> The only safe way is to use a transformer that is correctly wired. US mains
is
> 120 - neutral - 120. which gives 240 between the 2 hots. European mains are
> 240 - neutral. Both expect the neutral to be at / near ground potential. If
> you wire it to US 240 directly what was neutral in Europe will be at 120v. I
> have several isolation / step up transformers that I routinely use to
accomplish
> this. You need to be careful how they are wired in order to have both primary
> and secondary tied correctly to neutral and the grounds MUST be kept separate
> from the neutral to meet US electrical code.
In Europe, many outlets (German ones spring to mind) are mechanically
symmetrical with respect to swapping hot ("Live") and Neutral pins, and AFAIK
all equipment has to be able to operate whichever side of the input is earthy
(=close to ground potential). I would have no qualms at all about running
European equipment of 120 - 0 - 120, although for travel I have made my own
autotransformer from a 100VA split-primary mains transformer.
Some old (before about 1960) equipment bonds neutral to equipment chassis. This
is of course dangerous to run except with the correct side earthy. I have heard
rumours of German equipment with a neon between chassis and earth that lights up
if you get this wrong!
When wiring a step-up transformer:
Single winding (autotransformer) should have neutral one end, 120V in the middle
and you get 240V out of the other end. Ground (earth) goes straight through
without connecting to the windings.
Dual winding (isolating transformer) Input across Neutral and 120V - output
gives you 240V. Bond one side of the output to Ground, NOT neutral, and this
becomes the new neutral [*]. This ensures that a fault to the chassis is
guaranteed to trip earth leakage protection downstream of the isolation
transformer (which I doubt you have) but not upstream of it. This is much safer
if (for example) hot and neutral are reversed on the input. (Note: if you wire
one side of the output to neutral, you have something no worse than the
autotransformer.)
I don't know whether this latter meets the US electrical code, but it is correct
:-)
As regards strange sockets, I don't imagine 3-phase would be at all likely in a
US domestic installation, but 220V happens occasionally. The usual plug is just
like the standard US 3-pin plug, but with the two flat pins at 90 degrees to
their familiar position. This can roughly be represented in ascii-art thus:
o o
120V _ _ 220V
| |
DEC used it on a lot of their European equipment...
Philip
[*] For full isolation, don't link ground on the two sides of the transformer.
Link chassis etc. on the output to the virtual ground terminal that's bonded to
the supply rail you've chosen as neutral. This means that you have to touch
both a hot wire _and_ the chassis befor it will zap you. For some testing,
don't bond at all - you can't detect a fault to chassis, but you need a fault
_and_ to touch the wrong wire before you get a shock...
Some more ascii-art, for the record.
Autotransformer:
________ 240
C
C
120 ____C
C
C
N ______C_______ N
Isolating Transformer wired as Autotransformer:
_____ 240
||C
||C
120 _____||C
C||C
C||C
N ______C||C____ N
|____|
Isolating transformer:
_____ 240
||C
||C
120 _____||C
C||C
C||C
N ______C||C____ N - Bond this to earth and/or chassis as necessary
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Received on Thu Oct 07 1999 - 11:56:41 BST