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

 

[Baker1988]

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.

If u look at the bottom of the blue switch on the rcd it has a little off mark on it which would say it is off

 

[Baker1988]

As I was saying a rcd is basically made of 3 seperate coils wrapped around a circle iron core there is a live coil which the live goes through and a neutral coil where the neutral goes through then there is a third trip coild that detects when there is a difference in voltage going through the live and returning through the neutral if there is a difference then this will create a magnetic flux round the core and that will induce a current into the trip coil which will trip the rcd, well thats what I can remember off the top of my head haha

 

[Paul.M]

Yep as you have said the rcd is not connected!!!!

The customer told me that 4 years ago an "electrician" they called out to find a fault that tripped the stand alone up front rcd. It was an old cooker with an high earth leakage that did it. Because he could not isolate that circuit away from the rcd (it is/was on the down stairs rfc, plug in oven) he just pulled the rcd cables out and left the whole house without any form of rcd protection!!!!! THIS IS A MASSIVE NO NO. Regs state that you can not leave an installation worse off then when you found it, and this chap just fooked all that up.

If that was me at the time I would give the customer the options of:-

1. Buy a new oven or get this one repaired so not to trip the rcd and in the mean time disconnect the oven (unplug it lol).

2. Upgrade the board to an integral board (dual rcd with a none rcd busbar for L + N) so the oven will not trip any rcd's as it will not be on one.

Now saying that about option 2 we are dancing around the regs a little bit. At the time of the circuit being installed it was on the 15th ed regs so rcd was not needed BUT since then they have had a new 16th ed board put in and rcd is needed. What do we do??? If the cable run is greater than 50mm from the surface of the walls, ceilings and floors OR has mechanical protection in the form of PVC or steel conduit OR SWA cable we can put that circuit on a none rcd side of a cu. The chances of this happening is none existent in my opinion so option 2 becomes redundant.


Now that we have seen all our possibilities we are only left with option 1. Get a new oven and in the mean time pull the plug out and issue a warning notice in writing to cover your arse. After all you are the Inspector and Tester of this install now and it will be you in court if any of the circuits that you have worked on causes injury, death or property damage.

 

[Baker1988]

Yep as you have said the rcd is not connected!!!!

The customer told me that 4 years ago an "electrician" they called out to find a fault that tripped the stand alone up front rcd. It was an old cooker with an high earth leakage that did it. Because he could not isolate that circuit away from the rcd (it is/was on the down stairs rfc, plug in oven) he just pulled the rcd cables out and left the whole house without any form of rcd protection!!!!! THIS IS A MASSIVE NO NO. Regs state that you can not leave an installation worse off then when you found it, and this chap just fooked all that up.

If that was me at the time I would give the customer the options of:-

1. Buy a new oven or get this one repaired so not to trip the rcd and in the mean time disconnect the oven (unplug it lol).

2. Upgrade the board to an integral board (dual rcd with a none rcd busbar for L + N) so the oven will not trip any rcd's as it will not be on one.

Now saying that about option 2 we are dancing around the regs a little bit. At the time of the circuit being installed it was on the 15th ed regs so rcd was not needed BUT since then they have had a new 16th ed board put in and rcd is needed. What do we do??? If the cable run is greater than 50mm from the surface of the walls, ceilings and floors OR has mechanical protection in the form of PVC or steel conduit OR SWA cable we can put that circuit on a none rcd side of a cu. The chances of this happening is none existent in my opinion so option 2 becomes redundant.


Now that we have seen all our possibilities we are only left with option 1. Get a new oven and in the mean time pull the plug out and issue a warning notice in writing to cover your arse. After all you are the Inspector and Tester of this install now and it will be you in court if any of the circuits that you have worked on causes injury, death or property damage.

what is/creates earth leakage? why doesnt it happen on this oven and not others?
i new the cable had too be 50mm from the surface ofthe wall to not need rcd but i didnt know it also had to be 50mm from the celing and i didnt know that pvc trunking was classed as mechanical protection i always thought that it had to be metal/steel/galv

 

[Paul.M]

what is/creates earth leakage? why doesnt it happen on this oven and not others?

Without going into too much detail its the element that deteriorates and dumps the extra current down the earth return path.

i knew the cable had too be 50mm from the surface of the wall to not need rcd but i didnt know it also had to be 50mm from the celing

The regs state "50mm from a finished surface" so any wall, ceiling and floor counts. Thats why under part P you have to drill and route cables in the middle of the joist under the floor and it will be below a ceiling. 50mm is deemed to be an adequate depth for anyone drilling a hole or hitting a nail into the "fabric of the buiding".

and i didnt know that pvc trunking was classed as mechanical protection.....

Sorry my mistake, its not unless its surface mounted in which case surface mounted super-seeds the 50mm regs. Whoops


i always thought that it had to be metal/0mm 5steel/galv

Yes mechanical protection when buried in walls etc needs to be SWA or steel conduit/galv. Sorry if I misguided you.

My reply is in blue.

 

[Baker1988]

oooo so thats the reason we alway get told to drill in the center of the joist thanks for that thats made it much clearer thanks

 

[Paul.M]

Have a read of the Part P regs on drilling joinst's. Although it skips by the reasons why, thats the reason why we have to do it to be >50mm

 

[Baker1988]

i havnt read the part p side of stuff but i have read in my college books about where to drill joists but doesnt say why we have to drill them there

 

[Paul.M]

i havnt read the part p side of stuff but i have read in my college books about where to drill joists but doesnt say why we have to drill them there

The regs state "50mm from a finished surface". What is a finished surface? Walls, ceilings and floors!

Is'nt it great how we are told how to do things through the regs without an explanation of why???

 

[Baker1988]

The regs state "50mm from a finished surface". What is a finished surface? Walls, ceilings and floors!

Is'nt it great how we are told how to do things through the regs without an explanation of why???

well i will have remember that i always thought it was just 50mm for the surface of the walls, yeah its great:banghead: