Electrics question

[MassiveJim]

We currently have the main CU under our stairs and the kitchen socket circuit is separated on its own MCB(32a) on this circuit is a fused switch spur (13a) which runs via 2.5mm armoured cable to the shed where it connects to a separate CU, the sockets in the shed are all connected to their own MCB (16a)
both mcbs on this circuit are "b" type

The situation is this, a repairman came out to fix a faulty tumble dryer in the shed and he turned it on to test once he had fixed it, however our working tumble dryer was also switched on at the time.
as a rough calculation I expect this put somewhere around 20 - 24a load on the circuit.
What I would have expected to happen was either the fuse or one of the MCBs to trip. however what actually happened was some plastic in the fused switch spur melted and broke the circuit, then when I came to try and fault find and turned the switch on/off it was arcing because of the melted plastic preventing the switch from making proper contact.

My question is
what should I change in this circuit to ensure it trips in future whilst still allowing me to run a single tumble dryer in the shed and any power tool I might need to use.

 

[MassiveJim]

nobody ?

 

[Beer_MonsterUK]

OK, so fuses don't instantly blow at their rating current, for instance a 15amp fuse will happily take 30 amps for 5 minutes or so before deciding to blow whereas a type B 16 MCB will trip within 40 seconds (according to the time current graphs in the regulations).

I suspect that either there was an over current on the circuit which damaged the spur before the fused decided to blow, or perhaps even more likely that there was a high resistive connection on the fused spur, perhaps the screw wasn't tight, or that a bit of insulation was trapped, which will cause heat & damage.

Is the whole circuit protected by an RCD in the main CU? If not ,I'd suggest swapping out the fused spur for an RCBO in an enclosure.

 

[MassiveJim]

the fuse in the spur didn't actually blow.
And yes there is an RCD in the main CU

 

[JohnnyG]

...or perhaps even more likely that there was a high resistive connection on the fused spur, perhaps the screw wasn't tight, or that a bit of insulation was trapped, which will cause heat & damage.

This is more than likely what happened.
A 13a BS1362 fuse can withstand around 20a+ for ages whereas the fused spur might not be able to handle the heat that comes with a loose connection especially if it's a cheap plastic one.

 

[Lumeycom]

Loose neutral 99% of the time.

I run a tumble and washing machine behind a single 13a spur and have never had a problem nor am I likely to. Diversity allows for both behind a 13a.

 

[MassiveJim]

I can't remember the exact brand but it is/was a brushed chrome switch.
Inside the switch were 2 small pieces of plastic on springs that sit inside the rocker part of the switch that you press that move the actual switch inside which makes the contact.

one of these pieces of plastic had melted which meant the switch was no longer able to make contact and I could hear it arcing when I tried to turn the switch on.

 

[Beer_MonsterUK]

Yeah, that sounds like it's just a poor make of spur.

I wouldn't use any other make other than MK. Sure they cost more, but the quality is 2nd to none.

 

[Lumeycom]

Less so these days Beer.. Its not the quality that it used to be but still better than council spec crap.

Hager, Legrand or MK for something decent. If you want something pretty then Hamilton is where its at.

 

[MassiveJim]

funnily enough it has been replaced with a plain white MK spur. it is hidden behind the microwave so doesn't matter that it doesn't match the rest of the sockets