EICR question

[Wilko]

Putting on my student's tin hat, I'll give it a bash :

For bonding we are concerned about touch voltages and so the path must be comparable to the resistance of Cu and so we use resistivity ratios (>8). For use as CPC we use the adiabatic eqn and its k factors that include temp rise and thermal capacity of the materials. This gives a reduced steel requirement compared with bonding calcs and so SWA armour is often ok for CPC but not for bonding.

 

[HandySparks]

So by calculation you would need a 70mm 4 core to get the equivalent of 16mmcpc. How come we see so many sub mains being ran with the steel being the cpc if it doesn't comply?

They may be perfectly OK for a CPC, just not for bonding.

 

[davesparks]

So by calculation you would need a 70mm 4 core to get the equivalent of 16mmcpc. How come we see so many sub mains being ran with the steel being the cpc if it doesn't comply?

Because CPC size is calculated by the adiabatic equation and not by the equivalent conductivity method used for bonding. the CPC only has t carry fault current for a specified time which is limited by the fault protection device and so can be a lot smaller than the bonding conductor required on TNCS.

 

[davesparks]

The cpc also needs upgrading for the 100a supply to the sub board

The armour will be fine as the CPC, it almost always is up to something like 120mm I think from rough memory

 

[bigspark17]

The cpc also needs upgrading for the 100a supply to the sub board

What makes you ask this? Bonding and cpc requirements of swa armour are very diferent.

 

[akwoody2]

Sorry, a combination of me not following the thread correctly, reading it in my phone with a cracked screen and getting completely mixed up. Cpc is correct and won't need to be altered. As for the bonding it's not man enough so a recommendation will need to be put down

 

[Pete999]

Thanks. Out of interest how did you get to it being the equivalent of <10mm?

There is a comparison chart in the OSG
file:///C:/Users/Pete/Desktop/Electrical%202/SWA%20CPC.pdf

 

[Pat H]

Pete that's a link to the pdf copy in a folder on your PC desktop. So hopefully inaccessible to the outside world.
And I suspect admins will not allow a copy on the forum of any of its content due to copyright.

 

[Pete999]

Pete that's a link to the pdf copy in a folder on your PC desktop. So hopefully inaccessible to the outside world.
And I suspect admins will not allow a copy on the forum of any of its content due to copyright.

Oh sorry I thought I had posted a copy of the chart, can admin please delete

There is a comparison chart in the OSG
file:///C:/Users/Pete/Desktop/Electrical%202/SWA%20CPC.pdf

Admin can you please delete my reply