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.

 

[Gavin John Hyde]

its poor design to not achieve the Zs for the protective device when installing but sometimes it can not be avoided which is where the rcd comes in as for a 30mA the max Zs is 1667 ohms. Its not often I come across a circuit with the Zs for the mcb and circuit being outside that allowed but the RCD does provide a get out clause

 

[spud1]

its poor design to not achieve the Zs for the protective device when installing but sometimes it can not be avoided which is where the rcd comes in as for a 30mA the max Zs is 1667 ohms. Its not often I come across a circuit with the Zs for the mcb and circuit being outside that allowed but the RCD does provide a get out clause

So how would you report on it in the EICR, if at all?

 

[buzzlightyear]

high readings on Zs needs investigation.

 

[Gavin John Hyde]

So how would you report on it in the EICR, if at all?

FI if it is not possible to carry out investigation there and then. personally I would not code it as whilst its not an ideal design the regulations - although i cant recall it word for word does permit the use of rcd for fault protection when the circuit Zs is too high for the circuit protective device rather than additional protection.

 

[DPG]

Is the Zs high due to bad design, or deterioration /bad connections etc?

 

[telectrix]

what's the earthing system and what's the Ze?

 

[spud1]

Is the Zs high due to bad design, or deterioration /bad connections etc?

Bad design. The final circuit is fed from a sub distribution board on a long sub-main. The readings make sense when you tot up the circuit lengths and resistances.

 

[Nigel]

If the Ze is a decent reading and there is no external reason the Zs should be higher than the max permitted then it is a non-compliant install as the RCD should be for additional protection not fault protection.

As this is a periodic and RCDs are installed I would only code this as a C3 as the RCD is providing fault protection so there is no immediate danger but the installation is not to BS 7671. I would clarify within notes on the report.

If the Ze is high and the high Zs could not be avoided then no code as it is acceptable to use an RCD as fault protection in this instance.

 

[spud1]

what's the earthing system and what's the Ze?

Its TNCS and the Ze is 0.23 ohms

 

[telectrix]

so as per essex boy's post. a C3 maybe or just a comment.

 

[happysteve]

When you say, "slightly exceed," was the circuit designed prior to the introduction of the Cmin fudge-factor, and would it have complied at the time of design? Not that it really should influence your decision on coding as you're comparing the results to those required by the current edition of BS7671, but it might explain it. Sounds like poor design though, anyway.

 

[spud1]

When you say, "slightly exceed," was the circuit designed prior to the introduction of the Cmin fudge-factor, and would it have complied at the time of design? Not that it really should influence your decision on coding as you're comparing the results to those required by the current edition of BS7671, but it might explain it. Sounds like poor design though, anyway.

Yes very possibly complied at the time of install. Probably installed in late 15th maybe early 16th edition. Readings are within 0.10 ohms of compliance with current standards

 

[DPG]

Yes very possibly complied at the time of install. Probably installed in late 15th maybe early 16th edition. Readings are within 0.10 ohms of compliance with current standards

0.1 Ohms is officially a gnat's cock I believe.

 

[spud1]

0.1 Ohms is officially a gnat's cock I believe.

I couldn't comment, my hands arent steady enough and probes arent small enough!

 

[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.