Zs reading high on external lighting

[cliffed]

Fitted 2 new external lights onto existing cabling, their on a remote wall quite a distance from house.
My mistake was not doing a Zs before I started the works.
Finished & tested Zs reading 14 ohms.
Its a Tncs supply quite a big install…the lights are supplied by a 6 amp C type MCB.
Reported this to the Client & my Boss…still no feedback on what to do.
I”ve suggested a Rcbo to protect this circuit & FI TO take place.
Windering your thoughts on this.

 

[timhoward]

I'd add without trying to be dramatic that this is very close to the famous recent pub car park death scenario. Please don't leave external lights with Zs of 13 ohms connected. For your sake!

 

[cliffed]

There is exceeding a protective devices capability and there is exceeding it due to a circuit fault. Can you not see 14 ohm is indicating a fault which cannot be ignored by fitting an rcbo. Did you loop between line and neutral.

Yes of course I do…

 

[cliffed]

I'd add without trying to be dramatic that this is very close to the famous recent pub car park death scenario. Please don't leave external lights with Zs of 13 ohms connected. For your sake!

Yea I know scary …how long this as been like this I have no idea

 

[westward10]

Well if you are still there do some dead tests or a loop between line and neutral.

 

[cliffed]

Well if you are still there do some dead tests or a loop between line and neutral.

The report was given to my Company & the client weeks ago… still no actions

 

[cliffed]

My instinct here will be to replace the MCB With a Rcbo, tell the Client a FI will be required,& appropriate tests to be made on that circuit.
At least for now an Earth fault will be protected by the Rcbo…..which will be better than it is now.
My attachment above states that reasoning too.
I cannot quite understand why a few of you disagree with that option…& I fully understand the short circuit requirements…& that is typically of the high Zs reading on circuits where the Zs are high.
My concerns will also be directed @ the Company I work for…

 

[loz2754]

I have to agree that attempting to temporarily mitigate this CPC fault by installing an RCBO is not an acceptable approach.

The allowance in the regs for using an RCD as supplementary protection is a design consideration, not a fault correction provision.

If you had not been given time to locate and correct the fault, the circuit should have been disconnected as it is potentially dangerous.

At least that's the way I look at it...

 

[westward10]

My instinct here will be to replace the MCB With a Rcbo, tell the Client a FI will be required,& appropriate tests to be made on that circuit.
At least for now an Earth fault will be protected by the Rcbo…..which will be better than it is now.
My attachment above states that reasoning too.
I cannot quite understand why a few of you disagree with that option…& I fully understand the short circuit requirements…& that is typically of the high Zs reading on circuits where the Zs are high.
My concerns will also be directed @ the Company I work for…

It is you that doesn't understand. Further dead tests should have been undertaken to establish the issue. How do you know the rcbo will protect under earth fault conditions the earth may totally break down under fault.

 

[cliffed]

It is you that doesn't understand. Further dead tests should have been undertaken to establish the issue. How do you know the rcbo will protect under earth fault conditions the earth may totally break down under fault.

There is a earth loop impedance of 14 ohms.

 

[cliffed]

I have to agree that attempting to temporarily mitigate this CPC fault by installing an RCBO is not an acceptable approach.

The allowance in the regs for using an RCD as supplementary protection is a design consideration, not a fault correction provision.

If you had not been given time to locate and correct the fault, the circuit should have been disconnected as it is potentially dangerous.

At least that's the way I look at it...

Agree… so what about the high loop impedances found on other citcuits that do not comply eg ring main circuits just stick a Rcbo on it…

 

[westward10]

There is a earth loop impedance of 14 ohms.

So from the circuit configuration is this 14 ohm to be expected.

 

[loz2754]

Agree… so what about the high loop impedances found on other citcuits that do not comply eg ring main circuits just stick a Rcbo on it…

They would have been designed that way.
Unlike the circuit with 14 ohms Zs.

 

[cliffed]

They would have been designed that way.
Unlike the circuit with 14 ohms Zs.

Agree & thanks.. will be looking @ this circuit & will decide if this circuit’s R1+R2 is the culprit

 

[Vortigern]

When I have trainess/improvers working with me I always insist they work out the expected value of R1/R2 etc. Also for Ring Final Circuit tests, always calculate the expected value before measuring. So in the case above you would have an expected value derived from the estimated length of the circuit and so on. This would the verify if the circuit is too long of there a comparable value which can indicate a definite fault. I guess I would be out with the wander lead going light to light getting R2, or even between lights if the circuit is too long.

 

[cliffed]

When I have trainess/improvers working with me I always insist they work out the expected value of R1/R2 etc. Also for Ring Final Circuit tests, always calculate the expected value before measuring. So in the case above you would have an expected value derived from the estimated length of the circuit and so on. This would the verify if the circuit is too long of there a comparable value which can indicate a definite fault. I guess I would be out with the wander lead going light to light getting R2, or even between lights if the circuit is too long.

Thanks