"Jeff" wrote in message
. com...

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



Jeff

You are quite correct in what you say, however working in a Faraday cage is
inherently unsafe. You are surrounded on all sides by earth potential and in
the event of any fault, the path is more than likely going to go through you
to earth. Not a desirable situation. If mains supplies are floating, then
there should be no return path to provide a lethal shock.

Further provisions in the BS 7671 regs suggest that items of equipment are
separated so that a person cannot be in contact with two items of equipment
for precisely the reasons you give, to eliminate the risk of shock if two
items of equipment are faulty with the case of each item being connected to
a different side or phase of the mains.

The Faraday cage is designed to prevent all external influences from
affecting experiments and tests inside the cage. Careful design and
consideration of the inherent risks need to be taken into account at every
stage. A lot of stuff in commercial labs is hidden behind panels or
incorporated into the building design at the planning stage, so may not be
apparent in everyday use. Commercial labs also have the resorces to put
screening materials behind insulated walls, floors and ceilings and to
employ special techniques around window and door openings. These techniques
are simply too time consuming and expensive for most people to use in a
domestic shed type environment. The standard insulation tests for floors
using heavy, sharp, pointy objects with high voltage applied are just not
practical for a shed, the floor would collapse under the stress.

It is quite feasible to build a Faraday cage in a room or shed within a
domestic environment. It will be inherently unsafe to work in the shed with
live equipment exhibiting certain fault conditions unless additional
precautions are taken to ensure that no contact whatsoever can be made with
the screening material. Floating power supplies are a requirement due to the
need to maintain isolation of the interior of the cage from the outside
conditions.

None of these regulations are a statutory requirement in the UK but are
based on best practice as identified by the IET (formerly IEE). My concern
is to point out the safety implications and not dictate how the project is
finally built. Under UK law, electrical contractors are not allowed to
disconnect a customer's supply no matter how unsafe it may be. They can only
advise the customer not to use the installation/equipment and advise the
electrical supply company of the defects. The supply company can disconnect
a supply, but rarely do so as long as they are getting paid.

Mike G0ULI