Earthing question - plastic in metal?

[Andy Dingley]

Suppose a piece of kit is Class II double insulated and doesn't need an earth wire (proper factory stuff with magic stickers & everything).

It's now placed inside a big tin can.

This tin can is placed temporarily in an environment that's full of high power electrics, ad hoc connections, extreme hazard for shock or falls from a height. The culture here (on paper at least) is that everything has to be carefully earthed and PAT tested before use, especially if it's foreign kit coming in from outside.

If you're wondering, it's stage lighting gear.

What's the situation here, both practically and legislatively? An argument can be made that a class II device that is safe without an earth doesn't (or does it?) become unsafe if it's inside a conductive metal outer housing. Despite this though, any sensible spark with responsibility for incoming kit will presumably see a metal housing and want to see an earth connection to it - they have neither time nor credible ability to open it up and demonstrate that the insides are safe without?

Thanks for any comments.

[Chris J Dixon]

The biggest credible hazard is that the wires to your "safe" kit become damaged as they pass through the metal housing. If this is not earthed, it could be nasty.

I once hired a carpet cleaner which was double insulated, but had a metal outer case, presumably for durability. Unfortunately, the cable had slipped through the grip, whilst still functioning, and the live conductor had made contact with the casing. I only realised there was a problem when it sparked to a radiator.

Luckily no harm done, and hire fee waived, but it could so easily have been much more serious.

Chris

[robert]

.....

The protection provided by "double or reinforced" insulation ( which allows the double insulation label and no earth wire needed) probably no longer exists where the mains flex enters the "big tin can", therefore the can should be earthed.

Double insulated items with metal externals are not uncommon e.g. Kenwood Chef mixers.

[Fredxx]

The mains flex will have insulation which is sufficient so that it can be brought in close proximity to a tin can. Otherwise it would be unsafe to touch. My main concern would be any sharp edges which could cut into the cable, but even that could be overcome using grommets.

[newshound]

You need effective cable clamping too, otherwise a good tug on the cable could roll out the grommet