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

 

[Robbo]

It’s in a sheltered housing block, every socket on each floor in on a radial, this socket is next to stairs on the landing, same on each level.
I’m only adding a fused spur to it because I’m installing stairlifts for back ups to the main lifts.

when I did the Zs it came back a 0.79. Which is higher then the current tabulated results you fined in minor works book.

When carrying out trip times, it’s all satisfactory, 28.6 Ms at x 5 and 18.5 at x1. Cables run in 2.5 singles. Zsdb is 0.20.

Iv not installed the circuit, nor have i designed it. I’m only adding about 2 1/2 meters of cable. Problem is The jobs not been priced for extra work, hence the reason I asked do I just mention on the cert, that the current installation doesn’t meet current regs and advise the chainge the RCBO for a B .

 

[Deleted member 9648]

It’s in a sheltered housing block, every socket on each floor in on a radial, this socket is next to stairs on the landing, same on each level.
I’m only adding a fused spur to it because I’m installing stairlifts for back ups to the main lifts.

when I did the Zs it came back a 0.79. Which is higher then the current tabulated results you fined in minor works book.

When carrying out trip times, it’s all satisfactory, 28.6 Ms at x 5 and 18.5 at x1. Cables run in 2.5 singles. Zsdb is 0.20.

Iv not installed the circuit, nor have i designed it. I’m only adding about 2 1/2 meters of cable. Problem is The jobs not been priced for extra work, hence the reason I asked do I just mention on the cert, that the current installation doesn’t meet current regs and advise the chainge the RCBO for a B .

It does meet regs mate. Disconnection time is taken care of by a device permitted for fault protection. An RCD.

 

[Deleted member 9648]

Which regulation are you referring to Essex?

 

[Nigel]

Which regulation are you referring to Essex?

Nowhere near a Regs book currently I am afraid.

 

[Deleted member 9648]

Would appreciate it if you'd let me know when you find the reg no you are basing your disagree on. If I've got it wrong I'd like to know which regulation I've missed.

 

[Deleted member 9648]

Any chance of that reg no Essex?

 

[Nigel]

Any chance of that reg no Essex?

Nope. I am on holiday.

 

[mrloy99]

The increase in loop impedance from. 0.20 at the DB to 0.79 on a 10m run sounds like something not right ,loose connection ?As another poster suggested. Try testing with probes at the socket terminals. But having the RCD ..it will be fine. ..

 

[brianmoooore]

..it will be fine. ..

Don't agree. You are adding to this circuit, so you are taking responsibility for the whole of that circuit, including the bit installed by others before you. If the test results don't roughly agree with the figures you would expect, then you can't sign this off without finding out why and correcting it, whether it conforms to the letter of the regs. or not.
The fact that this job was a fixed quote isn't relevant, and shows the dangers of quoting blind before any tests. You can appeal to the client's better nature and ask them to contribute to correcting the pre existing problem, but if they won't, it's out of your pocket and write it off as experience.

 

[Deleted member 9648]

Don't agree. You are adding to this circuit, so you are taking responsibility for the whole of that circuit, including the bit installed by others before you. If the test results don't roughly agree with the figures you would expect, then you can't sign this off without finding out why and correcting it, whether it conforms to the letter of the regs. or not.
The fact that this job was a fixed quote isn't relevant, and shows the dangers of quoting blind before any tests. You can appeal to the client's better nature and ask them to contribute to correcting the pre existing problem, but if they won't, it's out of your pocket and write it off as experience.

The circuit will remain the same regardless of the device used for earth fault protection, it will remain the same whether a type C or B device is used. There is not a perceived problem with the circuit, only a perceived problem with the choice of fault protection device. The apparent purpose of changing the device is to meet disconnection times when the present device already meets disconnection times, beyond stupid.

 

[Deleted member 9648]

Which regulation are you referring to Essex?

Would appreciate it if you'd let me know when you find the reg no you are basing your disagree on. If I've got it wrong I'd like to know which regulation I've missed.

Any chance of that reg no Essex?

Nope. I am on holiday.

Any sign of that reg number yet Essex?

 

[Deleted member 26818]

Seems to be a lot of disagrees flying about on this thread.
From what the OP says, the original circuit complied at the time of design.
So not poorly designed at all.

The OP is intending to alter the existing circuit.
The alteration must comply with current Regulations.
Using the RCD for fault protection is allowed, though short circuit protection still needs to be provided.

To my mind, whilst the Zs is too high to achieve ADS, this may be because a smaller CSA CPC has been used.
It may be that the impedance of R1 + Rn, is low enough for the device to operate in the event of a short circuit.

If however the CSA of the CPC is the same as the Line and Neutral conductors, and short circuit protection will not be provided, the OP will have to consider other options.

Swap the type C device for a type B, rewire the circuit with larger CSA conductors.

 

[remedial]

good points spin. it seems like short circuit protection often gets overlooked.

however, as others have pointed out, calculated R1+R2 would be very high for the stated length of cable. perhaps something as simple as high contact resistance on the socket during Zs testing would explain the whole problem?

p.s. i just realised this is an old thread.
p.p.s. wheres that reg number essex?