Max Zs entry on EIC for an RCD circuit

[equitum]

As pointed out at my last inspection, this should be 1667ohm not 1.5ohm for say a 32A mcb circuit protected by a 30mA RCD. (IR < 50V)

Was I alone in entering anything other than 1667 for rcd circuits previously ?

[edit: in a domestic installation]

 

[GaryMo]

I enter figures from table 41.3 calculated at 80%.
So, for a B32 MCB I would put 1.15ohms irrespective of an RCD protecting the circuit.

Table 41.3 is for circuit breakers to BS EN 60898 and the overcurrent characteristics of RCBO's to BS EN 61009-1.
These figures give compliance with the 0.4s disconnection time set out by 411.3.2.2 and 411.3.2.3

 

[equitum]

But surely GaryMo you are not entering the MAXIMUM figure permitted by BS7671 ! And even for a non RCD circuit, why do you enter 80% figure? Thats an NICEIC recommended figure, not BS7671

This reminds me of another inspection where the NICEIC guy said I'd incorrectly entered 5s MAXIMUM disconnection time for a shower (16th edition) I stated that the maximum time is 5s. Firstly he said that was what the regs stated, and when he couldn't find it in the regs (as its not there) He said... well, I'd want it do disconnect within 0.4 so thats what i should put. I said, thats not what the form is asking for, its the maximum permitted, not the desired figure. Didn't go down too well with him !!

 

[GaryMo]

80% figures were agreed by an ECA assessor a couple of years ago.
Always done it that way since.

 

[equitum]

80% figures were agreed by an ECA assessor a couple of years ago. Always done it that way since.

So none of your certs are compliant with bs7671 then?

 

[GaryMo]

Ive just called the ECA technical department and have been told that i should indeed be entering the value from table 41.3 and not corrected values. Its a shame the assessment guy we had a couple of years ago doesnt sing from the same hymn sheet!

 

[equitum]

Nice one. It goes to say even the assessors get gt wrong sometimes, like mine did with the shower (see above)

 

[davelx]

Interestingly, there is a section in the ECA's guide to the 17th entitled "the futility of testing Zs on an RCD protected circuit" or something very similar.

They say that RCD protected circuits do not have to meet the Zs requirements as the RCD guarantees that the required disconnection time is met. All that is needed is to test the RCD.

 

[holmslaw]

..

 

[securespark]

Holms, don't forget, the voltage now used for these calcs is 230, as opoosed to 240 under the 16th.

 

[Lectrician]

But surely GaryMo you are not entering the MAXIMUM figure permitted by BS7671 ! And even for a non RCD circuit, why do you enter 80% figure? Thats an NICEIC recommended figure, not BS7671

This reminds me of another inspection where the NICEIC guy said I'd incorrectly entered 5s MAXIMUM disconnection time for a shower (16th edition) I stated that the maximum time is 5s. Firstly he said that was what the regs stated, and when he couldn't find it in the regs (as its not there) He said... well, I'd want it do disconnect within 0.4 so thats what i should put. I said, thats not what the form is asking for, its the maximum permitted, not the desired figure. Didn't go down too well with him !!

Anything in a bathroom did used to be 0.4 seconds. That was a regular thing they tried to catch you out on...... The upstairs lighting circuit

 

[Spark123]

For a new installation, under the 16th the maximum eli should not be more than 75% of the permitted maximum.

Under the 17th the figure is 80% for any instalation.

Wasn't the 75% just a rule of thumb? Whereas if you worked it out for a 70ºC copper cable measured at 20ºC then 80% was about right?

 

[Spark123]

Anything in a bathroom did used to be 0.4 seconds. That was a regular thing they tried to catch you out on...... The upstairs lighting circuit

Sure did, I think they dropped it in the 2001 ammendment.

 

[securespark]

Interestingly, there is a section in the ECA's guide to the 17th entitled "the futility of testing Zs on an RCD protected circuit" or something very similar.

They say that RCD protected circuits do not have to meet the Zs requirements as the RCD guarantees that the required disconnection time is met. All that is needed is to test the RCD.

That's ridiculous. Here's why:

1. The regs demand circuits to be designed correctly: The breaker type would have to suit the EFLI, regardless of whether the circuit was RCD protected or not.

2. A 30mA RCD is only good up to 1666 Ohms. What if you tested the RCD which was fine but you did not test the Zs, which just happened to be >1666 Ohms?

Then the RCD would not be guaranteed to give the required protection.

 

[Spark123]

It also doesn't address electronic RCDs which might not operate if the supply voltage collapses.