Black cable for 110v site extension leads?

[JohnW2]

Interesting, John. I certainly recall working in a house where when I touched the neutral it tripped the RCD - without having moved any wiring or anything, so I'm pretty sure I wasn't bridging it to earth except through me. It sticks in my mind because having reset the RCD I stupidly did exactly the same a second time!

I really think you must have somehow brought the neutral into contact with earth.

It hadn't occurred to me that I should have felt a shock. Possibly with it being the neutral the potential may have only barely been high enough to cause 30mA through my forearm ...

Unless there was some major fault somewhere in the installation, the neutral would be pretty close to earth potential - not remotely high enough to send 30mA through you. In any event, as I said, if 30mA did go through any part of you, you would 'feel' that as a very severe shock.

EDIT: I'm almost tempted to try touching a live just to compare, but that way lies 'asking for it', so instead I'll just try to take special note if/when I trip anything again by accident.

I presume you're not serious. That would obviously be very silly and, because RCDs are definitely not a foolproof panacea, it could be the last thing you ever did.

Kind Regards, John

 

[Risteard]

Not really, because as said only around half of the voltage will appear on exposed conductive parts in a fault (half of either 55V for a single phase system or 63.5V for a three-phase system) which still should not pose a shock risk. ....

Do I take it that you didn't really intend to type the words I've emboldened?

Kind Regards, John

No, because in the leads to the appliance the cpc will be of the same csa as the phase conductors, and therefore a fault with either phase conductor to an exposed conductive part will only expose it to half of the voltage. So there won't be anything like the 55V/63.5V available to touch in a fault condition (after all anything exceeding 50V would be considered unsafe!).

 

[KevDIY]

Hmm its a mystery then, John. I don't see how it could have been a direct neutral/earth bridge as I hadn't even loosened any wires by that stage, just removed the covers. But I agree that 30mA is chosen to be 'usually survivable' rather than comfortable, so I'm not doubting you. I never really considered it strange until now.

Yes just a small joke, to illustrate my surprise - I would 'never' intentionally touch a live wire no matter how many RCDs were on it. What really amazes me is that if one gets a real belt in the process of tripping an RCD then clearly I must have avoided being in the circuit all these years. Which of course is excellent and certainly what I've tried hard to achieve, but I just assumed at some point I must have done it and so not felt much. Though before I get too smug it also means that I've reserved my few genuinely careless moves for situations in which the RCD didn't trip or wasn't present

Cheers
Kev

 

[JohnW2]

No, because in the leads to the appliance the cpc will be of the same csa as the phase conductors, and therefore a fault with either phase conductor to an exposed conductive part will only expose it to half of the voltage.

Oh, fair enough - if there is an intact CPC and it is satisfactorily connected to the exposed part, then I agree. Given that the original comment was in the context of the pros/cons of earthed vs floating, I was thinking of the relative merits of earthed/floating when there was a failure of the CPC connection - in which case, with an earthed supply, a fault could result in a 55V touch voltage (relative to earth), whilst in the absence of any earth referencing, the 'touch voltage' should be of no consequence.

Your comment obviously applies equally whether the supply is earth-referenced or floating, provided only that the exposed part is connected via a CPC to the CT of the tranny secondary. However, if the supply is floating, the risk of getting even a significant 27.5V 'shock' is obviously very low.

Kind Regards, John

 

[JohnW2]

Hmm its a mystery then, John. I don't see how it could have been a direct neutral/earth bridge as I hadn't even loosened any wires by that stage, just removed the covers. But I agree that 30mA is chosen to be 'usually survivable' rather than comfortable, so I'm not doubting you. I never really considered it strange until now.

I obviously cannot know what you managed to do but, believe me, if 30mA had gone through any part of you, even if only for a few tens of milliseconds, you would certainly have known all about it

Kind Regards, John

 

[Risteard]

However, if the supply is floating, the risk of getting even a significant 27.5V 'shock' is obviously very low.

In an unearthed supply, however, were there a fault on two separate appliances then one would be exposed to a potentially dangerous fault.

 

[JohnW2]

In an unearthed supply, however, were there a fault on two separate appliances then one would be exposed to a potentially dangerous fault.

I keep hearing and reading (including in the oft-quoted IET's 'Wiring Matters' article) the equivalent of that in relation to (the similar situation of) generator supplies but I don't see (in either case) what it's got to do with whether or or not the supply is earth-referenced. Either I'm missing something, or else some folk are getting two issues muddled up.

As I understand it, the (extremely low) risk being referred to arises if two Class 1 appliances are fed from the same (transformer or generator) supply, but with no CPCs connecting their exposed parts to (each other and) some common point of the supply (N or CT), and then both appliances develop faults between live conductors and exposed parts, the live conductor being different for the two appliances (L and N, or 'L1' & 'L2'). In that situation, the two faults will result in a pd appearing between the two exposed parts which is equal to the full supply voltage (110V or 230V).

However, what I've just said is equally true whether the supply (hence CPCs) is earth-referenced or not. In fact, I would have thought that the most potentially dangerous situation (in the absence of RCDprotection of the secondary side of tranny) would be for the supply (N or CT) to be 'earthed' (connected to true earth) but for their not to be effective CPC connections from exposed parts to (each other and) that 'supply earth'.

I therefore wonder whether when people say (as you do above), that it is a potential problem of an 'unearthed supply', I wonder whether they are really referring to 'a supply without effective CPCs', regardless of whether the supply is earth-referenced or not? ... or, as I said, am I missing something?

Kind Regards, John

 

[KevDIY]

Isn't it simply that with an earthed supply, the exposed parts of properly made class 1 appliances will also be earthed, and so in theory will be unable to get appreciably above earth potential in the direction of either L1 or L2?

EDIT: Though I suppose they could get around half way to L1 or L2 potential (assuming similar earth/L impedances) if there were no over-current protection, but that would only be around 27.5v on a site supply presumably.

Cheers
Kev

 

[JohnW2]

Isn't it simply that with an earthed supply, the exposed parts of properly made class 1 appliances will also be earthed, and so in theory will be unable to get appreciably above earth potential in the direction of either L1 or L2?

As I just wrote, when there are two or more items of equipment, the important point is that the exposed parts be connected together (and preferably to the 'common point' of the supply) by CPCs - that minimises the potential difference which can exist between the exposed parts, regardless of whether or not the CPCs/supply are ('truly') 'earthed'. If the supply is not earth-referenced at all (i.e. 'floating'), then potential differences relative to earth are irrelevant, and non-harmful (unable to supply significant current).

Earthing a supply turns an otherwise 'harmless' potential between a live conductor and earth into a potentially dangerous one, but that is partially mitigated by the fact that, with a earth-referenced supply, RCDs can give some protection against L-earth shocks.

Kind Regards, John

 

[KevDIY]

Oops sorry. Yes you were quite clear, I misconstrued no earth reference for no protective conductor. I'm used to them only being protective conductors if they're earthed.

Its sort of like the argument between equipotential bonding and actual earthing of things in bathrooms & kitchens, then?

Cheers
Kev

 

[EFLImpudence]

Its sort of like the argument between equipotential bonding and actual earthing of things in bathrooms & kitchens,

No, it isn't. There is no argument.

 

[KevDIY]

True, but not everyone gets it; I have seen it taken all the way back to the CU's earth block in 10mm green and yellow

 

[JohnW2]

Its sort of like the argument between equipotential bonding and actual earthing of things in bathrooms & kitchens, then?

As EFLI has said, with the exception of the matter I'm about to raise with him, that's a different issue. In the case of a domestic installation, the supply is earth-referenced (neutral connected to true earth at transformer, and also at other places in the case of a TN-C-S supply, and the installation itself has to have a connection to earth. Hence, any CPC connected to the installation's Main Earthing Terminal is also, thereby, connected to true earth.

However, if you are saying that we may be dealing with an issue of terminology, you may be right. Just as 'earthing' and 'bonding' are two different (but sometimes confused) concepts, maybe some people are talking about two different meanings of 'earthing' - some meaning 'true earthing' (connecting to true earth - e.g. via a domestic installation's MET) and others meaning "connecting to the 'earthing system' {'MET'}, hence 'return path') of a local generator- or transformer-derived supply, which, as we have been discussing, is not necessarily connected to true earth.

Kind Regards, John

 

[JohnW2]

Its sort of like the argument between equipotential bonding and actual earthing of things in bathrooms & kitchens,

No, it isn't. There is no argument.

I agree that there's no argument in relation to bathrooms (no longer kitchens!) in a (necessarily earth-referenced) domestic installation, but I think I see Kev's point, in that it's all rather different if one is dealing with a supply derived from a local generator or tranny which is not earth referenced. In that situation, the risk mentioned by Risteard (and also often mentioned by eric) could be mitigated by 'equipotential bonding' (in the true sense) via 'CPCs' connected to two items of equipment, even if those CPCs were not connected to anything (N or CT) at the supply source. Indeed, as I've been saying, if those CPCs were connected to the genny or tranny, I wonder if some people might then call it 'earthing', even if there were no connection to true earth? Is that terminology issue perhaps behind some the the apparent 'confusion'?

Furthermore, although the primary purpose of the connections I mentioned above would be 'bonding' (to minimise PDs between exposed parts of the two items, in the event of the extraordinarily unlikely pair of faults postulated!), they could/should also, in that very improbably scenario, also result in operation of any OPD on the output of the source - which, again, could perhaps be interpreted by some as 'earthing' (even though there were no connection to true earth!)

Kind Regards, John

 

[EFLImpudence]

Isn't that just the difference between true earth, the supply earth and CPCs which 'we' call earth but none of which is bonding?