A call at 4am resulted in me talking someone through stopping a possible fire. A contactor had overheated and the arc chute was glowing bright red. I told them to turn all the breakers on that particular circuit off this removing the load.
When I got there I found this.
Obviously that’s been burning. I couldn’t get one quickly so for safety I removed the outgoing power from the left hand phase of the left hand contactor so it could not be loaded up even if someone accidentally turned on the breakers.
Sadly, the replacement is going to cost in excess of £500. It’s an AC1 300A one (basically AC1 is the highest load rating out there) and because it’s running heating it needs to be AC1 for sure.
I’ve ordered a replacement but it won’t be here till Tuesday.
I know this is nerdy stuff, but today I encountered the highest PSC (Prospective Short-Circuit Current) that I’ve ever encountered on an installation.
23kA was the figure I found, with an external loop impedance of just 0.01A
So, why you ask am I sharing this?
Let me briefly explain.
All breakers have a maximum “breaking capacity”. It’s a number, usually in a box, on the front of the device. Most domestic ones are 6kA, industrial tend to be 10, 12, 15 or even 25.
If a short circuit occurs which takes the fault current above the breaking capacity of the breaker then the breaker may not actually trip, or it might fail, or the it may trip but the spark will then “hop the gap”.
So, what do we do in this circumstance?
Well we could protect the installation with BS1361 fuses, however, that is not guaranteed to get your 0.4 second trip time.
My chosen method is to put an MCCB (Moulded case circuit breaker) with a 25kA breaking capacity into the meter tails. Not only do you then get your 0.4s trip time, but you get a guaranteed trip at high PSC’s.
When a system is getting false alarms it’s always really irritating. Thankfully this place has CCTV so we searched through the system and found sunlight streaming through a shutter door to be the likely cause.
We now have lots of new sockets, and lights working throughout the house again after I replaced sections of this stuff (pictured) with modern PVC cable.
The stuff I replaced was in a really sorry state, with the insulation cracking off as soon as I touched it. Rather scarily – this stuff has no earth (it carries just Live and Neutral) which means that the lights in the front of the house have never been earthed… something I did suspect to be honest.
The work is complete for now, but I’ve left the floorboards loosely fitted in readiness for more new sockets at the front of the house this coming year when we decorate the living room, taking in the front bedroom and small 3rd bedroom thus allowing me to disconnect and turn off the upstairs ring main and replace it with a much safer radial circuit.
Yesterday and today I’ve been doing general electrical work on our own house (yes… our own house). I’ve fitted 4 new power points (kitchen, living room x 4,in 2 places, landing) and have had to disconnect two lights in the house because the wiring is the old fashioned rubber cable and is in a pretty rough state.
I’ve also fitted a new radiator in the bedroom (replaced the old one because it was too small).
Christmas was very nice… we went to Hannah’s parents for Christmas Dinner and Boxing Day. I got Zelda – Spirit Tracks and Professor Layton – Pandora’s Box, as well as many other presents.
Now what to do… well eat pie and peas first of all cos they’ll be done soon!