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?

 

[UKMeterman]

To get an idea of what I mean get some thin card, toblerone box has worked in the past, and put two pieces of cooking foil, on top of each other with one layer of card in between. Set the megger to 1KV and test between the foils, taking care not to go ouch yourself!!