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GWC Sparky

Sub-Main supply to DB, Main Earthing Conductor seizing quarry

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GWC Sparky

Please could I have some opinions of a job I am looking at.

 

This is a commercial property, with multiple office suits on a 1st floor supplied by individual mains supplies from the DNO. individual meter cupboards at the back of the building supply each office.

One of the office suit supplies are as follows but all are supplied similarly:

The supply service head has a 60A main fuse and 16mm meter tails to the electricity meter and to a Switch Fuse SP+N isolator with 60A cartridge fuse protecting the 16mm squ TW&E sub-main cable that supplies the office suit DB on the 1st floor above the meter cupboard.

The CPC of the sub-main cable is 6mm squ. The Main Earthing Conductor from the service head to the Switch fuse MET is 10mm. in this case but some are 16mm.

The only Extraneous conductive parts that I believe require Main equipotential bonding in this office suit would be the gas central heating system in the kitchen of the office suit. However the gas meter for this boiler is located outside of the this office suit area in a communal toilet area. and is bonded to another office supplies earthing system MET.

 

My initial feeling is that the CPC of the TW&E cable is undersized for this situation with reference to Table54.7 and Table 54.8, however it could also be proved adequate buy calculation using the Adiabatic equation 543.1.3.

 

A constructive discussion would be gratefully received, thank you.

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GWC Sparky

Please excuse my hilarious spelling, that was Query not Quarry but sometimes I do feel like digging a hole.

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binky

never have liked jobs like this! 

 

Every cable is made with a CPC suitably sized for it's maximum load current under fault conditions, so in my book, the sub main 16mm is fine as it is as you have fused down to suit the cable size.

 

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Phoenix
10 minutes ago, GWC Sparky said:

Please could I have some opinions of a job I am looking at.

 

The only Extraneous conductive parts that I believe require Main equipotential bonding in this office suit would be the gas central heating system in the kitchen of the office suit. However the gas meter for this boiler is located outside of the this office suit area in a communal toilet area. and is bonded to another office supplies earthing system MET.

 

My initial feeling is that the CPC of the TW&E cable is undersized for this situation with reference to Table54.7 and Table 54.8, however it could also be proved adequate buy calculation using the Adiabatic equation 543.1.3.

 

A constructive discussion would be gratefully received, thank you.

 

 

First of all, the adiabatic can not be used as an alterative to table 54.8 (used to size bonding conductors on a PME system), it can however be an alterative to a CPC sized from 54.7. So you have to decide whether your earth in your T&E is a conding conductor as well as a CPC, or simply just a CPC. It sounds like you are heading for the later as you are saying in some cases stuff is bonded back to the DB (if bonding ran back to the intakes then the earth of the T&E would simply be submain only)

 

But, and this is a big but, when you say "However the gas meter for this boiler is located outside of the this office suit area in a communal toilet area. and is bonded to another office supplies earthing system MET." it jumps out at me that the equipotential zone is not established correctly, if you are treating each unit as an individual equipotential zone but some were heated by boilers elsewhere, then you'd have to bond the heating pipework flow and return and possibly DHW as well as it entered each unit, then you'd have to ensure that you couldn't be in contact with more than one EPZ at a time.

 

Assuming this is a smallish multi occupancy block then, I think I'd be looking to establish one equipotentail zone for the whole lot; big earth bar near the intake, pick up all the incomming services (which are probabaly all off a ryefield or similar?) and then pick up all other metalic services entering the block,

 

 

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GWC Sparky

Should the gas boiler pipework (Gas Pipe) be main bonded to the MET of the office DB, as it is in that offices kitchen area??

 

I can totally see the sense of what you have said about the cable if it was being used to supply a final circuit for example, but as a sub-main to a DB MET?? is that still a correct interpretation 

7 minutes ago, binky said:

never have liked jobs like this! 

 

Every cable is made with a CPC suitably sized for it's maximum load current under fault conditions, so in my book, the sub main 16mm is fine as it is as you have fused down to suit the cable size.

 

 

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GWC Sparky
4 minutes ago, Phoenix said:

 

 

First of all, the adiabatic can not be used as an alterative to table 54.8 (used to size bonding conductors on a PME system), it can however be an alterative to a CPC sized from 54.7. So you have to decide whether your earth in your T&E is a conding conductor as well as a CPC, or simply just a CPC. It sounds like you are heading for the later as you are saying in some cases stuff is bonded back to the DB (if bonding ran back to the intakes then the earth of the T&E would simply be submain only)

 

But, and this is a big but, when you say "However the gas meter for this boiler is located outside of the this office suit area in a communal toilet area. and is bonded to another office supplies earthing system MET." it jumps out at me that the equipotential zone is not established correctly, if you are treating each unit as an individual equipotential zone but some were heated by boilers elsewhere, then you'd have to bond the heating pipework flow and return and possibly DHW as well as it entered each unit, then you'd have to ensure that you couldn't be in contact with more than one EPZ at a time.

 

Assuming this is a smallish multi occupancy block then, I think I'd be looking to establish one equipotentail zone for the whole lot; big earth bar near the intake, pick up all the incomming services (which are probabaly all off a ryefield or similar?) and then pick up all other metalic services entering the block,

 

 

 

Thank you for making me feel even more terrified of this job, that's exactly what I think I'm looking at here, a complete mess that should be addressed as a hole building.

my understanding is that a submain that would then require Main equipotential bonding to its supply DB MET would need to have a CPC of at least 10mm.

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binky
10 minutes ago, Phoenix said:

 

Assuming this is a smallish multi occupancy block then, I think I'd be looking to establish one equipotentail zone for the whole lot; big earth bar near the intake, pick up all the incomming services (which are probabaly all off a ryefield or similar?) and then pick up all other metalic services entering the block,

 

 

 

this is what I would be looking to do

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binky
11 minutes ago, GWC Sparky said:

 

I can totally see the sense of what you have said about the cable if it was being used to supply a final circuit for example, but as a sub-main to a DB MET?? is that still a correct interpretation 

 

 

 a submain is just a cct. Many people run an extra bond to the board, but I don't believe that is actually necessary

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Fleeting

Is it a single DNO supply common to all the separate supplies in other words just one supply cable, if so it isn't overly important where the main protective bond connects. The question is if your supply has cpc which will not support bonding does the same apply at the office where it is connected.

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GWC Sparky
Just now, Fleeting said:

Is it a single DNO supply common to all the separate supplies in other words just one supply cable, if so it isn't overly important where the main protective bond connects. The question is if your supply has cpc which will not support bonding does the same apply at the office where it is connected.

 

This is were I'm feeling uncomfortable about the current installation. the suites are supplied from individual DNO supplies, I've found five so fare. it dose not have a common supply at all. 

It has a landlords supplie for the services of communal outside lighting, etc. This did have a 10mm bond wire that could possibly go to the main point of entry for the metal gas main. water would be PVC pipe? this I would need further investigation

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Fleeting

Multiple TN-C-S supplies sharing a single gas service is far from ideal. Network Operators may differ in their opinion but I would have thought the supplies need bonding together.

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binky

so you have multiple supplies but a common boiler / heating system that means you have a common  'extraneous conductive parts' in every suite. 

 

Think I would go with @Fleeting on this one.  MET at supply, link earthing to each fused switch and bonding for gas and water meter to this MET. Within each suite I think I would be looking to supplementary bond the heating/ water pipes, if metal, to the board for each suite. 

 

415.2 note 3 supplementary protective bonding may involve the entire installation, a part of an installation etc. 

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GWC Sparky

My conclusion from all the help given today, further reading and a hard thing is that, The 16mm T&E cable with 6mm CPC sub-main to each of office boards is in fact an extension of the main earth conductor from the DNO's supply to the MET of each DB and not just a CPC and hence table 54.8 has relevance to start with with regards to bonding to the PME supply. This I think is correct as none of the supplies have main protective conductors connected at the meter cabinet. ie the MET is not at that point but at the DB earth terminal. hence wouldn't normal practice  be to use 16mm earth cable to your MET??? Table 54.7

The next point is the possible problem of multiple EPZ's and that they should all be referenced from one central point or in this case cross bonded possibly.

this is like having multiple house hold supplies with interconnecting copper pipework possibly bonded in one house but none of the houses having correctly sized Main earth conductors for a PME supply

hard think that was 😂

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Fleeting

You only have one earthing conductor the earth in the 16.0 cable is a cpc. Providing it is of adequate csa to carry fault current it's size will become a factor if it is required to support main protective bonding above this size, this is issue 1.

Issue 2 is multiple TN-C-S supplies sharing a common gas supply and in my opinion the main earth terminals of all these supplies should be connected together.

Edited by Fleeting

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binky

The CPC is not a main earth, it's sized for the max fault current of that cable when correctly fused down to its max amapage or less. You should have a main earth to the switch fuse, but once fused down, it's just a cable with  a CPC, so you end up with eg a 10mm earth to the switch fuse, and the CPC afterwards. As Fleeting has said, you need to link the main earths for the supplies to an MET, Main equipotential bonding also to that MET. By linking everything any fault with one supply will raise the voltage to 'safe touch levels' ie 50V max across the entire installation. You're getting tied up with the idea that it's a DB on the end of the 16mm - think of it as just being a bigger garage board

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