Advantages of a type C MCB

[ban-all-sheds]

But still raises the question when the neutral - line impedance is so important why no provision in the paperwork to record it?

IIRC NICEIC EIC schedule sheets have a column for it.

 

[JohnW2]

With true TT, if Zs (i.e. EFLI) is all one measures,

But it's not.

one has no idea whether the requirements for disconnection times for a L-N fault are satisfied or not - they certainly wouldn't be on the basis of the Zs measurements. Only an L-N loop impedance could demonstrate that.

L - N loop has to be measured to determine the PFC.

I was very unclear in what I wrote. Apologies.

As you say, L-N loop impedance and PFC are effectively the same thing, provided one knows the voltage, so you could interchange them in what I wrote above. Although I wrote "if one only measured Zs...", what I really meant was along the lones of "if disconnection requirements are only based on measurements of Zs....".

It may well be that I am missing or forgetting something - is there a requirement in the regs for disconnection times based on PFC (or L-N loop impedance), rather than Zs?

Kind Regards, John.

 

[JohnW2]

I would say that it's anyone's guess as to how the authors intended "cannot be satisfied" to be interpreted ....

There is no ambiguity or imprecision in the meaning of "cannot" - look it up.

Whatever the dictionary definition, I think that, in practice, there is often plenty of scope for interpretation, because of the way in which the word is used -often with implicit qualifications and/or 'not mentioned'(but intended) qualifications.

In other words, in present context, "cannot be satisfied" could be interpreted as meaning "cannot be satisfied with the cable which has been installed".

Kind Regards, John

 

[EFLImpudence]

It may well be that I am missing or forgetting something - is there a requirement in the regs for disconnection times based on PFC (or L-N loop impedance), rather than Zs?

I don't think so, as such.

However, as PEFC is almost always lower than PSCC (hence PFC) the disconnection times of PSCC (PFC) will be better than PEFC.

It's the worse case scenario noted on the certificate (as Zs at farthest accessory - all the others will be better).

PFC is recorded on certificates and so calculations could be made.

 

[JohnW2]

It may well be that I am missing or forgetting something - is there a requirement in the regs for disconnection times based on PFC (or L-N loop impedance), rather than Zs?

I don't think so, as such.
However, as PEFC is almost always lower than PSCC (hence PFC) the disconnection times of PSCC (PFC) will be better than PEFC.

Yes, but how much better? For TN systems, what you say makes sense - if the PEFC disconnection times are satisfactory, then the PFC ones almost certainly will be. However, as I said, in a 'pure' TT system (i.e. not aided by the neighbour's TN-C-S!), PEFC disconnection times would be pitifully inadequate. You're obvious right in saying that the PFC disconnections times would be much better, but (if all one is paying attention to are Zs measurements) how does one know whether they would necessarily be better enough to satisfy the requirements (given that, as we seem to agree, the only requirements in the regs appear to be based on Zs, not PFC?)?

Kind Regards, John.

 

[Risteard]

But still raises the question when the neutral - line impedance is so important why no provision in the paperwork to record it?

IIRC NICEIC EIC schedule sheets have a column for it.

News to me.

 

[ericmark]

What worries me is my neighbours house is just 6 foot from mine. So if his earth system is different to mine then the gradient between the two houses could be rather steep.

Where we have 20 meters between properties then for the owner to select the earth system may be acceptable but when close then it needs to be selected by the DNO so the chance of our earths being at a different voltage is as low as possible.

This worries me with caravans. On a caravan site the fire regulations mean the caravans are not too close and having the main building as a TN-C-S and caravans as TT is not a problem. But when stored at home maybe two foot from the house with earthed cast iron drain pipes. Then to put the caravan on TT while house is TN-C-S could be dangerous.

NOTE: In the UK the ESQCR prohibit the use of a TN-C-S system for the supply to a caravan or similar construction.

OK so how does one keep batteries charged while in storage? Only safe way would be to ban TN-C-S total. The problem is does not matter how many RCD's or isolators one uses they don't disconnect the earth so if there is a problem you can't even turn it off.

 

[westie101]

And how many are kept on supply at home by an extension lead from a 13A socket.



Some more info

Permanent site buildings such as conventional living accommodation, shops, bars, etc, can be connected to a PME earthing terminal provided the installation meets the requirements of Section 11 of this CP. Particular care shall be taken to ensure that bonded metalwork does not exist in a position where it can be touched from outside the building. In particular, due to the probability of persons being barefooted on caravan sites, outside taps and the like shall be provided with insulated inserts.
Other bonded metalwork that may be accessible from the outside of the building shall be covered with permanent insulation. The extension
of PME to toilet and amenity blocks is not recommended.

The method of connecting caravans is via phase and neutral wiring only. No earthing terminal shall be provided and the customer shall be advised in writing to install a RCD in accordance with Figure C4, Appendix C. A copy of this drawing and its attached notes may be used in any correspondence with the customer advising the use of this type of protection. The responsibility for maintaining this equipment is entirely that of the customer. The earth electrode values and RCD sensitivity quoted in Figure C4, Appendix C, are based on limiting the sustained voltage on the protective conductor to 50V.

 

[JohnW2]

This worries me with caravans. On a caravan site the fire regulations mean the caravans are not too close and having the main building as a TN-C-S and caravans as TT is not a problem. But when stored at home maybe two foot from the house with earthed cast iron drain pipes. Then to put the caravan on TT while house is TN-C-S could be dangerous.

If you don't export the TN-C-S earth, but have local TT (or no earth, whilst in the drive!), then the cast iron drain pipe etc. surely would not, per se, present a significant hazard.

However, if you did export the TN-C-S earth, then you're into the same extensive debate (which resulted in no consensus) we've had about exporting such earths to outbuildings etc., and the risk would presumably be no different from the risk arising from that, or from (naughtily!) using a Class I tool/appliance on an extension lead in the garden (or near that drain pipe).

Kind Regards, John

 

[bernardgreen]

Particular care shall be taken to ensure that bonded metalwork does not exist in a position where it can be touched from outside the building. In particular, due to the probability of persons being barefooted on caravan sites, outside taps and the like shall be provided with insulated inserts.

Why does that particular care not ( apparently ) apply to domestic installations where it seems external water taps on a pipe through the house wall are not required to be insulated. People can be bare foot in the garden of a private house.

 

[westie101]

Don't know!

 

[JohnW2]

Why does that particular care not ( apparently ) apply to domestic installations where it seems external water taps on a pipe through the house wall are not required to be insulated. People can be bare foot in the garden of a private house.

Exactly. As I recently pointed out in my response to eric's message, the 'caravan hazard' situation being discussed is essentially no different from the potential hazard of TN-C-S we've often discussed which exists in many/most of the gardens in this country - yet the regs seem to treat the two situations very differently.

The probably should be a corollary of bonding built into the regs in the case of TN-C-S supplies. In other words, whilst the regs rightly have requirements (e.g. main bonding) to ensure an equipotential zone (equipotential to the supply neutral) within a building, one probably should be prohibited from 'exporting' that equipotential zone to outside of the building (except, perhaps, into an outbuilding which was itself consituted as an equipotential zone, if that were possible) - i.e. there should be a requirement for things like outside pipework and taps to be electrically insulated from the (bonded) pipework within the building (and/or fully insulated such that no metal could be touched). From a safety point of view, such a prohibition would seem to make total sense.

Kind Regards, John.