ZS Fail RCBO Coding Question

[tank]

Hi all, i'm doing an eicr on a school and am live testing ZS for ring final circuits. The supply is TNCS. i have two results and three questions if i may. i'm getting

c32 rcbo ZS 0.64 ohms. ( Max ZS allowed 0.55 ohms ) Should this be coded a C3 because although ZS fail, the rcd will catch an earth fault? Or C2 because we are now relying on the rcd for fault protection as opposed to additional protection? i would remedy both codes with a b32 rcbo.

c32 rcbo ZS 1.01 ohms. The rcd works. As the ZS so high I tested L-N loop and got 0.65 ohms. Would I be correct in thinking this a C2 as the rcbo will now not disconnect a short circuit before cable damage?

Finally, tin hat on, if the first result can be coded C3, could i remedy the C2 by swapping for a b32 rcbo? This would downgrade C2 to a C3 which could then only be fixed with a circuit redesign. Any thoughts?

 

[The Ghost]

Do you have previous tests to look at? Why do you think they put C type on the RFC?

 

[Richard Burns]

You would initially need to know why the C type breakers have been used, if it is to handle inrush currents from installed equipment then a B type may not be suitable.

Then you should assess the circuit to see if the resistance is sensible based on an estimate of the circuit length.
From the values given the circuit may have been designed originally with B32 breakers and they have been changed to C32 without proper testing.
However if there are loose connections or corrosion on the circuit causing the higher resistance then this should be repaired not covered over.
If the Zs values are valid then consideration should be given to the possibility of rearranging the circuit so that the Zs can be lowered.
If all this fails then you have to consider the circuit is compliant for earth faults though not ideal.
You would have to do the calculations based on the k²S² and I²t to assess if the circuit is suitably protected for L to N faults.
If it is not protected then remedial action is required, which may be changing to a B32 or increasing the csa of the circuit conductors.

 

[Octopus]

When you do EICR’s you report ... not plan fixes ......

 

[Wilko]

If you wanted to really dig - it is possible the low current (no trip) earth loop test records the Zs a little higher than it really is. There are ways to negate this effect for a comparison but it would take a bit of re-arrangement.

 

[Toneyz]

What is the wiring method?
I assume that you are testing on no trip what is Ze or Zdb +(R1+R2)?

 

[tank]

You would initially need to know why the C type breakers have been used, if it is to handle inrush currents from installed equipment then a B type may not be suitable.

Then you should assess the circuit to see if the resistance is sensible based on an estimate of the circuit length.
From the values given the circuit may have been designed originally with B32 breakers and they have been changed to C32 without proper testing.
However if there are loose connections or corrosion on the circuit causing the higher resistance then this should be repaired not covered over.
If the Zs values are valid then consideration should be given to the possibility of rearranging the circuit so that the Zs can be lowered.
If all this fails then you have to consider the circuit is compliant for earth faults though not ideal.
You would have to do the calculations based on the k²S² and I²t to assess if the circuit is suitably protected for L to N faults.
If it is not protected then remedial action is required, which may be changing to a B32 or increasing the csa of the circuit conductors.

thanks for the reply. i don't know why C types specifically were used. there is no documentation available, however the circuits are for general use socket outlets, no fixed loads, motors or IT suites.
i believe the Zs values are accurate and are the result of long circuits rather than loose connections or corrosion.
that's a good point about were the circuits designed to be protected by B types and then changed to C's without proper testing. the school was constructed in 2006, maybe originally the sockets were on B32 mcbs, and then changed to C32 rcbo's without proper testing. i'll try to find out the answer.

 

[Ian1981]

There’s a good chance you might be getting rcd uplift when testing on low current zs testing.
You may find your zs is in the max allowed.
Have you done an R1+R2 test on the circuits and separate zs test at the DB if it’s a sub board?
You may find they comply by calculation tho not an essential test on an eicr as it’s more an initial verification requirement

 

[Nigel]

What is the Ze?

 

[Zdb]

What's the maximum Zs from the manufacturer? It's usually a tad higher than that stated in bs7671.

 

[Strima]

What’s the R1+R2? What the Zs measured using no trip test?