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32amp Commando Socket Problem.


Biggles

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Hi all. 

 

It's been a while. 

 

I have a problem with a 32amp commando socket I've installed at home. I'm not a complete numpty around electrical installations like this, but this one has me scratching my head. 

 

When I build my boat I had a 16amp extension cable comming out of the consumer unit to power the boat and build. 

 

I have an electric car now and decided to upgrade this setup to 32amp with armour cable and a switchable Commando socket. 

 

I used the same terminals and MCB in the consume unit as the 16amp were on. 

 

Everything checks out on test on plug but as soon as a even very small load is applied the 63amp RCD trips. 

 

This installation of the original 16amp cable was always a bit gash thing but it worked so stayed. 

 

I'm not using the armour for the earth as I am using one of the 3 cables inside. 

 

Has anyone with electrical experience any suggestions as to why its tripping? 

 

Kevin 

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If it trips with a slight load but doesn’t trip with zero load, I’d suggest it is because somewhere downstream of the RCD, the neutral and earth are connected together. This means that some of the load current that should only go via the neutral, is going via the earth as well. This will trip the RCD. The tester can’t detect neutral connected to earth, since upstream of the RCD at some point neutral and earth will be connected together anyway.

Edited by nicknorman
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I live in a house with overhead supply. I only have 2 wires comming in with no specific earth so I am guessing my neutral is my earth also. 

 

Out of curiosity why should an exact replacement upgrade to the cable change how the earth trip responds. 

 

The previous wire and connection to the CU was seriously abused and over loaded at times. 

 

What is my option? An earth spike? 

 

Kevin 

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16 minutes ago, Biggles said:

I live in a house with overhead supply. I only have 2 wires comming in with no specific earth so I am guessing my neutral is my earth also. 

 

Out of curiosity why should an exact replacement upgrade to the cable change how the earth trip responds. 

 

The previous wire and connection to the CU was seriously abused and over loaded at times. 

 

What is my option? An earth spike? 

 

Kevin 

You should connect incoming live and neutral to live and neutral on the connector. You should connect the earth on the connector to an earth spike or equivalent. The important thing (with regard to the problem tripping) is to ensure that neutral and earth aren’t connected together after the RCD.

 

Oh and to answer the question in the second sentence, it’s because in some way it isn’t an exact replacement. Without being able to see it, I say say in what way it isn’t identical,.

Edited by nicknorman
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17 minutes ago, Biggles said:

Ok I understand that. So further question is does the rest of the house lack an earth? Certainly things do trip from time to time. 

 

Sorry to need a basic lesson. 

 

One would expect the rest of the house to have some sort of earth, spike in the ground etc. Normally this is done at the local transformer but maybe not always. You certainly wouldn’t want to have a house where things like the copper pipework wasn’t earthed.

 

However you don’t necessarily have to have a “proper” earth for things to trip. For example if you or a damp spider etc touch live whilst also being connected to earth (feet on concrete floor etc) then the RCD should trip regardless of whether or not things are “properly” earthed.

 

The advantage of having a proper earth is that, eg if your metal-cased appliance has a fault which puts live onto the case, the over-current MCB will trip. It won’t be lurking there for ages awaiting you to touch it and trip the RCD.

 

Remember that using your body to trip an RCD is bloody painful and “shocking” and it’s only probably not fatal, not definitely not fatal.

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Yes, typically as I understand the neutral is bonded to ground at the transformer, goes to the house as two cables, and then splits back into three at the house. At which point they are at the same voltage/potential difference.

 

However as said, if you then rejoin them later, any voltage drop over that cable will create a potential difference which will then trip the RDC. 

 

All that said, I'm slightly at a loss as to why you have swapped the socket from 16A to 32A if it's still fed from the same cable and MCB, presumably a 16A man at most. 

 

 

Daniel

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A proper earth used to be called a company earth - insofar as the supply company presented a suitable terminal for the use of.  But  there were various types depending on just how it was provided.  If not,  the iron incoming water main was used, but that became less acceptable with the adoption of plastic pipes.  A dedicated spike driven into the ground is now more common in the absence of something better.

 

It would be unusual nowadays not to have a terminal at the distribution board - but just how it is earthed can vary.

 

If you do have a earth-neutral connection downstream of the RCD, it can be tripped by current being drawn on other circuits, which can be very confusing.  E.g. the single pole fuse/mcb on the live feeding the commando socket is OFF, but when the immersion heather comes on, the RCD trips due to an earth-neutral fault on the commando line.

 

Edited by Tacet
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1 hour ago, Biggles said:

Ok I understand that. So further question is does the rest of the house lack an earth? Certainly things do trip from time to time. 

 

Sorry to need a basic lesson. 

 

This might explain TNC-S earthing 

 

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10 hours ago, DHutch said:

Yes, typically as I understand the neutral is bonded to ground at the transformer, goes to the house as two cables, and then splits back into three at the house. At which point they are at the same voltage/potential difference.

 

However as said, if you then rejoin them later, any voltage drop over that cable will create a potential difference which will then trip the RDC. 

 

All that said, I'm slightly at a loss as to why you have swapped the socket from 16A to 32A if it's still fed from the same cable and MCB, presumably a 16A man at most. 

 

 

Daniel

Hello DHutch. I have replaced the cable with 6mm armoured from the rather gash 16amp orange caravan / boat that became a permanent temporary ?. The reason for the the complete change from the CU is to permit charging my electric car at 7kw. The actual terminals and MCB at the CU are same as the orange cable used. The MCB is 40amp type D. 

 

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9 hours ago, Loddon said:

This might explain TNC-S earthing 

 

I don't have TNC-S earthing. At least not that I can see. I don't have that earth split off on the supply side from the electric company. Would it be a good idea to go around and make some PME connections and put in an earth spike near to the Commando box and give it a direct earth? 

 

I know my water main is lead under ground. I could earth down to this very easily. 

5 minutes ago, Boater Sam said:

The steel armour should be earthed at least at the incoming end else some odd capacity effects may occur.

I l'll do that. 

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  9 hours ago, Loddon said:

I don't have TNC-S earthing. At least not that I can see. I don't have that earth split off on the supply side from the electric company. Would it be a good idea to go around and make some PME connections and put in an earth spike near to the Commando box and give it a direct earth? 

 

                I know my water main is lead under ground. I could earth down to this very easily. 

 

 

You don't have any equipotential earth bonding to company earth/spike and water and gas mains then?

Odd sort of wiring if so.

I would suggest remedial work is required in that case for your own safety.

Is the installation very old?

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7 minutes ago, Boater Sam said:
  9 hours ago, Loddon said:

I don't have TNC-S earthing. At least not that I can see. I don't have that earth split off on the supply side from the electric company. Would it be a good idea to go around and make some PME connections and put in an earth spike near to the Commando box and give it a direct earth? 

 

                I know my water main is lead under ground. I could earth down to this very easily. 

 

 

You don't have any equipotential earth bonding to company earth/spike and water and gas mains then?

Odd sort of wiring if so.

I would suggest remedial work is required in that case for your own safety.

Is the installation very old?

On better understanding and looking again I am TNC-S. It's about 15 years old this lot. 

 

Should I still put in an earth spike and PME to the water main. I could easily do the gas main as well but I know its plastic from the road to the meter. 

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10 minutes ago, Boater Sam said:

All extraneous metal work, i.e. water, hot and cold and  central heating,  and gas pipes etc should be earth bonded to that company earth with 10mm cable.  Its in the regs.

That’s definitely not the case. How would that work now we have plastic plumbing I need to parts of the house? 

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5 minutes ago, Biggles said:

That’s definitely not the case. How would that work now we have plastic plumbing I need to parts of the house? 

Also bear in mind that the gas & electric incomers may well be via plastic pipe....my unit was rewired by the landlords (presumably registered) sparky....he earth bonded all the blue HDPE water pipe.....

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Just now, frangar said:

Also bear in mind that the gas & electric incomers may well be via plastic pipe....my unit was rewired by the landlords (presumably registered) sparky....he earth bonded all the blue HDPE water pipe.....

In our workshop it was bonded to the brass stop tap - I asked why, when the pipe was plastic, and told that the brass stop tap was in direct contact with the water, and the water will act as the earth connection all the way back to the pumping station.

 

Made sense.

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Just now, Alan de Enfield said:

In our workshop it was bonded to the brass stop tap - I asked why, when the pipe was plastic, and told that the brass stop tap was in direct contact with the water, and the water will act as the earth connection all the way back to the pumping station.

 

Made sense.

Except that it’s rubbish. Water in a plastic pipe is a very poor conductor.

 

https://www.bpfpipesgroup.com/media/1036/earthbonding.pdf

 

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14 minutes ago, Biggles said:

That’s definitely not the case. How would that work now we have plastic plumbing I need to parts of the house? 

Obviously you don’t need to bond the plastic pipes and you should not consider the water inside a plastic pipe to be conductive. However if the plastic pipes connect to copper pipes downstream, these should be earth bonded. It is to cater for a case of some wiring in the interstices of the house chafing on a pipe and making it live. If the plastic pipe goes all the way to the metal tap, there is no need to bond the metal tap. If there is a short run of metal pipe eg 1/2 metre, and no feasible means of it becoming unexpectedly live, there is no need to bond. But if there is significant run of metal pipe after the plastic pipe, it should be bonded.

 

(same link I posted in my previous reply)

 

https://www.bpfpipesgroup.com/media/1036/earthbonding.pdf

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10 minutes ago, nicknorman said:

Except that it’s rubbish. Water in a plastic pipe is a very poor conductor.

 

https://www.bpfpipesgroup.com/media/1036/earthbonding.pdf

 

Even worse when the eathing strap is just round the plastic pipe!! On the plus side it’s very neatly done and the separate earth was run all the way back to the board....did snigger.....

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12 minutes ago, nicknorman said:

Except that it’s rubbish. Water in a plastic pipe is a very poor conductor.

 

https://www.bpfpipesgroup.com/media/1036/earthbonding.pdf

 

Interesting read - thank you.

 

Plastic supply pipe to the incoming stop-cock and then copper pipe in the building still requires bonding tho', but plastic pipe within the building does not.

Every day is a school-day.

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