Garden lighting project

[Km1777]

Hi guys . Doing a garden lighting project for a friend . The house supply is tn-s the lighting circuit is on its own 6amp type A rcbo. Armoured cable feeding 8 type 1 led spike lights via ip rated junction boxes via photocell. My question is. As it’s a tn-s system would it be best practice to use the earth from main cu. Or TT it. My only worry was if the grass was soaking wet and someone touched the metal light fitting bare footed . Would this cause a problem as there would be two different earth paths . Does that make sense or have I got that one wrong . I am a qualified electrician but don’t really do much domestic work so don’t really have to think about this sort of thing often.

 

[timhoward]

I’d use the TNS earth. I’d prefer them to have a reliable fault clearing earth than relying on an RCD which will never get tested and might stick one day.
I’d also consider class II lights, or extra low voltage units as 12v LED spike lights exist.

 

[Km1777]

I’d use the TNS earth. I’d prefer them to have a reliable fault clearing earth than relying on an RCD which will never get tested and might stick one day.
I’d also consider class II lights, or extra low voltage units as 12v LED spike lights exist.

Thanks for that I would have advised to get 12 volt lights in the first place . It’s one of them jobs where he’s got all the ligts before I even looked at the job. Stupid thing to do really. My only worry was could there be potentially a shock risk if I used tns earth . And someone was on the wet grass and touched the wet light fitting . Or am I overthinking that.

 

[timhoward]

My only worry was could there be potentially a shock risk if I used tns earth . And someone was on the wet grass and touched the wet light fitting . Or am I overthinking that.

a) If the casing was live and TNS earthed it would cause the RCBO over-current component to trip before anyone touches it.
b) If the casing was live and TT earthed it should cause the RCBO to trip.
I trust a) more than b) as I encounter RCDs don't work a LOT more than over current devices that fail.

With a TNS setup, there is usually less chance of perceived tingle that you get with TNCS due to return current path being a higher potential than true earth. It rather depends where the nearest PME point is.

All that to say, in my opinion you are over-thinking it.
(If it had been a hot tub where people have bare feet, soggy grass, and TNCS then I'd start to think a little bit about tingles etc. But for garden lights on TNS I'd crack on.)

 

[Km1777]

a) If the casing was live and TNS earthed it would cause the RCBO over-current component to trip before anyone touches it.
b) If the casing was live and TT earthed it should cause the RCBO to trip.
I trust a) more than b) as I encounter RCDs don't work a LOT more than over current devices that fail.

With a TNS setup, there is usually less chance of perceived tingle that you get with TNCS due to return current path being a higher potential than true earth. It rather depends where the nearest PME point is.

All that to say, in my opinion you are over-thinking it.
(If it had been a hot tub where people have bare feet, soggy grass, and TNCS then I'd start to think a little bit about tingles etc. But for garden lights on TNS I'd crack on.)

Thanks for that Tim . In the event of a tnc-s system for outside lighting . Would you still use the pme earth . Or TT the lighting install . Just for future reference if I ever needed it . Just thinking I can’t say I’ve seen many if any scenarios where people have TT lighting installation in a garden.