Fault Find # 4

[Paul.M]

Call out, customer says his down stairs lights trip out after only being on for a max of ten minuets. Once the mcb is reset it trips about 5 minuets later and after that only a few minuets?

The facts:-

It is a none RCD board.
Circuit is protected by a 6A type B MCB. (60898)
No other circuits are affected.
It does not matter in which order the lights are turned on it always trips the mcb (so its not a fitting at fault here, save you some time lads )
There are no outside lights on this circuit.
The circuit is not over loaded.


Time to scratch your heads lads

 

[Floody]

Ok so the the circuit isn't over loaded, but MCB's trip over time with heat.

so it's possibly something to do with the MCB getting very warm in the space of 10 minutes. I'd like to say the electromagnet in the MCB is nakerd. Or is that just an easy way out of the question?

 

[Paul.M]

Ok so the the circuit isn't over loaded, but MCB's trip over time with heat.

so it's possibly something to do with the MCB getting very warm in the space of 10 minutes. I'd like to say the electromagnet in the MCB is nakerd. Or is that just an easy way out of the question?

Almost there Floody, 9/10

What 2 functions does a MCB do?

 

[Floody]

Trip under fault conditions

And.....

Bloody hell...I only thought it had 1job.

 

[Paul.M]

1. It is an overload device via a bi metal strip which heats up when a current is higher than the current ratting for the device.

2. A magnetic strip which operates when a fault to earth occurs.



Why would the mcb be getting warm enough to to trip the bi metal thermal strip?

 

[Paul.M]

I've got to shoot off Floody, call out. I will leave you with the picture of the fault, I've removed the offending mcb but what else can you see in that photo that is wrong? The fault was a loose mcb connection to the busbar as you can see. This generated enough heat to trip the mcb, it was simply doing its job and prevented the cu going up in flames. Btw I put the two lighting circuits on the upstairs mcb as a temporary measure until I could get a new mcb for that board the following day. All the lights in the house was under 1000w if on at full load so no problems there, just dont have discrimination in bertween up and down lighting circuits but it was only for a day.

 

[Baker1988]

Hi I was going to say its the bi metalic strip heating up and this usually happens when the circuit is pulling a little bit more current then it should as a 6 amp mcb can take about 8/9 amp for a hour or so as if it is only a little bit over the mcb size current the strip will slowly heat up and then trip

 

[Baker1988]

Ahh so the loose connection was causing arcing or something which was causing some sort of oxidation what made excessive heat so the mcb tripped thats good I love these fault finding posts

 

[Floody]

Looks to me as if the busbar where the old MCB sat is corroded/melted. So possibly a new busbar is needed before you fit the new MCB. As I wouldn't be happy with sitting a new MCB on top of that bar, the way it looks in the picture.

 

[Baker1988]

Looks to me as if the busbar where the old MCB sat is corroded/melted. So possibly a new busbar is needed before you fit the new MCB. As I wouldn't be happy with sitting a new MCB on top of that bar, the way it looks in the picture.

Yeah I agree probilly will be replacing the busbar lol thats what loose connections do alot of people dont understand what happens if you dont correctly terminate stuff and this is what happens haha

 

[Paul.M]

Yes gentlemen you are correct. Now look at the picture and at the RCD, what is missing? What is the problem with the rcd?

 

[Floody]

To be honest I'm not all that knowledgable about how an RCD is meant to look as I haven't covered it in college yet so I've never handled one.... BUT I can tell you how they work and what they are ther for.

so ill let Baker or someone else have a stab at that.

 

[Baker1988]

The rcd doesn't even look like it is wired in it has no wires coming out it so pretty sure it wont work isnt a rcd suppose to go over live and neutral that looks as though it is just connected to the live busbas and nothing else at all

 

[Floody]

Will agree with Baker on that. Can't see no wires.

but looking at other RCD's on google, this one in the pic seems to be missing something on the front to tell you whether it's on/off. Like a little indicator.

 

[Baker1988]

To be honest I'm not all that knowledgable about how an RCD is meant to look as I haven't covered it in college yet so I've never handled one.... BUT I can tell you how they work and what they are ther for.

so ill let Baker or someone else have a stab at that.

The rcd needs a live and neutral which it monitors to see if there is a difference in the voltage which which travels down the live and back through the neutral and if there is a difference the third trip coil would detect the difference and diconnect the rcd bus as I said it need both live and neutral but this one has the live bus bar connected to both the live and neutral point so it wont work and also there are no connections coming out of the rcd