earthing a container shed

[VoltzElectrical]

Please bear with me on this, as I have become totally confused and would like some clarification.

Scenario (hypothetical).

A 32 amp supply is required to supply a metal container 5m away from the main building. There is a spare way on the mains distribution board available. Is it permissible to use the supplier's earth (PME) either in the form of the cable sheath, or a separate core SWA for example, or is it a requirement that the container use only it's own earth in the form of a rod?

I.e. is it permissible to extend the equipotential zone of the building to include the container or not? I don't mean extend the PME, but use TNC-S from the board to the container and bonding too perhaps?

If anyone can advise on where I can find the answer that would be appreciated. I have been informed that the NICEIC don't like the supplier's earth used in this example, but I haven't dealt with them and have been led to believe from some posts on here that they like to make up their own rules?

Voltz.

 

[Parker27]

Have a read of this: Wiring Matters - 2005 issues - IET Electrical

Issue 16 Autumn 2005 - Electrical Installations Outdoors: a supply to a detached building.

 

[TJ Anderson]

Please bear with me on this, as I have become totally confused and would like some clarification.

Scenario (hypothetical).

A 32 amp supply is required to supply a metal container 5m away from the main building. There is a spare way on the mains distribution board available. Is it permissible to use the supplier's earth (PME) either in the form of the cable sheath, or a separate core SWA for example, or is it a requirement that the container use only it's own earth in the form of a rod?

I.e. is it permissible to extend the equipotential zone of the building to include the container or not? I don't mean extend the PME, but use TNC-S from the board to the container and bonding too perhaps?

If anyone can advise on where I can find the answer that would be appreciated. I have been informed that the NICEIC don't like the supplier's earth used in this example, but I haven't dealt with them and have been led to believe from some posts on here that they like to make up their own rules?

Voltz.

Wouldn't worry about the NICEIC mate, they don't write regulations, just refer to BS7671. If you require bonding or not may sway you as it would affect conductor size of your SWA for your 32A circuit e.g. 4/6mm and a Rod or 10mm if extending the PME.

 

[Leesparkykent]

Wouldn't worry about the NICEIC mate, they don't write regulations, just refer to BS7671. If you require bonding or not may sway you as it would affect conductor size of your SWA for your 32A circuit e.g. 4/6mm and a Rod or 10mm if extending the PME.

Long time no see mate. You need to get back to posting more regular.

 

[VoltzElectrical]

Thanks, it came up during a discussion the other day and I said I had recently connected one up. They asked 'how did you do it?' and I said the first way, i.e. by extending the equipotential zone. I was then told this was frowned on by the NIC who were the governing body.

 

[TJ Anderson]

Long time no see mate. You need to get back to posting more regular.

Thanks Lee, been so busy this year, I have merely been an observer on here for so long, I genuinely had almost forgot my password

 

[Darkwood]

Wouldn't worry about the NICEIC mate, they don't write regulations, just refer to BS7671. If you require bonding or not may sway you as it would affect conductor size of your SWA for your 32A circuit e.g. 4/6mm and a Rod or 10mm if extending the PME.

Remember the 10mm (if thats the size required for bonding) must be in addition to the earthing requirements for the submain and not inclusive of it so a seperate cable, additional core or larger cable may be needed to allow for it.

EDIT - As Davesparks has made me aware(Thanks ) I left out the vital info to this post.. I'm discussing using a SWA supply cable and the Steel wire cannot be used for bonding purposes therefore the above applies. It can be inclusive if a core or seperate but has to be additional if the steel is used as the circuit earth.

 

[Darkwood]

There is a scenario that can arise here when extending the equipotential zone - consider a metal container raised so insulated from the earth, you could see especially in a network fault a potential difference between the DNO earth and the true ground where the container sits... a dangerous situe for anyone that may create a path between the ground and container.. this may be why the NICEIC have reservations of it.. in such a case I would also drive a rod into the ground in addition to the extended equipotential bond.

 

[davesparks]

Remember the 10mm (if thats the size required for bonding) must be in addition to the earthing requirements for the submain and not inclusive of it so a seperate cable, additional core or larger cable may be needed to allow for it.

Eh? A single conductor can satisfy both functions if it meets the required csa of whichever is the larger (usually the bond)

 

[telectrix]

it's a metal container. that precludes the use of the PME in case there's a supply neutral fault. it's got to be made TT as you would for metal skinned caravans.

 

[TJ Anderson]

There is a scenario that can arise here when extending the equipotential zone - consider a metal container raised so insulated from the earth, you could see especially in a network fault a potential difference between the DNO earth and the true ground where the container sits... a dangerous situe for anyone that may create a path between the ground and container.. this may be why the NICEIC have reservations of it.. in such a case I would also drive a rod into the ground in addition to the extended equipotential bond.

I can see the logic with that, maybe something BS7671 should address. Would you do the same if the container was not insulated from ground and was extraneous?

 

[Darkwood]

Eh? A single conductor can satisfy both functions if it meets the required csa of whichever is the larger (usually the bond)

Nice pick up dave - I was refering to SWA where the armour cannot be used for the purposes of bonding so additional conductor is required or larger cable to ensure the earth core is big enough... I missed the key part of it out that its regarding swa Doh .. yep even I mess up :dozey:

 

[TJ Anderson]

Nice pick up dave - I was refering to SWA where the armour cannot be used for the purposes of bonding so additional conductor is required or larger cable to ensure the earth core is big enough... I missed the key part of it out that its regarding swa Doh .. yep even I mess up :dozey:

Tbh I assumed you were referring to SWA sheath too, till Davesparks said that.

 

[netblindpaul]

... I was refering to SWA where the armour cannot be used for the purposes of bonding so additional conductor is required or larger cable to ensure the earth core is big enough...

DW, can you please provide a reg that precludes the SWA acting as a bonding conductor?

 

[davesparks]

It'll almost certainly never be possible in a TNCS supply, but if it's TT or TNS in a larger installation you may find the armour can comply as a bond.

 

[Richard Burns]

If you have a normal building which is generally an insulated area then it is not too difficult to maintain a reasonable equipotential zone within the building, however for almost all areas using a TNCS supply it is very difficult to maintain the equipotential zone effectively outside of the building.
In the main this is not a major problem and the risks are low. Once you have a conductive material as the building then it is much more likely that in the case of a fault or a potential difference from system earth and true earth that you may have a shock risk as there is easy contact from the electrical system to the ground, this is why caravan sites and marinas try and avoid such situations and the DNOs do not like the idea of extending the equipotential zone beyond the walls of a building.
However then you want to look at the superior earthing system and better chance of fault protection from TNCS versus TT, balancing the risk factors would be site dependent, but in general I would go for the superior earthing system.
As Darkwood has said pinning the TNCS earth to the potential of true earth by the use of a rod can reduce the risk of shock when attempting to establish an external equipotential zone.

 

[netblindpaul]

It'll almost certainly never be possible in a TNCS supply, but if it's TT or TNS in a larger installation you may find the armour can comply as a bond.

Only a possible issue if it is a PME supply, TN-C-S could be OK.
It's not as cut and dried as many seem to assume.

 

[davesparks]

Only a possible issue if it is a PME supply, TN-C-S could be OK.
It's not as cut and dried as many seem to assume.

Yeah, I fell in to the usual trap there of writing TNCS when I meant PME.

 

[Darkwood]

DW, can you please provide a reg that precludes the SWA acting as a bonding conductor?

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.

 

[VoltzElectrical]

Is it any wonder I'm confused? It seems to be a nuanced scenario. Thanks for all the advice so far!

 

[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.