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.

 

[westward10]

Have you done a Ze and dead testing of the circuit as you need to confirm if 14 ohm is actually a realistic reading for the circuit before you fit an rcbo which may be an inappropriate course of action.

 

[timhoward]

Yes Ze and Zs at Db as well as R1+R2 for circuit as you need to know where the high Zs is coming from.

 

[Pete999]

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.

What other tests have you carried out? why a C type mcb?

 

[cliffed]

It’s a like for like replacement of new lights, doing a Zs revealed this high reading, this circuit supplies the car park lighting too all Swa cabling,.

 

[cliffed]

Have you done a Ze and dead testing of the circuit as you need to confirm if 14 ohm is actually a realistic reading for the circuit before you fit an rcbo which may be an inappropriate course of action.

Agree but @ this moment there is no earth protection on that circuit.
Or never ever been any earth protectio….regarding the high loop impedance.

 

[westward10]

You can't just fit an rcbo as it will not compensate for the lack of an effective cpc.

 

[cliffed]

You can't just fit an rcbo as it will not compensate for the lack of an effective cpc.

There is an earth but not acceptable for the MCB installed.

 

[timhoward]

1 - Are you happy with the earthing in general? You need to be sure that the supply is fine before you consider the circuit. Hence request to do Ze or at a minimum if you can't isolate Zs@DB.

2- Is it realistic that you have (quick smoke packet maths) around 300m of 1mm or almost 500m of 1.5mm on this circuit?

 

[westward10]

There is an earth but not acceptable for the MCB installed.

14 ohm needs investigating by dead testing. What is the Ze.

 

[cliffed]

14 ohm needs investigating by dead testing. What is the Ze.

Ze 0,23 ohms

 

[cliffed]

Regarding the high loop impedance

 

[cliffed]

1 - Are you happy with the earthing in general? You need to be sure that the supply is fine before you consider the circuit. Hence request to do Ze or at a minimum if you can't isolate Zs@DB.

2- Is it realistic that you have (quick smoke packet maths) around 300m of 1mm or almost 500m of 1.5mm on this circuit?

Not sure about the length of circuit. this was a like for like replacement I’ve done all I can reported to all…asked for existing Certification & basically no feedback whatsoever.
Im not happy with the Zs & suggested an Rcbo to give protection & FI will be required.

 

[timhoward]

This needs a sample of R1+R2 readings on the circuit at various points along it. Even 2 or three more would tell you a lot.
Unless it's truly a giant circuit and the readings progressively increase along it's length there is probably loose connection somewhere.
(If it really is a circuit that big I'd also be running some volt-drop calculations, it may not comply with max 3% volt drop on lighting circuits.)

 

[westward10]

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.

 

[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

 

[Dave OCD]

Is this an older SWA install with the armour as the sole cpc ? If so I'd suspect corroded armour at one or more termination points.

 

[i=p/u]

As said needs a good looking at, and rcbo would temporary protect until powers that be sort out a plan. But doesn't sound good. Like for like, many would swap and walk away without testing.

 

[Pete999]

I did ask what test had been done earlier but the OP has not as yet answered.

 

[cliffed]

Is this an older SWA install with the armour as the sole cpc ? If so I'd suspect corroded armour at one or more termination points.

Yes the Swa been in some time, 2 core only,armourping used for cpc.

 

[cliffed]

I did ask what test had been done earlier but the OP has not as yet answered.

Zs test only @ present..& that was documented

 

[cliffed]

As said needs a good looking at, and rcbo would temporary protect until powers that be sort out a plan. But doesn't sound good. Like for like, many would swap and walk away without testing.

Thank you…it’s now become a major problem & who is going to pay.

 

[timhoward]

Personally I'd stick the live in the earth bar, open up a few car park lights, measure R1+R2 and form a picture of where the resistance jumps high. It shouldn't take that long to get a better understanding of what's going on.
Leaving the live in the earth bar is also an option I'd prefer to installing an RCBO. Leaving part of the circuit functional is also an option if you find good Zs for part of it, while the remainder is investigated more.
You need to talk to your boss again about this. Mistakes happen. He or she will know that.

 

[i=p/u]

Walk in turn the breaker off or remove the Live or if you have time do as tim says, find that car crash and disconnect at that point.

 

[cliffed]

Cheers everyone…lesson learnt check Zs before replacing lights.

 

[telectrix]

Yes the Swa been in some time, 2 core only,armourping used for cpc.

that is most likely the problem. as someone has already stated, corroded armour. also depends on if the SWA has been correctly glanded in the first place, not just bodged. definitley needs investigation.