NICEIC Certification Scheme Rcd’s in series in communal area

[newfutile]

During an eicr i have discovered a 30 ma (RCD) Residual Current Device feeding a dual RCD consumer unit. The source rcd trips before either in the Consumer Unit .

i would remove the source rcd or replace with a type s but i cant be certain about buried cables in walls, at least some of it is in trunking.

anyone have any ideas?

 

[DPG]

During an eicr i have discovered a 30 ma (RCD) Residual Current Device feeding a dual RCD consumer unit. The source rcd trips before either in the Consumer Unit .

i would remove the source rcd or replace with a type s but i cant be certain about buried cables in walls, at least some of it is in trunking.

anyone have any ideas?

I don't really see how you can remove it unless you are absolutely certain it is not needed.

 

[MJPD29]

It may not be possible to remove it. Unless you are absolutley certain it is not required and you can see the entire cable run I would leave it there, it may be essential.

 

[newfutile]

This is what i thought as well there will be no way to examine all the existing cable for routing.testing the dual rcd’s in the cu is a problem too , i think gn3 shows a way round this but i have never tried it.

 

[mainline]

This is what i thought as well there will be no way to examine all the existing cable for routing.testing the dual rcd’s in the cu is a problem too , i think gn3 shows a way round this but i have never tried it.

How is the DB supplied ? Cable type, distance etc.

 

[pc1966]

This is what i thought as well there will be no way to examine all the existing cable for routing.testing the dual rcd’s in the cu is a problem too , i think gn3 shows a way round this but i have never tried it.

The trick with cascaded RCD that lack selectivity is you put the earth test lead on the supply neutral of the RCD to be tested, then your MFT diverts test current around the RCD under test, but not as far as the upstream RCD is concerned.

 

[newfutile]

Thanks for that , im not back at that job till after new year now , i will practise the technique ,presumably this keeps the balance in the first rcd but unbalances the second?

 

[pc1966]

Thanks for that , im not back at that job till after new year now , i will practise the technique ,presumably this keeps the balance in the first rcd but unbalances the second?

Exactly, the RCD detects an imbalance, it is not really caring where the stray current has gone.

That was one of the big problems with the old VOELCB that it required any stray current to be going back via the supply Earth alone where it sat in line to detect it. So it would not detect a fault to the true Earth (sadly a common way to get electrocuted in the garden using mains powered tools such as mower) nor a fault to CPC if there was another low impedance path (e.g. CPC linked to earthed service pipe so bypassing the VOELCB coil).