# What does isolation actually mean?

Discussion in 'Electronic Basics' started by Abstract Dissonance, Jan 11, 2006.

1. ### Abstract DissonanceGuest

Just about every time I see the word transformer I see the work "electrical
isolation" but I really have no idea what it means. Obviously there are is
no direct electrical connection but whats the big deal with this? Why is it
important to isolate the mains, say, with an isolation transformer when it
decreases efficiency?

I just don't see the point of isolating something just to isolate it.. there
has to be a very good reason but no one seems to mention it. Like one could
use AC to power a motor that powers another motor that generates AC and
hence there is an electrical isolation... but what good is it?

Thanks,
AD

2. ### Anthony FremontGuest

Primarily, it stops you from getting electrocuted by ensuring that there
is no DC path for current. Take TVs for example. Being the cheapest
possible design, they rarely if ever include a transformer. Typically,
the chassis is connected directly to one side of the AC (it's called a
"hot chassis" so you can google it). Since they usually only have a
two-prong plug, there's a good chance that the chassis could be
connected to the "hot" side of the AC line. Let's say, being the
curious sort that you are, that you decide to take your new scope and
examine some of the signals on the circuit board. You boldly grab the
ground connection to the scope and clip it to the metal chassis in
preparation for probing. After the sparks and blinding flash of light
subside and you hopefully are still breathing, you then fully understand
what a "hot chassis" is and then go out and promptly buy yourself an
isolation transformer to use on the next TV that you decide to poke
inside. ;-)

Another place that I can think of needing an isolation transformer is
when trying to hook a scope to a phone line. The ground at your house
is not quite the same potential as the ground connection at the CO. On
top of that the CO uses a negative power supply (meaning that their 48V
batteries have the positive terminals connected to ground) something
like the system that very old motor vehicles used.

3. ### David HarmonGuest

On Wed, 11 Jan 2006 16:47:13 GMT in sci.electronics.basics, "Anthony
Or rather, it eliminates ONE possible path by which you might be
electrocuted.
Take TVs for example. Even if you plug it in through an isolation
transformer while you work on it, there is still plenty of high
voltage present inside to kill you if you do the wrong thing.

4. ### Guest

Power from a standard electrical outlet is referenced to ground. This
means that if you are standing on damp ground the electrical current
from an outlet will happily flow through your finger, through your
body, and out your feet. WIth an isolation transformer you would have
to poke two fingers into the outlet and even then the current would
flow from one finger to the other. This is a much less dangerous case
since current that reaches your heart is most deadly.

5. ### phaetonGuest

Not to hijack the thread, but I have a question regarding isolation
transformers too....

Isolation transformers protect you from DC voltage, but AC voltage
would just set up an identical AC sinewave on the other set of windings
and zzzap! right?

6. ### Anthony FremontGuest

Which just happens to be the most dangerous one IMO, because it's not
always an obvious danger.
I figured that part went without saying. ;-)

7. ### Anthony FremontGuest

"phaeton" wrote
Well of course. It's just that touching one lead of the secondary
windings while standing on the damp ground with bare feet will not
electrocute you.

8. ### John GGuest

This is the point of ISOLATION.
The normal power supply in any country is connected to ground or at
least by its large spread is referenced to ground and so one side of the
line is at a potential that could well kill you.
With an isolation transformer the output side, although it has probably
line voltage between the ends, neither end is connected to ground giving
you and perhaps you oscilliscope a degree of safety.

Contrary to what another poster said it has nothing to do with DC as
there is not normally any significant DC on power distribution systems.
(Could happen in peculiar cicumstances I suppose.)

9. ### Anthony FremontGuest

"John G"
I meant that in the sense that there is no low resistance, direct path
to earth ground. I didn't intend to imply that the threat was DC
current that was flowing around. I thought it was pretty much standard
terminology, my mistake I guess.

10. ### John GGuest

Sorry I did not look back to see that it was you who mentioned DC.

In most counties there is a solid connection between the power system
and the ground(earth)

In the US (and other 60hz countries) it is the centre of the 240 volt
supply giving 120 either side for use by normal appliances and 240
across the 2 for bigger appliances.

In most 50 hz countries the centre of the Wye (neutral) is coneccted to
ground at every building and appliance use 240 volts from one phase to
Neutral or big things (Air con) use 3 phases.

11. ### Phil AllisonGuest

"Abstract Dissonance"

** It is almost entirely a human safety matter.

Isolation transformers make mains powered electronic stuff safe for us
humans to use.

** It is a basic safety issue that engineers deal with when designing many
appliances.

......... Phil

12. ### Alan BGuest

Well, the avoidance of untimely death can be handy sometimes.

13. ### Rich GriseGuest

Right, if you get across the terminals.

But, if you're just standing on the ground, there's no path for a complete
circuit. Without an isolation transformer, you yourself complete the
circuit, just because your feet are grounded.

The 120V is still there, but relative only to its own return, not
to the ground. Both sides are "floating" with respect to ground, so
at worst, theoretically, you'd feel a little tickle just from capacitive
leakage.

Unless you have your other hand on the chassis, in which case you're
still dead.

Good Luck!
Rich

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