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Hi All

I have around 30 garden lights which turn on and off with one switch in the house. The problem is, if water gets in one light it a) stops all lights working and b) requires troubleshooting 30 lights to find the problem.

Therefore, I was planning to have a second distribution board installed in my garden shed and run multiple circuits with 10 lights on each. Therefore, in case of a fault, 20 lights are still working and troubleshooting is reduced by 2/3rds.

Now the snag is that the switch is inside the house and to wire in 2 more live switches is a MAJOR headache. My electrician is trying to convince me he can wire it so the switch will work on the 3 new circuits but he can't explain how. My concern is if they use the same switch then a fault will trip all 3 circuits and we are back to square one. I have included a before / after picture to explain what I am trying to achieve. Is this possible? Thanks in advance.
 

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pc1966

Esteemed
Arms
Supporter
Yes, it is possible to do what you are suggesting. However, there are some limitations in how well it will work that you need to consider:
  1. The supply to outdoor lights should be on a RCD protected circuit, that is not shown in your diagram (but might already be in place)
  2. You won't get any useful selectivity between the supply MCB protection in the house and the per-set MCB in the garage CU. So if a fault occurs both will fire, but when you reset the house MCB 2/3 of your lights should come back on and the faulty set remain isolated.
  3. Usually with outdoor light it is water ingress that damages them. That will often trip the supply RCD before it reaches enough current to trip the MCB, but in this case it will not trip the garage MCB. So to fault-find you would need to manually switch off one of the 3 garage MCB in turn and try resetting the house RCD to find which one is causing problems.
  4. One difficult, though less common, sort of fault to traces is a neutral to earth short causing RCD to trip. With MCB switching only the line conductor you would not be able to isolate that sort of fault in this arrangement.
You would need double-pole switching of each set of lights to allow N-E faults to be isolated. If your garage CU was fitted with DP RCBOs such as the Wylex NHXS1B06, or similar, you would be able to do so.
https://www.screwfix.com/p/wylex-6a-30ma-dp-type-b-mini-rcbo/3505x
If your house supply is not RCD protected then you definitely should be using this sort of RCBO in the garage CU!
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Quick question, why the 5-core SWA cable from the house?

Usually you would run sets of lights in 2 core (with the armour as earth) or 3 core (easier to connect up with one core also earthed in addition to the armour).
 
Hi pc1966

Thanks for your extensive reply. You raise a very good point re all three tripping and then resetting one by one to find the culprit. That also works for me.

I have simplified the diagram a bit and the outdoor distribution board comes off an RCBO protected circuit indoors but you are absolutely right. I was actually planning to use an RCBO (32A) in the outdoor board and 6A MCB for each lighting circuit (also need a power circuit for the shed).

Why is it 5 core SWA? That is what the originally installer installed some years ago. It has been a PITA as working with 5 core SWA for each junction for each light is unnecessarily troublesome. I was planning on 2 core or 3 core as you suggest for the runs with the lights.

For the external junction boxes, previously used waterproof boxes with choccy block connectors inside to run each light and continue the run and then fill the box with nearly a whole tube of silicon to stop water ingress. Of course this means no changes are possible. Is there a better way to do it?
 

telectrix

-
Mentor
Esteemed
Arms
if you can split the lights into 3 separate sections with the 5 core SWA, use the switch to energise a 3 pole contactor, 1 pole for each section of lights. then each section can be dissed from the contactor for fault finding.
 

pirate

-
Esteemed
Arms
I'm tempted to suggest a 3 gang quinetic switch in the house, and 3 separate circuits fed from the garage, each with its own receiver
 
Thank you. I was thinking a relay could be another option.

if you can split the lights into 3 separate sections with the 5 core SWA, use the switch to energise a 3 pole contactor, 1 pole for each section of lights. then each section can be dissed from the contactor for fault finding.
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Ok - this is a new technology to me! Thanks for the suggestion. Will pass that on to the contractor.

I'm tempted to suggest a 3 gang quinetic switch in the house, and 3 separate circuits fed from the garage, each with its own receiver
 
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Had a go at putting my thoughts on paper. Here is a simple diagram for the shed consumer unit fed from the existing 5 core SWA cable from the house. Is this in theory workable?

Shed Consumer Unit.JPG
 
If one circuit has a fault only that particular circuit will remain tripped - allowing quicker trouble shooting and the remaining 2 circuits to keep working. Well, that was the plan at least. Could it be achieved without the 3 MCBs?

3 unnecessay MCBs.
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Hi westward10 - this is the crux of the issue.

I have one switch in my house and want to split the existing light circuit of ~30 lights into 3 circuits of 10 lights each. At the moment water ingress causes one of the 30 lights to stop all lights working. I don't have an easy option to add more switches indoors as there is no route to put in the additional cable without a lot of work.

Why are the outgoing lines from the three mcbs linked together.
 

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