Re: Isolation transformer

On Sat, 05 May 2007 09:35:41 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Sat, 05 May 2007 21:59:33 +0900, spamfree@xxxxxxxxxxx wrote:

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


<spamfree@xxxxxxxxxxx> wrote in message
news:v12o33tslue4l3ejsh35knts2b4gbbjnra@xxxxxxxxxx
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

---
View in Courier:

Here's a circuit of the "safe" way to use an un-isolated mains
supply:
HV!
/
HOT-----< <-----[DIODE>]--+--------+
|+ |
[BFC] [LOAD]
| |
NEUT--+-< <---------------+--------+ <-----[YOU]---+
| | |
GND>--+ CH GND [WATER PUDDLE]
|
[GND]



GND is the earth ground to which the neutral conductor is eventually
connected, and CH GND is the chassis to which the low side of the
load and the supply are connected. With the circuit connected to
the mains in this way you could be standing in a puddle of water
barefooted, and if you touched chassis ground you wouldn't get much
of a shock. Theoretically, that is, if all the mains wiring was
perfect. _Don't try it at home!!!_

Notice that the circuit will produce positive high voltage
regardless of which way it's connected to the mains. That is,
whether the diode is connected to HOT or NEUT, and CH GND is
connected to the other terminal, the circuit will work.

Now, however, if the circuit is plugged in backwards it'll look like
this:

HV!
/
NEUT--+-< <-----[DIODE>]--+--------+
| |+ |
GND---+ [BFC] [LOAD]
| |
HOT-----< <---------------+--------+ <-----[YOU]---+
| |
CH GND [WATER PUDDLE]
|
GND

This time, if you're standing in the puddle of water, barefooted,
and you touch CH GND it might very well be goodbye.

That's why polarized plugs and receptacles are used when there's a
risk of shock if the user can come in contact with metallic portions
of the device that are carrying current. Otherwise, the device is
designed to be so completely insulated that there's no way a user
can accidentally be shocked. The polarized plug idea is OK, but
there's no guarantee that an ungrounded mains receptacle _can't_ be
miswired.

The safest way around all these problems is to use a transformer to
isolate the circuitry from the mains, like this:


HV!
/
HOT-----< <--+ +---[DIODE>]--+--------+
P||S |+ |
R||E [BFC] [LOAD]
I||C | |
NEUT--+-< <--+ +-------------+--------+ <---[YOU]---+
| | |
GND>--+ CH GND [WATER PUDDLE]
|
[GND]

In that manner, since the primary and secondary windings are
electrically insulated from each other, current can't flow through
you to get to GND because there's no connection between the circuit
on the secondary side of the transformer and the mains.

Similarly, if the mains connections to the primary were reversed:


HV!
/
NEUT--+-< <--+ +---[DIODE>]--+--------+
| P||S |+ |
GND---+ R||E [BFC] [LOAD]
I||C | |
HOT---+-< <--+ +-------------+--------+ <---[YOU]---+
| |
CH GND [WATER PUDDLE]
|
[GND]

there would still be no galvanic connection between the mains and
you, so you'd be safe. Theoretically, if everything was perfect.
But, in any case, don't try this at home.

---
PS

You could also do this and still be safe: (Same caveats as
before...)

HV!
/
HOT-----< <--+ +---[DIODE>]--+--------+ <---[YOU]---+
P||S |+ | |
R||E [BFC] [LOAD] |
I||C | | |
NEUT--+-< <--+ +-------------+--------+ |
| | |
GND---+ CH GND [WATER PUDDLE]
|
GND


--
JF
.

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