Can anyone help me out here? I am not a safety engineer but an military
avionics/radar/systems engineer with a safety role.
A while ago I recounted, in this group, the tale of an item of test
equipment provided to my U.K company from a very large company in the
States. This item of test equipment was provided to us an an "off the
shelf" associated item of test equipment relating to some other military
equipment we had procured.
The test equipment is designed (supposedly) to operate from single phase
240v.It was brought over to the U.K and plugged into a 240v power
distribution system in the company where the 240v is supplied from an
isolation transformer with centre tap connected to ground. Thus the 240v
supplied to the equipment is basically +/- 120v.
The test equipment promptly tripped protection devices, and, I think, got
damaged. It has remained unpowered since, until matters are resolved.The
immediate response of the U.S engineers who brought the equipment over to
the U.K (subserquently backed up by their management in the U.S) was that
this was our fault because the 240v neutral was connected to chassis in
their test equipment, that this was normal practice and so on.
The U.S company were told that such a design was unacceptable and did not
meet anybody's safety standards (we are expecting the unit to comply with
U.S and U.K standards). Eventually the U.S company backed down, stated that
they had made a mistake and that, of course, neutral was not connected to
chassis.
We are now awaiting their next explanation. This will be, I am fairly sure,
something along the lines that they have components between neutral and
chassis that are not rated for anything other than low voltages and that we
have damaged their test equipment by connecting 120v between their chassis
and their neutral.
If this is their response, I will state that as far as I am concerned the
neutral is to be treated as being as "live" as the live line, that
components connected between neutral and earth must be rated to operate at
a voltage at least as great as the line to neutral voltage - and that such
components as "Y" capacitors (if used) are supposed to also have
significant voltage withstand capability.In short, I shall state that had
they designed the test equipment properly in respect of what they would
term a "normal" 240v supply, then there would have been no consequences of
connecting their test equipment to a +/_ 120v supply.
I would very much appreciate the views of this user group community in
regard to the above. Do U.S safety standards not dictate what I believe to
be necessary? Does anyone have any view on how this (possible) design could
be justified?
Regards
Dave Palmer
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