I have been reading on the benefits of using an isolation transformer for bench work but I am not sure why I would need one if my house has safety switch on it already. Would the safety switch still trigger if I was using an isolation transformer or would it cancel it's operation.

I have read that use of an isolation transformer won't prevent electrocution because you can still get electrocuted by the secondary but for that to hapen, would you have to for example, touch a live chassis with both hands? Presumably if you only have one hand in the chassis and touched something with B+ on it you wouldn't get zapped provided you are not grounded? I am trying to understand the principles behind this so any advice is appreciated. Regards, Angelo

There are traps with Isolation transformers & variacs

The isolation transformer is useful for AC/DC sets and those without transformers. These are trecherous. "Live chasses". It helps In Neutral Earth Return (SWER) by stopping you being part of the circuit. if you are careless.

I have not tested an RCD on an AC / DC set yet and I have a collector that would like one fixed. It will not be connected to the mains directly. And I doubt that an RCD will function with a fault after the rectifier.

I see no reason why the house RCD would trip wth the fault on the secondary side of the transformer. This is easily checked as I have a plug in RCD and an isolation transformer as well as an RCD on the bench supply.

Not at all a risky exercise.

A variac can be a trap a 240V RCD requires 30mA to trip. if you test the RCD with its built in test and it's on the output side the test resistor will be below 8K and not likely to "test" trip at a around 130V

It is at all times with mains operated equipment Cavet Actor. Just be careful.

There are some regulatory geniuses that think that you can repair equipment without it being alive. Sometimes I wonder if their brain is. Quite a bit like mains operated radio's & TV's just has to be alive.

There are some secondarys that can kill & DC grabs. It is the current that kills

Most domestic receivers are under 200mA . The line amp I have is around 320V DC on the Plates and the transformer, I suspect is good for 500mA. That will kill.

Marc

Thanks Marc. So unless I am working on transformerless radios, there is no safety benefit in plugging a radio whilst I am working on it into an isolation transformer?

I understand there is also an issue with test equipment being plugged into an isolation transformer.

Both RCDs and isolation transformers will always work to a degree. The problem is that there is more than one way to be electrocuted.

If your finger comes into contact a frayed power cord with exposed wiring or the primary side of the receiver's power transformer then either an RCD or isolation transformer will provide some protection.

However if your fingers bridge between a component lead and the chassis then neither device will save you.

It is good practice to include as many safety deviced as can reasonably be expected to provide protection though ulimately there is no substitute to simply being careful. If one treats all parts of any radio chassis as if they were live then there should be no problems.

Most electric shocks are due to two things, inexperienced people digging too deep and those who want to finish a refurbishment in five minutes and thus take shortcuts.

There are some regulatory geniuses that think that you can repair equipment without it being alive. Sometimes I wonder if their brain is.

I think the OH&S ban on live working relates mainly to sparkys who in my experience would often replace GPOs on live circuits in offices, etc, rather than find the breaker and isolate the circuit, not only as a time saver to them but also to avoid shutting down operational equipment on the circuit.

As for electronic equipment on the bench, yes, it's a bit hard to measure voltages and currents with the power off, LOL!

Most electric shocks are due to two things, inexperienced people digging too deep and those who want to finish a refurbishment in five minutes and thus take shortcuts.

... to that I'd add: and lapses of concentration -- often due to fatigue/tiredness.

My motto is: If you're yawning, switch off and leave it alone.

Having been severely zapped by an AC/DC radio as a schoolkid, I have a great respect for mains voltage and double check whether the power is connected before I touch anything. I have neon lights on my bench GPOs which I am in the habit of watching and I pull the plug out as well as well switch off between tests., which some people may think is being too careful but to me there's no such thing!

Another point on safety taught to me by my grandfather who was a motor mechanic: If somebody starts talking to you while you're working on a task, down tools until the conversation is over. That rule has stood me in good stead over the years.

Do any of you use an isolation transformer for working on sets that have a transformer or is it really only of benefit for US AC/DC sets?

I don't use an isolating transformer for any repair work though I have recently upgraded my switchboard and have allowed for RCDs on all light general power circuits.

When I am at the workbench I always wear workboots to provide isolation from the concrete floor.

With the normal domestic transformer sets I see little benefit in using an isolation transformer. But make sure the RCD works. I have had one go faulty.

There was an accident on one of the company sites, of a group I worked for,when one failed to trip.

Sparky's can help kill themselves, I saw one alcoholic have two accidents, one trying to measure the ohms of a 415V feeder (his boss also tried that... same result... smoke & blue flash) and the same one getting tossed several feet cutting off another 415 as he was too lazy to walk about 80metres to isolate it. To my knowledge he lost his drivers licence, for being pickled, but is not fried yet

I saw another recently where a house light had been wired to a power circuit .?

Also be careful with solid state regulated PSU's. Directly hooking them to ground can cause them to go beserk.

That also applies to them, when hooking mains grounded CRO leads etc. and antenna earths on aerial coils that are grounded to chassis. Isolate the earth with a 0.01mfd HV cap.

Marc

[was] pickled, but is not fried yet

Nicely put.

An alcoholic electrician. The mind boggles. A pretty sure candidate for a Darwin Award.

I saw another recently where a house light had been wired to a power circuit .?

Mixed circuits are permitted by the SAA Wiring Rules however it must be labelled as such and the whole circuit has to be wired in 2.5mm² (7/0.67 in the old money) twin and earth.

I'd agree that a mixed circuit in a dwelling is VERY unusual though. Mixed circuits are usually reserved for lockups and the backyard dunny.

The issue I had was the CB that light was found on was labelled "Pump".

With the Darwin awards it is preferable that the candidate has removed themselves from the gene pool in the most spectacular fashion. Eg putting rocket boosters on a car (been done... nothing bigger than an 11/2" found)

Somewhere I had an article on why these people should be left to their own devices,as that purifies the gene pool, by them being removed from it, hopefully before they have had a chance to breed another Moron.

http://www.youtube.com/watch?v=VJghe0ofkj0&feature=youtu.be

A post to 2011 winner

Marc

With the Darwin awards it is preferable that the candidate has removed themselves from the gene pool in the most spectacular fashion.

Yes, perhaps I should have said potential entrant into the awards. With alco-sparky's penchant for stupidity with 415v, it's only a matter of time ...

First time I have ever seen a ramraid on a lift. Just when one thinks they've seen it all...

There will be no repeat performance by him. He literarally shaf(killed)ed himself.

Marc