PME, to rod or not - college assignment

[hightower]

Okay, I know this question pops up a lot but I want to be really specific, and ask in the trainee bit for your thoughts.

I've done my first inspect & test assignment, and one of the questions is "show using the help of a diagram the earth fault loop path for an earth fault in the lighting circuit of the workshop".

So I drew this rather crude drawing (attached)

As you can see, the path goes through the relevant DBs, using the armouring of the distribution circuit as the earth to the workshop, and then eventually through the PEN conductor back to the transformer.

On Monday the tutor asked me how I was doing, and me being a clever sod said I was about done. He said he'd bet any money I'd done this question wrong and when I asked him he explained it was because the workshop needed a rod.

I argued it didn't because the armour of the SWA was sufficient to act as the CPC of the circuit (16mm SWA, 33A total load, 75m run, armour is much more than sufficient). There are no extraneous items in the workshop that need bonding, so my view (heavily based on the opinions of this forum) is that it is perfectly safe to take the supply earth out to the workshop. For info, the workshop is wooden construction on a concrete base.

Now I think I can see where my tutor is coming from about safety. If there was a lost neutral all earthed parts would become live. This might not be a big issue in the main building, but maybe there is a chance of someone using a metal tool outside the workshop and is stood directly on the ground. Then we'd have an issue.

So, I was thinking that a rod is not strictly needed by the letter of the law, but the more I ponder it the more I think it might be a sensible idea to put a rod in.

Just wanting peoples thoughts?

 

[FatAlan]

Seems to be a hot topic of conversation on other threads.

 

[Lucien Nunes]

A 'lost neutral' in the installation is not a problem; you are specifically considering the case of failure of the PEN which is part of the suppliers works. Would the hazard resulting from that be any different in the workshop to how it would be in the main building? E.g. you mention use of equipment outside, connected to a supply in the workshop. Is the safety of that compromised by connecting to supply in the workshop rather than the main building, provided that the normal requirements for the use of LV equipment outside are met?

Finally, consider a normal rod of normal resistance. What would actually happen if the PEN of a substantial distribution system that the rod is tied to, or even the service cable, goes O/C?

 

[hightower]

A 'lost neutral' in th installation is not a problem; you are specifically considering the case of failure of the PEN which is part of the suppliers works. Would the hazard resulting from that be any different in the workshop to how it would be in the main building? E.g. you mention use of equipment outside, connected to a supply in the workshop. Is the safety of that compromised by connecting to supply in the workshop rather than the main building, provided that the normal requirements for the use of LV equipment outside are met?

Finally, consider a normal rod of normal resistance. What would actually happen if the PEN of a substantial distribution system that the rod is tied to, or even the service cable, goes O/C?

Yes, I was meaning a lost PEN, apologies for my poor terminology. I suppose the hazard would be the same regardless of whether the appliance was plugged in to the main building or the workshop, but I guess the risk of someone doing it will be somewhat higher in the workshop.

As I understand it the reason caravans aren't allowed a TNCS supply is because there's the potential for the whole caravan to become live with a lost PEN. Obviously the door handles of the workshop aren't going to be tied to earth, and metal tools etc are a rare find nowadays, but surely there's still the potential for it happening. I can't see how RCD protection would help in the case of someone getting a shock because of a lost PEN - the RCD would see the current going through the neutral as it expects because the join to earth is on the supply side.

I've reread your last point a few times and I am having trouble with what you mean, but I think it's that you are saying any rod installed in the workshop would rise to a certain potential in the case of a broken PEN in which case the installation is no safer than if the suppliers earth was taken to the workshop?

 

[Richard Burns]

A complex question.
However a brief summary is that within the equipotential zone of a house it is easy (ish) to assess the risks and control the distribution of voltages across circuits and conductive parts during a fault.
Once you extend the equipotential zone out of this controlled environment then the risks become greater and the control and balancing become more onerous. The DNO do not like to have extended equipotential zones as they can cause problems.

In your example the shed has a concrete base which can be considered to be true earth and so an effective equipotential zone is difficult to establish without using a local earth reference.
If this local earth rod were connected to the TNCS supply, then on loss of neutral your earth rod (and other earth references) would be the neutral / earth path for all the supply current being used in the area after the neutral break. It would limit the potential rise on conductive parts to some extent but would be overloaded on current flow.
using your diagram to illustrate the idea:

 

[gazdkw82]

I apologies in advance if my understanding isn't correct here but, Isn't there 2 issues here

1. The loss/break PEN
2. complying with disconnection times

 

[hightower]

A complex question.....

So I think what you're saying is that rodding, but not seperating the rod (ie making the workshop an island with it's own TT system) would help the workshop maintain the equipotential zone of the intake building and minimize touch voltage?

So if I'm thinking about this the right way, the three options would read something like this:

1) No rod, use the supply earth in outbuilding - broken PEN means earthed work becomes live. The potential between this live earthwork and the true earth of the concrete floor means there's a good risk of a belt

2) Rod, separate from supply earth - broken PEN means earthed work in workshop remains at 'zero' as the supply earth stops at the DB or just before. However, couldn't the rod still pick up a possible voltage from the earthed PEN, but this would make the workshop it's own equipotential zone and quite safe?

3) Rod, installed alongside supply earth - broken PEN means earthed work in the workshop becomes live but because it also has a local reference to earth there is going to be reduced touch voltage between this earthed work and the true earth of the concrete floor.

My statements above might be far from the truth so please correct me, but based on this I'm looking at it and thinking it makes sense to go with option 2 or 3?

 

[Richard Burns]

I apologies in advance if my understanding isn't correct here but, Isn't there 2 issues here

1. The loss/break PEN
2. complying with disconnection times

The break in the supply PEN is the worst case scenario that we woudl have to deal with and so this is considered in detail. Covering this should also address other less serious touch voltage situations.
Compliance with disconnection times in the case of a single fault ought to be in place no matter what you are doing with the earthing system. Whether you are relying on an RCD or an MCB is a decision to make, generally based on the earthing system.

 

[Richard Burns]

So I think what you're saying is that rodding, but not separating the rod (i.e. making the workshop an island with it's own TT system) would help the workshop maintain the equipotential zone of the intake building and minimise touch voltage?
Yes, but it would also increase the risk in the remote building as the cpc may not be (but probably is) rated for the current that may flow through it

So if I'm thinking about this the right way, the three options would read something like this:

1) No rod, use the supply earth in outbuilding - broken PEN means earthed work becomes live. The potential between this live earthwork and the true earth of the concrete floor means there's a good risk of a belt

2) Rod, separate from supply earth - broken PEN means earthed work in workshop remains at 'zero' as the supply earth stops at the DB or just before. However, couldn't the rod still pick up a possible voltage from the earthed PEN, but this would make the workshop it's own equipotential zone and quite safe?

3) Rod, installed alongside supply earth - broken PEN means earthed work in the workshop becomes live but because it also has a local reference to earth there is going to be reduced touch voltage between this earthed work and the true earth of the concrete floor.

My statements above might be far from the truth so please correct me, but based on this I'm looking at it and thinking it makes sense to go with option 2 or 3?

1) yes that is it in a nutshell
2) The TT rod for the outbuilding means that the earthed and bonded parts in the outbuilding will not rise to the line potential on the loss of the PME PEN (although the neutral in the outbuilding will rise it will not be connected to the local earth) therefore there would be no particular shock risk.
The fact that there will be a potential rise in the earth zone around the supply building when the PEN is lost might cause an effect on the remote rod but not a significant one (unless the rod is very close to an extraneous part from the main building)
3) a low resistance rod at the supply incomer could reduce the touch voltage to an acceptable level, a bit like the low resistance earthing at the substation, and this would apply across the installation where the earthing system was maintained.