two circuit breakers needed to turn off lights?

Robbie71

DIYr,
I wanted to change some lights fittings outside house and after some to fro I have to throw one of the "light" & the "Alarm" circuit breakers to ensure the power is cut off.

any ideas on why this might be?

In case yer wondering, i did not touch any wiring yet

Bruthal

So what your saying is, if either of these 2 breakers are on, that outside light will be on?

Robbie71

Yep!
TBH I was lucky as the lights are sensored and went off themselves when I thought one breaker had turned them off . I tested the wiring in the fitting again just before starting and saw that it was still live.:eek:

Bruthal

Robbie71 wrote: »

Yep!
TBH I was lucky as the lights are sensored and went off themselves when I thought one breaker had turned them off . I tested the wiring in the fitting again just before starting and saw that it was still live.:eek:

This can cause confusion though, with sensor lights. You must test the light fitting properly with a suitable tester, such as a multimeter to test between L and N.

When confirmed off, then switch on the MCB you think powers it, and re test. Switch this off, re-test. then switch on the other breaker and re-test.

It would be very unusual for 2 MCBs to opeate 1 light or group of lights. One cause would be 2 switches from 2 different circuits being linked by a loop feed by accident, or a 2 or 3 gang switch with 2 circuits at it being linked when they shouldnt be.

Another (unlikely one) would be 2 damaged circuit wires conductors in contact with each other.

Robbie71

thanks,
I will double check again tomorrow but am pretty certain both are needed.
I turned all lighting mcbs off together - power disconnected
Then trying to find the individual mcb I tried to turned off the lighting mcbs individually and but there was still power.
Then I started started doing combinations and finally got two together that turned off the power .
I checked with multimeter.

If there is some cross wiring do i have to get elecrician to test / find it . This looks like a time consuming job €€€€

Bruthal

Robbie71 wrote: »

thanks,
I will double check again tomorrow but am pretty certain both are needed.
I turned all lighting mcbs off together - power disconnected
Then trying to find the individual mcb I tried to turned off the lighting mcbs individually and but there was still power.
Then I started started doing combinations and finally got two together that turned off the power .
I checked with multimeter.

If there is some cross wiring do i have to get elecrician to test / find it . This looks like a time consuming job €€€€

If it is a problem like your saying, it could be a hard one to find, especially if it was a fault on 2 wires causing it. I would positively prove it is 2 MCBs supplying one light first though. Going in and out on your own, testing, its easy to lose track of what you just tested. Add in security lights going off on their own and that adds to it.

You really need someone going through MCB by MCB while you test. All MCBs off, and the go through each one switched on, you test, then that MCB off, and onto the next one, to see are 2 supplying that light.

TPM

robbie7730 wrote: »

It would be very unusual for 2 MCBs to opeate 1 light or group of lights. One cause would be 2 switches from 2 different circuits being linked by a loop feed by accident, or a 2 or 3 gang switch with 2 circuits at it being linked when they shouldnt be.

Have seen this happen once in a hallway where the hall lights and outside lights were off the same 6 gang switch and different mcb's, and they got linked together.

Robbie71 wrote: »

DIYr,
I wanted to change some lights fittings outside house and after some to fro I have to throw one of the "light" & the "Alarm" circuit breakers to ensure the power is cut off.

any ideas on why this might be?

In case yer wondering, i did not touch any wiring yet

One very obscure thing I came across once was where a sensor light was wired to come if the alarm activated, but the sensor light was also wired to a different circuit to operate using the sensor :eek:

Bruthal

TPM wrote: »

Have seen this happen once in a hallway where the hall lights and outside lights were off the same 6 gang switch and different mcb's, and they got linked together.




One very obscure thing I came across once was where a sensor light was wired to come if the alarm activated, but the sensor light was also wired to a different circuit to operate using the sensor :eek:

Yea you just never know what someone might have done there. Nothing surprises me.

TPM

robbie7730 wrote: »

Yea you just never know what someone might have done there. Nothing surprises me.

Its amazing or should I say frightening some of the stuff you find.

Robbie71 If your not familiar/ experienced with electrics it might be worth considering getting an Electrician to give things a once over, If there is an issue with the lights in this case it may just be a once off mistake, or there could be other problems with other circuits.
I am not saying this is the case but you know what they say, "better to be safe than sorry"

M cebee

it does happen alright-i see it occasionally when replacing fuseboards etc .

there should be a switch for the sensor lighting as well

Stoner

it does sound like two circuits to the same switch box and linked out indeed