Lights Tripping MCB

[sl1210]

Hi guys,

Got to go back to a job tomorrow where the downstairs lights in property trip the mcb after 30 minutes or so. I was there briefly for an hour or so on friday and done an insulation resistance test on the cables going out from the fuseboard. I was getting a dead short from the line cable to the main earth bar with the lighting earth cable disconnected.

Any ideas on why this is. The feeds are at the switches, and i disconnected a couple of them and was still getting a bad reading. All I can think is the downstairs earths are connected at the 2 way light switch with the upstairs lights earth and this is what is coming back to the fuseboard. Will need to do more investigation for sure.

Any thoughts??

Cheers

 

[i=p/u]

dissconnect lighting cirucit at points and keep trying till yo get clear reading

 

[yellowvanman]

Hi guys,

Got to go back to a job tomorrow where the downstairs lights in property trip the mcb after 30 minutes or so. I was there briefly for an hour or so on friday and done an insulation resistance test on the cables going out from the fuseboard. I was getting a dead short from the line cable to the main earth bar with the lighting earth cable disconnected.

Any ideas on why this is. The feeds are at the switches, and i disconnected a couple of them and was still getting a bad reading. All I can think is the downstairs earths are connected at the 2 way light switch with the upstairs lights earth and this is what is coming back to the fuseboard. Will need to do more investigation for sure.

Any thoughts??

Cheers

You may still complete the circuit between line and earth via the neutral, if you haven't turned off the main switch!

Insulation readings only go down to something like 20K, make a reading with your continuity tester and see what the actual reading is.

If there was a dead short the MCB would trip instantaneously, not in 30 minutes as you have described. You may want to put a current clamp over the line of the circuit and see whats being drawn, and then see whats loading the circuit.

 

[sl1210]

the lights are on a 6a breaker and the total lamp loads are under 1000w, so it can't be an overload ;0))

 

[malcolmsanford]

But it could be an old breaker that is giving up the ghost.......................have you tried a new breaker just to see?

 

[sl1210]

its worth a try i suppose or maybe just put in on a 10a too

the thing is, once its trips the first time after like 20 minutes, when you put it back on again it trips within a couple of minutes. so it could well be some kind of overload on the 6a

 

[vernam616]

Are you an homeowner/apprentice or electrician?

 

[bigspark17]

it could be overload but in this case doesnt sound to likely if you say theres max 1kw.! what sort of lighting is on the circuit, isit ground floor .? how old is the wiring.??

 

[tony mc]

IMO I would not just replace the breaker with a 10 A breaker especially if you have less than a Amp on the circuit!

Dead test the circuit and make sure that its not leaking to earth or a fault between line & neutral!

If the breaker is faulted then your tests will come back OK .

As yellowvanman has already pointed out you need to remove the neutral of the circuit or you will get a dead short to earth !

If you do get a dead short on Mohms (with neutral disconnected) switch to low ohms and then get you detective head on and do a bit of calculating & head scratching!!

Let us know how you get on!

 

[sl1210]

i'm a recently qualified electrician, still working on my fault finding.

its ground floor, i think they have had mouse problems before. mainly wall lights about 5x 40w bc's, bathroom wall light energy bc, kitchen track light 4x 50w low voltage, 2x wall lights double ended halogens 150w, kitchen cupboard lights low volatge 20w pin lamps.

thing is it was tripping with most of the lights off

what do you mean by the calculating part tony are we still talking insulation resistance???

 

[tony mc]

No you need to find out what the resistance is on the faulted cables using your meter on low ohms!

You are looking for the fault so the nearer you get to it in theory the reading should reduce!

As it gets low enough you should have a general idea as to the loction of the fault by dividing the resistance of the known cables into the reading that you get!

Say your reading is 0.10 ohms on faulted cables. Your cables are 1mm twin and earth and the fault is between line and earth say.

The resistance of the conductors is 18.10 m ohms/m each so double that gives you 36.20m ohms /m multiply your 0.10 ms be 1000 to bring it to milli ohms and divide your reading into it to give you a rough distance to the fault .

So .10 x 1000 = 100 dived by 36.20 = 2.76 meters.

Its a guide and hope this makes sense !

 

[sl1210]

yeah it definitely makes sense. can't remember if i was ever taught this at college in my 3 years. lol

hopefully when i go back tomorrow the cables test ok with the neutral dis-connected as well, and it is just a case of replacing the faulty 6a breaker. otherwise i could be there a while.

i just want an easy life. haha

thanks tony

 

[Brad_88]

are the lights wired in twin and earth or in conduit

 

[i=p/u]

i just want an easy life too, when scratching my head

 

[sl1210]

twin and earth mate