Main earth bonding protective conductors

[buzzlightyear]

They are 400v TPN DB’s, Swa into the DB,Rcd could be before main switch,T/D 100ma
The buildings are Steel fabrication some have extraneous conductive parts entering Water/Gas others just the structure.
MEBPC would be 35mm if Tncs

he should have put above in the first post so that we are on the first page not the last .

 

[cliffed]

The issue I wonder about is whether you can actually TT the outer building in this situation? If the outer building shares extraneous
metalwork with the main house (water pipe), then that metalwork will still become live even if the shed is TT. So to effectively TT the outer building would it not require capping the extraneous part entering the shed with plastic??

Would you say the structure is extraneous too…

 

[buzzlightyear]

The buildings are Steel fabrication some have extraneous conductive parts

Would you say the structure is extraneous too

The begging question is it. Then it should be bonded is this a cow shed or other.

 

[buzzlightyear]

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ecatoday.co.uk/blog/1483/Outbuildings-and-protective-conductors-a-closer-lo&ved=2ahUKEwiR8rfYz8z0AhVaQkEAHahnDeMQFnoECAQQAQ&usg=AOvVaw3LZ8Vz3p88r4cPEJI7xQon

 

[pc1966]

I'm going to throw me tin hat in the ring GN8 page69 shows
fig 5.14 for out buildings ,considering it a TNCS and going to the out buildings so that now makes that a TT ,for both building

Good example, but my copy is on page78 as Figure 5.15 for multi-building TN-C-S case.

The guidance notes are a handy thing!

say a pen fault arises then both buildings would now be live
has PC states ,so to the outer building needs to be TT and not reliant on tother end @cliffed you're option is either RCD IT or make it TT.
@ tiny tim howard I'm on the same page has you.

None of the options for fully meet the requirements are easy and/or cheap and/or low-disruption.

Changing the main bonds to 35mm is probably fairly easy to do and certainly should be done.

Going TT opens up a lot of building blackout risks if, as here, it was never designed that way as it is very unlikely that the rod(s) and building extraneous are low enough Ra to clear on the sub-DB OCPDs so even with a delay RCD then most faults, except probably socket outlets on instant RCDs, will trip the incomer delay TT.

What really is the risk of roasting the SWA sub-mains on an open-PEN fault? Hard to say, but they are likely around 10mm copper equivalent though a higher heat dissipation capacity (physically larger cable so more area to get heat out) so you are probably needing significantly north of 100A average fault current to cause problems. Unless the main building (where you can meet 35mm, etc) is very poor in terms of extraneous Ra and one of the external buildings is very good, I would doubt that you will see that. Even with a L-N short and open PEN (so stiff 230V on the CPCs) you would be needing less than 2.3 ohms Ra on an out building (probably under half that), to damage the SWA.

EICR is not my business really, but if asked on a random forum what to code this as I would put C2 for the mains DB bonds being far too small, and C3 for the sub-mains as not meeting the current open-PEN case equivalent copper sizing.

Over to you folks for another round of discussions!

 

[pc1966]

The issue I wonder about is whether you can actually TT the outer building in this situation? If the outer building shares extraneous
metalwork with the main house (water pipe), then that metalwork will still become live even if the shed is TT. So to effectively TT the outer building would it not require capping the extraneous part entering the shed with plastic??

That is another consideration with open-PEN cases, etc, but if you are looking at low enough extraneous Ra values to worry the sub-mains I doubt you are looking at much in terms of externally accessible touch potentials.

I don't think that is typically noted on an EICR unless somewhere falls in to the special areas aspect of the regs (so swimming pool, agricultural building, marina supplies, etc) but you could find examples where the open PEN risk is enough for close together metalwork that you really would not want one TT and the other TN.

 

[LastManOnline]

Would you say the structure is extraneous too…

If the structure is metal and in contact with the ground than yes it is extraneous. But the next key question that needs to asked is "is it high resistance extraneous or is it low resistance extraneous? The low resistance extraneous is the concerning one. In both situations fault current can be exported and so there is a potential shock risk. However in the low resistance scenario fault current can also be imported (open PEN) and you now have the added danger of having heat damaging currents flowing in the electrical earthing infrastructure

 

[buzzlightyear]

Over to you folks for another round of discussions

this rounder about is spinning fast I'm just about to get off.

 

[timhoward]

To throw a random thought into the ring, there are reportedly now more open pen detection devices on the market since the advent of EV chargers.
I have no idea if it is economical to install open pen protection at the origin and whether that would mitigate the scenario discussed above sufficiently.
It might be possible to have more conventional bonding at the outbuildings then if the services are/ can be insulated.

 

[pc1966]

I guess the BPG #4 is not really intended to cover industrial situations, but what is relevant under C2 are these:

• Absence of effective main protective bonding of extraneous-conductive-parts entering the premises
• Inadequate cross-sectional area of a main protective bonding conductor where the conductor is less than 6 mm2 or where there is evidence of thermal damage

Now the 6mm given in the 2nd point is obviously related to the typical domestic case of 100A max / 25mm tails so some sort of scaling to the OP's sizes (120mm tails, presumably around 300A supply?) would be reasonably. Hence 10mm is certainly looking a bit 'inadequate' to me.

 

[cliffed]

he should have put above in the first post so that we are on the first page not the last .

Sorry

If the structure is metal and in contact with the ground than yes it is extraneous. But the next key question that needs to asked is "is it high resistance extraneous or is it low resistance extraneous? The low resistance extraneous is the concerning one. In both situations fault current can be exported and so there is a potential shock risk. However in the low resistance scenario fault current can also be imported (open PEN) and you now have the added danger of having heat damaging currents flowing in the electrical earthing infrastructure

Agree just making the point of plastic entry to building, you can’t put plastic all around the metal structure, it’s all going pear shaped or even earth cable shaped

 

[pc1966]

To throw a random thought into the ring, there are reportedly now more open pen detection devices on the market since the advent of EV chargers.
I have no idea if it is economical to install open pen protection at the origin and whether that would mitigate the scenario discussed above sufficiently.
It might be possible to have more conventional bonding at the outbuildings then if the services are/ can be insulated.

I would doubt that any are up for the sort of current levels on this sort of a system, but it is an idea that would also reduce the risk of damage due to high voltages on phase-neutral arising from unbalanced loads, etc, during such a fault.

But I suspect that some reels of 35mm wire are a simpler, cheaper, and more reliable solution!