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

[Mighty Chum]

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?

 

[Lucien Nunes]

There's no reason to expect the LV circuit protection to trip, as the output of the transformer is self-limiting by design. Like many kinds of discharge lamp, neon tubes must be driven by (approximately) a constant current rather than a constant voltage. To provide this, a traditional wirewound neon sign transformer is different from a straight-forward step-up transformer as it has a magnetic shunt to limit the secondary current to the correct value for the type of tube in use, just as a ballast limits the current through a sodium or metal halide lamp.

That could be say 50mA at 5kV - enough to cause a hot steady arc dissipating 250W and easily capable of setting fire to stuff - but consuming only 1A from the mains.

 

[Darkwood]

You express your a trainee and with all respect the answers have been informative and tailored for your knowledge.... was your overviewer aware of the cause and very high risk associated with this fault... letting you get that close with the camera(phone) was a risk in itself as these faults can track far and tend to burn their return routes out leaving exposed metal at very high potential waiting for the unsuspecting.. hope you had some distance and that is zoomed in!!!!!!

 

[davesparks]

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

It may be now, but originally it was based on the policeman as above. Firemen have much longer poles they can use for such things

 

[Marvo]

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)

Yep same here, not law as such but sans10142 installation regs. There might also be an EWAR reg about these firemans/firefighters switches I'm not sure TBH and we're not so PC in Africa, we still call them firemen

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 story sounds plausible about being in reach by a truncheon or even a firepersons axe, I'm not sure what the exact height requirement is here or how it was decided cause the police don't have truncheons, maybe using an R5 rifle instead

You express your a trainee and with all respect the answers have been informative and tailored for your knowledge.... was your overviewer aware of the cause and very high risk associated with this fault... letting you get that close with the camera(phone) was a risk in itself as these faults can track far and tend to burn their return routes out leaving exposed metal at very high potential waiting for the unsuspecting.. hope you had some distance and that is zoomed in!!!!!!

I think the danger with these is that whilst they can certainly deliver enough energy at 10 or 15KV to defibrillate you, you're also as likely to get thrown off a roof or off a ladder by one if you come into contact.

 

[Knobhead]

I think the danger with these is that whilst they can certainly deliver enough energy at 10 or 15KV to defibrillate you, you're also as likely to get thrown off a roof or off a ladder by one if you come into contact.

You bet!

Try getting 12KV up your arm.

I was knocked unconscious when I hit the wall but walked away.

 

[Silly Sausage]

You bet!

Try getting 12KV up your arm.

I was knocked unconscious when I hit the wall but walked away.

Like coming home from pub

 

[Knobhead]

I'm used to it.

 

[GMES]

You bet!

Try getting 12KV up your arm.

I was knocked unconscious when I hit the wall but walked away.

What about the poor wall.

 

[Darkwood]

Not the same but I got 1million volt static discharge through me when I was in a textile mill, the machines had grounding rods that you discharged the materials with before walking beyond a point, thing is I was carrying a ladder and negotiating the machines when it lept a few feet to my ladders....... took me down like a lead weight - I tell you now, you know when you been hit by that static charge.... only a few lads there had received it and they say you never get complacent ever again.

 

[Silly Sausage]

I'm used to it.

Me and you both!

 

[PEG]

Was it a all male school then?

It certainly was...but for a handful of lady teachers...only one being the content of a young pottery students dreams.

Sadly,her interest in my hormone filled teenage shell,was limited to derision of my glaze application...:icon12:

 

[Mighty Chum]

My sparks asked me to watch the sign whilst he got the manager, having stood watching it for a few moments and taking the photos of my own accord I decided to turn it off of my own volition. Having read this thread I am realising what a silly thing that may have ended up being. I'm 28 years old so do most of the work we are offered, not to the same speed as my sparks but to a high enough quality he's willing to cert it once tested. I think that may have created a bit of hubris. Did not realise the transformer would kick out such a high voltage. To think I was a bit worried about changing lights in an 11lkv room I feel a bit silly just walking over and turning the isolater with no one around.

Chalk that up to experience.

 

[Marvo]

Yeah as Darkwood pointed out earlier in the thread when arcing faults occur at that voltage there's a high likelyhood that anything in the area could give you an unwelcome whack and even if the supply is from an RCD it won't protect you. Your mentor should really have been aware of the unusual hazards of such an installation but at least there was no incident or injury and hopefully you'll recognise the dangers immediately yourself in similar circumstances future.

 

[UKMeterman]

Another thing to learn is that if you hear anything HV start hissing, leg it.. Corona discharge is when the insulation is starting to fail in a big way, and if it is a HV supply you don't want to be near it when it fails!

 

[Engineer54]

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.

View attachment 28110

That used to be the case in the UK, but found to my amazement on the last UK project i worked on, that the fire chief can negate that regulation. This happened on a large shopping complex (over 400 shopping outlets) where i am convinced the local fire chief took a back hander by allowing banks of these firemans switches covering several shopping outlets, be positioned in unmarked cupboards located in a service corridors. I tried everything to get that decision revoked/reversed, but all to no avail. I did make an official protest copied to all parties concerned, but it seemed that aesthetics means far more than the safety of firemen, shopworkers and the general public as a whole... That project opened my eyes in several other areas as well, just how much standards had fallen overall in the 20 odd years since i had last worked in the UK!!