Zs measurements in the Cert column

[cliffed]

Still having a discussion @ work about this…TNCS system DB/CU is a split 30ma RCD configuration,5/5
MCB’S are 60898 what Max Zs would you put in the column for Max measurement for the MCB

 

[buzzlightyear]

what Max Zs would you put in the column for Max measurement for the MCB

What would you put.and see if can get correct.

 

[cliffed]

I would put 1666 for the Rcd measurement & the normal Zs measurements for the MCB’s

 

[buzzlightyear]

I would put 1666 for the Rcd measurement & the normal Zs measurements for the MCB’s

Are you testing TT or PME.
in your tick box for Max measurement tested.

 

[cliffed]

Are you testing TT or PME.
in your tick box for Max measurement tested.

PME Cheers

 

[cliffed]

It’s actually in the column Max permitted Zs for installed device ie: MCB 60898

 

[buzzlightyear]

It’s actually in the column Max permitted Zs for installed device ie: MCB 60898
What the regs say what mcb you have installed.

 

[Pretty Mouth]

1667 ohms for any circuit protected by a 30mA RCD

 

[pc1966]

On a TN system I would normally base it on the MCB Zs value.

Yes, you can disconnect according to regs on 1667 ohms with 30mA RCD, but on TN-C-S it ought to be pretty low and unless it is an unusual situation, anything above the usual OSG sort of tabulated values suggests a fault in the circuit implementation which needs fixing, not a planned use of the RCD.

High current sub-main, or TT earthing, are a very different kettle of fish.

 

[cliffed]

1667 ohms for any circuit protected by a 30mA RCD

Well that’s the dilemma & some disagreement here… To be fair I’ve seen both measurements recorded on Certs… personally I prefer the Zs for the MCB, with the 1666 only for the MCB

 

[Pretty Mouth]

Well that’s the dilemma & some disagreement here… To be fair I’ve seen both measurements recorded on Certs… personally I prefer the Zs for the MCB, with the 1666 only for the MCB

I personally don't see any value in using the MCB max permitted Zs values when there is an RCD protecting the circuit. Following 411.4.204, table 41.5, and 643.7.1 (a) and (b) means that I don't have to provide a Zs value, either measured or calculated, to prove ADS, assuming I have confirmed continuity of CPC. It frees me up to focus on other parts of the testing.

While I agree that a Zs test might show a problem with a loose connection in the circuit, I find the results to be inaccurate when compared to a continuity test (particularly no trip tests and/or TT earthing). IMO, better to take an R1+R2 reading, which should give a more accurate picture of the circuit's health.

EDIT: it also cuts down on the amount of live testing

 

[Vortigern]

I would put in the max for the MCB. It is often the case that the RCD does not trip under testing due to no-one ever testing the RCD since it was installed. If the RCD is inoperable I would like to know the MCB will achieve ADS as per regs despite faulty RCD. I have quite often found RCD not operating, a quick few switches on and off "cures" this, but it was faulty until then.

 

[Rockingit]

I would argue that the RCD component of the circuit is there for additional protection - therefore you'd always use the appropriate Zs to achieve a disconnection time under ADS except if the default stated values by the manufacturer are those for RCD anyway.

 

[cliffed]

I would argue that the RCD component of the circuit is there for additional protection - therefore you'd always use the appropriate Zs to achieve a disconnection time under ADS except if the default stated values by the manufacturer are those for RCD anyway.

Yea that’s my thinking, scares me looking @ those 1666 measurements

 

[Rockingit]

Yea that’s my thinking, scares me looking @ those 1666 measurements

In theory, a max impedance of 1666 on a 230v nominal supply does give you a current of slightly less than 5x but vastly exceeds 1x so it shouldn't matter..... but ..... we press the test button on RCD's for a reason whereas ADS just dependably sits there.

But frankly if you're even anywhere close to working with numbers that high then you've got bigger issues anyway, not least of which is Vd.