enclosed fuse will allow it to pass ~9A for a couple hours or more before it gets hot and melts.

Has anything been changed/altered/added or work carried out before fuse blew?

You say lights work when on but in morning the fuse has blown is there and outside light on a PIR?

JohnD · Posted: Mon Apr 30, 2007 3:28 pm Post Subject:

Might be mice gnawing the cable. Might be a nail through the wall or floor touching a cable. might be a water leak getting into a fitting (esp. under the bathroom floor). Might be a problem with an external lamp running off this circuit. Might be overloading of the circuit (too many bulbs). Might be old and damaged wiring shorting out as it cools down. Might be incorrect wiring running some other appliance off the lighting circuit (e.g. a bathroom heater). Does anything stop working except the lights?

Do you know how to open a ceiling rose and disconnect some wires and test with a multimeter, and are you willing to do that? If not, ask around friends and neighbours for a recommended local electrician.

As you have a 5a rewirable fuse, I imagine your electrical installation is quite old. What are the cables made of?

DESL · Posted: Mon Apr 30, 2007 3:30 pm Post Subject:

Quote:

Might be overloading of the circuit (too many bulbs)

That would be a lot of lamps at 100W each unless there are 2 or more 500W floods!!

JohnD · Posted: Mon Apr 30, 2007 3:32 pm Post Subject:

One of those # plantations in the loft?

DESL · Posted: Mon Apr 30, 2007 3:40 pm Post Subject:

Quote:

One of those # plantations in the loft?

Hadn't thought of that

dh · Posted: Mon Apr 30, 2007 3:46 pm Post Subject:

Thanks for the help.

DESL - I appreciate the explanation of how the circuit can operate for a limited time - that definitely agrees with my problem. There is no outside light (the "melting" seems to take place even if no lights are on, i.e. overnight). Nothing was changed or added before the fuse blew. But a bathroom fan runs off the same switch as the bathroom light - didn't have problems with this before but perhaps this is the source?

DESL · Posted: Mon Apr 30, 2007 4:59 pm Post Subject:

Can't see the bathroom fan being a problem as I can't see it drawing 9A without the circuit board being fried. I could be proved wrong and if the fan has an isolator switch, then switch it off over night and see if problem persists.
Is it an over run fan i.e. carrys on running after light is switched off?

bernardgreen · Posted: Mon Apr 30, 2007 6:29 pm Post Subject:

Are there any sockets that you have recently started using and also do not work when the light fuse has blown ? If so that socket has been wired to the light circuit which is of course wrong.

RF Lighting · Posted: Mon Apr 30, 2007 7:03 pm Post Subject:

When the fuse has gone, does it look like the fuse wire has melted (top), or like it has blasted away (bottom)?

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downlights

dh · Posted: Wed May 02, 2007 11:27 am Post Subject:

hello - it's not an over run fan, and the fan works fine when the circuit is operational.

There are no new sockets. Everything is set up as before.

It looks like the fuse is blasted away. I have heard it.

I still don't understand how this happens when nothing is actually on.

DESL · Posted: Wed May 02, 2007 12:42 pm Post Subject:

If, as you say, there is nothing switched on and the fuse is popping then there is an intermittent short circuit somewhere in the lighting circuit.

As it's the upstairs lighting circuit you will be able to do a visual check for a damaged/chewed cable WITH THE POWER OFF!! If loft is boarded out look for nails in cables and or squashed cables under boards.

Check around where the cables pass through ceiling especially where you have non standard ceiling roses. i.e. dreaded connection strip wrapped in tape.

If there is nothing visually obvious and also if you don't feel confident to carry out these checks get an electrician in.

JohnD · Posted: Wed May 02, 2007 1:11 pm Post Subject:

One possibility worth checking...

It seems to blow during the night, when (presumably) the occupants are asleep in bed.

At this time, they will not be running taps, flushing toilets. One of the things that can happen when water is not being run, is that a dripping ball cock may cause a cistern (in the loft or a WC cistern) to overfill. The water will (usually) run out of the overflow pipe. There can also be drips or spillage. If this happens in the loft it might drip water onto an electrical connection and cause a short.

Just a thought.

Have as look in the loft for any signs of dripping, and look at the water mark in the tank and any cisterns to see if there are signs of overfilling.

You can do this while you are up there looking for wiring problems. Use a torch so that you can turn off the power before poking about at potentially faulty wiring.

If the house is frequently left unoccupied during the day, and the fuse does not blow at those times, the fault is probably unconnected.

jbalkid · Posted: Wed May 02, 2007 1:48 pm Post Subject:

Which light switch is the last one to be operated (either on or off) at night and is it always the same one?

When you heard the fuse "pop", which light (or appliance) was being switched on at the time.

dh · Posted: Wed May 02, 2007 2:21 pm Post Subject:

Nothing was switched on when it "popped" last night. Interestingly, I notice now that the length of time before the short circuiting takes place is reducing (i.e. no longer in the night but later on the same evening).

Usually it's the bathroom light/fan that is used last at night.

Thanks for all the suggestions & advice - it is much appreciated.