Problem with Ring Main

[Scott Baxter]

Hi All,

Looking for a bit of advice on an issue I'm having with my RCD constantly tripping .

I moved into the house November 2014 and haven't had a problem until now. One day RCD started tripping no matter what was plugged/unplugged. Tried removing all the appliances but couldn't diagnose the fault.
It's in a new build house so I called the housebuilder who arranged for an electrician they use to come out.
They have found an issue with the downstairs sockets ring between a socket in my living room and a socket in my utility room (which are at opposite sides of the house).
The electrician said it was going to be a bit of a nightmare as we have concrete floors....
Now I've never built a house but surely cables are dropped from the roof?

To fix the problem they have suggested splitting the ring in two. Now I understand this will fix the problem but I'm worried this might have an adverse effect of the value of the house if we were ever to sell. I wouldn't want a surveyor to pickup on a "fix" and include this in the home report.

I am still under warranty at the moment so I'd like the issue resolved properly even if that means ripping up the floorboards and replacing cables.
Is this unrealistic and the solution they have provided is suitable?
Any help/advise is greatly appreciated.

Thanks,
Scott

 

[flameport]

Cables can be installed in a variety of places, from above is one of them but under the floor is just as acceptable. Generally depends on what is easier at the time.

Converting the ring into 2 radials is perfectly acceptable. No surveyor would ever comment on it, as it would just be 2 separate circuits, just like any other.

 

[endecotp]

I think your electrician probably knows what they are doing.

I would worry that whatever has caused this problem might also occur elsewhere, e.g. if the under-floor cables were incompetently installed. Sod's law says the next one to fail will be a week after whatever warranty you have expires. Ask the electrician to do insulation resistance tests everywhere and see if there are any other suspicious readings.

 

[ban-all-sheds]

And write to your MP asking for the Consumer Rights Act to be extended to cover houses. With 15-20% profit margins, the builders ought to be able to stand by their products...

 

[Taylortwocities]

Converting the ring into 2 radials is perfectly acceptable.

Except that there are limitations.

Assuming the ring is cabled in 2.5mm² cable, a ring final is protected by a 32A CPD - fine for most applications.


If it is converted to a radial in 2.5mm² cable it will usually have a 20A CPD* - and that may be tight if the radial has power hungry appliances on it (ie kitchen = washing machine/tumble + kettle + toaster) and the circuit could trip at breakfast time.
If the (soon to be) 2 x radials are for living areas, then it should be fine.

But I agree with BAS

the builders ought to be able to stand by their products...

Is there no NHBC?




* 2.5mm² can support up to 27A if installed properly. The nearest MCB that you can get is 25A, but not many CUs have this value available.

 

[ban-all-sheds]

Is there no NHBC?

There is.

It stands for No Hope of Being Credible.

 

[ericmark]

I have seen ring finals split many times. However there are problems.
1. Maximum for radial often 20A where ring final is 32A this should not cause a problem as items over 2kW should have their own dedicated supply so in theroy no washing machines, tumble driers, dish washers, ovens, or immersion heaters are powered off the ring final, but in practice this is often not the case.

2. The split may not be central.
a) Central in respect to load.
b) Central in respect to cable used.
3. The amount of cable permitted for a ring final is more than what is permitted for two 20A radials.

So we have loop impedance, loading and volt drop considerations. Splitting a ring final is not the same as installing a radial system. Also post installation not as easy to ensure the split is where it is within regulations. We had a post on this forum where we considered if we should calculate the volt drop when doing an inspection and test, we came to the consideration that although in theroy we can measure volt drop and work out cable lengths using our loop impedance testers, in real terms the instruments are not really accurate enough to be sure enough of the readings in most cases to be able to fault an installation on volt drop. We may suspect too much cable used, but proving would be hard.

When we install a ring we take a new role of 100 meters of cable and install all from that role, we know we are allowed 106 meters so very easy to work out if within the limits. With a radial we are allowed 32 meters of cable on a 20A MCB so a ring is using the same about of cable as three radials not two. In theroy we can take the impedance of the circuit (Line - Neutral) and if it is more than 0.35Ω plus the incoming Ze then it can't be split. But as a ring we can have 0.59Ω plus Ze. It would be great if we could simply look at the installation certificate, but traditionally we measure Line - Earth not Line - Neutral so it is not shown on the certificate.

So much depends on size of house and number of rings. A house with three rings one for kitchen, one for ground floor and one for first floor then with the ground floor earth loop impedance being around the 0.80Ω mark or less and around equal sockets on each leg then unlikely to be a problem. However if there is not a separate kitchen ring or the earth loop impedance is around the 0.95Ω mark or there is a silly split of sockets like 5 on one leg and 20 on the other then not really a good idea.

Now I will put myself in the position of the company electrician. There is no way that I want to dig up cables laid in concrete, the house may be expensive, but often the wiring is sub contracted, and this electrician is likely expecting to find some other tradesman has damaged his cables. So either a quick fix and run, or a very careful removal so he can show it was not his fault, but some one with cob hopper boots who has damaged the cable so he can claim off the builders.

So now as house owner, I want to know why it failed. If it failed due to a brick being dropped on the cable before the floor when in then unlikely all cables damaged that way, but if shoddy workmanship then now is the time to find out. Also I would be thinking why should I take the risk? Now if the builder says sorry we can fix by splitting into two circuits and here is £500 for the inconvenience then I may be tempted to allow the circuit to be split. But if he is not going to make a good will gesture then sorry do the job as originally agreed I want a ring final thank you very much. My house built 1980 and builder wanted £25 per extra socket a clear rip off. I am told still a silly price for extras so why should the reverse not also be true?

 

[Scott Baxter]

Thanks folks, while I realise splitting is probably a perfectly good solution (and one I'd be happy with if I was having to pay!) the fact that it's not standard and not the "ideal" solution makes me want them to fix this properly.
I want things the way they were meant to be, it's still covered under warranty so from my point of view it's their responsibility to fix.
I'll speak to the housebuilder and see what they have to say.

Any chance they will have plans of where the cables are (or at least should be)?

 

[Mikefromlondon]

Already some very good advice has been given, there is not much more to add, except beware the situation where the exact location of the fault is, it could be half way, quarter way, as erickmark stated, or it could be between the first or the last socket and CU, in which case if the fault laden cable was isolated from the CU and the first or the last socket, that would leave all the sockets wired on a single radial circuit with almost all the sockets on it, so chance of a possible overloading by a new owner who may not have been aware of this and buys a house as is, and later on beefs up the 20A breaker to 32A. Let us hope you or your electrician finds that the problem is more towards mid point, then you don't have any real worries.

 

[ban-all-sheds]

items over 2kW should have their own dedicated supply

Why do you say that?

 

[winston1]

1. Maximum for radial often 20A where ring final is 32A this should not cause a problem as items over 2kW should have their own dedicated supply so in theroy no washing machines, tumble driers, dish washers, ovens, are powered off the ring final, but in practice this is often not the case.

That is because there is no such regulation. There is some vague guidance only. These items come with fitted 13A plugs so they are expected to be plugged in but no manufacturer says they cannot be plugged in to a ring final.

 

[ericmark]

items over 2kW should have their own dedicated supply

Why do you say that?

Because that is what it says in my BS7671:2008 OK may have changed I have not got amendment 3 and also it is not really followed, I consider a dedicated ring final for the kitchen is a dedicated supply for laundry equipment, never worked out why laundry and food go together! But point is if the house is designed as laid out in BS7671:2008 then all but the kitchen ring final have very little load so even 16A radials would do the job. Where the problem lies is where BS7671:2008 has not been followed or if the circuit in question is the kitchen ring final.

 

[securespark]

Not sure about the BYB, but App 15 (I know it says informative!!!) of the BGB says cooker, ovens and hobs >2kW should be on their own circuit.

There is some vague guidance only.

I don't see it as vague.

 

[ban-all-sheds]

Not sure about the BYB, but App 15 (I know it says informative!!!) of the BGB says cooker, ovens and hobs >2kW should be on their own circuit.

But not kettles, washing machines, tumble driers....

 

[OwainDIYer]

Not sure about the BYB, but App 15 (I know it says informative!!!) of the BGB says cooker, ovens and hobs >2kW should be on their own circuit.

But not kettles, washing machines, tumble driers....

And a tumble dryer may be running at full load for several hours continuously. It's the nastiest load in the typical house from that point of view.