What if a circuits Max Zs is exceeded but the circuit is protected by an RCD?

[spud1]

Just processing an EICR and noticed a couple of final circuits measured Zs values (fed from a sub distribution board) slightly exceed the Max permitted Zs values for the OCPDs that protect them.

Obviously I will note this as a design flaw in the observations, but I just wondered, from the perspective of ADS, as the circuits are also protected by a 30Ma RCD, does this negate the need to fully consider Maximum permitted Zs values for the OCPDs?

Is it not deemed that the fault protection in this event could be being provided by the RCD?

After all, if the earthing system was TT the measured Zs values would be way higher than the Max permitted for the devices...

Cheers

Spud.

 

[Strima]

On a TT system a do a Zn to ensure the circuit OCPD would disconnect in time for short circuit conditions, then Ze to ensure the circuit would meet the requirements for RCD protection.

On a TN if the readings were just over and the circuit was designed and installed to pre-amendment 3 of the 17th then I would lean towards a C3.

If it was post amendment 3 then C2 as it should have been done properly when installed.

 

[davesparks]

for a 30mA the max Zs is 1667 ohms.

This has gone up to 7667 in the 18th edition as in a TN system the permitted voltage rise has gone up from 50V to 230V

 

[happysteve]

This has gone up to 7667 in the 18th edition as in a TN system the permitted voltage rise has gone up from 50V to 230V

Nah. (Made me look, though! )
411.4.204.

 

[Ian1981]

This has gone up to 7667 in the 18th edition as in a TN system the permitted voltage rise has gone up from 50V to 230V

Don’t think it has Dave.
The tables still show 1667 ohms for a 30mA rcd
Table 41.5
The formula for a TN is still zs x ia < Uo x cmin
Also regulation 411.4.204

 

[spud1]

This has gone up to 7667 in the 18th edition as in a TN system the permitted voltage rise has gone up from 50V to 230V

Are you pulling our plonkers Dave?

 

[DefyG]

Could just be down to the RCD. Always find a difference in Zs before and after the RCD.
Measured one the other day 0.25ohms before the RCD, 0.85 ohms after the RCD resulting in a higher than expected Zs for the circuit.

 

[willy fixit]

By-pass the RCD and measure again! Or - r1 + r2 + Zdb (Ze)

 

[Zdb]

How much higher is slightly higher?

If only slightly then you may find that the Zs is still within the maximum permitted values stated by the manufacturers' instructions / data.

 

[Zdb]

Deleted

 

[Bobster]

0.1ohm is still likely in tolerance if you can alter the cmin factor a little.

Or look at manufacturer data for the MCB. BS7671 is sometimes overly cautious with the values it uses.