Trainee asking for an explanation on something I found today (arcing cable)

Mighty Chum · Feb 24, 2015

Hey there folks, just to let you know I am only a trainee, not actually started my NVQ yet but been working with most aspects of the trade for about 6 months now.

Today I was working on the roof of a cinema replacing some emergency lights. Whilst up there the spark I am working under approached me saying "There's a real weird sound up here" as he did so. I assumed he just meant the AC units and waved it off. As I went back to grab another fitting I heard it too, a constant popping or cracking sound. Cast my eyes over the large Hollywood style "Cinema" sign and saw this.

Electrical Stuff - Album on Upload the image directly to the thread

After some shared expletives between myself and my sparks, we took the photos and then killed the feed using the rotary isolater.

By the looks of it the earth cable had suffered a fire at some point, the unit developed a fault which resulted in the live or neutral becoming too hot, burning out and then the now exposed ends of the broken cable began arcing.

What I'm asking of you folks as I forgot to ask my sparks at the time, is in your opinions, why did the circuit not trip? Why would a burnt out earth cable then cause the main incoming to overheat so much?

Marvo · Feb 24, 2015

Looks like a high voltage neon transformer. If the fault was on the secondary side the overcurrent protection on the 230v supply won't trip. Secondary side fusing would be required.

For future reference if you see these types of transformers again anywhere be sure to give them a very wide berth and don't go anywhere near the wiring that supplies the neon tubes.

Mighty Chum · Feb 24, 2015

Thank you for the information. As far as I know it was supplying the lights that surrounded the letter so very likely a neon. I made sure not to touch any metal whatsoever, only the plastic isolater as I really didn't like the idea of it being on despite it looking in a fairly "stable" condition.

Also the original installation is so shoddy nothing is fed from the DB you would expect, all circuit lists are either wrong, obsolete due to later modifications or you will find other fittings etc spidered off them in a horrendous way. As the cinema was in operating hours we couldn't search for its breaker. We are back tomorrow for 5am and are planning to identify where they are being fed from and then completely remove their feeds from their breakers.

Gods only know when the numpty who turned them on did it and gods only know when the next numpty will do it, if they aren't connected at all, at least we eliminate numpty from the equation.

For my own curiousity, what would you guess the reason behind the incoming cable burning out so catastrophically and creating that arcing mess? Fire from the earth (perhaps it was a bonding cable actually looking at it) destroying the insulation in it?

Marvo · Feb 24, 2015

The installation is very shoddy and these things are a notorious fire hazard at the best of times. 9 times out of 10 any arcing fault will be on the secondary side that supplies the neon tube, not on the supply side. They can produce up to 20 KV or thereabouts which makes the secondary wiring very prone to arcing. If you get a whack off the secondary wiring it will be a defining moment in your life that you won't forget (assuming you live to tell the tale). Where I am it's law that this type of transformer is supplied by a firemans switch on the 230v supply. The switch has to be mounted in a visible location and easily accessible/operated with a 2 meter pole.

[ElectriciansForums.net] Trainee asking for an explanation on something I found today (arcing cable)

Mighty Chum · Feb 24, 2015

So if I'm understanding you correctly, the likelihood is that the arcing side isn't from the board but the feed from the transformer. As it kicks out such a high voltage the cable they've used to go the neon is woefully inadequate, creating lots of heat, burning the cable out and causing this abomination.

Would that also explain why it hasn't tripped the breaker, as the fault is occurring outside of the breakers sphere of influence? The transformer overriding it.

Thanks a load for the responses as well.

Marvo · Feb 24, 2015

Mighty Chum said:
So if I'm understanding you correctly, the likelihood is that the arcing side isn't from the board but the feed from the transformer.

Yes, most likely arcing on the high voltage feed that's coming from the transformer.

Mighty Chum said:
As it kicks out such a high voltage the cable they've used to go the neon is woefully inadequate, creating lots of heat, burning the cable out and causing this abomination.

The HV wiring has very thick insulation that prevents arcing. The insulation probably got damaged where the excuse for a gland fell out and it's rubbing on the cut edges of the plate it's going through.

Mighty Chum said:
Would that also explain why it hasn't tripped the breaker, as the fault is occurring outside of the breakers sphere of influence? The transformer overriding it.

'Sphere of influence' is a term usually referring to earthing arrangements. See page 5 of the linked doc. A circuit breaker on the final circuit suppling a transformer won't protect against faults on the secondary side. Circuit breakers are selected primarily according to the size of cable they're protecting, not the appliances or devices that are supplied by that circuit.

davesparks · Feb 24, 2015

As a trainee I realise you are just doing as you are instructed and not carrying the responsibility, but I would be wary of someone who takes it upon themselves to disconnect disconnect something at a DB without getting the permission/authorisation from the client or their representative.

Turning off the sign at the isolator then informing the client of the dangerous situation is a very sensible thing to do.

But when the management notice that the big illuminated sign is no longer working and then find out that it has been disconnected by a well meaning but unauthorised electrician, that electrician could be in a world of trouble!

Silly Sausage · Feb 24, 2015

Get the transformer removed and take it home.
You can make yourself a nice little Jacob's Ladder with it and a couple of straightened out coat hangers

Mighty Chum · Feb 25, 2015

Thanks a bunch for the information Marvo, looking forward to surprising my sparks on the way to the job tomorrow with my new found learning.

Dave, thanks for the warning, very nice to know for future reference. I feel I didn't explain that after consulting with the manager of the danger of the fault and the relative ease in which it could happen again, he agreed we should remove it. Also the sign itself is meant to be permanently turned off as it faces the wrong way, cannot be seen and costs them money for no benefit.

davesparks · Feb 25, 2015

Marvo said:
Where I am it's law that this type of transformer is supplied by a firemans switch on the 230v supply. The switch has to be mounted in a visible location and easily accessible/operated with a 2 meter pole.

View attachment 28110

I don't know about law, but it is certainly in the regulations to install a firefighters switch (not a firemans switch since it went all PC!) and not more than 2.75m above the hard standing beneath it (unless agreed by the fire service)

I'm not sure if that height has changed in recent years but in years gone by the maximum height was based on the height of a policeman! There was a minimum height requirement to join the police, the firemans switch had to be low enough to be operated by the shortest policeman using a standard length truncheon. This is also why our firemans switches must be operated by an upwards action!

PEG · Feb 25, 2015

...It's a good job we didn't get together as kids,i did this very project,at 15,and took it in to my physics lesson...everyone was impressed,and my teacher thanked me,before ensuring i took it home...

If you made it through school in the 70's,without electrocution or buggery...there really should be some kind of award...

Knobhead · Feb 25, 2015

davesparks said:
I don't know about law, but it is certainly in the regulations to install a firefighters switch (not a firemans switch since it went all PC!) and not more than 2.75m above the hard standing beneath it (unless agreed by the fire service)

I'm not sure if that height has changed in recent years but in years gone by the maximum height was based on the height of a policeman! There was a minimum height requirement to join the police, the firemans switch had to be low enough to be operated by the shortest policeman using a standard length truncheon. This is also why our firemans switches must be operated by an upwards action!

The height was lowered as of the 17[SUP]th[/SUP]. As you say, to account for short arsed coppers and even shorter firewomen.

Jonspark · Feb 25, 2015

Archy Styrigg said:
Get the transformer removed and take it home.
You can make yourself a nice little Jacob's Ladder with it and a couple of straightened out coat hangers

https://www.youtube.com/watch?v=PXiOQCRiSp0

DPG · Feb 25, 2015

davesparks said:
I don't know about law, but it is certainly in the regulations to install a firefighters switch (not a firemans switch since it went all PC!) and not more than 2.75m above the hard standing beneath it (unless agreed by the fire service)

I'm not sure if that height has changed in recent years but in years gone by the maximum height was based on the height of a policeman! There was a minimum height requirement to join the police, the firemans switch had to be low enough to be operated by the shortest policeman using a standard length truncheon. This is also why our firemans switches must be operated by an upwards action!

I'm pretty sure the height requirement is to allow for a fireman's short axe to be used to operate the isolator. Daz

Darkwood · Feb 25, 2015

PEG said:
...It's a good job we didn't get together as kids,i did this very project,at 15,and took it in to my physics lesson...everyone was impressed,and my teacher thanked me,before ensuring i took it home...

If you made it through school in the 70's,without electrocution or buggery...there really should be some kind of award...

Was it a all male school then?