Delayed action fault

[JamesP]

Hello Guys,

There

 

[Doc Hudson]

Welcome to the forum. The rate of rise of the fault current and thus the speed at which the fuse blows will be proportional to the resistance of the fault path (or connected loads) and the characteristics of the protective device, e.g. fuse wire or MCB switch on modern fuse boxes. Fuse wires generally take longer than MCB's. Also if there is damp involved in the fault path, (or a dead rodent across the circuit) it may take longer for the fault current to increase sufficienty to blow the fuse. It sounds to me as though you need your circuit cables tested first of all

Doc H.

 

[NozSpark]

As Doc says proper testing should be able to narrow down the fault to a single section of cable

 

[SteveT]

I agree with Doc

 

[ProDave]

My money goes on an intermittent short.

If it were overloading (i.e too many lamps) and it took 2 minutes to blow, then that would be the fuse wire heating up and melting, and I doubt it would make much of a bang.

the fact it goes with a bang, means a LOT of current is passing to blow the fuse.

Does it blow when you walk over a particular bit of florr? could be a pinched cable, and when you step in a particular place it shorts. If you can find a spot that triggers it, that narrows down how much floor to lift.

An electrician with an insulation tester should be able to find the faulty cable fairly quickly. If it was wired 35 years ago, it's very likely wired as a spider network back to one big junction box under the floor. If you can find that, then he will have access to all the cables and should be able to identify the faulty one.

 

[JamesP]

Thanks for that Doc H and all other responders

In my original question I avoided giving any clue as to what I thought was the problem

 

[sb_89]

Interesting fact about squirrels ..... They have a very thick layer of enamel on their teeth, effectively insulating them from the live cable they are gnawing through. Also I once opened a loft hatch to have one run full pelt at me...., I fell off my steps trying to get away lol

 

[NozSpark]

James, remember that if the tiled floor is an upstairs room that you can also get to any cables from below.. its often a cheaper and quicker way to get at cables/joints such as you have.

But don't forget that through testing should be able to narrow the fault down, do that before any destructive works.

 

[Doc Hudson]

As a point to note for future repairs. Excluding, solder, crimp or other specialist sealed joints, all joints, (junction boxes), should be accessible for inspection & testing and not buried or concealed in inaccessible locations even if wrapped in any sort of tape.

Doc H.

 

[JamesP]

Thanks Guys,

Still pondering this,

ProDave: Yes the fuse does go off with a bang, so it

 

[Andy™]

had a similar fault to this a while back. turned out to be a dodgy connection (helped along with some moisture) shorting L-N, but then blowing clear. then shorting. then blowing clear

 

[NozSpark]

If the circuit only serves one room then you could just isolate it and rewire the lights (in trunking) off the socket circuit via a fused spur,,,, I'd also consider an emergency light or two so they still have light if the socket circuit trips though

 

[betty swollocks]

As already said

If you suspect the fault lies in an inaccessible space it may be quicker to isolate that length of faulty cable and connect the remaining lighting circuit via another option

Perhaps joining from another lighting circuit if practical

Perhaps joining via fused spur off socket circuit

Perhaps rerunning a new length of cable as replacement

I have had a problem where a bathroom installer screwed through a cable which didn't blow any fuses so he thought it was old cable and so tiled over it

Come nighttime owner turns upstairs lights on, blows fuse

I took a look, whole house redecorated with no real option of running cable through house to loft space but then I found the old shower cable disconnected in fuse board, found other end in loft, quick test showed it to be usable so I used that as my new supply

This could be a quick repair if you use somebody who knows what he is doing

 

[mikep]

Had a cooker feed short tripping the Mcb, carried insulation test etc, 35 ohms across L-N cable buried in wall all the way to kitchen, solid floors so ran new SWA on outside wall all working now, 3 weeks later got a call for a water leak from shower through ceiling opened ceiling to gain access to soil pipe found that someone had joined old cooker feed with a JB which had filled up with water from the shower trap, so that was the original short on the cooker feed.

 

[M107]

I once opened a loft hatch to have one run full pelt at me...., I fell off my steps trying to get away

lol

I had the same happen with a rat...............then the pest control guy arrived (commercial property) gave me a right old ribbing & said it was cobblars a rat would run from you not at you, up the steps he goes pushes up the hatch with his head....................screams like a girl and falls down the steps......yep roland didnt know he was supposed to run away & not over the blokes shoulder. :slap

 

[boltonsparky]

If the wiring's 35 years old it might be time to rewire the property.

BS3036 (rewireable) fuses are very unforgiving and a situation has to be pretty bad for them to blow. A multitude of similar faults could be slowly developing. 

I'd recommend a full EICR and take it from there. 

 

[sellers]

If the wiring's 35 years old it might be time to rewire the property.

BS3036 (rewireable) fuses are very unforgiving and a situation has to be pretty bad for them to blow. A multitude of similar faults could be slowly developing. 

I'd recommend a full EICR and take it from there. 

Wise words, fully echo all of that.

 

[batty]

Four month old thread it makes you wonder what the outcome was.