A terrible terrible DIYer made a real mess of this installation.
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.
Changed all 27 lights in this building. Every single one had to have the hole tidied up as the old holes were really badly cut and not round.
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’ll see over the coming days.
Today project Tidy Up really got kicked off. An old busbar system needs stripping and the building part rewiring (particularly the old IT system) so I thought I’d start at the disboard.
A lovely Schneider Acti9 fitted to replace a pair of old Crabtree C15 units, complete with RCBO’s where the circuits would take them.
Next week I strip out a load of rotary isolators connected to the busbars, with the eventual goal of totally losing everything on the ladder frame and shortening the box trunking.
I hate crimping these things. It’s so annoying when you think it’s all good and then you realise a pair of wires have jumped over each other.
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.