"Jeff" wrote in message
. com...

" When working with a Faraday cage, remember that safety is of the utmost
importance. Any mains voltage electrical supply fed into the cage must
come
in via an isolating transformer so that the mains voltages are floating.
This will reduce the risk of electric shock if a chassis becomes live
with
respect to the earthed cage. It should not be possible to reach any
'ordinary' mains powered sockets, switches or devices outside the cage
area
when you are within the cage. No external mains earth should be brought
into
the cage area. The Faraday cage should be provided with its own
completely
separate electrical earth. Mains sockets within the cage area should not
contain an earth connection.


That is certainly not the way any screened room I have worked is has been
wired, and sounds extremely dangerous. Not having the earths connected
inside the room is potentially lethal should a case go live.This is a
particular problem with a lot of test equipment where the input filtering
causes the case to float to 1/2 mains voltage without an earth. An RCD
will
not protect you in this situation because of the isolating transformer.

Imaging the situation where you are conducting emc tests with the UUT on
an
earthed copper sheet but all the test equipment is floating at 120V!!

It is also a common requirement to have test equipment outside the
screened
room but connected to equipment inside, isolating transformers and the
lack
of mains earth would complicate this set up immensely.

All the installations that I have seen have not used isolating
transformers,
just filtering on the incoming mains with a mains earth connection.
Protection being provided by an RCD on the mains input.

Jeff



Jeff

If you are working surrounded on all sides by earth potential and you touch
a mains live wire, then the current has a path through you to earth. In
theory, only the phase (or live) wire represents a serious hazard but in the
event of a chassis going live, this could be fatal.

By having mains voltages floating, earthing either side of the isolated
mains accidently to earth should not result in a lethal shock as the
potential on the touched wire only will move down to earth. An RCD is not
needed because although one side of the mains has become earthed through
you, there should be no return path unless you are touching the other mains
wire somehow, in which case you would get a shock whether you were in
contact with earth or not!

110v Isolating transformers with their central point earthed are used on all
building sites in the UK to limit shock voltages to 55v AC. In this case the
transformer is earthed because it is impossible to work away from other
natural earth points, girders, trusses, rods, etc. To a large extent this
also prevents the risk of inappropriate equipment designed only for domestic
purposes being used on site.

For a Faraday cage installation, the mains supply should be completely
isolated and floating using a double insulated approved transformer, no
autotransformers.

I previously worked in a test environment where isolating transformers and a
rubber mat were regarded as adequate protection against shock. Times change.
A proper Faraday cage setup does represent quite a complex environment to
design, is potentially (sorry) very dangererous to work in and so it is
worth looking at the official documents available for their recommendations.
In the UK at least, the standards are recommendations rather than statutory
requirements although you will have a tough time in court if someone is
injured or killed as a result of non-compliance.

Before attempting to set up this sort of testing room, consider carefully
what your requirements are, the safety implications and design accordingly.
Sticking a load of earthed foil and mesh around the inside of a shed is only
the start.

Better safe than crispy fried critter.

Mike G0ULI