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House ring main failed - help!


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If it's only poor continuity that's better than failing on bad insulation where the cable would probably need replacing.

I had mine done in 2019 when the main board was replaced, 35 years old and the wiring passed with flying colours ?

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Yesterday afternoon: Called an electrician with a good reputation on the local Facebook page and, whilst he said he was just finishing up at the office, he would call in on his way home - I must admit I didnt expect him to be able to come until next week sometime. To save time, he suggested I lift some carpets near the Consumer Unit and see if I could find an inspection hatch. He also said that, given what I had done so far, it was almost certainly a junction box sitting in water on the ground under the floor, rather than having been screwed to a joist.

 

I found some short lengths of screwed down board that created a hatch in the hall so, when he arrived, he was able to look down, point out the cables sitting on the ground in puddles of water. He said he wouldn't go down until the water had cleared/been cleared, but he was certain that I would find a wet junction box.

 

He did check an outdoor socket which he hoped might be the cause, and a simple solution, but it was fine.

 

So he has left me with the job of clearing the water and, if I can, finding the wet junction box - he is happy to come back and do the job if I find I'm struggling. The hope is that the box is in the water under the floor that we can see so far, or under a couple of other floorboards that I can easily lift in the lounge. If so, I should be able to source and fit a junction box, and screw it to a joist, out of any future water. He did add that the fault could be anywhere on the ring, so I may have to lift more boards to see other place - (See the bit where I asked him about Meggars below).

 

So - all good so far. £40 for coming and, hopefully, identifying the likely cause, even if I end up fixing it :) 

 

However, I asked him if he had a Megger, and if he could use it to identify the approximate location of the fault by plugging it into each socket on the ring. He replied that he had a Megger, but it is a big job as it would involved lots of stripping back and "stuff". We left it at that, and off he went for his tea. I've got some lights and tools together and might get the wet and dry vacuum from the boat to pump out the water. The plan is to get stuck in tomorrow afternoon.

 

I have since watched a few videos on Youtube where a Megger is used in the way I expected - remove a few wires in the consumer unit and do some tests, then connect a few together, and plug the Megger into each socket in turn, and an unusual resistance suggests roughly where the fault might be.... so it doesnt seem to be quite the big deal that he made it out to be...... 

 

So I lost a bit of confidence in him at this point - I could appreciate him not wanting to do there and then on Friday afternoon on his way home, but he could have offered to come back next week - I stressed more than once that there was no hurry.

 

Am I right on the relative simplicity of using a Meggar, and it being sensible to use one to narrow down the fault? - my perception from the videos is that, if it isnt an issue within the socket with the unusual resistance, it is most likely in one of the cables entering/exiting it around the ring - and this is where I would find the wet junction box, if that is the cause.

 

Anyway - I might find what I am looking for down the two holes that I can open up, and all will be good - we'll see.

 

 

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6 minutes ago, Richard10002 said:

Yesterday afternoon: Called an electrician with a good reputation on the local Facebook page and, whilst he said he was just finishing up at the office, he would call in on his way home - I must admit I didnt expect him to be able to come until next week sometime. To save time, he suggested I lift some carpets near the Consumer Unit and see if I could find an inspection hatch. He also said that, given what I had done so far, it was almost certainly a junction box sitting in water on the ground under the floor, rather than having been screwed to a joist.

 

I found some short lengths of screwed down board that created a hatch in the hall so, when he arrived, he was able to look down, point out the cables sitting on the ground in puddles of water. He said he wouldn't go down until the water had cleared/been cleared, but he was certain that I would find a wet junction box.

 

He did check an outdoor socket which he hoped might be the cause, and a simple solution, but it was fine.

 

So he has left me with the job of clearing the water and, if I can, finding the wet junction box - he is happy to come back and do the job if I find I'm struggling. The hope is that the box is in the water under the floor that we can see so far, or under a couple of other floorboards that I can easily lift in the lounge. If so, I should be able to source and fit a junction box, and screw it to a joist, out of any future water. He did add that the fault could be anywhere on the ring, so I may have to lift more boards to see other place - (See the bit where I asked him about Meggars below).

 

So - all good so far. £40 for coming and, hopefully, identifying the likely cause, even if I end up fixing it :) 

 

However, I asked him if he had a Megger, and if he could use it to identify the approximate location of the fault by plugging it into each socket on the ring. He replied that he had a Megger, but it is a big job as it would involved lots of stripping back and "stuff". We left it at that, and off he went for his tea. I've got some lights and tools together and might get the wet and dry vacuum from the boat to pump out the water. The plan is to get stuck in tomorrow afternoon.

 

I have since watched a few videos on Youtube where a Megger is used in the way I expected - remove a few wires in the consumer unit and do some tests, then connect a few together, and plug the Megger into each socket in turn, and an unusual resistance suggests roughly where the fault might be.... so it doesnt seem to be quite the big deal that he made it out to be...... 

 

So I lost a bit of confidence in him at this point - I could appreciate him not wanting to do there and then on Friday afternoon on his way home, but he could have offered to come back next week - I stressed more than once that there was no hurry.

 

Am I right on the relative simplicity of using a Meggar, and it being sensible to use one to narrow down the fault? - my perception from the videos is that, if it isnt an issue within the socket with the unusual resistance, it is most likely in one of the cables entering/exiting it around the ring - and this is where I would find the wet junction box, if that is the cause.

 

Anyway - I might find what I am looking for down the two holes that I can open up, and all will be good - we'll see.

 

 

 

It would involve breaking into the circuit at various sockets in turn and testing so not a fast job. He may have been talking about of cables have to be replaced. That might require a degree of plaster removal.

 

Hope you find a box that you can get at. i would hope it would have its own access cut into the floor.

 

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My opinion too given the wet weather. I see you have found puddles in the subfloor as I guessed.

 

It is a faff finding the cable that is down, you need to remove as many of the sockets as you can, then megger the cables one at a time till you can say its this cable that goes from here to that socket over there.

 

Beware that a 500v megger hurts and that if you have anything electronic or electrical appliance connected it will destroy it. Do not megger any RCDs either.

May be easier to disconnect the live MCB for that ring at the consumer and get in under the floor and crawl around in the wet till you find the bare connections or JB.  Don't get in with the power on the ring as i the wet you could get electrocuted under the floor, not a good thought.

Edited by Tracy D'arth
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12 minutes ago, Richard10002 said:

I might find what I am looking for down the two holes that I can open up, and all will be good - we'll see.

 

An endoscope/inspection camera that plugs into your phone or laptop might be handy for finding the box - or indeed confirming there is only one in a puddle.

 

 

 

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2 minutes ago, Tony Brooks said:

 

Hope you find a box that you can get at. i would hope it would have its own access cut into the floor.

Ideally you wouldn't have any junction boxes under the floor. The ring main should run from socket to socket, with any spurs off the ring being taken from a socket (with three wires to each terminal at such sockets). That said, if any spurs have been added later they may be easier to connect into a plain piece of cable between sockets. Are there any sockets which are clearly later additions which might be a clue to where this has been done?

  • Greenie 1
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Taking up the floor is not much fun. 

I would go back to the suggestion of tracing the approximate position of the fault by breaking the ring at and seeing where it lies by process of elimination.   Even  with a megger, it would be a similar process , logically at least.

Taken extra care not to electrocute yourself.

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20 minutes ago, Tacet said:

Taking up the floor is not much fun. 

I would go back to the suggestion of tracing the approximate position of the fault by breaking the ring at and seeing where it lies by process of elimination.   Even  with a megger, it would be a similar process , logically at least.

Taken extra care not to electrocute yourself.

Still might have to go under the floor to fix the problem. Get a small electrician or a minor/miner ?

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1 minute ago, Tacet said:

True.  But you will have a better idea where to start looking

Can the poster comfirm how old the property is. How much room under the floor. Is the central heating been fitted after the house was built. No solid floors etc. 

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3 hours ago, Tacet said:

Taking up the floor is not much fun. 

No... but there are 3 places where it's easy, so I'm going to see how things look, and hope I get lucky.

 

3 hours ago, Tacet said:

I would go back to the suggestion of tracing the approximate position of the fault by breaking the ring at and seeing where it lies by process of elimination.   Even  with a megger, it would be a similar process , logically at least.

 

Yes... I liked that idea, but I'm concerned about messing about in the consumer unit as it's very tightly packed and there is a chance if I take some wires out of the switch things, I may not get them back in. Ditto the sockets.

3 hours ago, Tacet said:

Taken extra care not to electrocute yourself.

Very conscious of this, hence wary of going further than I'm comfortable with.

3 hours ago, Jon57 said:

Can the poster comfirm how old the property is. How much room under the floor. Is the central heating been fitted after the house was built. No solid floors etc. 

I think its around 1935, with a 1965 extension. The extension has solid floor. About 2ft to 2ft 6in below floor. CH must have been fitted after it was built.

 

Most of the affected ring, (Downstairs ring), is wired under the original floor/floorboards. There is a kitchen/garage ring which is is mostly the ground floor extension.

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1 hour ago, Tracy D'arth said:

You could be looking for just minor damage to a cable that is in the wet, a nick or cut would be all it takes to trip the RCD. Possibly Neutral to earth as it hasn't tripped a MCB.

 

I am hoping :) although if a junction box in water is obvious, that should be an easy find and fix.

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15 hours ago, TheBiscuits said:

 

An endoscope/inspection camera that plugs into your phone or laptop might be handy for finding the box - or indeed confirming there is only one in a puddle.

 

 

 

This is a good idea, they are very useful!

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Remember test all cables you work on to make sure they are not live, 240v kills meggers just wake you up.

Unfortunately you will have to disconnect the ring main cables at the consumer unit so that you have the two sides of the ring main separated. One twin and earth for one side and one twin and earth for the other side. Test with megger set on 500 volts between live and earth and then between neutral and earth so you can see which conductor the fault is on. You will be looking for a good reading of at least 1 Manything under this is a fault.

Now you have to guess how the ring main is wired in order to split it into manageable parts in order to find the length of cable the fault is on. Guess the socket at the middle of the ring main and then disconnect and separate all the cable at that socket. Test these cables back at the consumer unit to see which side you still have the fault on, hopefully you can see where the cable goes so you can guess another part to split and test again. Repeat until you find faulty cable.

Hope this makes sense, I’m not sure what the hardest part is, trying to write down what to do or actually doing it.

Instructor to apprentices let’s all join hands while I test this megger.  

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20 minutes ago, JonesBoy said:

Remember test all cables you work on to make sure they are not live, 240v kills meggers just wake you up.

 

Unfortunately you will have to disconnect the ring main cables at the consumer unit so that you have the two sides of the ring main separated. One twin and earth for one side and one twin and earth for the other side. Test with megger set on 500 volts between live and earth and then between neutral and earth so you can see which conductor the fault is on. You will be looking for a good reading of at least 1 Manything under this is a fault.

 

Now you have to guess how the ring main is wired in order to split it into manageable parts in order to find the length of cable the fault is on. Guess the socket at the middle of the ring main and then disconnect and separate all the cable at that socket. Test these cables back at the consumer unit to see which side you still have the fault on, hopefully you can see where the cable goes so you can guess another part to split and test again. Repeat until you find faulty cable.

 

Hope this makes sense, I’m not sure what the hardest part is, trying to write down what to do or actually doing it.

 

Instructor to apprentices let’s all join hands while I test this megger.  

 

 

Also on wnid up meggers charge up a vehicle condenser and toss it to a dopey student :giggles:

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5 minutes ago, Tony Brooks said:

 

Also on wnid up meggers charge up a vehicle condenser and toss it to a dopey student :giggles:

When I was an apprentice there was an enormous machine in the workshop, we called it ''The Octopus'' It could test everything, bulbs. clamp dynamos and starter motors on it and run them up, growler for armatures ect and load condensers 500v. We use to load one and holding it gingerly by its wire take it to the storeman for a new one, he always grasped hold of it, OUCH!!!! never learnt his lesson.

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2 hours ago, bizzard said:

When I was an apprentice there was an enormous machine in the workshop, we called it ''The Octopus'' It could test everything, bulbs. clamp dynamos and starter motors on it and run them up, growler for armatures ect and load condensers 500v. We use to load one and holding it gingerly by its wire take it to the storeman for a new one, he always grasped hold of it, OUCH!!!! never learnt his lesson.

The magneto connected to two brass screws set in the wooden toilet seat was a favourite garage game too.

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On 23/01/2021 at 22:07, Tracy D'arth said:

You could be looking for just minor damage to a cable that is in the wet, a nick or cut would be all it takes to trip the RCD. Possibly Neutral to earth as it hasn't tripped a MCB.


Opened up the hatch in the hall again yesterday. The water had soaked away and the cable sitting on the damp earth has no junction box, so I lifted it off the ground and clipped it to a joist.

 

I don’t have a Megger and didn’t fancy getting under the floor, when I wasn’t sure what it might achieve - other than me not being able to get back out again :) 

 

Opened up a couple of short pieces of floorboard in the lounge, and could see no issues, and that was as much as I could do.

 

This afternoon I called the electrician that deals with my letting agent, and he arranged to come and have a look tomorrow.

 

Having made the appointment, I thought I would try turning on the circuit, on the basis that the cable may have dried out. Didn’t expect any success, but it didn’t trip, and all seems to be working.

 

Electrician is still coming because there could obviously be a nick in the cable, and the insulation degraded, so the cable ought to be replaced. Said he would Megger the whole circuit to make sure it’s all OK.

 

Its a bit of a dogs breakfast under the floor, so I’m expecting to get him back later in the year when we’ve all been vaccinated.

 

Thanks to all for the help, info and advice.

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32 minutes ago, Richard10002 said:


Opened up the hatch in the hall again yesterday. The water had soaked away and the cable sitting on the damp earth has no junction box, so I lifted it off the ground and clipped it to a joist.

 

I don’t have a Megger and didn’t fancy getting under the floor, when I wasn’t sure what it might achieve - other than me not being able to get back out again :) 

 

Opened up a couple of short pieces of floorboard in the lounge, and could see no issues, and that was as much as I could do.

 

This afternoon I called the electrician that deals with my letting agent, and he arranged to come and have a look tomorrow.

 

Having made the appointment, I thought I would try turning on the circuit, on the basis that the cable may have dried out. Didn’t expect any success, but it didn’t trip, and all seems to be working.

 

Electrician is still coming because there could obviously be a nick in the cable, and the insulation degraded, so the cable ought to be replaced. Said he would Megger the whole circuit to make sure it’s all OK.

 

Its a bit of a dogs breakfast under the floor, so I’m expecting to get him back later in the year when we’ve all been vaccinated.

 

Thanks to all for the help, info and advice.

 

I have read that megger tests on circuits in new plaster can give a lower reading so suspect even intact PVC insulation might allow damp in.

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1 minute ago, Tony Brooks said:

 

I have read that megger tests on circuits in new plaster can give a lower reading so suspect even intact PVC insulation might allow damp in.

 

 

Suggesting that its water resistant but not resistant to being continually submerged?

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