where installations are supplied from a pme supply it is often impracticle to extend the earth equipotential zone to remote buildings because of the interviening ground which exhibits true earth potential. this potential will be negative with respect to th pme earthg( at least when some load is being supplied and for this reason extending the zone would be undisirable. (EXTRACT FROM PRACTICLE GUIDE TO 7671)
i was taught as long as you meet the disconnection times required then the earthing was deemed safe and acceptable?
also what does it mean by interviening ground which exhibits true earth potential. this potential will be negative with respect to th pme earthg( at least when some load is being supplied).
[This message has been edited by james S (edited 10-08-2003).]
james, generally, it is recommended to consider separate bonding arrangements for each remote building. Where load current is present in the neutral of a pme supply then the voltage drop across that neutral will be apparent between items connected to the main earth terminal and true earth. Perhaps not dangerous with respect to an actual shock but it may be possible to perceive a shock. Effectively, you have a higher resistance parallel path for that load current. Small but decernable currents can flow thus defeating the whole notion of an equipotential zone. Simply meeting disconnection times is not the only caveat for a safe installation!
I see what they're trying to explain, but I don't see the problem that they seem to think is undesirable.
If you have a PME supply to a building, the branch circuit earths and all other water pipes etc. are bonded to the supply neutral just ahead of the meter. In the event of the supply neutral being lost, all bonded metalwork therefore goes to 240V, the idea being that if everything exposed goes up in voltage there is nowhere for a difference of potential to exist and thus there is a reduced shock hazard. This bonded area constitutes the equipotential zone.
If you run a 3-wire feeder (line, neutral, earth) to a remote building, then all the bonded metalwork in that building will also be tied to the supply neutral, and the building therefore becomes an extension to the equipotential zone of the main building. In other words, any rise in potential of the equipotential bonding in the main building will be mirrored by a similar rise in the outbuilding.
The ground between the two buildings, if they are widely separated, will maintain true earth potential whereas the earth where metal pipework enters it under the buildings will be a part of the equipotential zone due to the resistance gradient.
During normal operation, the potential at the neutral terminal where the bonding is made will not be at true earth potential due to the small voltage drop along the cable when current is flowing. Therefore the building's bonding system will be at some small voltage slightly above true earth, typically under 1 volt.
That's what they are saying with the part you questioned.
I don't understand why that should be a problem. I would much rather have the outbuilding with a solid neutral-earth bond on the supply than the frequent alternative where the outbuilding gets its own ground rod and runs as a TT system.
Re: PME SUPPLIES#138940 10/10/0311:31 AM10/10/0311:31 AM
Yep i agree with Paul,, dont see the problem in using the earth supplied from the supply company.
An electrician i know wires camp sites.. the electric hook ups.. he installs the cable to the posts as a PME supply, then disconnects the earth from the socket, but connects it to an earth stake, making it a TT connection..
WHY>> i just dont see the point as long as you have you disconnection times etc..
Re: PME SUPPLIES#138941 10/11/0305:22 AM10/11/0305:22 AM
There is a good, valid reason why PME supplies are not allowed on camp sites. In the event of a supply neutral failure, diverted neutral current from loads in say a caravan with a metal frame would flow through any one making contact with that frame and earth. With respect to extending pme bonding to remote buildings, I think that the advice in the Guide is correct. It would appear that the thinking in that advice is that extending services to a remote building WITHOUT bonding in the remote building would place those services at a different potential with respect to earth, (consider the services being routed above ground). Such an arrangement, as the Guide suggests is not desirable. As Pauluk suggests, the potential difference under normal load conditions is likely to be low. Abnormal conditions however could see such potentials rise to as much as one half of the single phase supply voltage. However, there is nothing wrong with a supply to a remote building being derived from a pme source in a main building providing that exposed and extraneous conductive parts in the remote building are bonded. The disadvantage of such an arrangement is that the live conductors of the supply to the remote building may be required to be relatively small yet the accompanying earth must also act as an equipotential bond and therefore be sized on the main building supply neutral which may result in a substantial cpc. There are many instances where it would be simply undesirable to derive a supply to a remote building from a pme source in a main building. This is usually to avoid diverted neutral current rather than concerns about bonding arrangements. Sometimes conversion of the remote building to a TT system is the most prudent option. Bringing your own pme supply to a remote building used to require special permission from the Secretary of State and certainly currently requires strict compliance with Electricity Safety, Quality and Continuity Regulations so it is usually not an option!
take your point lyle, but in that instance, a neutral failure would not automatically mean a caravan chassis would become live, you have assumed the caravan has its earth and neutral connected at its incomer, this would not be the case, it usualy would just have a flex of some description feeding a small consumer unit. so the problem would not occur, also the RCD should have tripped anyway.
The thing is we all interpret the regs in different ways, i think most sparks just have to use a lot of common sense.
The caravan would never have neutral and earth connected at any point, but the CPC (earth) from every branch circuit on the site is connected to main earth busbar, which is in turn bonded to the neutral just ahead of the meter.
Loss of the incoming neutral therefore results in current flowing back to the supply xfmr neutral via the earth conductors and whatever extraneous paths to earth are formed by the whole installation. Depending upon the load and the impedance to earth, that will result in the site neutral (and thus the earths and caravans) all rising to some potential above ground.