Alan said:
Having had experience of ground loops and general interference
I wanted to design a system that was as isolated as possible, but
still safe. To start with the mains power feed I researched around
and I believe it boils down to this...
Is this How you use an Isolation Transformer and RCD to protect equipment
and break ground loops?
Primary | Secondary
--------------
L -------- | --------| |------------- L2
( | ( | |
)| ) | |
( | ( | |
)| ) | RCD 30mA |
( | ( | |
N -------- | ----+---| |------------- N2
| | | |
| ----| |------------- E2
E ---------- --------------
^
|
Isolation transformer
Also can charge build up be solved with a 1M resister between the
two earths?
Cheers
Alan
Sorry Alan, but you use EITHER an RCD or an isolating transformer. As it
stands, your isolating transformer adds weight, cost and NO PROTECTION
WHATSOEVER to the user. Here is why:
An isolation transformer is used to minimise electric shock hazards in
MEN (Multiple Earthed Neutral) power systems. With the MEN system,
distribution transformer secondary star-points (ie neutral connections)
are connected to "the body of the earth" at multiple points. There are
thus 2 ways of getting an electric shock:
1) hook yourself up across Phase & Neutral
2) hook youself between phase and the planet (eg wet concrete floor, or
muddy paddock etc).
Obviously the latter is the most common technique for electrocution, as
in general we are always in good contact with the earth. MEN is used to
ensure a low impedance to fault currents (which are correspondingly
high) thereby ensuring protective devices quickly trip. This is a good
thing; the downside is the increased risk of electrocution.
The use of an isolation transformer removes the connection between the
neutral conductor and earth. This removes #2 as a means for getting
zapped - now you must hook up to both phase and neutral, which takes
considerably more effort - but is achievable, if you try hard enough
Alas in your schematic the second "earth" terminal is of course not
connected to earth, but is actually the neutral conductor. In the case
of metallic equipment, you have now ensured that the case is securely
connected to neutral - ie you just achieved half of the necessary
connections for electrocution to occur. Its worth noting that an
isolation transformer has no "phase" and "neutral" outputs - they are
interchangeable (hey, its AC...). Iso trannys generally have a fuse in
each output lead.
An iso tranny with 1 outlet has NO CONNECTION to the earth pin at all.
An iso tranny with 2 or more outlets will have all of the earth pins
connected together, but going nowhere. The reason for this is as
follows: imagine two appliances, one having a phase-to-case fault, the
other with a neutral-to-case fault. If you plug the first one into a
dual-output iso tranny (with earth unconnected), the case now sits at
the phase conductor potential. The operator will not get a shock, and
the equipment will continue to operate. However if they plug the second
faulty device into the same iso tranny, one case is at phase potential,
the other at neutral potential. one hand on either appliance WILL KILL
YOU. OTOH if the two outlet earths are connected together, then as soon
as you plug in the 2nd appliance, one or both output fuses will rupture
(the first appliance means the phase conductor is at earth potential,
the 2nd shorts phase to neutral thru the earth terminal link).
an RCD is a controlled-electrocution device, and will trip an isolating
device very rapidly (< 300ms) when earth current (>= 30mA) is detected.
Of course this earth current must flow (for a little while). If faulty
equipment causes the earth leakage current, then the RCD will trip when
you plug in said equipment. If it is the operator getting a belt that
causes the earth leakage current, then the operator must get said belt
before the RCD will trip.
cheers
Terry
