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-   -   Earthing question - plastic in metal? (https://www.diybanter.com/uk-diy/325668-earthing-question-plastic-metal.html)

Andy Dingley July 6th 11 11:30 PM

Earthing question - plastic in metal?
 
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 July 7th 11 07:31 AM

Earthing question - plastic in metal?
 
Andy Dingley wrote:

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?

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
--
Chris J Dixon Nottingham UK


Have dancing shoes, will ceilidh.

robert July 7th 11 09:48 PM

Earthing question - plastic in metal?
 
On 07/07/2011 07:31, Chris J Dixon wrote:
Andy Dingley wrote:

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?

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.


......

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[_3_] July 7th 11 10:27 PM

Earthing question - plastic in metal?
 
On 07/07/2011 21:48, robert wrote:
On 07/07/2011 07:31, Chris J Dixon wrote:
Andy Dingley wrote:

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?

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.


.....

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.


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 July 7th 11 10:36 PM

Earthing question - plastic in metal?
 

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.


.....

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.


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.


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



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