DNO removed RCD on TT system

[finalcoat]

I've been doing some (non-electrical) odd jobs at a large country house nearby.
Recently, they had to call out the DNO as there was smoke and arcing sounds coming from inside the CU cupboard. The DNO cleared a slug from the RCD box - However, they have replaced the RCD with a isolation switch.
Should the DNO have replaced the RCD with another RCD?
All the bathrooms and outside circuits are protected by RCBOs but there is now no RCD protection on any other sockets.

Untitled

• finalcoat
• 13 Nov 2014
• 1

Untitled

• finalcoat
• 13 Nov 2014
• 1

Untitled

• finalcoat
• 13 Nov 2014
• 1

 

[JohnW2]

The DNO cleared a slug from the RCD box - However, they have replaced the RCD with a isolation switch. ... Should the DNO have replaced the RCD with another RCD? ... All the bathrooms and outside circuits are protected by RCBOs but there is now no RCD protection on any other sockets.

If, per the title of this thread, it is really is a TT system then, as your surmise, every circuit must have RCD (or RCBO) protection - so, no, if there are final circuits with no such protection,then that up-front (probably 100mA, time delayed) RCD should not have been removed (or should have been replaced with another).

Under current regs, nearly all sockets (and some other things) should have 30mA RCD/RCBO protection, but that's a different matter, and does not apply retrospectively. However, the need for at least 100mA RCD protection on all circuits is essential for a TT system.

Kind Regards, John

 

[ban-all-sheds]

No "retrospective" requirement - that's true.

But that raises an interesting question of intent.

What if this were not a TT installation, and somebody had put an RCD before the CU(s) because they wanted, voluntarily, to provide RCD protection for buried cables and bathroom circuits? Putting to one side the arguments over compliance with 314.1, if the intent of installing the RCD was to bring the existing installation into compliance with the 17th, then it could not be replaced by a switch.

Or ditto if it was already there, and was left in place on a rewire or CU change.

 

[westie101]

It all depends in a way who the device belonged to, in older installations it was common for the electricity boards to provide Voltage operated protection if earthin was poor, this was often wired where the isolator now is.

If the RCD was the customers should the DNO be called to it or should it have been an electrician?

Consider also that DNO staff in the main are not trained in the finer aspects of BS7671 so will have seen it as an isolator so changed it.
It could also indicate that they did this in order to maintain supply and didn't make it clear to the customer it would need an RCD fitted.

 

[JohnW2]

What if this were not a TT installation, and somebody had put an RCD before the CU(s) because they wanted, voluntarily, to provide RCD protection for buried cables and bathroom circuits? Putting to one side the arguments over compliance with 314.1, if the intent of installing the RCD was to bring the existing installation into compliance with the 17th, then it could not be replaced by a switch. ... Or ditto if it was already there, and was left in place on a rewire or CU change.

Indeed, that's why I prefaced my comments with "if it really is a TT installation".

Of course, one way to get a handle on the likely intent is to look at the In of the RCD which was removed. If, as probably is the more likely, it was 100mA, then one might reasonably conclude that the intent was not to bring circuits up to current reg requirements but, rather, was put there because, at elast at the time, it was a TT installation. That obviously wouldn't prove that it was still TT - but I have tosay that I can see no evidence in the photos that is is now TN - so it's probably still TT.

Kind Regards, John

 

[finalcoat]

Many thanks for your quick replies
I believe it is a TT system as it is a rural property with overhead conductors. Also there is an earth spike connected to the earth connection going to the bottom LH corner.
I'm presuming this is hazardous as without knowing the earth impedance you cannot guarantee that the MCB will trip on a L-E fault.
I will tell the owners to get the DNO back in as a matter of urgency.

As an aside - can anyone tell me why there are two live conductors into the isolator but only one neutral?

 

[JohnW2]

If the RCD was the customers should the DNO be called to it or should it have been an electrician? ... Consider also that DNO staff in the main are not trained in the finer aspects of BS7671 so will have seen it as an isolator so changed it. ... It could also indicate that they did this in order to maintain supply and didn't make it clear to the customer it would need an RCD fitted.

That's the main point, isn't it? Yes, if the RCD was not the DNOs, and it was not really them who should have been called out (although DNO would presumably also have had to attend to pull the cutout fuses), but they did attend and replaced the RCD with an isolator 'to maintain a supply', that could explain things. However, in such a situation, if it is (as seems to be the case) TT, and if (as we've been told) there are some circuits which are not otherwise RCD protected, then one would surely have expected the DNO person to have made it very clear that it was essential to get an electrician to fit an RCD as soon as possible?

Kind Regards, John

 

[Screech20]

Sorry if this answer has been said, as I'll be honest, I've not read them all...

I don't see why the DNO touched the rcd. If that was me (I work for a DNO) then I would have advised to contact a domestic electrician as its the customers equipment.

Who is your DNO?

 

[JohnW2]

Many thanks for your quick replies I believe it is a TT system as it is a rural property with overhead conductors. Also there is an earth spike connected to the earth connection going to the bottom LH corner.

Fair enough - and, as I've said, your photos certainly seem to confirm that it's TT.

I'm presuming this is hazardous as without knowing the earth impedance you cannot guarantee that the MCB will trip on a L-E fault.

Worse that that ... unless it's a very unusually low TT earth impedance, you can almost guarantee that MCBs will not trip in the event of and L-E fault. A typical 'fairly good' TT earth would have an impedance of, say, 50Ω - that means that even a zero impedance L-E fault would only result in a fault current of about 4.6A - not enough to trip even a B6 lighting MCB (ever, let alone within the required disconnection time), let alone 32A MCBs on a sockets etc. circuit.

I will tell the owners to get the DNO back in as a matter of urgency.

Indeed. The DNO may well tell them that it is 'not their problem' and that they need to get an electrician to do it (easy, now that there is an isolator). IMO, it certainly needs to be addressed as a matter of urgency.

As an aside - can anyone tell me why there are two live conductors into the isolator but only one neutral?

There are only two Ls going into that property (only two main fuses) - either two phases of a 3-phase supply or, possibly, a 'split-phase' (sort-of 2-phase) supply. ... which leads me to something else I was going to suggest. It could be better to get the DNO/electrician to put separate 2-pole RCDs into each of those two live feeds (rather than a 4-pole one serving both) - so that the entire installation will not be taken out if the RCD trips.

Kind Regards, John

 

[JohnW2]

I don't see why the DNO touched the rcd. If that was me (I work for a DNO) then I would have advised to contact a domestic electrician as its the customers equipment.

Indeed. However, they probably had to open the RCD to diagnose the problem and, having got that far, replaced it with an isolator 'as a kindness' to facilitate fitting of an RCD (but seemingly forgot to tell the householder about the urgent need for a replacement RCD).

I say 'as a kindness' but they might also have been thinking that, if the electrician did things correctly, had they not installed the isolator they (the DNO) would have to attend once or twice more - to pull, and subsequently replace, the cutout fuse!

Kind Regards, John

 

[JohnW2]

As an aside - can anyone tell me why there are two live conductors into the isolator but only one neutral?

There are only two Ls going into that property (only two main fuses) - either two phases of a 3-phase supply or, possibly, a 'split-phase' (sort-of 2-phase) supply. ... which leads me to something else I was going to suggest. It could be better to get the DNO/electrician to put separate 2-pole RCDs into each of those two live feeds (rather than a 4-pole one serving both) - so that the entire installation will not be taken out if the RCD trips.

I've just realised that I didn't read the question properly! Whether it's a 3-phase or 'split-phase' supply, there will be only one neutral, common to all the live 'phases'. A 3-phase supply is 3 phases (lines) plus (usually) one neutral - not three phases and three neutrals. Such is the nature of a 3-phase supply!

Kind Regards, John

 

[finalcoat]

The DNO was SSE
I was assuming the DNO owned the RCD but I could be wrong. I have a TT system at my own house but here the RCD is fitted between the main fuse and the meter so this would be DNO property.
They probably fitted the isolator as they didn't have a 4 pole RCD in the van -
but they should have notified the owner of the consequences.
It may be easier and quicker and less hassle to get an electrician in and fit 1 or 2 RCDs after the isolator/cut out

 

[ban-all-sheds]

All the bathrooms and outside circuits are protected by RCBOs but there is now no RCD protection on any other sockets.

I'll take your word about the RCBOs (pictures too blurred for me to see them), but it does not look to me as if those DBs were installed by a numpty DI - it all looks too professional. Is there anything there to say when it was done?

I'm quite surprised that whoever did it left a single RCD supplying an entire large house.

It could be better to get the DNO/electrician to put separate 2-pole RCDs into each of those two live feeds (rather than a 4-pole one serving both) - so that the entire installation will not be taken out if the RCD trips.

Or if a single-pole RCBO trips on a N-E fault.

 

[JohnW2]

It could be better to get the DNO/electrician to put separate 2-pole RCDs into each of those two live feeds (rather than a 4-pole one serving both) - so that the entire installation will not be taken out if the RCD trips.

Or if a single-pole RCBO trips on a N-E fault.

...or if (as would usually be the case with TT) an MCB failed to trip on a L-E fault! I imagine that what you're getting at is that, although the MCB wouldn't usually trip (so that a TD RCD would trip) in response to an L-E fault, at least one can manually operate the MCB to allow the RCD to be reset, whereas the same is not true with an N-E fault if one only has a single pole device (MCB or RCBO) on the circuit.

Kind Regards, John

 

[finalcoat]

The house was refurbished about 15 years ago
The electrical system is overkill - each room has it own circuit for power and lighting - each bathroom has a separate RCBO for UFH, heated towel rail and spa bath and an MCB for lighting.