Making a fireproof insulator

[Matty F]

I need to make an insulator that is subject to electrical arcing. The

100 year old originals appear to be machined from an asbestos compound of some kind. I don't have any asbestos and I don't want to machine asbestos anyway. I've made an insulator from epoxy resin with loads of chopped fibreglass in it. It's the middle one here: panels fit into the slots.

Unfortunately I have discovered that I can set the epoxy alight after a few seconds using a blowtorch, and that is no good. There is likely to be arcing and bits of molten copper flying around nearby.

What else could I make the insulator from? I doubt that Formica would catch alight very quickly so I could machine it from that. Or maybe concrete reinforced with fibreglass. Or pottery. Any other ideas?

[John Walliker]

How about machinable glass-ceramic? Look up Macor.

Although it is flammable, perspex can also be good in this type of application, because it does not suffer from surface tracking. (Rather than carbonising, the surface layer just evaporates.)

John

[Matty F]

That looks ideal thanks. There's even a warehouse a few miles away selling it.

I'd rather not have anything that could feed a fire. Most of what's in the picture that isn't metal is asbestos.

[James Salisbury]

What is the machine?

[Matty F]

Macor is very expensive. 25x25x100mm is about GBP175 and they don't have a bigger size. If I used clay and fibreglass I could use the mould that I made for the epoxy, then I won't even need to machine it.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Matty F saying something like:

Find what one of these is flame-retardent.

[Matty F]

It's a motor controller for a tram.

Here's one I prepared earlier!

's what that one looked like when I was given the job of putting it together: by sorting out all the bits:

[Donwill]

I would think you need to mould a ceramic material (clay?) and have it fired and glazed like HT insulators. Don

[Chris J Dixon]

I don't know about price or availability, but back when I had to do this sort of thing professionally, Arclex is one of the materials we used:

that the mental mist is clearing, this may be the one we used to line some tube train fuse boxes, and the machine shop went through lots of tungsten-tipped tools trying to cut holes in it.

Nomex is quite popular

looks interesting

[The Natural Philosopher]

Masterboard/multiboard. Its a gyspum and glass fibre composite IIRC.

[newshound]

I was just going to say that! I suspect that Aquapanel might be fire resistant too. You can buy blocks of compressed vermiculite for making brazing hearths, etc but they probably don't have the mechanical strength.

[Andy Dingley]

Tufnol. The basic stuff is linen fabric with phenolic. Easy to machine, well behaved. Tufnol themselves in the UK are also helpful with advice.

2F/14 is the "electrical" grade, but this is mostly about permittivity at frequencies far above trams care about. For general use they offer many grades. If you need better self-extinguishing or low-fume performance against fire, then they have other grades with glass fibre reinforcement and epoxy.

Epoxy can be self-extinguishing, but you need good layup technique and a vacuum rig to cure it. It's about getting a much better mat/resin ratio.

100 years ago, it was probably slate. AFAIK manufactured asbestos boards weren't good enough until the 1920s, owing to lack of suitable resins. Personally I'd use offcuts of Marinite, which is industrial switchgear panelling. Trespa works too.

[dennis]

Concrete moulds well but I don't know how well it resists arcing even when dried out.

[Dave]

Don't personally know but it may be worthwhile asking a Tesla coil enthusiast such as

[Matty F]

I suspect that I didn't get the mixture exact enough. It's a 1:5 ratio which is hard to measure accurately in small quantities. I've used epoxy before and it seemed reasonably fire resistant.

We have a box of similar material. It wasn't thick enough but I imagine I could epoxy a couple of bits together and machine it. I'll test what we have for fire resistance. I'm in New Zealand so don't have easy access to odd materials.

[Andy Dingley]

permittivity and little about fire hazards. Mine's largely polypropylene.