150m submain to multiple outbuildings earthing (PME)

[Wilko]

... I noted a passage that said in a PME installation 'the armouring (or a core) should not be used as a main protective bonding conductor' ... as may have to carry sustained high currents which may result in heating of the bonding conductor ... unless the electrical installation designer determines that the heat produced in the armouring or core due to its use will not cause overheating of the live conductors of the cable when on full load'.
So my question (could be embarrassing) is how does the designer make that determination?

Never embarrassing, it's a good one MW !
Sounds like a hint not do it
Never one to be shy : if the sub main cable was selected as if the included bonding conductor was a live conductor then any long duration earth current should not overheat the cable (?). But I can't make the same statement about the armour as it hasn't been tested that way.

The next problem is the bonding conductor to the outbuildings could be required to be quite big with PME, bigger than the submain conductors (gulp). There's a good example of this in GN8 p70 Fig 5.15. Another reason to TT the outbuildings I guess.

 

[jeremy]

Is there any livestock on the farm?

 

[Aurai]

You cannot rely on PME for socket outlets on Jetty and Pontoon fixtures and in Caravan Parks, so I suspect a similar regulation will apply in a farm installation as described??

 

[johnboy6083]

To add to this, as a rule, don't supply sub boards from an MCB. You won't get full discrimination, and MCB'S have much lower max Zs values than an equivalent fuse.

 

[Midwest]

As this thread seems to have stalled, I will ask my question, and it may be pertinent in any case.

I was reading through the December issue of Professional Electrician, and there's a piece by Elecsa 'The earthing functions that can be performed by the armouring of a cable'.

On reading through it, I noted a passage that said in a PME installation 'the armouring (or a core) should not be used as a main protective bonding conductor', may have to carry sustained high currents which may result in heating of the bonding conductor.

It's a lift from GN8 (page 54) 'For installations forming part of a TN-C-S system in which the supply is earthed at multiple points (PME conditions applying), and it is intended to utilise the armouring of a cable or a core of an
armoured cable as a protective bonding conductor the designer needs to consider the effects of the currents that may flow in the conductor due to network conditions. The armouring or a core of such a cable is normally not used as
a protective equipotential bonding conductor where PME conditions apply unless the electrical installation designer determines that the heat produced in the armouring or core due to its use will not cause overheating of the live conductors of the cable when on full load'.

So my question (could be embarrassing) is how does the designer make that determination?

For anyone interested, I posed my question to Elecsa technical and received this reply;

As you have already stated one needs to consider the possibility of the conductors overheating.
'Where PME conditions apply, the use of the armouring of the cable as a main bonding conductor may result in overheating of the live conductor where they are to be operated at significant proportion of their current-carrying capacity for sustained periods or at higher ambient temperatures.

Overheating of a live conductor in this context is where the conductor operating temperature exceeds the rated value appropriate to the insulation material such as 70® for thermoplastic (general purpose PVC)which can lead to premature ageing and deterioration of the insulation

Such over heading may be due to the diverted neutral currents or network circulating currents

The probability of the live conductor overheating due to diverted neutral currents is likely to be reduced if

(1) Where the cable is lightly loaded, bearing in mind that the heat produced in a conductor is proportional to the current squared and the ambient temperature is in the normal

(2) Where it is expected that the armouring will not carry a significant magnitude of diverted neutral current

Where the electrical installation designer considers that a reasonable probability exist of overheating being caused to the live conductor of the cable due to diverted neutral currents in the armouring, the armouring should not be used as a main bonding conductor.

Where after careful consideration the electrical installation designer decides that using the armouring of the cables as a main bonding conductor would not result in overheating of any of the live conductors, the armouring may be put to use provided its cross sectional area if sufficient to meet the requirements of regulation 544.1.1.'

Still none the wiser, but guess it's only going to be an issue in high load situations, unless someone can tell me differently?
PS I did ask about 'the armouring (or a core)'.

 

[Wilko]

I'm interested

I think the SWA armour is pretty much never going to be suitable for bonding an outbuilding in a TNCS installation. Here's my reasons : we need the steel armour of the SWA to have the same resistance as the copper from Table 54.8 and that means x8(ish). And from Reg 544.1.1 if it's TNCS at the origin of the installation then it's the incoming DNO neutral size thats used in Table 54.8 and it sets the bonding size for any outbuildings using the origin TNCS. A good example is in GN8 figure 5.15.

 

[westward10]

The whole point of assessing the size of the armour is to ensure it is adequate to carry any diverted current. This is why it often is not practical because the csa of the cable is often larger than essentially required.

 

[Midwest]

Elecsa response was about using the armouring not a core. GN8 says 'The armouring or a core of such a cable is normally not used as a protective equipotential bonding conductor where PME conditions'?

 

[westward10]

Elecsa response was about using the armouring not a core. GN8 says 'The armouring or a core of such a cable is normally not used as a protective equipotential bonding conductor where PME conditions'?

"Normally not" would not mean that you could not as long as it is correctly sized.

 

[Wilko]

"Normally not" would not mean that you could not as long as it is correctly sized.

Agree.
Not prohibited, just not likely to make sense.

 

[sparkydaryl]

This is not really something I have ever had to deal with. Always done commercial submains which have been tns. But as an electrician are we allowed to just change the earthing system as it suits us? I thought that kind of thing was down to dno.

 

[westward10]

This is not really something I have ever had to deal with. Always done commercial submains which have been tns. But as an electrician are we allowed to just change the earthing system as it suits us? I thought that kind of thing was down to dno.

The same would apply with TN-S the armour must be suitably sized. To overcome the problem of armour inadequacies you can create a TT earthing system.

 

[Wilko]

I think it's the owner's choice whether to use DNO means of earthing (if they've provided one) or not. Normal to use DNO means of earthing for the first building of the installation, but may be different answer if it's a larger site with multiple small outbuildings that also need bonding.

 

[Midwest]

"Normally not" would not mean that you could not as long as it is correctly sized.

Too many 'not's' in there for me to make sense of it