finding fault in metal underground conduit

[Emmett Jenner]

hi there, first post on here.

I've got a shed in my garden which I connected to one of the feeds to a wall socket in the house.

I hooked it up in June and it worked fine until about a week ago. The power went off in the house. I turned it back on again. About a week after that it went off again and couldn't be turned on as it would turn off immediately.

The cable to the shed runs down from the attic on the outside of the house through a metal conduit. When it gets to the ground it runs along a fence and then goes under the lawn to the shed.

When it gets to the shed it splits up via nylon connector block and conduit junction boxes to feed 4 sockets. All under the shed floor.

So I'm sure this question has been asked a thousand times before... but is there a way to find the fault without digging up the lawn? I've got to take up the shed floor anyway to do some other work, the floor was only put down temporarily.

Can I use a multi-meter to test resistance and somehow work out where the fault is like that?

My dad does not do much DIY himself but pointed out that where the conduit has been welded to the shed frame it is possible I've burnt through the insulation on the cable inside the conduit. It's also possible water has got into the conduit. I pumped the junctions with silicone to prevent water damage. It is probably water related as the fault took some time to appear.

So yes, main question is how to find out which branch of the wiring to investigate. I don't want to open up everything?

 

[shaun1]

Some thoughts/questions to get the ball roling;

If you welded conduit to something with the cable inside then I would think it's pretty likely you have damaged the insulation. Probably quite severely.

What kind of cable is in the conduit?

When you say it is fed from a socket in the house, how is that socket protected/fused. Is there additional protection to the shed?

How are the joint boxes under the shed protected from water ingress?

What is the earthing arrangement of the incoming supply?

I think the main response you will probably get here is to seek the services of an electrician to find the fault and check the safety of the instalation. But in answer to your question, testing continuity will show which piece of cable is at fault, but not where along it's length the fault lies. And you may need an insulation resistance tester at 500v to show the fault if it is intermittent.

 

[buzzlightyear]

Sheldon from the big bang, employ a local spark to put it right without the proper gear you could make a danger to you self!

 

[shaun1]

Also, have you earthed the conduit and confirmed earth continuity along it's length?

 

[Des 56]

An electrician may access the first of the conduit box/connector block joints under the shed floor,disconnect and test to determine if its shed side or house side
Without asking details the installation itself sounds a little hokey pokey

 

[Octopus]

There's very little the op can do without proper test kit and the knowledge to use it .....

 

[Emmett Jenner]

The conduit is earthed to the shed. There is a continuous earth circuit through all the terminals and connectors.

The circuit now has its own breaker in the consumer unit. Hasn't made any difference to whether or not there is a fault, the fault is still there.

RE welded conduit, I'm pretty sure I did this without the cables inside for the obvious reasons pointed out above. What may have happened was that the weld went through the conduit and created a spike on the inside which has torn some insulation as the cable was being fed through.

So what is the 'proper test kit and the knowledge to use it'? I'm not a household electrician, I'm multi-skilled so work with PCBs, re-wiring motor vehicles. I'm actually an IT guy during the day.

 

[SparkyChick]

So what is the 'proper test kit and the knowledge to use it'? I'm not a household electrician, I'm multi-skilled so work with PCBs, re-wiring motor vehicles. I'm actually an IT guy during the day.

In this case, I would suggest an insulation resistance tester is probably going to be your friend, but a low range continuity tester and earth loop impedance tester would also be useful. Or an multi function installation tester.

As for the knowledge... you need to know how to use the test gear and more importantly, how to interpret the test results to establish whether they are good, bad, what you expect, way off etc. etc.

 

[davesparks]

The conduit is earthed to the shed. There is a continuous earth circuit through all the terminals and connectors.

The circuit now has its own breaker in the consumer unit. Hasn't made any difference to whether or not there is a fault, the fault is still there.

RE welded conduit, I'm pretty sure I did this without the cables inside for the obvious reasons pointed out above. What may have happened was that the weld went through the conduit and created a spike on the inside which has torn some insulation as the cable was being fed through.

So what is the 'proper test kit and the knowledge to use it'? I'm not a household electrician, I'm multi-skilled so work with PCBs, re-wiring motor vehicles. I'm actually an IT guy during the day.

what size earth connection do you have between the house's incoming earth supply and the shed? Is it the correct size to be a main bonding conductor or have you set up a separate earthing system for the shed? This buried conduit will be an extraneous part to the shed, and will be forming an earth electrode, presumably you consider the implications of this when designing the installation?

What type of cable have you installed in the conduit and along the fence?

The proper test kit is an insulation resistance tester for starters, though other tests will be required before re-energising the circuit. The knowledge to use it is both knowing the safe working procedures and logical fault finding process and also good working knowledge of the wiring regulations to ensure that the existing installation and any repair is carried out in a safe and compliant manner.

 

[telectrix]

be nice to him dave and he might offer to rewire your landy. .

 

[davesparks]

be nice to him dave and he might offer to rewire your landy. .

There's nothing wrong with her wiring, it's just got a slight problem where the patches which are patching up the rust on the patches on the rear crossmember have gone a bit too rusty to pass an MOT.

 

[mickfred]

Its most likely damged insulation and if it keeps tripping then the short may well be able to be detected by a multimeter. If this is the case fish new wires in (dad may have to give a hand) and check where possible that the edges are smooth .

 

[Emmett Jenner]

I could definitely re-wire the Land Rover. That's right up my street.

With these electrics for my shed I had the feeling at the time that it was difficult to get the cable through the conduit but after wiring it up and turning it on I was satisfied that there hadn't been any snags during the installation. I'd say it has to be water related as the fault took some time to appear. The question is where to look first.

 

[Octopus]

The question is where to look first.

Yellow pages for a local spark?

 

[buzzlightyear]

The question is where to look first.

yeah Sheldon ,in to space!