How essential is an isolation transformer?

[pwdrive]

"In fairness, the sets on soak probably wouldn't have had earthed test gear connected to them"


Maybe not at the BBC  

The death lead...no prizes for guessing (no googling)

Those that were/are in the radio/tv servicing trade will probably know.

An isolating transformer if used correctly isolates the operator from the mains as well.

Lawrence.

[SurreyNick]

Mm, this was a good question to pose. I may reassess my first instinct to employ one. What the replies on this thread demonstrate, plus those on the link David (Yorkie) gave, is that I should reconsider and properly plan my workbench arrangement. This I shall now do

Any additional information and advice will be very welcome.

Thank you

Nick

[ppppenguin]

"I'd love to know what the "death lead" was!"

Mains plug to croc clips:-)

The other version is mains plug to mains plug. Occasionally useful for bridging power from a live circuit to a dead one.

Both versions are dangerous. I have used both. I don't have either version at present.

[Mark Hennessy]

"In fairness, the sets on soak probably wouldn't have had earthed test gear connected to them"


Maybe not at the BBC  

The death lead...no prizes for guessing (no googling)

Those that were/are in the radio/tv servicing trade will probably know.

An isolating transformer if used correctly isolates the operator from the mains as well.

Lawrence.

Well, if the sets did have earthed test gear connected, and everything was working with no fireworks, then no problem! An isolation transformer is only needed for a small number of scenarios - such as when connecting to a live chassis or the "hot" side of a switched-mode PSU. In most cases the isolation afforded by the DUT itself is perfectly fine - and an RCD offers protection for when it isn't. Did you at least use RCDs back then?

Yes, I have worked for the Beeb for a long time, but I have worked elsewhere too, and have probably done more hours at service benches outside of the BBC. I've seen a lot of very bad practice first-hand. Against that, within the BBC, engineers are much better educated about the risks, and are all the safer for it. The funny thing is that it doesn't cost the business any more to be safe - you just need to get engineers to think about things properly. I'll never understand why this subject causes so much apparent confusion, given that we're talking about incredibly simple stuff here - I mean, how can someone successfully fault-find line output stages or PAL decoders yet not understand the basics of voltage sources?

Perhaps an extension lead with the earth lifted? Usually for the 'scope - seen that done countless times! For the benefit of anyone reading (and the Forum Owner!), this is a Bad Idea. Keep all test gear firmly earthed!

Perhaps not quite as dangerous as the lead a friend uses to connect his generator to his house (a BS1363 mains plug on each end!) (also, a Bad Idea!)

So how does an isolation transformer isolate the operator, given that said operator must be assumed to be earthed because he or she will be reaching out to touch test gear, etc, or perhaps standing on the floor? Or am I confusing semantics?

[pwdrive]

Correct!

There were two types of croc clip death leads in our workshop.

The Lesser Death Lead (plastic shrouded clips" and the Greater Death Lead, no shrouding at all...

Lawrence.

[Mark Hennessy]

"I'd love to know what the "death lead" was!"

Mains plug to croc clips:-)

Oh yes - I didn't think of that. Not warped enough

The oh-so-ironically named "Safebloc" mounted on my variac will allow you to use 4mm plugs - the lid closes just sufficiently if you're careful - but I can't possibly explain how I know that

[Mark Hennessy]

Lawrence, between that and the chainsaws, how come you are still alive

[pwdrive]

The most common use for a death lead in our workshop was to use it for the old Philips tv's when the outside engineer had forgotten to bring it in with the customers tv, the spare Philips leads in the workshop always seemed to "vanish"

Lawrence.

[Mark Hennessy]

Mm, this was a good question to pose. I may reassess my first instinct to employ one. What the replies on this thread demonstrate, plus those on the link David (Yorkie) gave, is that I should reconsider and properly plan my workbench arrangement. This I shall now do

Any additional information and advice will be very welcome.

Thank you

Nick

Nick, to get this slightly back on topic (not that being off-topic is a problem, of course), please do ask away. Perhaps start by describing what you currently have in mind?

For what it's worth, here's what I have always done in a workshop (as you might have already seen in that other thread):

• Sockets that are powered 24/7, for computers, clocks and similar items. These have a double-pole isolator switch, but this is left on.
  
• Sockets that are powered when the workshop is in use. Also switched by a double-pole isolator, and fed via an RCD.
  
• Sockets that are isolated. If you only have one isolation transformer, you'll only have one of these. I have a pair of transformers that I built into a case. I only switch these on when needed (not very often) as they run warm and are a little bit noisy.
  

All sockets are clearly differentiated - coloured sockets are useful here.

Where is your workshop? Is it in the house, or is it in an outbuilding? Do you have an RCD in the consumer unit?

In my case, the workshop is in the cellar, and the sockets in the cellar are on the main house ringmain, so already have RCD protection. If a fault was to occur on the bench, there's no way of telling whether the bench RCD would trip before the house one, but the odds of the former are higher than if there was no bench RCD. If the bench RCD trips first, that saves the minor hassle of resetting clocks and computers around the house. I've yet to cause the RCD to trip, so can't being to guess...

Having double-pole isolators is a good plan because you can quickly hit them if your DUT starts emitting its magic smoke. Or, someone else can knock the power off should you be receiving an electric shock that the RCD doesn't know about (Live to Neutral). Put them as near to the door and the bench as you sensibly can.

Good lighting is essential, of course. Depending on how your house is wired, it will hopefully stay on should the RCD supplying sockets trip in the consumer unit. If there is a chance of being plunged into complete darkness in that event, consider emergency lighting (I really must get around to installing mine). I recently picked up an LED fitting from CPC for £12 - bargain!

Perhaps the best advice is this: always *think* - never fall into auto-pilot. Never assume the safety you think you have provided will work - it is only there for when all else fails, and it won't always help.

[pwdrive]

"So how does an isolation transformer isolate the operator, given that said operator must be assumed to be earthed because he or she will be reaching out to touch test gear, etc, or perhaps standing on the floor? Or am I confusing semantics?"

It depends how far you want to take it, it won't isolate you from earth but it will isolate you from live if the isolation hasn't been compromised, live and neutral feeding a load is a simple circuit, if your not part of it then no shock.

Lawrence.