On 27/04/2018 08:08, Ian wrote:
On 2018-04-27, Harry Bloomfield wrote:
Dave Plowman (News) formulated the question :
Good grief. I'd expect a transformer like that to have a thermal fuse
inside the winding which would rupture long before the transformer
overheated.

Thermal fuses I think are a recent thing, I rather think is a rather
old transformer.

Actually the transformer itself looks to be ok, and probably wasn't too
bothered by the overload. Is it possible the LV bell wire got hot enough
to melt through the PVC insulation and cause a short on some of that mains
wiring it was tangled with?

It was almost certainly the bell wire that got red hot from a dead short
and started to burn without seriously inconveniencing the transformer.

They are very lucky that the MCB did eventually trip to cut the power.
It must have been touch and go at one point judging by the charring.

I have seen a similar near miss where a malfunctioning oil fired boiler
vent was allowing flames and/or very hot smoke to escape onto wood above
it. Luckily the escaping gasses also heated up the mains feed to the
boiler and when the charring was sufficient it took out the MCB.

The other interesting one was CH valve failed with water dripping down a
wall and getting a mains socket with a plug in it wet. The charring from
that was impressive - although the carpet underneath was soaking wet and
so able to cope with the shower of red hot carbon embers dropping down
onto it (old style house wiring with 30A fused ring main and no MCB).

--
Regards,
Martin Brown