Faultfinding #6

[High Tower]

Ok guys, if were getting these out too quick and overwhelming you guys then say!

So........

Im working in a florist, its in a arcade in the city centre, each unit is supplied with a 16mm SWA from a Ryefield board in the plant room, each board is a metalclad 10 way Merlin Gerin, main switch only CU. circuit arrangement is not important in this case.

I have to fit new 4x2 mods, in the suspended ceiling. Turn off MCBfor the circuit. the lights go off, but placing a volt stick on the cable still shows voltage. i must stress that i hadnt got to the point of safe isolation by this point.

Why??

 

[Paul.M]

^^^^ This is a good one. I've come across this problem before and on two occasions it was two different faults with the same symptoms as you have posted HT. I'll keep quiet on this one

 

[Baker1988]

There could be a few problems here
1: volt sticks are very unreliable they some times glow saying things are live when they actually are not you should use a proper voltage indicator.
2: there could be a shared neutral with another circuit so the neutral could still be carrying current
3: you could of turned the wrong mcb off

 

[High Tower]

Ok, don't believe everything you hear about volts ticks, they are not that bad. They are like a lot of things, there's a time and a place.

Volt sticks should never be used for proving dead, I have my fluke T5 for that, but they are brilliant for finding circuits etc before testing for dead. When it comes to fault finding they are priceless.

Good point about the wrong MCB, but I had got the right one as the lights went out when I turned it off.

 

[Floody]

Was it still connected to the RCD?


edit: never mind, can see its a main switch only.

 

[High Tower]

No rcd, main switch only mate

 

[Floody]

Could it have been the lights already there? Ones that hold energy after being switched off. Running lights or whatever.

 

[High Tower]

Oh most discharge lighting will hold a charge in the cap, but no its not that, dont over complicate, draw yourself a circuit diagram, simple circuit, mcb-switch (neutral looped at the switch)-MOD!-MOD"-MOD3-MOD4-MOD5-Mod6

then try and figure how when the mcb is in the off position there can be voltage indicated.

Incase you dont know what a mod looks like this is one:

 

[Paul.M]

I had a picture in my head that all you Mods looked like this



From left to right we have Phil (he is going to kill me for saying that but I need to get my own back on him lol), Sintra (in green of course mate),Marvo, High Tower, Jason and Dan.

Another slap on the wrist for me I think lol :rofl:

 

[Marvo]

Phil won't be happy.

 

[High Tower]

Absolutely priceless, thanks for the laugh !!!

 

[Paul.M]

Phil won't be happy.

Mr Pink actually looks likes Phil if he had my goatee.

 

[High Tower]

Any more thoughts guys ???

 

[Happy Jack]

It sounds like some kind of phantom voltage / capacitive coupling, as a consequence of its close proximity to other energized circuits. Although post # 8 seems to suggest the problem is a little more straightforward...

 

[High Tower]

Jack I like your pattern of thought, I'll give it a little while longer and then let the moggy out the bag!