earthing a container shed

[davesparks]

To simplify it, installations in metal containers shouldn't be fed from a PME supply without an additional earth electrode of suitably low Ra being connected to mitigate the dangers should a supply fault occur

 

[Dave OCD]

And what would you class as 'suitably low' in this scenario Dave ?

 

[davesparks]

And what would you class as 'suitably low' in this scenario Dave ?

The only guidance which appears in bs7671 on the subject of additional earth electrodes connected to a PME supply is contained within the swimming polls section and gives an Ra of 20ohm as the upper limit.

My experience of installations in steel containers is limited to the ones on our storage site. As a requirement of the licence each one has a minimum of 2x electrodes connected to opposing corners with 3mmx25mm copper tape to connect the container to the electrode. The resistance from the highest point on any part of the containers to earth is <10ohms. I also have to fit 50mm flexible bonds to the container doors before our next inspection.
This is all for lightning protection, but it also serves quite well for an electrical earth.

 

[netblindpaul]

Ill give you the reason first then I need to look it up ... (PME systems)

The SWA as an earthing conductor is fine and is sized according to the requirements of the circuit thus in a fault it will trip the protective device in the required time, with PME set-ups the Bonding is sized in accordance to the possiblity of carrying N currents in a Network fault for long periods of time, the long term reliability of the SWA and its glanding was deemed not to be good enough for such a scenario especially with buried cables with infrequent testing.. I had this downloaded somewhere but that was on my old laptop that bit the dust, I now question whether it was a BS7671 requirement or the NICEIC stipulation on one of our jobs... I remember the DNO also got involved and requested a larger earth than our regs required due to network set-ups...

My problem is I use more than just the BS7671 and with the fact some of the sites I have been on have special requirements too I occasionally get a blurred lines between them all.. because I can see the scenario and the dangers I assume the BS7671 has them covered but seems they do fall short in areas.

DW,
I'll go with the NICEIC making up their own rules over and above BS7671, some of which make no real engineering sense based on fundamental principles.
Yes the SWA could carry circulating network currents, which could cause an increase in temperatures which impact on the calculations under BS7671, however, all this is detailed in guidance. So you are right there, but there is still no reg no. that precludes either.
I also use much more than BS7671, and a lot of what I work with is statute law as you well know.
The requirement to meet DNO specifications is, in BS7671, however, the requirement comes from the DNO, not BS7671.
So, almost everything you say is correct, but not "legislated for" under BS7671, as you say, it does fall short in many areas.
However, it is the basic level that fixed installs must comply with.
Some understand much more than BS7671, others don't.

This is one of the major issues that causes things like the steel consumer units dabarcle.
Those who don't understand should blindly follow the rules, that way, they do the job right, if they don't fully understand, don't question it, just follow the rules and you'll be fine.
If you do then if you are competent and insured, you can go off and really DESIGN your own installation...

There is however, nothing in any of what I have said, nor anything in what you have said that precludes the use of the SWA of a cable being used as a bonding conductor, as long as, it meets the requirements of the relevant "bodies", excluding the NICEIC, I refer to BS7671 & ESQCR, along with other statute law & British Standards, such as BS7430.
Whilst I "am" an NICEIC "AC" I don't take their word as "Gospel", they are not "that" correct, a lot of the time IMHO.

 

[VoltzElectrical]

To simplify it, installations in metal containers shouldn't be fed from a PME supply without an additional earth electrode of suitably low Ra being connected to mitigate the dangers should a supply fault occur

Can you clarify that you mean that the supply to the metal shed should not be PME (exported PME from the distribution Board) in which case I agree, or:

that the earthing supply from the distribution board should not be TN-S? I.E. From the distribution board which is PME, then run a sub-main with separate earth and neutral with additional bind of the appropriate size.

I only ask this as I want to be clear that I am not suggesting exporting the PME from the DB to the shed.

 

[telectrix]

TN-S is fine to use. If you are using PME, then rod/s should be added at the container with a Ra < 20 ohms. this is to limit the PD between the container structure and earth.