Home Assignment 308

JuniorSparky · Nov 24, 2013

Hello everybody,

I was working on my fault finding home assignment, but I'm struggling a bit with one of the questions. It says:

"A fault has occurred on the cooker circuit. It was reported that
· The cooker was switched on
· a cracking sound was heard from the cooker
· the kitchen lights flickered and dimmed
· the cooker went off.
Describe the likely faults that could produce these symptoms."

IMO it is short circuit as a result of an arc. The arc can occur due to deteriorated insulation (thanks to high ambient temperature of the cooker), or maybe due to dirty connections.

The flickering may be a result of voltage drop. The only way I can explain it to myself is that it is caused by the cooker circuit. It is drawing more current during the fault and the lights flicker for the time before the CB trips and then they dim.

Though I might be completely wrong. I'm confused I've not heard that short circuit draws extra current.

Is overcurrent actually the fault I am looking for? Is it possible that the cooker draws so much current that it makes the light to flicker and the arc to occur?

Thank you.

telectrix · Nov 25, 2013

you're pretty well correct. a breakdown in insulation can cause a short, resulting in a VD to the rest of the installation.

Marvo · Nov 25, 2013

JuniorSparky said:
Though I might be completely wrong. I'm confused I've not heard that short circuit draws extra current.

Is overcurrent actually the fault I am looking for? Is it possible that the cooker draws so much current that it makes the light to flicker and the arc to occur?

Thank you.

Overcurrent is when a load is too large for the circuit it's connected to. Most overcurrent (more commonly known as overload) faults are caused by the user of the installation plugging too many high current appliances into sockets. I wouldn't consider this as a fault as such, more as misuse unless it's as a result of poor installation design.

Short circuit is another ball game entirely, when a short circuit occurs thousands of amps can flow momentarily to the neutral or the earth as a fault current. Electrical installations are designed so that when a direct short circuit occurs the fault current is large enough to ensure that the protective device (fuse or MCB) disconnects the circuit very quickly. For the few milliseconds this fault current is flowing the supply voltage will drop, this would cause the crackling noise and the flickering lights.