Rcd

Hi the max Zs value in the regs  for mccb and rcbo  if you have a rcd or rcbo can the max zs be higher than in the regs 

ie say b32 is 1.10 ohm if I but the circuit on a rcbo can it be higher to pass on a cert 

You shouldn't be relying on a RCD to achieve disconnection.

If your circuit has a higher Zs then I would be investigating why (unless its a TT system)

IMHO

On a TN system, 

No.

I would say using that as a get out clause is poor design.

RCDs and RCBOs are not there to achieve disconnection times on TN systems, they are installed to protect people.

I have just accessed by stroma and one question he asked me was the max zs times for mccb / rcbo and then he said if it is back up by a rcbo it can be higher is this right coz I was always under the impression it should not go over the value shown in bs7671 

Hum .................. if its higher than you think it should be you should investigate why.

if Zs is too high on TN system for an MCB to operate then chances are something is wrong

it's abit like TT systems, in theory 1667 Ohms (or whatever that figure is)  is an allowable Zs, but in reality I would expect the same R1+R2 figures as any other system, the add the rod. If you aren't getting those sort of numbers then as said above, the cct isn't good.

Russ nichol said:

Hi the max Zs value in the regs  for mccb and rcbo  if you have a rcd or rcbo can the max zs be higher than in the regs 

ie say b32 is 1.10 ohm if I but the circuit on a rcbo can it be higher to pass on a cert 

Click to expand...

If you can find a regulation to back up your proposed values then yes.....

If not...

I would just stick to the vales from the tables in Part-4 with ref to the protective device selected and the earthing arrangement applicable.

and I'd be double checking that my R1+R2s are proportional to the lengths of circuits you are testing.

Guinness

Can you give some examples of the R1 + R2 and Zs reading, and earthing arrangement and Ze?

Hi on a tncs system 

house skt  on a b32 rcbo rated at 1.10 ohms

when checked all  insulation and r1 x r2 the reading 1.28  due to length of run is big   

Can you get away with it due to rcd protection 

Cheers all

Russ nichol said:

Hi on a tncs system 

house skt  on a b32 rcbo rated at 1.10 ohms

when checked all  insulation and r1 x r2 the reading 1.28  due to length of run is big   

Can you get away with it due to rcd protection 

Click to expand...

does your volt drop comply?

did you get those figures testing through socket front of by testing direct onto the cables themselves?

possible option, stick a 20A MCB on cct (you can get 25A in some makes) if loading is light.

Russ nichol said:

Hi on a tncs system 

house skt  on a b32 rcbo rated at 1.10 ohms

when checked all  insulation and r1 x r2 the reading 1.28  due to length of run is big   

Can you get away with it due to rcd protection 

Click to expand...

what were the end to end readings?