Tim W wrote:
Dave Osborne
wibbled on Thursday 17 December 2009 17:43

Tim W wrote:
Earthing the car will be a better bet IMHO than
trying to insure that there is no way that a mains cable entering the car
cannot become damaged on a sharp with of metal and become live.

Another way would be to take the double insulated approach, but to use
SY flex to take the mains in and earth the braid.
As long as you're not planning to take the SY flex into the battery
charger, which would defeat the double-insulated properties of the
charger and be more dangerous!

No, I was thinking of the SY flex being the flying lead used to plug the
charger into the house supply - that being the bit that is vulnerable to
chafing. The charger and its immediate wiring can be secured and protected.

I'm not sure I entirely agree that feeding an earth shielded flex into a
Class II appliance makes it dangerous though. What failure scenarios could
that cause that would result in a dangerous condition?


Class II appliances are designed such that if a live wire goes astray
inside the unit, there is no external metalwork which can become live.
(this is not, I stress, saying that there would be no external metalwork
at all).

In the event of a fault the unit will simply stop working or pop a fuse
(thermal or over-current), rather than become hazardous to touch. This
allows the manufacturer of the product to make the internal chassis
live, to have various different bits of internal metalwork at different
potentials, etc.

This, in turn, facilitates value engineering the design for minimal
production costs. It also allows the product designer to create a
product with mixed metal and plastic components (e.g. a hand-held
electric drill) without needing to ensure that there is contiguous earth
bonding within the unit.

By introducing an earth wire (or indeed armour/screening) into the
interior of the unit, you would then effectively make it a Class I
appliance. You would then have to make an electrical safety reassessment
of the product design. Depending on the nature of the product this could
be straightforward or it could be virtually impossible to achieve
without extensive design reworking.