150m submain to multiple outbuildings earthing (PME)

[Woodbeare1]

Hi there can I describe an installation and I would love to hear some opinions as to weather the earthing is up to standard. Its quite a long run so I'm concerned about the difference between true earth and system earth!

Submain comes direct from meter cupboard in farmhouse through 100a main switch and 63a mcb

Sub main is 25mm2 swa 3 core using the 3rd core as cpc (also bonded to swa outer protection) it runs as follows

25m to adaptable box two CUs for cottage and workshop

Another 25m now goes on from the box to a 2nd shed with a AB and garage unit

From here it goes another 45m but reduces to 16mm2 swa 3 core to a barn with another AB and Garage unit

Here it reduces again to 10mm2 swa 3 core and goes through a 63a rotary isolator before running another 50m to the last shed.

I'm now about 150m from the meter cupboard and although the earth is sizeable I'm along way from the PME supply, should I be putting in earthing rods on a pme system??

Any help and constructive abuse greatly welcomed!!

 

[Midwest]

Hmmm, quite bizarre. I was about to start a thread (hypothetical one!) on a similar tangent, but I'll let this one run in case the answer to my question comes up, and I save myself any unpleasant embarrassment.

To start the ball rolling for the OP, I would suggest the resistance of your protective bonding cable to any extraneous pipework might be high at the furthest point, or am I getting that completely wrong?

 

[westward10]

What type is the 63A mcb, what is the earthing system, what is the Ze, what size is the earthing conductor at the origin (to size bonding).
Is this a new years wind up.

 

[The Ghost]

What type of supply is it. Not enough info.

 

[westward10]

What type of supply is it. Not enough info.

I asked what type of supply and just noticed PME in the post. Not enough info though.

 

[Toneyz]

is the farmhouse off the same supply? I think that further to exact calcs you will exceed max Zs that's only one aspect what is the aprox average load at each building as for volt drop.From what you have posted this distance is not in a staight line but each building has been added in turn.A better set up would of been out to each building from D.B. at farmhouse what would the longest run of been then ?.

 

[westward10]

Is this an existing installation or are you proposing it for a new installation.

 

[Toneyz]

Is this an existing installation or are you proposing it for a new installation.

I think it is a existing installation Westward 10,hopefully they will come back and let us know as they want to know what we think

 

[Woodbeare1]

Hi all sorry about the patchy details the 63a mcb is a B type and the cpc is 10mm2 and yes the farmhouse is on the same supply but the sub main comes off between the meter and the farmhouse CU

 

[Woodbeare1]

Yep this is existing but the last 50m is just being put together

 

[Wilko]

I'm assuming farmhouse is TNS or TNCS to its MET and DB1 is attached to that. And you are concerned about just how far away you can rely on the DNO earth (?).
One problem with the described daisy chaining of supplies between buildings has already been mentioned by AB in #6 - the cumulative voltage drop.
That led me to think about daisy chaining the bonding. I think the cable sizes described in reg 544 apply to each building and they are intended to link back to the installation MET (in farmhouse) individually from each building. Not daisy chained.
If that is correct, buildings after the farmhouse + 1 may need to be converted to TT (?). That may also address the increasing difference between the exported earth potential and the local. Just a thought

 

[westward10]

I assume main bonding is required in the cottage and the separate core of the 25mm cable, without other information should be adequate. Is bonding required in the other buildings.

 

[Woodbeare1]

I'm assuming farmhouse is TNS or TNCS to its MET and DB1 is attached to that. And you are concerned about just how far away you can rely on the DNO earth (?).

spot on Wilko am beginning to think converting everything after the workshop to TT might be the way to go!

 

[Toneyz]

I think that if you are happy with the loads not rising to give a volt drop issue and you do a TT after the point of the workshop the only problem to overcome would be the max Zs for the linked supply cable by what you have posted the existing buildings are daisy chained and there is not a practical way of supply other than the A.box at garage. what is your Zdb there ?

 

[westward10]

Deleted

 

[bigspark17]

Pme should not be used in agricultural buildings unless a metalic earth grid system with multiple access points for testing is installed.
What type of buildings are these, metal framed.? If im reading your posts correctly that the 1st submain goes into a cottage then alsong as that zs figure is ok and all bonding in place i see no problem with that, i would then be thinking the first farm building should be tt with upfront s type rcd, then each building daisy chained off the buildings one to the next with suitable ocpd. Bonding each building frame work as you go along, aslong as 10mm cpc loops from one building to the next. If cpc/swa are smaller or only 2c is usedyou may need to individual tt each building.

 

[bigspark17]

is the farmhouse off the same supply? I think that further to exact calcs you will exceed max Zs that's only one aspect what is the aprox average load at each building as for volt drop.From what you have posted this distance is not in a staight line but each building has been added in turn.A better set up would of been out to each building from D.B. at farmhouse what would the longest run of been then ?.

Not sure ive ever encountered a set up where multiple outbuildings have individual supplies, usually theseare add on after add on!

 

[westward10]

It's a farm you only imagine the set up.

 

[Toneyz]

So the 50Mtr run is being added from the garage therefore what is the Zs at this point can the 63A B mcb disconnect in time.

 

[Midwest]

Hmmm, quite bizarre. I was about to start a thread (hypothetical one!) on a similar tangent, but I'll let this one run in case the answer to my question comes up, and I save myself any unpleasant embarrassment.

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?

 

[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