What does isolation actually mean?

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.

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?

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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.

On Wed, 11 Jan 2006 08:22:51 -0600, via , "Abstract Dissonance" spake thusly:

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

On Wed, 11 Jan 2006 16:47:13 GMT in sci.electronics.basics, "Anthony Fremont" wrote,

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.

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Which just happens to be the most dangerous one IMO, because it's not always an obvious danger.

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I figured that part went without saying. ;-)

"phaeton" wrote

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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.

"John G"

systems.

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.

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.)

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.

--
John G

Wot\'s Your Real Problem?


>

"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

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