Bang!

[Delboybully]

So I go to a job a couple of days ago in a small shop. One of the things to do is fix an RCD that isnt working in a dual RCD board (not that neat but ok, no labels), everything works but if you turn the RCD off it doesnt disconnect the supply to the 4 MCBs it covers. Also the test button doesnt work. Take the cover off the board and the RCD isnt bypassed, it looks like it has failed closed.

I do a quick Ze, turn off the MCBs, RCDs and the main switch, test for dead and replace the RCD. I then power up and the replacement RCD trips. So I then start doing combined IR test L+N to E for each circuit to find the offending circuit. MCBs 1,2 and 3 or ok

Which leaves circuit 4, I connect my jump lead to neutral and the live and BANG!!!!! Nice pirotechnics with a few bits of hot metal spraying around!
Luckily my jump leads have a plastic covering on them or I could have got a belt!

I can tell you I wasnt expecting that. Luckily my jump leads have a plastic covering on them or I would have got a belt! My apprentice missed the fireworks as he was getting stuff from the van but he was impressed with the jump lead afterwards with a bit missing and the charring on the live cable

So I do some more investigating and it turns out the cable coming into the MCB was live and coming from the upstairs flat and was feeding the other 3 circuits in the shop through the bus bar. The guy in the flat above (luckily he was in) told me he has been getting electric bills of roughly a grand every 3 months, no wonder he had been paying for half the electric bill for a small cafe.

Now I followed the safe isolation procedure as I always do but this still happened. I phoned the Elecsa technical helpline and the guy said you followed the safe isolation procedure and its just one of those things you occasional see. He suggested waving a volt stick around the board in future. I then needed to phone the technical helpline again today on another matter so I asked the new guy the same thing, he again said that there isnt much you can do about these things.

I am one of the hated 5WWs, been doing this for approx 2.5 years. I am prepared for a slating, however I would be interested to learn how any of you would of approached this problem and would you of ended up in my situation. Obviously I followed the correct isolation procedure but still ended up in an undesirable spot. Any suggestions would be appreciated

Del

 

[Hawkmoon]

Obviously it is a lesson learnt for me and I shall be doing as Tel suggested from now on. Thanks to clive P for an honest reply.

That's good because Full safe isolation procedure is the absolute minimum you should do before touching anything...

...coupled with the knowledge and experience to know that sometimes even this will not fully eliminate risk (timed supplies/borrowed neutrals etc).

 

[MDJ]

I must be the only guy here with a MFT, when I stick mine on to the IR test scale it won't let me press the button if the circuit is live, so either OP didn't have the right equipment or his was broken. I understand what he said about what happened, however when he put his meggar across the live cables to test for continuity why didn't his tester let him know not to test due to a live circuit.

 

[Hawkmoon]

I must be the only guy here with a MFT, when I stick mine on to the IR test scale it won't let me press the button if the circuit is live, so either OP didn't have the right equipment or his was broken. I understand what he said about what happened, however when he put his meggar across the live cables to test for continuity why didn't his tester let him know not to test due to a live circuit.

Think he was putting in a temp link to join L-N for a combined IR test to earth (if I read OP correctly)

 

[scammell man]

Hence my comment re not doing live neutral test. oP States he was putting a jump lead on, ie shorting live and neural of a live supply. Tester on PFC would be more appropriate!

 

[shanky887614]

after safe isolation if your not sure on what the cable is etc then don't touch the copper.

simples (obviously the guy got caught out because there was no warning labels etc stating dual supply)

use vde pliers/long nose and you cant go far wrong. no need to touch the copper imho

 

[MDJ]

Think he was putting in a temp link to join L-N for a combined IR test to earth (if I read OP correctly)

Oh right, I usually meggar between cores first before inserting a link just to see if everything is okay, if he would have done that then the tester would have shown a live circuit and the bang would have never occured, anyhow I am done here, you guys are doing a good job by yourselves lol.

 

[gnuuser]

Hi,

To be honest, I think most of us will have carried out isolations this way.
Turn off all the MCB'S , RCD's and main switch and test. I can't believe that EVERYBODY checks every single circuit on every isolation. For my part I think the OP was getting a bit of a slating; when it's not really due. Good point is, it has reminded us all that you can't take everything for granted.

Regards.

and why not? I do!
(went to change a 250 hp 3 phase motor one time and identified and locked out the disconnect, only to find out it was fed from two different panels after metering the leads)
have been careful to check everything since then!

 

[shanky887614]

Oh right, I usually meggar between cores first before inserting a link just to see if everything is okay, if he would have done that then the tester would have shown a live circuit and the bang would have never occured, anyhow I am done here, you guys are doing a good job by yourselves lol.

there is an option on megger and fluke to ir at 230v

why no one uses it i will never know

 

[ifindoubtdont]

MDJ that's called experience and if all who read this thread can learn and take away a little bit more knowledge for next time, then that's what this forum is all about.

 

[Guest55]

all this talk of safe isolation makes me chuckle when i remember back to a hotel mains change a few years ago.....

the elecky cupboard was a mess , it was all coming out , db's , tails en all.

so with no messing about i pull the dno fuses and i give everything a waft over with my fluke stick , which beeped ??....
how could this be ???

a 16mm earth was live !!!!
with the dno fuses out !!!
and all tails removed !!!

after 10mins of head scratching , i'd worked out that this pvc single was feeding the electric heaters CU and had entered through the rear of the backboard , into the bottom of the cut-out and straight into the incoming terminals !

nice lol.

 

[itselectric]

I had to rewire a ward in a local hospital, our first job was to isolate all circuits and remove wiring before new install. The hospital engineer was on site and confirmed the wards circuits were dead. We worked through the ward one room at a time and when it came to the sluice there was a pyro feeding a bedpan washer, so I chopped through it, bearing in mind we had been on the job for over 4 hours and worked without incident. I cut the pyro with my bolster chisel and it set of crackling like a sparkler for the 10 metres I could see it until it disappeared into the roof space. Lucky for me the fuse must have blown at that point. After checking for possible fire, I contacted the engineer who sent one of his guys up, and he located the circuit fuse, it was down a corridor about 20m away from the ward.
What made it worse the cable was 1.5mm 2core on a 50amp fuse.
All because I took someone else's word that all circuits were dead.
This happened in the 1970's and since then I have taken no-ones word that a circuit was isolated.

 

[SPARTYKUS]

Love the sparkler description itselectric.

- - - Updated - - -

So even pyro has its limits!

 

[itselectric]

Love the sparkler description itselectric.

- - - Updated - - -

So even pyro has its limits!

Change of underwear was almost required that day. I thought I was going to have a bit of explaining to do to the fire chief.

 

[MDJ]

Change of underwear was almost required that day. I thought I was going to have a bit of explaining to do to the fire chief.

make sure you install a fan in your toilet then, so if it ever happens again your visitors won't know you did it lol

 

[Guest55]

make sure you install a fan in your toilet then, so if it ever happens again your visitors won't know you did it lol

dont be ridiculous , you cant fit a toilet fan !!!!!!!!!!!



....theres no shower circuit to connect it to. :bucktooth: