Isolation transformer

Thanks for a great explanation, John. I will copy and paste this to Word in the morning and change to courier and study it. I used to be able to do this in Agent, but I can't quite see where at the moment, nevertheless your piece here is a keeper for my files. Thanks once again, jack

--
PS

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

                                        HV!
                                        /
HOT-----< ]--+--------+

** Go get f***ed - you asinine cunt head !!

My god - how can ANYONE be that f****ng STUPID !!

Better this brain dead CUNT were electrocuted right NOW.

What a pile of vile sub human SHIT !!!!!!!!!!!!!!!!!!

Must still vote for the long deceased Bob Menzies ......

Maggot are feasting on his putrid brain.

Eeeeeeyuuuccccckkkk

....... Phil

** Go get f***ed - you asinine cunt head !!

My god - how can ANYONE be that f****ng STUPID !!

Better this brain dead CUNT were electrocuted right NOW.

What a pile of vile sub human SHIT !!!!!!!!!!!!!!!!!!

Must still vote for the long deceased Bob Menzies ......

Maggot are feasting on his putrid brain.

Eeeeeeyuuuccccckkkk

....... Phil

** Go get f***ed - you asinine cunt head !!

My god - how can ANYONE be that f****ng STUPID !!

Better this brain dead CUNT were electrocuted right NOW.

What a pile of vile sub human SHIT !!!!!!!!!!!!!!!!!!

Must still vote for the long deceased Bob Menzies ......

Maggot are feasting on his putrid brain.

Eeeeeeyuuuccccckkkk

....... Phil

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.

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.

It still helps if you touch the mains power wires or wires and parts that aren't isolated from ground - while standing on damp concrete with leather shoes or leaning on a grounded scope - both sides of the line to the equipment being repaired are isolated from ground so you could theoretically grab hold of the hot mains wire and still keep breathing.

That's essentially what wikipedia and your other responders are telling you.

Draw the circuit using a stick figure, ground his leg, have him touch the hot incoming un isolated mains voltage. See the path from hand to ground? Now put an isolation transformer in the circuit with the same stick figure - no path to ground, and as long as he touches only one conductor on the secondary side of the mains, he's breathing.

The reason they suggest keeping one hand in your pocket while working around live circuits. You ain't lived till you've gotten across the B+ of a tube transmitter - it will change your perspective on a lot of things if you survive the first mistake.

One of the ironies is very high voltages MAY leave you alive. I watched a navy tech open the door on a transmitter and he shut it down, we all heard the safety relays slam down when he hit the disconnect (just opening the door would have also done it) - supposedly shorting the power supply, he then put the grounding stick on the HV cap according to SOP - and drew an arc from his ass to the scope cart where he was kneeling. He joked about it for about 5 minutes, then the reality hit him and he turned white and went into shock, and pissed all over himself.

Polarized plugs grounded chassis and all are great things for safety - you can add more safety if you want (and given that you are having trouble with a relatively simple concept, perhaps you should - unless you worship Darwin or something).

The +12 volt terminal on battery in a boat can be mighty uncomfortable with salt water all over your hands, kneeling on a wet deck - and a small cut you didn't know was there, will really bring the idea home fast.

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Yeah, thanks, Andrew. It's starting to sink in, and I will be a full bottle on the subject in a few days when I've absorbed all the great advice I've received here. Pity about the abusive clown, but I suppose every group has to have one. I've copied a bunch of his messages including headers to his ISP and his news account. Perhaps they can show him the error of his ways. Ever in hope, jack

Me, I'm not game to go anywhere near a plugged in TV set with the back off. I won't even touch anything until the set's been off for a day to allow all the caps to depolarise. I guess the problems I'm having with this concept is that in Australia, there are no hot chassis that I'm aware of. My old man had an old valve radio from at least the 40s, and it had the chassis exposed at the back. I suspect that it had a transformer as it ran on

240V. We have never had non-polarised outlets, or even unearthed ones (well legally) and I recall the problem with an electric heater that was run with a single pole switch and an illegally wired extension lead (only licenced folk can do this in Australia) such that when switched off, the neutral was disconnected, but the thing was still live. From vague memory, this was remedied with two pole switches on all such equipment. I might be wrong here on some things like light fittings, but I would only ever (illegally) install a two pole switch where needed, and check all wiring that I do (again illegally) with an outlet tester that tells me if all wires are in the correct place and are all functional. I'd never make a living as a sparkie, but I sleep at night :) jack

The key with an IT is that the earth is NOT the "other side of the circuit". Without an IT, you need only touch ONE wire in the box to potentially become part of the circuit. With an IT, you need to touch TWO wires in the box.

Remember, current must flow in a loop - you aren't part of the circuit until the current can do that. That's why birds can land on the high voltage transmission wires without getting electrocuted, but squirrels trying the same thing get fried as soon as they try to go from one wire to the next (one wire OK, two wires ZAP).

The IT guarantees that every amp of current leaving the IT on one wire

*must* return through the other wire. It doesn't have the option of returning through the earth, like house mains do.

Similarly, people using arc welders don't usually get electrocuted, even though theres 100+ amps going through the metal they're welding. The transformer in the welder forces the current to return through the ground strap, not the person.

As an aside, it's pointless to try to think of the "potential" of the isolated side of the IT relative to earth ground. Current can't flow between the isolated circuit and earth ground, therefor there can't be a potential between them. Easy to test with a standard 12v power transformer - measure output A to earth ground, nothing. Measure output B to ground, nothing. Measure A to B, you get 12v. What happens if you measure A-Gnd and B-Gnd at the same time? *Now* you have a circuit, and each branch (each meter) sees half the voltage (6v). But - not because anything is going through the earth, you're just using the local ground as a wire to connect your two meters together. You could just as easily have connected the two meters in series and gotten the same results, for the same reason.

yes.

yup zap!

Never ever put both hands inside a powered TV set (or other mains-powered device)

The isolation transformer only protects you from the zap you get from touching the live wire while touching the ground.

:>

Bye. Jasen

the picture tube will hold a charge for longer than that.

Many TVs in australia are hot chassis, something Phil will no doubt point out if he's following this thread.

things have changed a bit since then :)

Often a TV power supply section has a bridge rectifier across the mains with the negative connected to the chassis. plug polarisation won't help you there.

more recently they are putting isolation into the powersupply section again so that they can have AV inputs grounded to the chassis, but if you have a TV from the 80s or 90s it's quite likely to have a live chassis.

Bye. Jasen

You are connected to ground. If you touch a hot wire, you may die or say many bad words.

If the equipment is isolated from ground, it is much harder to get a shock since you are no longer connected to the circuit.

Thanks to everyone who helped me with this very basic conceptual problem that I had (Except Phil Allison, who should get a life, or refrain from posting to usenet when he's pissed -- I've sent a sample of his foul abuse to his ISP and newsfeed provider to see what they have to say about having him as a customer)

My original misconception appears to have been that I could not see an intuitive difference between an inductive link with the mains power and a copper link with it. I was using the Van de Graaf generator analogy, and not the "240VDC lead acid battery" analogy of the IT secondary circuit.

When trying to explain my dilemma to my wife (very intelligent girl, not much interested in technology), I thought of this large battery and what would happen if I touched the positive terminal to my water pipes. Nothing, of course, was the instant reply I gave myself :) So I then thought of the secondary terminals of the IT as like the terminals of a battery, and not the balls on the top of a Van de Graaf generator. I think I'm right now until my next problem of understanding crops up :) jack

** YOU are nothing other than a steaming great TURD - " jack " .

I supplied a correct explanation and good advice on how you could work it out with a diagram - in my first reply.

To which you replied by pouring brain dead scorn on over it.

Doing THAT made you " 100% fair game " for any and all abuse.

What an utter arsehole you are - scum of the planet.

....... Phil

Thanks Ed I think I get it now. My concept of the secondary circuit from the IT was rather wrong. jack

Sort of like this?

" ** Go get f***ed - you asinine cunt head !!

My god - how can ANYONE be that f****ng STUPID !!

Better this brain dead CUNT were electrocuted right NOW.

What a pile of vile sub human SHIT !!!!!!!!!!!!!!!!!!

Must still vote for the long deceased Bob Menzies ......

Maggot are feasting on his putrid brain.

Eeeeeeyuuuccccckkkk

....... Phil "

We'll see what your ISP and news provider have to say about it, huh? jack

ps

" ** So you nave no idea what an electrical circuit is ??

** Not until YOU go figure what an electrical circuit is and how come a broken one is not one. Draw yourself some diagrams. Then figure out the CIRCUIT by which how folk get electric shocks from the AC power."

Is not at all helpful!!! It even got chipped by another regular.

You started out talking about servicing mains connected equipment. If it is powered through an IT, it is isolated from ground. That makes it impossible for you to get a shock by being in the mains _ground path_ whatever you touch in the equipment. You can still get shocked via being in other paths, but one very common path for shocks is eliminated.

As to chassis not being hot - you are _servicing_ the equipment. That means there might be a defect in it that could make the chassis hot where it would not be if there were no defect. In addition, I'd be willing to bet there's plenty of older equipment in Australia that does have a hot chassis, even if there is no defect.

Ed

Only Philips made sets with no tranny IIRC. They used quite complex methods to ensure safety since the AC supply is 240 V. These included having the knobs attached via a tapped through grub screw with insulation forced in on top of the screw.