More on meters and 544.1.2

[JohnW2]

In my opinion, the reg has no ambiguity and the commas make it's requiremnt quite clear.

Thanks for your opinion. As you will have seen, there are a variety of opinions on this issue.

In John's first post he stated it as an either or scenario. I don't know what is inside a meter, but asuming that there are insulating parts inside would make bonding after the meter an understanable proposition.

I don't fully understand what you're saying there. If the meter does represent an electrical interuption (i.e. no electrical continuity across it), then a 'bond' attached after the meter would not fulfil the function of Main Protective/Equipotential Bonding. Like you, I don't know whether meters always/sometimes/never represent electrical interruptions (and I doubt that one can guarantee that the same is, and always will be, true of all meters).

I am a bit confused by John's quote
'Are you not overlooking the (one and only) reason for main bonding - namely to prevent a PD arising within the house by virtue of a difference in potential between the house's MET and the potential introduced (from outside the premises) by an incoming service pipe?'

Why are you confused? Do you disagree with my statement? If so, perhaps you can explain why.

As far as I am concerned, the one and only purpose of Main Protective/ Equipotential Bonding is to ensure an equipotential zone within the premises - which is achieved by ensuring that there are no 'touchable' parts within the premises which are at a potential appreciably different from that of the electrical installation's MET. The only thing which can compromise that equipotential zone is an extraneous-conductive-part (as defined in regs) which enters the premises. Bonding them to the MET minimises the PD (relative to MET) which they otherwise might result in. Bonding anything which is not such an extraneous-c-p (such as a water service pipe after an insulating interruption) does nothing to produce/ensure an equipotential zone, as is therefore not MPB/MEB. Do you disagree with some/all of that?

Kind Regards, John

 

[JohnW2]

There is an article in Wiring Matters from Summer 2012 which may be useful reading about this subject.

OK, having looked at that article again, I think I can perhaps see what your thinking, but I don’t think it invalidates the statement I made.

I suspect what you are thinking about is that, as explained at length in the article, in a TT installation, the main purpose of MPB/MEB (given that the purpose in relation to PME supply faults obviously doesn’t apply) is that of reducing the ‘touch’ PD between exposed-c-ps and extraneous-c-ps in the event of an ‘L-E’ fault within the installation. The article actually says that this effect is ‘most noticeable’ in TT systems but, frankly, the beneficial effect on these touch voltages in TN systems will be negligible (probably no more than a volt or two).

In fact, one doesn’t really need their maths to make the point. All they are saying is that, in a TT installation, in the event of an L-E fault in the installation, the potential of the installation’s earthing system (Earth rod, MET, CPCs and exposed-c-ps) will rise to a high level relative to true earth, not that much lower than line voltage. There will thus be a dangerous PD between any exposed-c-ps and anything which represents a route to true earth, such as an extraneous-c-p. I think we all knew that – and, indeed, that it is the main (dare I say ‘one and only, with TT?!!) reason for needing main bonding in a TT installation.

However, I don’t see this is inconsistent with my statement “the (one and only) reason for main bonding [is] to prevent a PD arising within the house by virtue of a difference in potential between the house’s MET and the potential introduced (from outside of the premises) by an incoming service pipe” – that latter potential being ‘true earth potential’. It’s only because that extraneous-c-p introduces that true earth potential that the hazard discussed above (and in the article) exists.

Whatever, I don’t think any of this alters the arguments or discussion. In particular, bonding after an insulating section (or insulating meter) still achieves little/nothing, and still does not correspond with what I would call MEB/MPB - not the least because what one is then bonding is not an extraneous-c-p! If it’s not an extraneous-c-p (i.e. a route to earth other than the installation’s earth), then it does not present the risk of high exposed-extraneous touch PDs which the article talks about, even in a TT installation. If the pipework is literally ‘floating’ (very rare) there is no serious ‘touch PD’ shock risk. If it’s not floating, that’s because it is connected to the MET and installation CPCs – which, again, means that the high PDs between it and exposed-c-ps of which the article speaks will not occur. Whatever, if it doesn’t represent a path to ‘true earth’, the high touch PDs we’re talking about cannot arise.

The article is, rightly, stressing the significant danger which exists if, in a TT installation, something which is an extraneous-c-p does not have MEB, but that’s a totally different matter, and not something we’ve been discussing in this thread. No-one has ever suggested other than that a metal pipe which enters a property and remains metal (presumably the most common situation, certainly in the past) must be main bonded - for the reason discussed above. Interestingly, from the viewpoint of this ‘touch voltage’ function, there is no obvious need for the bonding to be applied ‘as close as practicable to the point of entry’ – in fact, its benefit is theoretically at its greatest when bonding is close to where the fault occurs, wherever that may be. However, the whole discussion here has been about the different situation, in which there is (or may be) an ‘insulating interruption’ in the service pipe – in which case, of course, the extraneous-c-p ceases to exist,and ceases to pose any hazard, beyond that interruption.

Kind Regards, John

 

[bernardgreen]

Wouldn't it be so much easier to understand things if the word "earth" was replaced by "protector" in all areas of the installation.

Main PROTECTIVE Terminal instead of Main Earth Terminal.

Only in a TT system is that terminal an earth ( ground ) terminal, in all varieties of PME that terminal is NOT an earth ( ground ) terminal. In PME it is the NEUTRAL of the supply. Voltage drop along the neutral means that terminal known as the MET is never at ground potential.

The difference in voltage between MET and ground may be very small ( in normal conditions but not in some fault conditions ) due to the voltage drop along the neutral from cut out to street cable. Any current in the neutral along the street cable to sub-station will further affect the voltage difference between MET and ground.

Sockets should be wired with Live Neutral and PROTECTIVE as the CPC is already known as. The function of the "earth" pin is not to "earth" the appliance but to protect it from becoming Live in the event of a fault.

( leave the word "earth" for appliances and their plugs, too much to change in the public's language )

If the MET was ensured to be at ground potential then metallic water pipes could only import a potential ( from the ground ) that was the same as that of the MET. But with PME the MET cannot be ensured to be at ground potential and therefore anything at ground potential is a possible hazard to any one touching an item connected to the MET. ( the reason why the use of "earthed" tools is the garden is considered dangerous ).

 

[JohnW2]

Wouldn't it be so much easier to understand things if the word "earth" was replaced by "protector" in all areas of the installation.

At first sight, that might make sense (and would reduce the need to write 'earth' so often), but it's so well established that its not going to change, and probably would not be very helpful. The reality is that, under some fault conditions, no 'earth' is going to be anywhere near earth potential. You tend to major in your discussions on PME 'earths', but TT ones are probably even worse in this respect. In the presence of an L-E fault in the installation, the potential of a TT 'earth' is likely to rise to close to line potential (until, hopefully, some protective device operates'). We therefore already know and accept that an 'earth' may not be at earth potential under fault conditions, and I'm not sure that changing the word would make any difference to our thinking.

Sockets should be wired with Live Neutral and PROTECTIVE as the CPC is already known as. The function of the "earth" pin is not to "earth" the appliance but to protect it from becoming Live in the event of a fault.

...and, at least as important, to facilitate operation of a protective device in the event of an L-CPC fault - but, again, I don't see that changing the words would alter anything.

If the MET was ensured to be at ground potential then metallic water pipes could only import a potential ( from the ground ) that was the same as that of the MET. But with PME the MET cannot be ensured to be at ground potential and therefore anything at ground potential is a possible hazard to any one touching an item connected to the MET.

As above, although you always talk about PME, the risk is probably greater (certainly, the risk of a very high MET potential much more likely) in a TT installation - per my most recent post.

Kind Regards, John

 

[bernardgreen]

John

With a TT earth and a Live -CPC fault the potential on the CPC will rise until the current into the ground rod is enough to trip the RCD be it a 30 mA in the CU or the 100 mA specific for the TT installation. The same with a Neutral to CPC fault. Provided the ground rod has a low enough impedance then the voltage on the CPC relative to ground is very unlikely to reach 50 volts above ground. A failed network neutral cannot force the potentialon the CPC above 50 volts.

With PME and a defective network neutral there is no way to cut the supply neutral before its connection to the MET ( the CPC ) and thus there is no way remove hazardous voltages on the neutral from the CPC. This is still safe in regards to electric shock PROVIDED there is no means to touch the CPC and ground at the same time. It does mean there will be current flowing from CPC to ground through any and all paths from CPC to ground. High much current will depend on the impedance of that path. If there is a low impedance path, such as metal service pipe to a metal distribution network a very high current could flow and possible burnt out bonding cables.

Limiting the CPC potential above ground to less than 50 volts was the idea behind the Voltage operated earth leakage circuit breaker. Any fault that raised the CPC to more than 50 volts above ground tripped the VOELCB. The VOELCB worked on very low earth leakage current provided all the current going to the CPC went through the coil of the VOELCB. 50 volts on a ( typical ) 500 ohm coil meant the device would trip on 100 mA of leakage. Both Live and Neutral were isolated and the CPC remained connected to true ground via the coil of the VOLECB

The VOELCB system was compromised when alternative paths from CPC to ground came into existance. A 100 ohm parallel path from CPC to ground would have to be taking 500 mA of current before the voltage on the CPC reached 50 and tripped the VOELCB. Still safe voltage wise but a fire risk from high currents in the CPC to the alternative path to ground.

 

[JohnW2]

With a TT earth and a Live -CPC fault the potential on the CPC will rise until the current into the ground rod is enough to trip the RCD be it a 30 mA in the CU or the 100 mA specific for the TT installation. ... Provided the ground rod has a low enough impedance then the voltage on the CPC relative to ground is very unlikely to reach 50 volts above ground.

I agree that there will hopefully (as required) be an RCD which will limit the duration of the high potetial of the TT earthing system (MET, CPCs etc.) but during that period prior to RCD operation, the potential will usually be an awful lot more than 50V above earth - as I said, 'approaching' full line voltage. Given that RCDs fail (there is this story afoot that 1 in 7 {or is it 7%?} of those in service are faulty), this underlines the importance of adequate main bonding in a TT installation - since, otherwise, persistant 'touch voltages' much greater than those which will commonly be seen as a result of a PME supply fault are then likely to be present.

I agree that with PME is that, although the potential of the 'earth' will rarely rise to as high as it does with and L-E fault in a TT installation, there is no (reg-compliant) way that a protective device could render the situation safe if the PME potential rises. A (true) voltage operated device, with its own earth rod, could be used to disconnect the L and N (after splitting of the PNE feed), but (as you say) there is no reg-compliant way that it could disconnect the MET from the 'PME earth'. Main bonding is really all one can do to minimise risks.

Given that L-E faults within installations are presumably much more common than PME supply faults that result in very high MET-earth PDs, and also given the apparently fairly high failure rates of RCDs, I would suggest that the risks resulting from inadequate main bonding in a TT installation are probably, in fact, numerically appreciably higher than the risks associated from inadequate main bonding in a PME installation. Adequate main bonding should minimise risks (within the created equipoential zone, not necessarily outdoors) with either supply type.

Kind Regards, John

 

[bernardgreen]

I accept that a transient voltage CPC to ground greater than 50 volts could exist when the fault occurs but ( provided at least one of the RCDs operate ) it will be short duration. Never the less it is a shock hazard.

It is the risk of over loaded bonding cables catching fire when the CPC of a PME system is bonded to a very low impedance earth. My first experience of this was the damage at a hill top transmitter site which was supplied by an overhead four wire 3 phase + neutral 440 volt supply. Most of the equipment was 230 volt between a phase and neutral. The network transformer was about a mile away at the bottom of the hill. The transmitter earthing and lightning system had a very large ground rod and mat system.

A fault occured in the supply, probably a snapped wire between poles.

The damage was confined ( other than smoke damage ) to those items of equipment that had the supply earth connected to them and also used the ground mat as a functional earth. Those that only had the ground mat as earth survived.

At the insistance of one of the users ( an organisation with a lot of influence and technical knowledge ) the supply was altered to be 11 Kv (?) to a three phase transformer located at the top of the hill adjacent to the equipment buildings.

 

[JohnW2]

I accept that a transient voltage CPC to ground greater than 50 volts could exist when the fault occurs ...

Indeed - but I think 'could' is a bit of understatement, at least in terms of most residential TT installations. The resistance of the rod is likely to be 50-100, and the remainder of the fault loop impedance probably <22&#937; (assuming a rod resistance of ~20&#937; at the transformer. That means that between ~69% and ~82% of line voltage is likely to appear between the CPC and true earth in the presence of an L-E fault.

( provided at least one of the RCDs operate ) it will be short duration. Never the less it is a shock hazard.

Indeed - but, as I said, the reliability of RCDs appears to be in doubt - and I assume that, with the increasing prevalence of 'all-RCD' CUs, many TT installations only have a single RCD in any one circuit.

It is the risk of over loaded bonding cables catching fire when the CPC of a PME system is bonded to a very low impedance earth.

Yes, that's possible, but it's obviously a reason for having adequate bonding conductors, not a reason for not having bonding. The 'standard' 10mm² main bonding cable is 'rated' at about 60A, and therefore, I suspect, will not come to any great harm (certainly not catch on fire) at less than 200A or so. If one has extraneous-c-ps with an impedance to earth less than about 1&#937; (which may sometimes happen), one might therefore need to consider a fatter bonding cable, in case neutral potential ever rises to near line potential. However, I do wonder how common it is for PME faults to result to neutral potential rising to anywhere near that high. In any event, the CNE conductor itself will commonly only be 16mm² - so if your 10mm² (or even 16mm²) bonding conductor is melting or bursting into flames, then that might be the least of the problems in terms of the big picture!

Kind Regards, John