New single RCD consumer unit

[nairb123]

Last February (2014) a long standing tenant moved away. I decided to upgrade the existing fuse box to a new RCD consumer box. I contacted a local known electrician who came and installed a single RCD box and mains connected heat & smoke detectors. Tested the circuits and left me with a certificate etc and all worked fine for a while....... New tenant had been in for a few weeks when the dreaded RCD trips started.

I have since had the electrician back and replaced the shower unit and tested all the circuits. The trip outs continue.

My question is..... I have been told that the single RCD box is incorrect and I should have a dual RCD consumer box (since the 17th edition?)
I know this may not fix the trip outs. (much effort has gone into tracing wires without a smoking gun yet) But when the single rcd trips everything goes out.

Should a Dual RCD unit have been fitted the first time. Should it have been fitted by law??

Thanks in advance for any info

nairb

 

[ericmark]

314.1 Every installation shall be divided into circuits, as necessary, to:-
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.

In other words if it does not trip you don't need multi-RCD protection if it does trip due to a fault then single is good enough but if nothing wrong then needs splitting into more than one trip.

It then continues:-
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit.

So if light comes in from outside or if supplied from battery as with an emergency light or caravan battery lights no problem but if losing the supply would cause danger then need multi-trips.

Of course if you have a general power cut it would produce same danger but that's outside your control.

In general a house always has multi-circuits the argument as to if all from one RCD is a circuit or all from MCB is a circuit was never talked about until Part P rules changed.

I have quoted from BS7671 which is NOT law but Part P and EAW act both mean in real terms it should be followed. Also convention is where a 30ma RCD is used other than with boats and caravans which have battery back-up you always use more than one.

Before 2008 we did use single RCD's but these were 100ma not 30ma.

With a new box cure is simple swap RCD for either a 300ma or simple isolator and swap MCB's for RCBO's exactly what method depends on earth type.

 

[PrenticeBoyofDerry]

I decided to upgrade the existing fuse box to a new RCD consumer box. I contacted a local known electrician who came and installed a single RCD box and mains connected heat & smoke detectors. Tested the circuits and left me with a certificate etc and all worked fine for a while.......

Did you get a certificate from building controls as well?

New tenant had been in for a few weeks when the dreaded RCD trips started.

Did they bring electrical equipment with them or is the let including these?
If so have they been PAT tested?

I have since had the electrician back and replaced the shower unit and tested all the circuits. The trip outs continue.

So what was the issue with the shower then?

My question is..... I have been told that the single RCD box is incorrect and I should have a dual RCD consumer box (since the 17th edition?)

That would depend on how the person who told you that, interpriates the guidance within BS7671.

I know this may not fix the trip outs.

Well maybe it will, the trips could be down to a build up of earth leakage over all the circuits that when added together is enough to trip the single RCD, if the loads were divided over two or more residual units, it could cure your problem.

Should a Dual RCD unit have been fitted the first time. Should it have been fitted by law??

By law no, but most electricians with anything about them would use a board that has two or more RCDs.
It could be possible that your installation was not suitable for a change without remedial work being carried out, did the alleged electrician do a report on your existing installation prior to the change?

 

[JohnW2]

Before 2008 we did use single RCD's but these were 100ma not 30ma.

CUs with a single 30mA RCDs were surely common in the years prior to 2008, weren't they? (with 'split load' ones {the single RCD protecting only some of the circuits} becoming gradually more prevalent during that period)? Several members of my family had them in their houses (and some still do!).

I realisethat my experience is limited, but I don't think I've personally ever seen a 100mA RCD in a domestic TN installation. TT is obviously different, and a 100mA RCD in such an installation is there for a different reason.

Kind Regards, John

 

[PrenticeBoyofDerry]

Before 2008 we did use single RCD's but these were 100ma not 30ma.

CUs with a single 30mA RCDs were surely common in the years prior to 2008, weren't they? (with 'split load' ones {the single RCD protecting only some of the circuits} becoming gradually more prevalent during that period)? Several members of my family had them in their houses (and some still do!).

I realise that my experience is limited, but I don't think I've personally ever seen a 100mA RCD in a domestic TN installation. TT is obviously different, and a 100mA RCD in such an installation is there for a different reason.

There were plenty of 30mA RCD units installed prior to 2008, some covering the whole board some covering a selection of circuits and the second split being MCBs only.

 

[JohnW2]

There were plenty of 30mA RCD units installed prior to 2008, some covering the whole board some covering a selection of circuits and the second split being MCBs only.

Thanks - as I said, that was certainly what I thought, and what I saw happening in houses I knew. It was also (again, in my experience) quite common during that period for people to have a single standalone 30mA RCD installed upstream of an existing non-RCD CU.

Kind Regards, John

 

[nairb123]

Thanks for the info. I pretty much confirms my thoughts. A dual RCD box should have been put in first. When the existing single RCD box trips ALL circuits are dead.
It also would help in the diagnostics to be able to split the loads.
At the moment the rcd trips when the tenant is in the shower.... except its one of the kitchen rings that trips its mcb and has to remain off for a couple of hours. When left off the rest of the house is stable and the shower is working fine.
Carpets and some flooring have been up and loft space inspected but so far no obvious fault has been found like water ingress or common connections on shower/kitchen ring.

From what I read from the post... since the electrician found no faults after the installation. Its not his problem and is justified in wanting a similar amount of money to fit a dual rcb.

The single rcd is rated at 100A 30mh

A follow up on a subsequent post.

I have not yet received any further certificate from building controls

They have brought all electrical equipment with them(tv/computer/fridge). BUT... all other circuits seem ok. The fault is intermittent but when it fails its always due to the shower being used. I have had a new shower put in. The floor space under the shower/bath is dry(1st thing I checked). Shower cable is of the correct rating(check by electrician). And roof space is dry and shower cable seems fine. I know this is not exhaustive testing.

The next step is to trace all the wiring from the rcd to the shower. The shower is the heaviest load I think (gas cooker)

ta
Nairb

 

[EFLImpudence]

You probably don't need another new CU.

Fitting one RCBO (or some RCBOs) could make it compliant and less inconvenient.

 

[PrenticeBoyofDerry]

If you unplugged all the appliances from the kitchen socket circuit, then run the shower does the RCD still trip?
And you should have received a document from building controls, as a new consumer unit is deemed notifiable work, is this electrician allowed to sign/notify their own work. I think some one may have broken the law!

 

[JohnW2]

From what I read from the post... since the electrician found no faults after the installation. Its not his problem and is justified in wanting a similar amount of money to fit a dual rcb.

The RCD trips are not his problem, and nor is it all that likely that fitting a dual-RCD CU, per se, will get rid of that problem. More generally, a lot probably depends upon what conversations you had with the electrician and what you instructed him to do. He really should had advised you that standard current practice would be to install a dual-RCD CU, and that many electricians would probably interpret the Wiring Regulations as indicating that a single-RCD CU was non-compliant with those regulations. If he gave no such advice, it could be argued that he is at least partially responsible for your having ended up with what many would regard as an 'inadequate' CU.

As EFLI has said, you may well not need a new CU. How many 'ways' does the new one have, and are there any spare 'ways'? Is there a 'main switch' as well as the single RCD, or just the RCD? The other issue for discussion with your electrician is:

I have not yet received any further certificate from building controls

Are you sure that the work actually was notified to your local authority's Buildings Control department? If it wasn't, the work will have been technically unlawful.

Kind Regards, John

 

[bernardgreen]

While the trips occur when the shower is running it is not always the case that the fault is in the shower or the cable to it.

A neutral to earth fault elsewhere and on a different circuit can divert some of the load current on the neutral away from the sensor in the RCD.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

The amount of current divert from a small load ( kettle 3 kW etc ) will not create enough un-balance to trip the RCD but that diverted from the shower may be enough.

 

[AndyPRK]

A picture of the consumer unit would be usful

 

[PrenticeBoyofDerry]

A picture of the consumer unit would be usful

A picture of the electrician would be more useful!

 

[nairb123]

The company who did the work was recommended by somebody who owns 20+ rental properties. I was satisfied by the work done. BUT there was no mention of a single or dual rcd. All other work done has so far worked fine. I assumed the certificate takes ages to drop thru the door. I will chase this.
Using a single rcd was at the discretion of the electrician.

Getting something rewired/changed is not an issue. Understanding the cause is.

The RCD is an WYLEX NHRS804 photo attached.

I have worked on large computer systems for years. Often fault finding. Like most systems ... start with the obvious and go down the fault tree. I also used to do a bit of electronics...ages ago. So I have some understanding of earth loops/induction/capacitance issues. The fact thats its the shower means its pulling loads of current, compared to the lights.
The house was occupied by the previous tenant for 12 years without any electrical problems that I am aware of. Now the house may go 3 weeks without a trip then get 3 in 2 days. Always caused by the shower. And I know that on some occasions there are no kettles/washers going at the same time.
Also... if the MCB that trips (kitchen socket) is not kept off then the rcd cannot be switched back on. So far a 2 hr wait is the best we have done before switching the MCB back on. I have asked that the tenant try after 1 hr. Once the MCB is back on it stays on until tripped later during a shower.(maybe days later)
Is it possible that a shower cable that runs in close contact with other cables could cause and induction loop?. But I thought this would quickly dissipate. The cables are all together for 1.8 m after leaving the rcd.

Untitled

• nairb123
• 27 May 2014
• 1

not the best pic. Shower is on 3. Smoke alarms are on lighting(4). The mcb that trips is on 2

Thanks

 

[PrenticeBoyofDerry]

I would expect a building control compliance cert, to turn up within a couple of months of completion, generally takes a couple of weeks after being notified. Did the electrician do a report on the installation prior to fitting the new unit?

Again have you tried taking off load all appliances in kitchen, then using shower?

If the circuit needs two hours to settle down, that to me would suggest a thermal problem.