Zs reading does not meet current regs

[Robbo]

Doing some prior tests befor installing a fused spur from a radial circuit, and have found the Zs reading higher then the current regs, obviously it complys with a past reg, but as I’ll be doing a minor works for the job, when filling out their cert would I just put it down as an observation in comments on current installation and grade it C3.

Cheers.

 

[Robbo]

Thanks for info guys.

 

[davesparks]

Using 1667 as a get out for poor design doesn’t sit right with me.

As asked before what rating is the RCBO, nearest C type is 32 amp at 0.68. What CSA is the cable and what is Ze and earthing arrangement?

I agree as a get out for poor design, but when assessing an existing installation it has to be considered.
I may have read the OP wrong but I thi k this is an existing circuit which is slightly over the Zs requirement.

 

[Zdb]

Manufacturer's data should also be taken into consideration with existing installations as their maximum Zs values are slightly higher than the tabulated values in BS7671.

If you comply with the manufacturer's values then in a way you are still complying with BS7671.

For example a Schneider Acti9 B6 MCB has a maximum Zs of 7.99 ohms. Where as BS7671 states that a B6 is only 7.28ohms.



Something to consider.

 

[Deleted member 9648]

In this case then due to bad design/change in Regs the high Zs is through no fault of yours.

In this case I would utilise the RCBO for fault protection although. This is not permitted in Bs7671 so will need to be recorded on the MWC as a deviation.

Changing it to a B Type may import nuisance tripping when the stairlift is starting.

Only a possibility but personally based on the info we have got I would leave it as a C Type and utilise the RCD for fault protection and note the deviation on the cert.

An RCD is a permitted means of fault protection, there is no deviation.

 

[ChrisElectrical88]

Whilst i put the point forward about the max Zs been 1667, i agree with the other that on a newly designed circuit it should meet the disconnection times for the particular protective devive.

On adding to a circuitn and going 0.10 or whatever over whikst its RCD protected then using the 1667 to get away with it isnt to bad IMO, its not like hes getting a reading over 10 Ohms or even close where therenmight be an obvious issue with the circuit even though it complies with the 1167.

Hope this makes sense

 

[Nigel]

An RCD is a permitted means of fault protection, there is no deviation.

Yes only when a suitable Zs is not obtainable. For example a high Ze. It is not permissible to cover up poor design which is the only reason for high Zs when the Ze is low.

This is the reason it should be recorded as a deviation because it was not the design intention to rely on the 30mA RCD max Zs of 1667 ohms.

Also if 30mA RCD was not a requirement for the circuit for any other reason then a 30mA RCD would not be the correct OCPD for fault protection in this instance. You would install a 500mA RCD.

 

[Deleted member 9648]

It complies with Bs7671 therefore it is not a deviation. Design intention or otherwise it complies. The type C RCBO will trip within the specified disconnection time under earth fault conditions, if it's changed for a type B it will still trip within the disconnection time. Nothing has changed. Utterly pointless.

 

[Nigel]

It complies with Bs7671 therefore it is not a deviation. Design intention or otherwise it complies.

Installing a circuit that does not meet max Zs does not comply with BS7671.

 

[Ian1981]

Yes only when a suitable Zs is not obtainable. For example a high Ze. It is not permissible to cover up poor design which is the only reason for high Zs when the Ze is low.

This is the reason it should be recorded as a deviation because it was not the design intention to rely on the 30mA RCD max Zs of 1667 ohms.

Also if 30mA RCD was not a requirement for the circuit for any other reason then a 30mA RCD would not be the correct OCPD for fault protection in this instance. You would install a 500mA RCD.

However if it’s protecting socket outlets then you would install 30mA rcd protection anyway so both fault protection and additional protection is covered.
The regulations allow the use of an rcd to be used for fault protection regardless of the Idelta N of the device provided that the maximum resistance is not exceeded to keep the touch voltage under 50 volts under fault conditions.
Also as long as the 0.4 or 5 second disconnection time is met then the requirements for fault protection is satisfied be it earth fault or short circuit conditions.
It maybe poor design on TN earthing arrangements however it is acceptable by the regulations so it can’t possibly be a departure if the regulations allow it.
Sometimes on existing installations it’s just how it is but If completing alterations I do not note it as a departure.
But yes it should not be designed like that if possible
Also it should be confirmed that short circuit protection is still afforded by the ocpd so I always do a live to neutral loop test If using the rcd for earth fault protection

 

[Deleted member 9648]

Installing a circuit that does not meet max Zs does not comply with BS7671.

There is no requirement to install a circuit to a maximum Zs for a fuse or MCB, as long as the device used for fault protection meets the required disconnection time with the measured Zs. Other factors such as length of the circuit/R1+R2 may be a deviation. But as long as other design factors of the circuit comply and the disconnection time is met by a permitted device there is no deviation

 

[Jdsparky]

Are you using a no trip loop test? As this will give a high reading usually, so what was the r1 r2 plus ze?

 

[Norah Nabino]

Yes it’s a 30mA bs 61009 type C RCBO.

Is there any reasons why you chose the C rated protective device??

 

[brianmoooore]

Why is a circuit that feeds a single socket outlet (albeit a double) fed by a 32A protective device in the first place?

 

[Norah Nabino]

Why is a circuit that feeds a single socket outlet (albeit a double) fed by a 32A protective device in the first place?

Exactly ,that is why one member on here was asking for more information like the Ib,cable size etc to confirm the design of the circuit.

 

[ChrisElectrical88]

Possibly an old cooker point turned double socket