Discuss PME supply block of flats in the Electrical Forum area at ElectriciansForums.net

chris day

London Sparks
Messages
370
Location
london
Hello Guys

I have a 200a 3-phase supply PME bolted to a large ryefield panel, the ryefiled panel then supplies 14 flats within the same building.

Each flat is supplied a 63a from ryefild via meters in the form of a 16mm 3core SWA cable.

The furthest away is flat 14 at around 25 meters, seeing as we are exporting the PME supply i would need a minimum 0.05 max between the main bonding in the flats and the MET at the origin (Ryefeild) because the CPC in the SWA is a combined CPC and main bond.

how can i calculate the R2 value alone as looking at table 11 OSG its R1+R2.. would i simply divide by two seeing as the line conductor is the same CSA.
 

Tony Reidy

Electrician's Arms
Messages
118
Location
bedfordshire
Hello Guys

I have a 200a 3-phase supply PME bolted to a large ryefield panel, the ryefiled panel then supplies 14 flats within the same building.

Each flat is supplied a 63a from ryefild via meters in the form of a 16mm 3core SWA cable.

The furthest away is flat 14 at around 25 meters, seeing as we are exporting the PME supply i would need a minimum 0.05 max between the main bonding in the flats and the MET at the origin (Ryefeild) because the CPC in the SWA is a combined CPC and main bond.

how can i calculate the R2 value alone as looking at table 11 OSG its R1+R2.. would i simply divide by two seeing as the line conductor is the same CSA.
Do a long lead test an you will have r2
 

cliffed

Regular EF Member
Messages
1,217
Location
Lancs
The resistance is not the issue here,the main bonding must be sized accordingly to the Neutral size.
 

Strima

Electrician's Arms
Messages
3,522
Location
St Neots
Can you show me where it states maximum of 0.05 ohms for bonding?
 
OP
C

chris day

London Sparks
Messages
370
Location
london
Thanks Cliffed... so your saying if i just look at table 54.8 will that be the neutral in the SWA sub mains of the neutral of the incoming main supply
 

Wilko

Electrician's Arms
Messages
5,385
Location
Berkshire
Hi - it’s the size of the incoming DNO supply at the last point where it’s still a PEN conductor, if I’ve understood correctly. Hopefully it’s 50mm or less and you’re good with the 16mm (values from Table 54.8 as you say). The 0.05 Ohms thing is an urban myth :) .
 

cliffed

Regular EF Member
Messages
1,217
Location
Lancs
Thanks Cliffed... so your saying if i just look at table 54.8 will that be the neutral in the SWA sub mains of the neutral of the incoming main supply
Exactly,could well be 25mm,for main bonding,also Main Earth should be sized accordingly,maybe for 200 amps 35mm,good luck.
 

mhar

Electrician's Arms
Messages
653
Location
devon
The BNO should have the spec of the supply so you can determine the size of the N, if not a quick phone call to your DNO would be the way to proceed.
Your DNO will have a policy document for their requirements for protection for multi way distribution units (Ryefields etc), ask for a copy
 

bigspark17

Electrician's Arms
Messages
1,309
Location
wales
I cant see why any bonding would be required internally to the flats? Any bonding would need to come from the ryfield/MET to water/gas/structural in prefumanly cellar/switchroom. As other have said, what is dno supply/tails?
 
OP
C

chris day

London Sparks
Messages
370
Location
london
The water and gas inside the flats are 10mm bonded ... the thing that I am quite confused about is the main bonds for the water and gas coming into the building is one huge 50mm cable.
Then inside the flats 10mm water and gas bonds

There are two main bonds?
 

davesparks

Forum Mentor
Messages
12,803
Location
guildford
Main bonding should be installed for each installation.
The installation in each flat is a seperate electrical installation and should have its own main bonding.
 

Bellendian

Regular EF Member
Messages
199
Location
Gatwick
Main bonding should be installed for each installation.
The installation in each flat is a seperate electrical installation and should have its own main bonding.
If they are being classed as a separate installation, then surely you would base the bonding conductor csa on the supply conductors for the individual installation?
 
OP
C

chris day

London Sparks
Messages
370
Location
london
Thanks Dave... so going with that and table 54.8, it states selection in accordance with the PEN conductor. So would I need to size the earthing arrangements for each flat according to the main PEN conductor, the PEN conductor at origin is 70mm which would render the CPC of the SWA too small at only 16mm to each flat
 

davesparks

Forum Mentor
Messages
12,803
Location
guildford
If they are being classed as a separate installation, then surely you would base the bonding conductor csa on the supply conductors for the individual installation?
No, the size of the bonding is based on the size of the DNO's combined neutral and earth conductor. Neutral current diverted via the bonding under fault conditions will depend on this, not on the size of the supply to each flat.

Ultimately for a job like this the DNO should be consulted.
 

cliffed

Regular EF Member
Messages
1,217
Location
Lancs
No, the size of the bonding is based on the size of the DNO's combined neutral and earth conductor. Neutral current diverted via the bonding under fault conditions will depend on this, not on the size of the supply to each flat.

Ultimately for a job like this the DNO should be consulted.
Yea that’s how I read it,if you export Pme,then you need to ensure its requirements.
 

Bellendian

Regular EF Member
Messages
199
Location
Gatwick
No, the size of the bonding is based on the size of the DNO's combined neutral and earth conductor. Neutral current diverted via the bonding under fault conditions will depend on this, not on the size of the supply to each flat.

Ultimately for a job like this the DNO should be consulted.
Agreed, but surely this is why the 50mm bond to structural steel from the ryefield is in place. Also, any of the diverted neutral current from the PEN conductor in fault would also be shared across a number of bonds due to the duplication across units?

In any event I suspect it's all completely hypothetical, if it's a new build with individual gas and water metering, there are unlikely to be any extraneous parts, and if it's an older block with common risers in Iron, then the 'main' main bonds from the ryefield would take care of any stray currents.

In reality, has anyone ever worked on a flat with a 50mm bond anchored to the stopcock?
 
OP
C

chris day

London Sparks
Messages
370
Location
london
yes this is true, i spoke with the NICEIC and they say use tabe 54.8 for the ryefield or consult DNO, then each flat cant have any less than 10mm secondary main bonds
 

Ian1981

Forum Mentor
Messages
2,969
Location
North east
You are not exporting pme at all, the zone is extended not exported which is neither here nore there if it’s all part of the same building complex.
The 0.05 max resistance from the Met to the protective bonding conductors is totally inaccurate and unfortunately is banded about within the industry as a requirement when in fact it’s not.
The 0.05 figure was an indication of measuring between pipe work to determine if the pipes were relatively connected together by a low resistance if the bonding/earthing clamps could not be seen and were ‘built in’.
It’s all covered in GN3 which points out they the 0.05 figure is not between the MET and the bonding conductors.
 
Last edited:

bigspark17

Electrician's Arms
Messages
1,309
Location
wales
Main bonding should be installed for each installation.
The installation in each flat is a seperate electrical installation and should have its own main bonding.
I disagree and you are contradicting yourself in both posts.. you could segregate one flat to a eicr or eic if works were on going but the OP asks about the ryefield & flats.

No, the size of the bonding is based on the size of the DNO's combined neutral and earth conductor. Neutral current diverted via the bonding under fault conditions will depend on this, not on the size of the supply to each flat.

Ultimately for a job like this the DNO should be consulted.
As there is one DNO supply to the OP property then bonding would need only be carried out from ryefield, in this case 16mm.

Dont know what @cliffed is on about exporting pme :/!!!
 

mhar

Electrician's Arms
Messages
653
Location
devon
yes this is true, i spoke with the NICEIC and they say use tabe 54.8 for the ryefield or consult DNO, then each flat cant have any less than 10mm secondary main bonds
You need to install in accordance with BS7671 AND read your DNO policy document such as I linked to in post 18 and comply with this.
This covers bonding requirements towards the end.

Locally my DNO supplies and installs the Ryefield boards because they have had so many instances of refusal to connect to customer installed installations because of non-conformity with their requirements. The NIC are not supplying the electricity, you need to conform with your suppliers requirements
 

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