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