Game on!

[brman]

You know this, but I'm going to say it anyway in case anyone else wants to have a go: You are now completely unprotected from electric shocks. What would previously (thanks to the annoying 30mA RCD) have resulted in nothing worse than a bout of swearing could now lead to a funeral.

Care to explain that? Because, like voicecoil I use an isolation transformer when developing 240V switching devices (in my case 3 phase pmsm drives) specifically to prevent myself getting an electric shock. That and stopping me melting 'scope probe leads anyway.... ;)

[Kitwn]

Care to explain that? Because, like voicecoil I use an isolation transformer when developing 240V switching devices (in my case 3 phase pmsm drives) specifically to prevent myself getting an electric shock. That and stopping me melting 'scope probe leads anyway.... ;)

Yes it does need qualifying.

If there is no earth reference connection anywhere following the transformer and you touch a live connection then that side of the supply will float to your potential and no current will flow. Same with your scope probes, the scope floats to the same potential as that side of the supply (or vice versa) and there is no problem.

The problem arises when one side of the supply is connected to exposed metalwork either deliberately or by accident such as touching a metal bench or via another piece of equipment plugged into a dis board fed from the transformer. This creates the risk of you contacting both sides of the supply at once.

You are quite right that an isolating transformer can protect you in certain circumstances but most electronic workshops have strict guidelines about the use of isolating transformers including only ever plugging in one piece of equipment at a time, covering metal benches with rubber mats etc. My concern was if the whole CNC machine and other gear was going to be isolated by the transformer something somewhere could be providing an earth reference and that creates a danger.

The primary danger I was concerned with is the fact that a transformer negates the protection provided by an RCD. If you connect yourself between a standard mains supply and earth it's the leakage to earth (which creates a small difference in the live and neutral currents at the RCD) that is picked up by the RCD and that trips the supply. Put a transformer downstream of the RCD and connect yourself across it's output and there is no earth leakage back to the mains supply so the RCD won't protect you.

[Voicecoil]

Y

The problem arises when one side of the supply is connected to exposed metalwork either deliberately or by accident such as touching a metal bench or via another piece of equipment plugged into a dis board fed from the transformer. This creates the risk of you contacting both sides of the supply at once.

......That's why SMPS engineers tend to work with one hand behind their back I guess!

Seriously though, the danger isn't that great as to get a shock off an isolated supply you'd basically need to be touching something live with one hand and something neutral with the other, which is likely a smaller risk than touching something live and leakage to earth via feet etc.. And of course as part of the procedure I've always gone through when working in such a way, you do a check that there's mains earth continuity through the iso. transformer, so that in this case the machine chassis is properly earthed.

[brman]

......That's why SMPS engineers tend to work with one hand behind their back I guess!

Seriously though, the danger isn't that great as to get a shock off an isolated supply you'd basically need to be touching something live with one hand and something neutral with the other, which is likely a smaller risk than touching something live and leakage to earth via feet etc.. And of course as part of the procedure I've always gone through when working in such a way, you do a check that there's mains earth continuity through the iso. transformer, so that in this case the machine chassis is properly earthed.

I could have misunderstood but I think kitwin was making the point that, even with a good chassis earth, you will not notice if you have a short from line or neutral to the chassis. So, if you then touch chassis and live you will get a belt the RCD might not protect you from.

[Voicecoil]

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[Voicecoil]

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[Voicecoil]

OK, I see what you mean, I must have read it wrong. But I guess in an industrial setup you're not going to have a 30mA RCD, so similarly it's going to be a matter of working carefully.Anyway it's a lot less scary than 370V DC on the primary side of a power supply (or on the braking terminals of a VFD for that matter )
Interesting to hear you work on PMSM drives, what sort of power? I've only ever come across quite small PMSM's

I could have misunderstood but I think kitwin was making the point that, even with a good chassis earth, you will not notice if you have a short from line or neutral to the chassis.

It's not beyond the wit of man to make a circuit that detects that, if I have a spare moment I might just add one to my isolation transformer. Not that the current arrangement is anything more than temporary until the workshop electrics get sorted out (type F RCCB maybe?), but I can see it might have other uses.

[brman]

OK, I see what you mean, I must have read it wrong. But I guess in an industrial setup you're not going to have a 30mA RCD, so similarly it's going to be a matter of working carefully.Anyway it's a lot less scary than 370V DC on the primary side of a power supply (or on the braking terminals of a VFD for that matter )
Interesting to hear you work on PMSM drives, what sort of power? I've only ever come across quite small PMSM's



It's not beyond the wit of man to make a circuit that detects that, if I have a spare moment I might just add one to my isolation transformer. Not that the current arrangement is anything more than temporary until the workshop electrics get sorted out (type F RCCB maybe?), but I can see it might have other uses.

up to about 1kW motors. It is not our main business though so I don't claim to be any sort of expert.

edit to add: yes, I agree. I am much more worried about being zapped by me putting my finger somewhere it should not be than worried about the possibility of a secondary fault invalidating the protection. But I think Kitwin is right with his warning. It is all to easy to forget that not everyone understands the consequences of what might happen and so might think that an isolation transformer is a guaranteed safe solution.

[Muzzer]

You may be overthinking or misunderstanding this. Having spent most of my career designing and developing mains powered SMPS products, I would endorse the use of an isolation transformer. It's true that the RCD no longer protects you against accidental contact with an HV circuit - but when it's floating, by definition one such point of contact isn't in itself an issue. On the other hand, if it's non-isolated mains, you can get a lethal (deadly) belt just by touching a live node unless you are fully enclosed in a heavy duty rubber gimp suit. Unfortunately these are expensive, uncomfortable and can get you arrested.

Most professional labs use an mains isolation transformer and a variable source, either a variac (variable autotransformer) or a modern switchmode AC source. There is generally an e-stop to completely kill the power remotely in case you observe anything untoward. And you don't let anyone near the stuff without full and formal training.

There's no protecting against complete idiocy and at some point you need to learn the importance of NOT touching 2 different nodes, particularly with different hands. I once did that across 415Vac and somehow live to tell the tale. It hurt like f*ck and I never did it again but might not have been so lucky. For that reason, only using one hand to attach probes etc, combined with an isolation transformer is an <almost> foolproof way to avoid unnecessary bouts of death. Nowadays, we are trained in CPR and have access to defibrillators but those are the ambulance at the bottom of the cliff rather than the fence at the top.

[Voicecoil]

On the other hand, if it's non-isolated mains, you can get a lethal (deadly) belt just by touching a live node unless you are fully enclosed in a heavy duty rubber gimp suit. Unfortunately these are expensive, uncomfortable and can get you arrested.

PMSL