MCB keeps tripping

[pburley]

Hi I worked on a lighting circuit in a flat yesterday ( two kitchen lights, four spots on a bar type, a hall light and a bathroom light) total 520 watts, running through a 6 amp B type MCB. Lights work fine but after about an hour the MCB trips. I found a couple of loose connections yesterday but not much else. Before I go back today, has anyone got any ideas. Ta.

 

[Hawk81]

faulty transformer. after an hour it heats up and causes the fault

 

[pburley]

Hi, Sorry I should have said, there are no transformers, it's all 240 volts.

 

[BillyBoBob]

Could be a dodgy circuit breaker.but not likely.
Have you done an IR test?
Are the kitchen lights flouresents? maybe the ballast units are old and starting to burn out, they could create a fault when they heat builds up?
Lets us know if you find it. as its a good thing to keep in mind

 

[UNG]

I try a clamp ammeter on the circuit to check the load

 

[keniff]

IR the light circuit L-N and L-E and do this with all the light switches on, failing that maybe try a new mcb, its only pulling just over 2 amps so shouldnt be overload, i hate these types of faults good luck!

 

[pburley]

Hi guys, did an IR test in circuit and found a dead short. Stripped circuit down into bits and tested again, finally found fault in switch wire on the light in the small room between kitchen and bathroom, line and earth were shorting out. It was only a short run in plastic conduit, so I changed the wire and all seems to be OK. I switched all lights on and sat there for two hours, all OK. Does this sound right? I thought a short would trip instantly?

 

[BillyBoBob]

How are your (R1+R2) readings, you may not be getting your Zs.
This would cause your Circuit breaker to take a longer time to trip.
It sound slim but it is a plausible explanation for your fault.
I agree you prob found what was causing it, but there is a chance your walking away from a circuit that will fail to get a 0•4s disconnection time.
It may be worth while looking at the rest of the cables on the circuit for signs of damage.
Hope this helps.

 

[pburley]

Hi, nice one I will pop back tomorrow and check Zs. Thanks guys, I love this site, it's like having twenty mates in the van with you.

 

[robsparky1975]

Hi, nice one I will pop back tomorrow and check Zs. Thanks guys, I love this site, it's like having twenty mates in the van with you.

yep great innit,like one big happy family