Re: Isolation transformer


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On Sat, 5 May 2007 14:47:06 +0100, "Andrew Holme" <andrew@xxxxxxxxxx>
wrote:


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On Sat, 5 May 2007 12:46:59 +0100, "Andrew Holme" <andrew@xxxxxxxxxx>
wrote:


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I'm trying to find out why these are used as safety precautions in
servicing mains connected equipment. Seems to me that not being
electrically connected to the mains is a moot point. Surely the pole
transformer isolates me from the generating dynamo at the power
station, but my outlet can surely still kill me. Induced currents can
surely be as dangerous as directly conducted currents?

Wikipedia states under this heading:

"In electronics testing, troubleshooting and servicing, an isolation
transformer is a 1:1 power transformer which is used as a safety
precaution. Since the neutral wire of an outlet is directly connected
to ground, grounded objects near the device under test (desk, lamp,
concrete floor, oscilloscope ground lead, etc.) may be at a hazardous
potential difference with respect to that device. By using an
isolation transformer, the bonding is eliminated, and the shock hazard
is entirely contained within the device."


Why would things connected to the same ground have dangerous potential
differences from that ground?
Could some kind soul 'splain this to me, please?

jack

If you put your fingers between live and neutral you get zapped. But
neutral is locally connected to ground, and your body has at least some
capacitance to ground, so you are connected to neutral via ground. If
you
touch live, it completes the circuit. Isoltation transformer neutral is
not
connected to ground.

Thanks Andrew.
Sorry, I still don't get it. How does the floating neutral of an
isolation transformer help in one's safety? You still get a shock if
you touch live and are in any way connected to earth.
Surely, however you place yourself between a high potential and a low
one is rather immaterial. If you allow electrons to be pushed through
your body with more than about 80V, you are flirting with death?
I understand the worst case (where an earth leakage won't protect you)
is to hold the live/active wire in one hand and the neutral in the
other. That is almost certain death in Australia (240V single phase).
Even worse to do it with two phases of a three phase supply (415V).
Does an isolating transformer perhaps only ground a live chassis with
a non-polarised, non-earthed supply? jack

ps could you explain that "capacitance to ground"? I would have
expected "electrical connection to ground". But then I'm on the steep
learning curve :)

The term "neutral" indicates which side of the mains supply is earthed.
There is no "live" and "neutral" on the secondary of an isolation
transformer: it is symmetrical (balanced) with respect to ground.

Aha. Now I see a glimmer of light. I understood that an IT was merely
a 1:1 transformer. That is, mains voltage in: mains voltage out. So
for the equipment to work, it still must have two wires entering that
are mains potential apart, I would have thought. These would now be
arranged to be + half mains potential and - half mains potential?

Yes, they are mains potential apart. It's not really meaningful to talk of
+/- half mains, because this is only relative to the voltage at the middle
of the secondary winding, which is floating relative to the rest of the
universe, unless you anchor it by grounding one end.

Very little current can flow if you touch one end of the secondary when
the
other end is open-circuit. If you connected the open-circuit end to
ground,
that would be a different story.

So you switch the equipment off after the IT? I understood the aim was
to work on the equipment while running.
So without the IT, the live wire can damage you, even when the neutral
is open circuit, whereas with an IT installed under the same
circumstances, the "live wire" out of the IT is almost impotent?

Yes; however, if you connect a ground to the isolated radio/TV (e.g. through
a 'scope probe) any live points in the circuit are just as dangerous as raw
mains; but the earthed chassis, which is the thing your hand is most likely
to touch, is safe.

In summary: you're safer with an isolating transformer; but not necessarily
safe.


.

Relevant Pages

  • Re: Isolation transformer
    ... electrically connected to the mains is a moot point. ... transformer is a 1:1 power transformer which is used as a safety ... capacitance to ground, so you are connected to neutral via ground. ... expected "electrical connection to ground". ...
    (sci.electronics.basics)
  • Re: Where to I put a Fuse ??
    ... >> barrier between whatever's plugged into the output of the transformer ... >I'm a little unclear on what the Galvanic barrier does. ... >I guess that not being directly connected to the mains is good. ... Are there more reasons to use the isolation transformer? ...
    (sci.electronics.basics)
  • Re: Isolation transformer
    ... electrically connected to the mains is a moot point. ... transformer is a 1:1 power transformer which is used as a safety ... GND is the earth ground to which the neutral conductor is eventually ... you to get to GND because there's no connection between the circuit ...
    (sci.electronics.basics)
  • Re: Isolation transformer
    ... electrically connected to the mains is a moot point. ... transformer is a 1:1 power transformer which is used as a safety ... GND is the earth ground to which the neutral conductor is eventually ... you to get to GND because there's no connection between the circuit ...
    (sci.electronics.basics)
  • Re: Aerial Amp - feeling a tingle
    ... induce 'half mains' voltage into an unearthed co-ax courtesy of the ... rules allow the manufacturer to avoid use of a safety earth connection. ... voltage effect on the case of a PC if the safety earth connection in the ...
    (uk.tech.digital-tv)