Diagnosing fault on new outside lights circuit

[Trevor McAlister]

Hi
I need help diagnosing fault on a new outside lights circuit, it keeps tripping the MCB or the RCD, and even some times the other RCD of the split consumer unit.

The circlet is brand new, cables were checked by electrician during 1st & 2nd fix. It wasn’t connected to the consumer unit by the electrician with the rest of the circuits because at the time the outside lights weren’t fitted due to ongoing outside work.

Every time I connect the lights and turn the circuit on it trips.
Just powering up the circuit with no lights attached to the ends doesn’t trip ether the MCB or the RCD, which seems to subject it’s not an earth leakage problem, or at least not in the cable. The 4 outside lights are new so there shouldn’t be a problem there.

I have also tried just connecting 1 of the 4 lights and it still trips. I tried 2 different lights one at a time so they can’t both be faulty.

I have tried swooping the 6A MCB with a spare I had, but it still trips.

The circuit layout is as fellows; signal 1.5mm T&E cable to upstairs landing, there splitting via junction box to light A 400w (on motion sensor) above front door. Second cable run to back of house, through internal light switch, to lights B 150w, C 150w & D 150w (only D is on motion sensor).

I have resemble access to junction box where the cable splits, having lifted the hall carpet. I have tried disconnecting each of the cable runs in turn, and with just one light on the end of each run it still trips. I didn’t have any spare cable with me this weekend to eliminate any faults on the cable between the junction box and the consumer unit, I will have to wait to next weekend to do that.

Any suggestions please on where else I should look for the fault.
The next things I was planning to do are:
Eliminate a cable fault between the junction box and the consumer unit.
Put a temporary plug on each outside light to check they all work.
Then I am out of idea’s!

The only cable ends I don’t have easy access to is the junction box where the lights B, C & D goes to the light switch, as this is now under the bathroom floor. But as the circuit still tripped with this run disconnected I won’t be lifting that floor just yet.

Grateful for any help.
Trevor

[ericmark]

Point one must be of course you need permission from local authority for lights over 150W so I would be looking to reduce the size of the 400W lamp which will reduce the load.

Since you say both trips first question are you sure the neutral is on the correct bar in the consumer unit.

There is no substitute for test equipment. Insulation resistance, and loop impedance should have been tested by the guy installing the cables and I would hope you have also tested the RCD's with a meter not just test button.

But before I go further again are you sure the neutral wire is in the neutral bar for that MCB?

[Trevor McAlister]

Hi
As I understand the over 150w rule, this only applies to new builds and some other special condition I can’t remember off the top of my head. so I am ok there.

Yes the neutral is on the correct neutral bar out of the two available. (It’s a split consumer unit.)

Trevor

[sparx]

Hi my thoughts were as Ericmarks, if it's a newish so called 17th ED. con/unit with main switch and 2 RCD's there are 3 Neutral bars! one not on RCD and used to join main sw. to RCD neutrals, please take another careful look.

[sparx]

Hi my thoughts were as Ericmarks, if it's a newish so called 17th ED. con/unit with main switch and 2 RCD's there are 3 Neutral bars! one not on RCD and used to join main sw. to RCD neutrals, please take another careful look.