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]

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?