mcb keeps blowing on lighting

[Wayne Hudson]

Hello sparks
I was at a Job earlier today,the customers upstairs lights would not turn on due to the 6A mcb operating so it's a short circuit so I disconnected the supply to the first light the supply disconnected from consumer unit ,then did a insulation resistance which came out clear tried the mcb without any connection it stayed on.so connected the supply at consumer unit and left the other end not connected to anything it stayed on so connected the loop out and switch live it all come back on for 2 poxy minutes then it would come back on .I had to leave disconnected and will be returning back tommorow . Any suggestions would be extremely grateful
Thanks

 

[telectrix]

fancy allowing a glaswegian in.

 

[Geordie Spark]

Oh didn't I tell you? I let a nice little man in with Jack boots and a swastika on his back........ he asked if I could show him the meters, but I wasn't sure what he meant, so I just showed him those things which whizz round, and have "GAS" and "ELECTRICITY" on them. He thinks you may need new ones as they are going backwards and he will try and catch you next time. :dita:

Thanks pet .. that was very helpful of you. :13:

 

[Knobhead]

I would like to see him explain why all the shiny new breakers he’s talked the customer in to buying are total crap.

 

[Wayne Hudson]

So why are they total crap. I will just admit my mistake what every one seems to no everything about and change the rcbos for dp rcbos .I don't think I'm a no it all one bit just because I thought I was right in what I was saying but I was wrong admittedly I can see how that makes me a no it all .

 

[Bobster]

Good luck finding proteus dp rcbos.

You going to provide these free of charge or re invoice the client?

 

[Wayne Hudson]

Will have to take rcbos out and install a 17th d/b and just take it as a lesson learned

 

[Geordie Spark]

So why are they total crap. I will just admit my mistake what every one seems to no everything about and change the rcbos for dp rcbos .I don't think I'm a no it all one bit just because I thought I was right in what I was saying but I was wrong admittedly I can see how that makes me a no it all .

I see you haven't got yourself an avatar yet son.

Take you pick from one of these: http://www.canstockphoto.com/images-photos/cowboy.html


Always happy to help. :crazy:

 

[davesparks]

I explained to the customer about the difference between rcbos and 17th edition d/b and the customer agreed on the rcbos.on the old d/b all the lighting was not protected a shed supply and cooker supply also was not protected by 30mA rcd .Any cables buried less 50mm has to be protectected by a 30mA rcd or rcbo,tt earthing arrangement zs=8.69 ohms thanks to all

TT system? You are aware that the standard single pole RCBOs are not allowed on a TT system aren't you?

But then as an intelligent electrician you have already realised the massive flaw with standard SP RCBOs and only ever fit SPSN RCBOs?

 

[Bobster]

Beat you this time Dave

 

[davesparks]

So why are they total crap. I will just admit my mistake what every one seems to no everything about and change the rcbos for dp rcbos .I don't think I'm a no it all one bit just because I thought I was right in what I was saying but I was wrong admittedly I can see how that makes me a no it all .

They are a problem because they will detect a neutral to earth fault but not disconnect it.

Think of it this way:

A fault occurs between N and E
A portion of the installation neutral current flows through the fault to earth thanks to the effect of parallel resistances.
The SP RCBO detects this fault.
The SP RCBO then disconnects the live of that circuit.
The neutral remains unbroken and so the diverted neutral current continues to flow through it.
The point of the fault and the cable leading up to it becomes very very warm as the diverted current is greater than its CCC.
FIRE!

So there you have it, the device designed to protect against faults to earth does not disconnect them, instead it gives a false sense of safety as to the untrained person it appears that the faulty circuit is 'off' and therefore safe.

 

[davesparks]

Beat you this time Dave

Well and truly!

But at least we still agree!

 

[Wayne Hudson]

Thanks for that davesparks that makes perfect sense . If the earthing was pme would the sp rcbo be ok to use

 

[davesparks]

Thanks for that davesparks that makes perfect sense . If the earthing was pme would the sp rcbo be ok to use

Think about it for a minute,

With PME the neutral and earth are combined externally and so their loop impedances must be equal, therefore with PME the portion of the neutral current flowing through the fault will be much higher than in a TT or even some TNS systems.

 

[Wayne Hudson]

Think about it for a minute,

With PME the neutral and earth are combined externally and so their loop impedances must be equal, therefore with PME the portion of the neutral current flowing through the fault will be much higher than in a TT or even some TNS systems.

So the sp rcbo would operate on a n & e fault being pme . I ave just ordered some dprcbos expensive lesson this is but needs to be done . Thanks for your help mostly appreciated

 

[davesparks]

So the sp rcbo would operate on a n & e fault being pme . I ave just ordered some dprcbos expensive lesson this is but needs to be done . Thanks for your help mostly appreciated

The SP RCBO will operate exactly as I described above, but the current through the fault which it does not disconnect will be much greater and therefore there is a higher chance of it resulting in a fire.

Why is it an expensive lesson? SPSN RCBOs can be bought for less than most known brands of SP RCBO.
Don't forget SPSN and DP aren't the same thing, they both will do the same job in this case but why pay for DP when SPSN is cheaper?