Builders and electrics PT2

[ARW]

OK so they removed a door chime to plaster a wall.

No big deal, however they did not know that it was powered by a mains powered transformer that was under insulation.

No attempt was made to isolate the bell wires and IMHO for the last two days the bell wires have been touching resulting in the transformer overheating and causing a small fire that took out several T&E lighting cables before the MCB tripped.

The yellowish insulation is the old stuff, the 300mm new brown stuff was placed over the old stuff back in November.

Lucky bastards looking at the scorch marks on that timbers.

One other point/question. The insulation has turned white where the most heat was. Any ideas why?

[Graham.]

Good greif, that's scary. Was there a secondary side fuse inside?

[Harry Bloomfield]

Any primary fuse could have avoided that. It looks like quite an old transformer..

[Roger Hayter]

So the transformer was covered with insulation originally, or just since November? Either way I think it would be gross (criminal) negligence if a professional did it?

[Harry Bloomfield]

Er, it doesn't look as if the mains supply cable was overloaded (as I first assumed), rather those mains cable were just damaged by the fire.

[Graham.]

Sorry Harry, I meant secondary side fuse.

That transformer must be nearly 50 years old. If they still make iron cored bell transformers, they are bound to have a thermal fuse embedded in the windings. Same goes for extractor fans.

[Dave Plowman (News)]

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.

[Fredxx]

That was my thought, and I thought there was a requirement for one. If not there ought to be.

[Harry Bloomfield]

Dave Plowman (News) formulated the question :

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

[alan_m]

Ash? Possibly for the same reason that partially burnt wood residue is black but fully burnt wood residue is white(ish) ash.

[Tim Watts]

That's like an article about how to do *everything wrong* installing a bell transformer :-|

Most of my SELV PSUs are screwed to a plasterboard panel next to the lighting junctions in free open air with the scenario in mind (one is on a bit of wood - that's historic).

I also buy PSUs from RS

eg

so I have some faith in the build quality as well has having "over-everything" protection.

[Ian]

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?

[Brian Gaff]

I do hope the builders were insured for this, or they could be a bit of an ex builder when the claim goes in. Brian

[Dave Plowman (News)]

ISTR an old bell transformer which had replaceable cartridge fuses for both input and output inside it. Otherwise, it would need to be 'large' enough to take the fuse on the lighting circuit they are normally on (at least at one time) in event of a failure.

[dennis]

8<

The EU requires one but don't worry when we leave we can go back to not having them.

[ARW]

Well the apprentice mistook "Leave the transformer in the van" for "throw it in the bin" however I shall retrieve it on Monday morning for closer examination.

[Fredxx]

Only an idiot Remoaner would contemplate such.

[dennis]

Lievers are the ones that say we are going to make cheaper stuff, how are we going to do that without ignoring safety? We won't be competing in the semi closed EU market but the world markets with all the other cheap manufacturers like india and china.

[Fredxx]

I don't recall this? If so a very small minority might have made such an assertion.

We don't currently compete that well now, in buying more from the EU than we sell.

Brexit was never about these things that sore loser Remoaners bang on about.

[Roger Hayter]

We'll have to learn to use aspirational (as opposed to tested) CE marking in that case. Everyone looks for the CE mark (except in the US where they have a similar mark).