Pendant from doomsville

[a1guvner]

Carried out a refurb on a posh house in Kensington, client calls to advise having an issue with some lights in a hallway, we attend to find a faulty dimmer module, rated upto 400w, single mod so no derating and mod suited for lamp type and 120w load, fit a replacement mod and starts having a meltdown again that evening, NO RCD/MCB tripping, attend again all cabling double checked visually all fine and another module fitted. further investigation into what is going on here when looking at the pendants the client supplied had found a fault on the fitting with some choice wiring,

Any ideas why the RCD/MCB did not trigger and burnt out the dimmer mod, Wired on a two way with the dimmer at the end of the strapper

All RCD tests rechecked and operate fine

 

[Specialist]

What was the Fault on the fitting ?

 

[a1guvner]

What was the Fault on the fitting ?

An old crumbling 3c flex letting by to earth

 

[Richard Burns]

Are you sure the flex was not leaking line to neutral?
If you had enough current flowing on a partial short then you could easily be taking over 1.7A which would burn out the dimmer but not trip the MCB. If there is no path to earth then the RCD will not trip.

 

[DNS1]

Wow, I've fully understood the reasoning and theory behind your solution to the problem Richard. This is a big deal for me! Looks like the hours spent with the books is beginning to pay off! Yaaaay! :willy_nilly:

Sorry, I'll go now... :biggrin:

 

[Richard Burns]

Wow, I've fully understood the reasoning and theory behind your solution to the problem Richard. This is a big deal for me! Looks like the hours spent with the books is beginning to pay off! Yaaaay! :willy_nilly:

Sorry, I'll go now... :biggrin:

Cool, glad that things are starting make sense, often helps, but not always easy to get to.

 

[GLENNSPARK]

you would have thought that the o/p would have IR tested this BEFORE fitting a dimmer though?....and whats this about a line-earth fault and the RCD hasn`t let go?...."letting by to earth"?<<<<eh???

 

[DNS1]

Can't rate this forum highly enough as a training aid for someone like me (keen and with too much time on my hands!)

Great to be able to read up on a problem, think about it for myself, then scroll down to see what the pros have to say.

 

[Paul.M]

Are you sure the flex was not leaking line to neutral?
If you had enough current flowing on a partial short then you could easily be taking over 1.7A which would burn out the dimmer but not trip the MCB. If there is no path to earth then the RCD will not trip.

Some times you just have to take your hat off to members. Richard you deserve more than a like + thanks for that. Top answear sir.

 

[DNS1]

What sort of partial short would be causing 1.7A or more to flow, but 6A or less (assuming the lighting is on a 6A MCB)?

Just curious

 

[GLENNSPARK]

What sort of partial short would be causing 1.7A or more to flow, but 6A or less (assuming the lighting is on a 6A MCB)?

Just curious

where theres a reduced resistance of the insulation..but it hasn`t completely given up the ghost....such as what can occur in elements and white goods etc......

 

[GLENNSPARK]

such as high resistance joints with the associated increase in conductor tempreture....starting a breakdown in insulation.....

 

[a1guvner]

Are you sure the flex was not leaking line to neutral?
If you had enough current flowing on a partial short then you could easily be taking over 1.7A which would burn out the dimmer but not trip the MCB. If there is no path to earth then the RCD will not trip.

Could of been line to neutral originally and when the fitting was taken down fault may moved line to earth

 

[a1guvner]

you would have thought that the o/p would have IR tested this BEFORE fitting a dimmer though?....and whats this about a line-earth fault and the RCD hasn`t let go?...."letting by to earth"?<<<<eh???

Letting by = leakage

These fittings were fitted post contract by another contractor and the original replacement dimmer was fitted by one of our sparks who was on a 1st fix containment job near the site so did not have his IR tester and he has been re-educated,

an RCD letting go = operating

 

[lloyd1980]

very good answer by the way