Fault find domestic lighting circuit

[parametrics]

Delayed blow.
Circuit blows when all lights are off.
Old circuit with 5 amp fuse wire.
Downstairs lighting circuit. No recent electrical work done on the house.
Question is how to test?
Suspect rodents in loft .... does a downstairs lighting circuit go up that far?
Suspect fault near the box. What could cause this to blow when not loaded? 

 

[UNG]

Sounds like it is an old installation and possibly the cable insulation is breaking down

Unless you have the right test equipment finding the fault will be difficult

If you post a location someone on here may be able to help you if you need a spark

 

[parametrics]

Location is Horsham, West Sussex. Yes I obviously need a sparky, preferably someone that fixes the problem with the minimum of fuss.

The main question is why would an unloaded circuit blow in the first place. It must be the main live lead out of the box.

 

[parametrics]

The mechanics of what is happening here. Yes I think you are correct that the insulation is on its way out but what is creating this slow blow effect. length of time = overnight.

 

[UNG]

While the circuit has no load being used the cables to the switches remain live from what you have said so far this installation sounds like it is quite old and it will probably be a cable or cable joint / junction breaking down. The fault has a high resistance and therefore does not instantaneously blow the replacement fuse wire

The old wire fuses are not that reliable and can under the right conditions carry a fault current for a number of hours before they blow, this would be noticeable by the fuse carrier becoming warm over a number of hours

It sounds like this fault is something that needs looking at sooner rather than later

 

[Doc Hudson]

Question is how to test?
 

Standard 'dead' circuit tests as described in BS7671 wiring regulations. i.e. continuity of conductors, polarity, insulation resistance tests, and/or splitting circuit down into smaller sections to prove/disprove where the fault/faults lie. leave it unresolved and it could start a fire IMHO.

Doc H.  

 

[Geoff1946]

If it was mine, still on rewireable fuses, and randomly blowing them, finding and understanding the precise fault mechanism would be my last thought.

Direct your thoughts towards  re-wiring, before your house burns down! 

 

[roys]

I’m with Geoff

Rewirable fuses, random blowing, I would be thinking how old is this wiring?

Maybe time to start preparing for the worst case that a total rewire might be required.

Whatever happens get it checked as soon as, and the sparky will soon tell you the condition of the general wiring.

 

[Murdoch]

I'd be thinking outside lights and / or water leaks / or recent diy

Without proper test kit you'd be not really investigating properly

How old is the property / wiring?