Zs reading high on external lighting

cliffed · May 31, 2021

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 · May 31, 2021

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 · May 31, 2021

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 · May 31, 2021

cliffed said:
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 · May 31, 2021

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 · May 31, 2021

westward10 said:
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 · May 31, 2021

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

cliffed · May 31, 2021

westward10 said:
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 · May 31, 2021

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 · May 31, 2021

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

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

cliffed · May 31, 2021

westward10 said:
14 ohm needs investigating by dead testing. What is the Ze.

Ze 0,23 ohms

cliffed · May 31, 2021

Regarding the high loop impedance

cliffed · May 31, 2021

timhoward said:
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 · May 31, 2021

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 · May 31, 2021

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.

Zs reading high on external lighting

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