Fault on existing lighting circuit

[bobsmate]

I've recently been called to a fault on a lighting circuit in a property, when asked if the occupier had done anything differently he told me that all he had done was change the lamps to the eco friendly ones.

I then disconnected all the lamps and when i then tested the circuit the only time i can see that there is a fault is when i test between the live and the neutreal when the neutral is connected into the main neutral bar in the fuseboard. When i then disconnect the cable completly and test between the live and neutral there is no fault showing up at all. But when i then test between the neutral and earth bar it is coming up as a dead short.

So my question is could a neutral to earth fault on another circuit be what is causing the fuse to blow on this lighting circuit when the lamps are in?

 

[ban-all-sheds]

I've recently been called to a fault on a lighting circuit in a property, when asked if the occupier had done anything differently he told me that all he had done was change the lamps to the eco friendly ones.

It is not unknown for people to think that it's a good idea to deny having done something which has caused a problem, then when you find that all the cables at a ceiling rose have been plaited together and dipped in chocolate they will swear blind that it's always been like that.

And get stroppy if you insist that there is no way it could possibly have been.

If all he did was change the lamps he could not have introduced faults unless one of the luminaires or ceiling roses was flaky and the mechanical action of changing the lamps broke something.

I then disconnected all the lamps and when i then tested the circuit the only time i can see that there is a fault is when i test between the live and the neutreal when the neutral is connected into the main neutral bar in the fuseboard.

What kind of fault?

What kind of test?

When i then disconnect the cable completly and test between the live and neutral there is no fault showing up at all.

What kind of test?

But when i then test between the neutral and earth bar it is coming up as a dead short.

Why shouldn't it?

So my question is could a neutral to earth fault on another circuit be what is causing the fuse to blow on this lighting circuit when the lamps are in?

No.

 

[bobsmate]

Sorry i've tried doing all the quotes thing but its not workin

In answer to your first question:
But there is no fault on the circuit when it is disconnected from the fuseboard.

Second Question:
A dead short

Third question:
Insulation resistance


Third question
Would it do this on a tns system?


and last question
Why not?

 

[RF Lighting]

In answer to your first question:
But there is no fault on the circuit when it is disconnected from the fuseboard.

There must be.
With the circuit disconnected from the CU, all lamps removed, and all switches in the on position, what exactly are your readings for insulation resistance at 500VDC between:
1) Live and neutral
2) Live and earth
3) Neutral and earth

Third question
Would it do this on a tns system?

Probably.

and last question
Why not?

Because thats not how electric works.

 

[bobsmate]

In answer to your first question:
But there is no fault on the circuit when it is disconnected from the fuseboard.

There must be.
With the circuit disconnected from the CU, all lamps removed, and all switches in the on position, what exactly are your readings for insulation resistance at 500VDC between:
1) Live and neutral
2) Live and earth
3) Neutral and earth

The reading on all of these is 299 ohms, which is why i cannot understand why the fuse keeps blowing.

 

[flameport]

If those readings are correct when the switches are all on, the only options left are a faulty lamp, or one of the lampholders is faulty in such a way that the fault only shows when a lamp is inserted.

 

[bobsmate]

If those readings are correct when the switches are all on, the only options left are a faulty lamp, or one of the lampholders is faulty in such a way that the fault only shows when a lamp is inserted.

Ok so what your saying is i should try and change the lampfittings? These lampfittings have no earth terminal at the moment but that shouldn't make a difference should it?

 

[londonboy]

Take one bulb and check each fitting individually, and while your at each one have a visual at the wiring, looking for loose/shorting connections.

 

[bobsmate]

Take one bulb and check each fitting individually, and while your at each one have a visual at the wiring, looking for loose/shorting connections.

When you say take one bulb and check each fitting seperatly, how do you mean?

 

[Johnmelad502]

If those readings are correct when the switches are all on, the only options left are a faulty lamp, or one of the lampholders is faulty in such a way that the fault only shows when a lamp is inserted.

Ok so what your saying is i should try and change the lampfittings? These lampfittings have no earth terminal at the moment but that shouldn't make a difference should it?

Not unless you have a floating/unconnected/un-isolated earth touching another

 

[londonboy]

Get a bulb.
Put it in fitting.
Check connections.
Turn on at switch.
Turn off at switch.
Remove bulb.

Go to next light and repeat!

 

[bobsmate]

Get a bulb.
Put it in fitting.
Check connections.
Turn on at switch.
Turn off at switch.
Remove bulb.

Go to next light and repeat!

Right ok perhaps i should've explained a bit better all the lamps will stay on for a certain amount of time and will then go off so seeing if one lamp works in one light will do me no good as i they are all working for a short time anyway. But thanks anyway.

 

[bobsmate]

If those readings are correct when the switches are all on, the only options left are a faulty lamp, or one of the lampholders is faulty in such a way that the fault only shows when a lamp is inserted.

Ok so what your saying is i should try and change the lampfittings? These lampfittings have no earth terminal at the moment but that shouldn't make a difference should it?

Not unless you have a floating/unconnected/un-isolated earth touching another

If you mean another cable (i.e neutral or live) then surely that would show up when i do the insulation resistance test?

 

[bobsmate]

One other thing which i've forgotten to mention is when i did the insulation resistance test at the fuseboard and got a dead short i then went to the first light split up the cables and tested everyone of them and all the readings came out fine.

 

[londonboy]

Ahhhhhhhhhhhhhhhhhhhhhhhhhh now he says!