RCD wiki + Common Misconceptions about RCDs

[bernardgreen]

However, the point Paul_C makes about the whole of the installation's 'earth' system then becoming live (without any protective devices operating) perhaps needs to me mentioned/addressed.

That is the one thing that really does concern me with PME systems. Having experience neutral bounce here on three occasions ( in thirty years ) when the CPC was definately more than 50 volts from ground I do feel that some extra protection may be required for that situation. The inside of the house is safe ( equipotential ) but external water taps, the gas meter and anything else outside but bonded is then a serious hazard.

Not using PME and having an adequate earth rod is one option.

Or having a voltage operated isolator that cuts Live Neutral and PME "earth" when ever the neutral is not within a few volts of true ground. Not point in cutting Live and Neutral if the "earth" is the hazardous conductor.

I am NOT advocating that until it is forced on us by regulations.

 

[JohnW2]

I can't help but think that we sometimes lose sight of the practicalities of safety. The phrase "reasonably practicable" is very common in safety terms (look it up it does have a legal definition). I rough terms it means that it is accepted that going to the far end of a fart to mitigate all risks is not really possible and that we have to accept a degree of risk.

I couldn't agree more, but some regulations don't seem to have heard about that sort of common sense. Take, for example, the case of LV and ELV cables in proximity/contact. A standard T&E cable, insulated and sheathed, is deemed to be well enough insulated for me to handle it when live, even if wet. However, if a ELV cable comes in contact with it, BS7671 seemingly requires the ELV cable to have insulation 'rated' for the LV voltage in the LV cable. What's that all about? The really frustrating thing is that the insulation of ELV cables (e.g. 'alarm cable') is almost certainly more than adequate for 230V (remember the experiment I undertook, even involving submersion in saline for several weeks?!), even though not officially 'rated' for 230V.

I see what youu mean about the nail and the water pipe, but in reality what are the chances of that? So I would suggest it is fair to accept these situations.

Exactly my point. As I implied, I would suggest that such a ridiculous scenario gets close to 'the far end of a fart' in terms of likelihood!

Kind Regards, John.

 

[westie101]

Is it because the authors of the "regulation" er guidelines, don't want to appear as liable for any advice they give.
The common disease at the moment disguise the fear liability as H&S.

I would suggest that in the case you state any properly qualified person would agree - but hang on a lot of the council building regs folk are not properly electrically qualified!

 

[JohnW2]

That is the one thing that really does concern me with PME systems. Having experience neutral bounce here on three occasions ( in thirty years ) when the CPC was definately more than 50 volts from ground I do feel that some extra protection may be required for that situation. The inside of the house is safe ( equipotential ) but external water taps, the gas meter and anything else outside but bonded is then a serious hazard.

Yes, we discussed and agreed that on a number of occasions.

Not using PME and having an adequate earth rod is one option.

Although some people regard me as mad for saying this, that's my current inclination. When the promised replacement of my supply cables take place, I'm probably going to be 'offered' (provided with) a PME earth - but I am very seriously contemplating declining that offer if/when it comes, ignore the PME 'earth' terminal and stick with my TT electrode. Those who regard that as mad will point out to me that I would then be totally reliant on RCDs for fault protection - but that's no different from my present situation and I still think that, on balance, I might decide to remain 'mad'

That Or having a voltage operated isolator that cuts Live Neutral and PME "earth" when ever the neutral is not within a few volts of true ground. Not point in cutting Live and Neutral if the "earth" is the hazardous conductor. I am NOT advocating that until it is forced on us by regulations.

Such would make good sense, but not only is it not yet forced on us, but having something capable of disconnecting the 'earth' would obvioulsy be a definite no-no under the regs. Furthermore, unless everyone was forced to do it (and that was rigidly policed), it might introduce as many potential hazards as it removed, because of the possible linkage between house's CPCs via primary bonding and service pipes.

Kind Regards, John.

 

[JohnW2]

Is it because the authors of the "regulation" er guidelines, don't want to appear as liable for any advice they give. The common disease at the moment disguise the fear liability as H&S.

Quite so - and the HSE are trying hard with their PR to point out that it is not them that is responsibe for a lot of the nonsense around.

I would suggest that in the case you state any properly qualified person would agree - but hang on a lot of the council building regs folk are not properly electrically qualified!

Exactly. And, needless to say, the matter I cited is but one of many examples of 'OTT silliness' than comes from the pens/typing fingers of these people.

Kind Regards, John.

 

[JohnW2]

With TT, there is the possibility of a fault on the incoming line to the casing of the unit, which obviously would not be detected by any RCD inside the unit since the fault is "upstream" of any such device. Such a fault would then not only make the casing live, but also everything else connected to it throughout the house. Hence the suggestion that with such an arrangement only a plastic casing be used. You might want to work something about that into the piece.

Indeed. I have already mentioned in the draft the need for an insulated enclosure for an up-front RCD, so I suppose I could slip that in as well, for the situation in which there is no such up-front RCD.

Kind Regards, John.

 

[ban-all-sheds]

Not using PME and having an adequate earth rod is one option.

Or using PME and adding your own rod to it.

Or having a voltage operated isolator that cuts Live Neutral and PME "earth" when ever the neutral is not within a few volts of true ground. Not point in cutting Live and Neutral if the "earth" is the hazardous conductor.

AFAIK you may not have a switching device of any sort in an earth conductor.

You could have your isolator in L&N just after the meter - Q is would you be allowed to move the earth connection to N downstream of that or does it have to be done at the cutout?

If you did move it you could use an earth fault relay looking at current in the conductor to your rod.

I think.

 

[JohnW2]

Not using PME and having an adequate earth rod is one option.

Or using PME and adding your own rod to it.

When contemplating the posible future of my installation, I considered that option, but decided it was inadequate. The impedance of the path resulting in a high voltage neutral could easily be so low that attaching an earth rod to it would have very little impact.

[AFAIK you may not have a switching device of any sort in an earth conductor.

Indeed - I've already pointed that out to Bernard. It's a definite no-on under current regs.

You could have your isolator in L&N just after the meter - Q is would you be allowed to move the earth connection to N downstream of that or does it have to be done at the cutout?

Good question - but I suspect the answer is that this would not be allowed, not the least because the earth fault path would then be via the meter.

If you did move it you could use an earth fault relay looking at current in the conductor to your rod. I think.

You could - but, as above, I suspect that such an arrangement would not currently be allowed - but I may be wong. Westie??

Kind Regards, John.

 

[Paul_C]

Good question - but I suspect the answer is that this would not be allowed, not the least because the earth fault path would then be via the meter.

An L-E fault on the installation already results in the fault path being via the meter anyway. All that would happen is that the return to the supply neutral would also pass through the N connections on the meter. As the two N terminals on the meter are just connected together internally, it wouldn't make much difference.

 

[westie101]

You could - but, as above, I suspect that such an arrangement would not currently be allowed - but I may be wong. Westie

Theoretically and technically I can't see why not.
But would ask the question (in view of earlier comments) why?

 

[JohnW2]

You could - but, as above, I suspect that such an arrangement would not currently be allowed - but I may be wong. Westie

Theoretically and technically I can't see why not.

Inteesting - not really what I expected you to say In fact, I have a feeling that, quite apart from 'your' regulations, BS7671 may not allow a combined neutral and earth conductor to exist in a domestic property beyond the service head - but I may be wrong (will look later).

But would ask the question (in view of earlier comments) why?

You may be able to help answer that. I don't know the facts, but there seems to be a perception that it is not 'incredibly rare' (i.e.'rare enough to ignore') for the potential of an incoming TN-C-S neutral to rise to a dangerous level above true earth under certain fault conditions. If that is the case (you tell me!), then it would seem reasonable to have a means of automatically disconnecting that source of danger, because of the issue of earthed/bonded metalwork outside of the equipoential zone (e.g. the outside taps which concern Bernard).

Kind Regards, John.

 

[JohnW2]

You could - but, as above, I suspect that such an arrangement would not currently be allowed - but I may be wong. Westie

Theoretically and technically I can't see why not.

543.4.1 of BS7671 says "PEN conductors shall not be used within an installation except ...."

... and, perhaps of more interest to you, a note attached to that says "In great Britain, regulation 8(4) of ESQCR 2002 prohibits the use of PEN conductors in consumers' installations".

Does that alter your view at all? I guess it depends upon where "the consumer's installation" starts.

Kind Regards, John.

 

[JohnW2]

An L-E fault on the installation already results in the fault path being via the meter anyway. All that would happen is that the return to the supply neutral would also pass through the N connections on the meter. As the two N terminals on the meter are just connected together internally, it wouldn't make much difference.

True - but see what I've just written to Westie about 543.4.1.

Kind Regards, John.

 

[westie101]

I perhaps mis-understood but was answering for the case of having a monitoring device on the connection from the MET to a privately owned earth rod.

In answer to the how often, perhaps we should look at the two main cases.
Loss of neutral on a 3 phase main in the street very, very rarely leads to reports of shocks etc. Lots of other effects but rarely that! These faults are fairly rare and usually only occur on one type of underground cable that is not in use in all the DNOs.

Loss of neutral on a single phase or 3 phase service to a property also occur , having said that in this patch (about 250,000 customers) 3 or 4 per year. This often leads to reports of shocks from internal metalwork, despite correct bonding being in place (possibly a whole new debate) these cases are always made safe, initially, by getting the customer to switch the main switch off.
I have never heard of a case where anyone received a severe (requiring hospital attention).

 

[JohnW2]

I perhaps mis-understood but was answering for the case of having a monitoring device on the connection from the MET to a privately owned earth rod.

Ah, no, that wasn't the question - and, like you I can't see why there would be any problem with that.

The issue was that of taking the combined neutral and earth of a PME supply through the meter before deriving the installation's 'earth' from it (i.e. before invoking the 'S' of TN-C-S). This was BAS's suggestion as a possibly way of implementing bernardgreen's idea about automatically disconnecting both N&E without (perhaps!) technically violating the prohibition of having a disconnecting device in the protective earth. I suspected that this delay in spilting the PEN conductor into N & E until after the meter would not be allowed. Do you have an answer to that question?

In answer to the how often, perhaps we should look at the two main cases.
Loss of neutral on a 3 phase main in the street very, very rarely leads to reports of shocks etc. Lots of other effects but rarely that! These faults are fairly rare and usually only occur on one type of underground cable that is not in use in all the DNOs.
Loss of neutral on a single phase or 3 phase service to a property also occur , having said that in this patch (about 250,000 customers) 3 or 4 per year. This often leads to reports of shocks from internal metalwork, despite correct bonding being in place (possibly a whole new debate) these cases are always made safe, initially, by getting the customer to switch the main switch off. I have never heard of a case where anyone received a severe (requiring hospital attention).

Thanks. Adequate bonding should, of course, prevent shocks occurring indoors, regardless of the potential of the PEN conductor relative to 'true earth' but, as bernardgreen says, this is not necessarily the case outdoors, where bonding is impossible. Do you have any idea what sort of pd relative to 'true gropund' can arise in the sort of situations you are mentioning?

Kind Regards, John.