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Theberylbloke
1st May 2018, 07:32 PM
Greetings everyone,

I was recently sent some photos, by an acquaintence, of issues at a house where things did not go quite as one might think.

I'll post some photos first then go into a bit of detail about the situation. I cannot tell you when, where or exactly the fault that caused the issues due to legal constraints.

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So first up above is a view behind the main switchboard. When the switchboard was opened those present twigged that there were problems! Notice the burnt earth wires.

Next a view of some wiring in the ceiling space.

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In the last of those you can see the singed edge of a piece of cardboard that would likely have kept right on burning if it had not been for the covering of insulation.

Finally here is an A/C isolator switch installed beside the outdoor part of a split A/C system
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Cheers

The Beryl Bloke

BobL
1st May 2018, 08:15 PM
Holy cheeses!

.RC.
1st May 2018, 08:15 PM
Well they were obviously drawing a decent amount of current through the earth. Which way was it going. Earth to switchboard, or switchboard to earth?

So was it a neutral issue?

Theberylbloke
1st May 2018, 08:23 PM
So Part 2.

What could cause this fault? A possible cause is an active to neutral short circuit where the neutral conductor burns in two i.e. goes open circuit and the active conductor then welds itself to the load side of the broken neutral and the protection device on the circuit (be it fuse or circuit breaker) does not operate.

The first thing that happens is you can get lots of current flow to earth, yes more than enough to melt the insulation off the wires. Secondly, any earthed metal in the premises (that's all machinery, appliances and likely taps) become electrically alive.

There are two other pieces of relevant information. This was on a metal framed house. (Most of us have metal framed and clad workshops, so this is very relevant) The second item relates to the earth stake and main earth wiring. The code of practice for this is AS:NZ3000 wiring rules. The resistance of the main earth wire is specified as no more than 0.5 ohms and the main earth conductor size scales up as the mains active conductors get larger. The is no standard for the resistance of the earth electrode to the actual ground. That figure can be pretty much anything. The earth electrode must be driven to get a better connection to earth rather than, say, just buried.

The above two facts have been a big part of the reason for the damage seen in the cables. The house sits on steel posts embedded in concrete and as they are all joined together by the conductive steel house frame their combined parallel resistance to earth is less than the driven earth stake. So fault current has flown through any of the circuit's earths that were connected to the steel frame rather than through the main earth conductor and stake. Thus the house frame was also live and had potentially lethal voltages on it.

There was at least one person in the house during this fault. It is estimated that the fault lasted for 10 to 12 hours before supply was disconnected.

The moral of this story is that electricians should do a "Fault loop impedance" test on all circuits they install the test tells them what the fault current for the circuit is as allows them to install an appropriately rated circuit breaker for the circuit. That breaker has to trip within a set timeframe in order to protect the cables of that circuit from heat damage from the fault current. Circuits with too smaller a cable or too long a cable may not be able to be adequately protected.

This is also relevant to recent discussions about creating three phases from single phase supplies and then running the three phase to various machines around a workshop. Whatever method is chosen to generate the three phases such a setup must be able to supply sufficient fault current to operate protective devices, keeping in mind lengths and sizes of cable runs. Failure in this regards can lead to the results in the above photos.

I should also add that there is no hint of substandard electrical work at the house where this happened.

The risk of a fatality was very real in this event. Electricity deserves everyone's respect.

Cheers

The Beryl Bloke

Theberylbloke
1st May 2018, 08:25 PM
Hi R.C.

Go to the top of the class. Yes a neutral problem.

Cheers

The Bery Bloke

eskimo
9th May 2018, 01:50 PM
I am guessing loose terminals on the aircon main switch.
happens often on aircons