Ok, I must have done something wrong but cannot work out what, 3 phase BANG!!!

Inteificio · Nov 29, 2011

Hi,

bit of a rush job, but when double checked by me, then another spark could not work out what I did wrong.

Needed a r1r2 reading on a feed from one of our DB boards to an isolator about 10m away. This circuit has been live for some time, just a double check was needed (d63 breaker was on it, we had no idea why, wanted to double check).

I isolated and locked off the supply at the DB board.
I linked l1,l2,l3,R2 at the isolator all at once (far side of the breaker as access to near side was very tight.)

When I went to do the r1,r2 at the DB board it worked fine on friday. Did it again yesterday (accidentally left paperwork at home so had to redo).

When I did it monday morning, when checking the resistance the breaker kept on tripping on the isolator. These wires are locked off and not connected to supply. The N was not touched at either end. The E was in contact at both ends (so not excluding parallel paths. breaker trips 3 times. hopefully unrelated there sounded like a electrical bacg from inside the wall and potentially at the same time a piece of large machinery on the other side of the factory trips the RCD.

I do not get what is going on!

What am I overlooking.

Inteificio · Nov 29, 2011

One other piece of info. We were getting 3v between phases, that I assume was induced voltage from other circuits in trunking.

Inteificio · Nov 29, 2011

seriously, nothing?

I thought I had overlooked something really obvious and was going to earn a few condescending remarks...

mark80 · Nov 29, 2011

What type of breaker is it that is tripping? If its an RCD then it may trip on continuity tests as even though your not checking the neutral, youre still measuring line to cpc so the RCD will still see an imbalance. I have a megger mft and that will trip rcds on continuity tests. As to the electrical bang, if youve safely isolated etc, then it should have nothing to do with you and must just be coincedence.

Clutching at straws an assuming its an RCD but only thing i can think of is that you measured from a different position this time, ie you measure from the feed side of RCD on friday and then the load side this time? As measuring through it would cause it to trip.
Sorry if ive got the total wrong end of the stick!

Inteificio · Nov 29, 2011

Hi,

Yeah it was an RCD, 40 amp. both times I measured the wrong side of the RCD (no way I wanted to touch the top, all 3 lines were 1 colour and very cramped, to much potential for time wasting and getting the phase rotation wrong).

I am assuming the bang and tripped machine on other side of the factory was a co-incidence....well hoping would be more appropriate word than assuming.

Not sure why the RCD went the second time and not the first.

vaughant · Dec 28, 2011

With 3 phase always do your continuity checks before and after you've made any alterations,it's par for the course with me now and takes seconds.
Just the usual phase to phase,phase to earth.
I got caught out 3/4 weeks ago on an intall job in work where there was quite literally a nick in the insulation putting red phase straight to earth,blew 2 fuses then I thought to investigate,it was only happening when the box lid was screwed back on.
As for unlabelled cables welcome to the wonderful world of old factory installations,that same job I was talking about had 3 phase in,into a j/b,up to a dol starter,back out to the fan motor,all red cores,none labelled,all pretty recent too.
Lazy barstewards.

Specialist · Dec 28, 2011

vaughant said:
With 3 phase always do your continuity checks before and after you've made any alterations,it's par for the course with me now and takes seconds.
Just the usual phase to phase,phase to earth.
I got caught out 3/4 weeks ago on an intall job in work where there was quite literally a nick in the insulation putting red phase straight to earth,blew 2 fuses then I thought to investigate,it was only happening when the box lid was screwed back on.
As for unlabelled cables welcome to the wonderful world of old factory installations,that same job I was talking about had 3 phase in,into a j/b,up to a dol starter,back out to the fan motor,all red cores,none labelled,all pretty recent too.
Lazy barstewards.

About Par for the course with Old Industrial stuff is'nt it, think the attitude was Labels cost money & time to fit and anyway any Electrician should know what's what lol.