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.

This is exactly the recommended situation in a 'normal' UK mains enviroment
where all exposed metal must be bonded to mains earth (equi-potential
bonding). An RCD is used to provide protection.


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!

Whilst that is true, not having a earth on equipment *inside* the screened
room is still a hazard.For Example; If due to a fault a case goes to mains
potential, the the case of adjacent equipment will be at some other
potential, that could be anything from 'real' ground (the earthed wall
potential of the cage) to perhaps 1/2 rail due the filtering components.
Either way there exists the possiblity of lethal shocks. It is not easy to
control what the case potential or connection will be. Commonly there will
be a connection to the screened room ground on at least some of the
equipment, and that will aslo be external mains ground by default. A lot of
equipment cases will float to 1/2 mains due to the input filtering. Once a
fault develops it is very easy for the isolation barrier provided by the
transformer to be by-passed leaving you with no protection. A neutral to
ground fault could go undetected forever until someone touches something at
line potential. At least with an earth and an rcd there is some protection
in this situation.

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.

True, 55V is considered 'safe', but it is not common to have 110V test
equipment and mains in a screened room in the UK, so it is not really
relavent.

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

As I said this has never been the case in the dozen or so screened rooms
that I have worked in!!

73
Jeff