RCD or Isolation Transformer?? What is best to use for safety

[Shock]

I have an isolation transformer I rarely use and definitely do not have it installed 24/7. Perhaps it is time I put it to use...  Is the ground pin lifted on that outlet (if so - why???) or do you keep it connected (unless you need to lift ground)? Also - do you short one of the secondary connections to earth (like safety transformers do)?

They aren't permanently wired at least not at the moment, though I've seen plenty of people do that. I have two isolation transformers one has no earth present on the secondary side, I can optionally add a flying earth lead direct to the DUT chassis. The other has an earth carried through to output of the isolation transformer. No I don't tie the earth on the secondary side to neutral or anything like that.

Dimbulb tester - read about it mainly for ramping up old tube devices. What is the wattage of the bulb (or is it a screw in e27 bulb with different wattage depending on the load)? I guess it is better for switching supplies and modern electronics (for which Variacs are not always appropriate for).

I use B22 bayonet style but same thing, I have a selection of different bulbs and two sockets so I can vary the wattage a little. I use it mainly on linear supplies, but it would be useful on switching supplies as long as it did not produce startup, low voltage, or confusing the voltage detection type problems.

The dimbulb tester works differently from the variac not really comparable aside that it can limit current to the DUT. If you leave something on your variac at full voltage and an intermittent short occurs it will draw as much current as it can and your variac will happily oblige until something gets too hot (variac, fuse, DUT) and fails. A dimbulb tester is a little more useful in those situations because you can leave something on test and reduce the chances of things burning up.

For tube amp initial evaluation power up you would use a variac monitoring the current as you slowly ramp up the voltage. There is a few different approaches from there depending if you intend on being gentle on the capacitors or not.

[Shock]

Safety is not a game....

It's often impossible to know the exact situation...

I think I mentioned this before somewhere but there is also a situation if the isolation transformer has a connected proper earth to the secondary and there is a neutral earth miswiring/fault present in the DUT (or power cable). It can cause a touch hazard on the DUTs metal chassis. Bet that one is a lot of fun to discover.

[IanMacdonald]

I have two isolation transformers one has no earth present on the secondary side, I can optionally add a flying earth lead direct to the DUT chassis. The other has an earth carried through to the secondary.

This is dangerous. and also pointless as it achieves nothing. If a transformer with an earth on any part of the secondary gets connected to a bridge rectifier whilst an earthed scope probe is connected to the -ve rectifier output,  this can result in a voltage doubler rectifier being created. Or, 150% supply  depending on where the earth is connected. The result, at the very least, will be a spectacular bang.

What you have effectively created, is an autotransformer.

[Brumby]

Using an isolation transformer adds an element of "abstraction", if you will, where safe and proper use requires an understanding and an advanced awareness that you can not expect a beginner to have.

The same could be said about an RCD.

I think it more likely a beginner will be able to understand (if they don't already) the role, function and issues of an RCD which is a device commonly found in many homes than the use of an isolation transformer - which is not a common sight ... even in EE circles!

Perhaps not in Australia, but isolation transformers are very common in the UK. Almost every bathroom has a small one powering the shaver socket.
https://www.victorianplumbing.co.uk/white-dual-voltage-shaver-socket-shas?campaign=googlebase&gclid=EAIaIQobChMI8fq19Nam1gIVrrftCh3GIAuOEAYYASABEgLs4_D_BwE&gclsrc=aw.ds
https://www.hafele.co.uk/en/product/isolating-transformer-20-va-240-v-50-hz-input/000000f800004d5e00010023/

How on earth is an "invisible" permanent installation like that even remotely applicable to the technician's workbench situation?  Most of the population won't even know there's an isolation transformer behind the wall plate - and grounding of any sort doesn't come into the discussion.  The other thing is that if some poor soul got themselves between the two output connections, there's not going to be any RCD to intercede.

In Australia, all the installations I have seen in bathrooms have been standard power points - ones that could run a hair dryer - with RCD the only safety protection.  I don't doubt the "shaver socket" could be around - I just haven't encountered any.

[oldway]

I never said that one end of the isolation transformer had to be earthed, but that a reference rail should be chosen in the circuit fed by the isolation transformer and that this rail must be earthed.
The most frequent case is the SMPS where the reference is the negative of the bridge rectifier.
What is important is knowing what is connected to the earth with 100% certainty ....
Leaving the circuit unearthed will result in a hazardous situation as the earth will move in the circuit where the oscilloscope ground clip will be connected, or if there is an insulation fault.

[Brumby]

One other thing about RCDs.

Their standard placement is in a fixed installation, where proper grounding of the circuit is certain.  In that situation, their operation is well defined and only failure of the device can create an additional hazard.  If one is incorporated into a test bench, I would hope it would be similarly placed.

However, by its very nature, an isolation transformer may NOT be placed in a known, consistent configuration every time it is used.  It is connected however the circumstance requires.  Safety in this instance now becomes more involved.  What is dangerous to touch?  What is not?  Certainly, an isolation transformer can present a risk if it fails, but it can also present an extra risk just by being connected to things.

Personally, I think there's a lot to be said for being able to point to one spot and say "that is live" and another "that is earthed" with 100% certainty.


I am not saying that isolation transformers don't have their place on the test bench - just that it requires more understanding of how it is configured to appreciate the risks.  If you use one regularly, this will have been something you have learned, but if you don't, then it's something you need to become familiar with in order to minimise unpleasant surprises.

[madires]

I am not saying that isolation transformers don't have their place on the test bench - just that it requires more understanding of how it is configured to appreciate the risks.  If you use one regularly, this will have been something you have learned, but if you don't, then it's something you need to become familiar with in order to minimise unpleasant surprises.

Yup! And there is no "this is the only way to use an isolation transformer safely" rule. It depends on the situation, the DUT, the T&M devices and what you are trying to measure or test.

[Brumby]

Exactly.

[Shock]

I have two isolation transformers one has no earth present on the secondary side, I can optionally add a flying earth lead direct to the DUT chassis. The other has an earth carried through to the secondary.

This is dangerous. and also pointless as it achieves nothing.

The earth is not tied to secondary winding at all if that is what you mean, I wrote that directly below it.  It's just present on the output side of the isolation transformer. I've adjusted my post to make it clearer, please adjust yours if required and I'll delete this.

[Vtile]

Here is a few good videos in "carlsons lab" I just ended up watching while checking my linquistics to confirm what is "dim bulb" setup (= typical series resistor/bulb )..

[Zero999]

Using an isolation transformer adds an element of "abstraction", if you will, where safe and proper use requires an understanding and an advanced awareness that you can not expect a beginner to have.

The same could be said about an RCD.

I think it more likely a beginner will be able to understand (if they don't already) the role, function and issues of an RCD which is a device commonly found in many homes than the use of an isolation transformer - which is not a common sight ... even in EE circles!

Perhaps not in Australia, but isolation transformers are very common in the UK. Almost every bathroom has a small one powering the shaver socket.
https://www.victorianplumbing.co.uk/white-dual-voltage-shaver-socket-shas?campaign=googlebase&gclid=EAIaIQobChMI8fq19Nam1gIVrrftCh3GIAuOEAYYASABEgLs4_D_BwE&gclsrc=aw.ds
https://www.hafele.co.uk/en/product/isolating-transformer-20-va-240-v-50-hz-input/000000f800004d5e00010023/

How on earth is an "invisible" permanent installation like that even remotely applicable to the technician's workbench situation?  Most of the population won't even know there's an isolation transformer behind the wall plate - and grounding of any sort doesn't come into the discussion.

Because an isolation transformer works on the same principle, irrespective or where it is installed, be it the bathroom or a technician's workbench. The power source is no longer bonded to earth, so making contact with a single conductor will not result in a shock. The majority of those who've done house wiring, in the UK, will be familiar with shaver sockets and how they protect against shock by isolating the circuit from the earth.

The other thing is that if some poor soul got themselves between the two output connections, there's not going to be any RCD to intercede.

The same is true for an RCD. It won't protect anyone from a shock between live and neutral.

In Australia, all the installations I have seen in bathrooms have been standard power points - ones that could run a hair dryer - with RCD the only safety protection.  I don't doubt the "shaver socket" could be around - I just haven't encountered any.

In the UK, these are the only sockets allowed to be installed in bathrooms. I suppose the passive protection, an isolation transformer offerers, is considered to be more reliable than an RCD. There are no moving parts to fail or contacts to weld shut.

I don't have a problem with isolation transformers in bathrooms but I do think the UK electrical regulations are over the top. I think ordinary UK sockets should be allowed, perhaps with an RCD with a lower tripping current of 5mA, in addition to the existing 30mA RCD which mandatory for all circuits in the home. That would give two levels of RCD protection.

[Brumby]

Because an isolation transformer works on the same principle, irrespective or where it is installed, be it the bathroom or a technician's workbench.

You are completely missing the point.  Where it is installed and how it is configured is exactly the issue.  It is not the principle - its the implementation.

Let me rephrase then .... It would be very difficult for someone to find themselves in a dangerous situation with a fixed, enclosed installation isolation transformer bathroom shaver socket.

It is nowhere near as difficult to imagine a dangerous situation arising on a workbench.

[Zero999]

Because an isolation transformer works on the same principle, irrespective or where it is installed, be it the bathroom or a technician's workbench.

You are completely missing the point.  Where it is installed and how it is configured is exactly the issue.  It is not the principle - its the implementation.

Let me rephrase then .... It would be very difficult for someone to find themselves in a dangerous situation with a fixed, enclosed installation isolation transformer bathroom shaver socket.

It is nowhere near as difficult to imagine a dangerous situation arising on a workbench.

I know. The bathroom sockets are two pin, for a start, no earth, but this isn't the same on a workbench, where there's an earthed oscilloscope. I have never said that this is not the case.

I just took issue with your statement that RCDs are easier, for the beginner to understand,  than isolation transformers. The reality is isolation transformers are no more difficult to understand, than RCDs. They are both fairly simple devices, which a beginner can easily understand.

I can see your point about an isolation transformer voiding any upstream RCD protection, when the secondary is earthed, thus it requires extra safety precautions, which a beginner may not be familiar with. I can appreciate that this is why, some people feel strongly about not using an isolation transformer, on the test bench.

[Brumby]

I just took issue with your statement that RCDs are easier, for the beginner to understand,  than isolation transformers. The reality is isolation transformers are no more difficult to understand, than RCDs. They are both fairly simple devices, which a beginner can easily understand.

Theoretically - as in explaining how each operates - I will agree with you, but the issue I have is with real-world usage.

An RCD will typically be implemented in a standard configuration where the protection and the risks can be simply defined and once in place is unlikely to be changed.

An isolation transformer can change it's risk profile by simply moving a scope's earth lead from one point to another.  The risk change is not obvious.  This makes it more dangerous - unless the operator is able to correctly assess the risk in any configuration.

Experienced technicians still have to think about that - and that's certainly not something I would expect from a beginner.

[Shock]

An RCD will typically be implemented in a standard configuration where the protection and the risks can be simply defined and once in place is unlikely to be changed.

Or will they? How can we tell?

I think it's better to do a thought exercise of "how safe will this be when I do this or this" rather than rely on an RCD type device because they think it's a set and forget safety. The RCD device may be 90ft away, bypassed, defective, inadequate or counterfeit (shun the thought) to start with.

It's a bit different if the person has no interest in electronics and is just diving to the unknown (who you can tell, don't do it!). For someone who intends learning about safety it's not up to anyone here to remove safety options from the table or say sorry you're too much a beginner to learn how to do that, btw good luck on that repair.

Many DUTs have transformers anyway, if you think that a bench 1:1 transformer is going to be bad what about an amp with windings coming out in all directions, how could you possibly deal with that?

Again, you need to be knowledgeable and methodical to identify risks and work in a manner that reduces mistakes. Once your knowledge increases you can make more informed decisions.

[Zero999]

An isolation transformer can change it's risk profile by simply moving a scope's earth lead from one point to another.  The risk change is not obvious.  This makes it more dangerous - unless the operator is able to correctly assess the risk in any configuration.

Experienced technicians still have to think about that - and that's certainly not something I would expect from a beginner.

I don't see how it's that complex.

Situation 1
Isolation transformer, powering a circuit, with neither side earthed. Exercise caution, as with any piece of mains operated equipment. Presume, both conductors are at a hazardous potential, as there could be a fault, causing one of them to be earth and the other to be live, which an upstream RCD won't protect against.

Situation 2
Clipping an oscilloscope scope probe's 0V onto a circuit, powered by an isolation transformer. Exercise extra caution, as the RCD upstream will not provide any protection now. Presume all conductors, connected to the mains, are at a hazardous potential, unless one is absolutely sure otherwise. The number one rule is the earth can only be connected to one part of the circuit. The earth on the signal generator and oscilloscope probes, all need to be connected to the same point.

Things go wrong when someone presumes it's safe to touch a live conductor, on a circuit powered off an isolation transformer, or they try to connect the earth to different parts of the circuit.

[Brumby]

RCDs can fail.  So can isolation transformers.  The failure of any safety related device brings a whole new world of "discovery".  Please note that I am not trying to attempt to cover failure modes here.

The scenarios I am addressing are predicated on functioning devices in a typical situation.

Many DUTs have transformers anyway, if you think that a bench 1:1 transformer is going to be bad what about an amp with windings coming out in all directions, how could you possibly deal with that?

Seriously?  It doesn't matter how many windings come out of a transformer within a piece of gear.  It doesn't change the risk of injury, per se.  It just makes the job of tracing through them longer.

Sorry - but that's just a rubbish argument.

Again, you need to be knowledgeable and methodical to identify risks and work in a manner that reduces mistakes. Once your knowledge increases you can make more informed decisions.

Absolutely.  I could not agree more.

An isolation transformer can change it's risk profile by simply moving a scope's earth lead from one point to another.  The risk change is not obvious.  This makes it more dangerous - unless the operator is able to correctly assess the risk in any configuration.

Experienced technicians still have to think about that - and that's certainly not something I would expect from a beginner.

I don't see how it's that complex.

Situation 1
Isolation transformer, powering a circuit, with neither side earthed. Exercise caution, as with any piece of mains operated equipment. Presume, both conductors are at a hazardous potential, as there could be a fault, causing one of them to be earth and the other to be live, which an upstream RCD won't protect against.

Situation 2
Clipping an oscilloscope scope probe's 0V onto a circuit, powered by an isolation transformer. Exercise extra caution, as the RCD upstream will not provide any protection now. Presume all conductors, connected to the mains, are at a hazardous potential, unless one is absolutely sure otherwise. The number one rule is the earth can only be connected to one part of the circuit. The earth on the signal generator and oscilloscope probes, all need to be connected to the same point.

To you and I, that's not a problem - but I envisage a beginner taking all the necessary precautions - then moving one wire which changes everything without them realising...

Things go wrong when someone presumes it's safe to touch a live conductor, on a circuit powered off an isolation transformer, or they try to connect the earth to different parts of the circuit.

Look at this through the eyes of a beginner .... Isn't this exactly what using an isolation transformer can offer?


If you want to encourage a beginner to be comfortable about using an isolation transformer, then I suggest you make sure they have some means of being properly educated and supervised.

Considering the range of "beginners" that have presented themselves on this forum alone, I cannot, in all conscience, give my blessing without some serious qualification.

[Shock]

RCDs can fail.  So can isolation transformers.  The failure of any safety related device brings a whole new world of "discovery".  Please note that I am not trying to attempt to cover failure modes here. The scenarios I am addressing are predicated on functioning devices in a typical situation.

There is no such thing as a typical situation. Nor a typical RCD or isolation transformer for that matter. You might be surprised at the estimated failure rates of RCD type devices, so it's still relevant just as it is with isolation transformers.

Seriously?  It doesn't matter how many windings come out of a transformer within a piece of gear.  It doesn't change the risk of injury, per se.  It just makes the job of tracing through them longer.
Sorry - but that's just a rubbish argument.

I was just pointing out in this scenario now your working on a transformer inside the DUT, now it has many windings and exposure to high voltages on the isolated side, how is that not a similar set of risks?

Sure the RCD type device may be working and if you make a mistake on the primary side touching the live circuit you will expect a jolt, hopefully not sustained just below the trip threshold or too far away, that would suck.

But on the secondary isolated side of the DUT you have a very similar scenario as with using an isolation transformer, you can run into high voltages, which an RCD will offer no protection against and when you hook up mains powered test equipment such as an oscilloscope you are back with the same problem.

So the argument of "too complex for beginners" doesn't fly with me. You can teach most of the concepts pertaining to RCDs and isolation transformers in 10 mins. I'm not saying don't use an RCD as I use them myself in certain situations, I'm just saying don't set a precedent which dumbs down safety.

If you want to encourage a beginner to be comfortable about using an isolation transformer, then I suggest you make sure they have some means of being properly educated and supervised. Considering the range of "beginners" that have presented themselves on this forum alone, I cannot, in all conscience, give my blessing without some serious qualification.

If you want to discourage beginners against learning about anything then I suggest you reconsider your motives. The more hazards you are taught to identify the better. If you don't understand about isolation transformers, measuring voltage potentials, identifying earth/grounding, sources of high voltage etc you probably shouldn't be sticking your fingers or equipment inside.

[Brumby]

I was just pointing out in this scenario now your working on a transformer inside the DUT, now it has many windings and exposure to high voltages on the isolated side, how is that not a similar set of risks?

Of course there are risks, but it is quite different.  I'm sure there are a lot of people here who can see what I've been saying - but have had the wisdom to just hang on to the popcorn.

Since you seem to be unable to understand that, it is pointless to continue.


This comment, however, needs direct addressing:

If you want to discourage beginners against learning about anything then I suggest you reconsider your motives. The more hazards you are taught to identify the better.

I am not, never have and never will discourage beginners from learning - especially when it comes to safety.

My concern relates to the attempt to try and do this via a text-based forum which can be viewed by anyone.  You not only have the identified party who asked the question, but you will have many others watching with interest.  We often have very little understanding of the skill level of those who do participate in the discussion, but we have absolutely no idea of the skill or experience of those who simply view what is written.

Unless fair consideration for the fact that any advice, suggestions or recommendations given on this site may be treated as coming from a credible source - and - that the potential audience (especially the unseen) may have little or no prior understanding as well as (more likely than not) having NO mentor to guide them, then anything stated or inferred as being a "safe" is an invitation for someone to give it that status, even though they do not properly understand the conditions required for that safety to be effective.  An RCD does not change the topology of the mains supply circuits in play.  An isolation transformer does - and it is not constrained to just a single configuration.  This needs clear instruction and proper explanation.

If you have a beginner who wishes to dive into isolation transformer use - then I have no problems as long as they have some mechanism to keep them safe.  A mentor is preferred - but they are not always available.  I would cautiously suggest that someone who has been able to follow this thread and can understand what has been said and why (on both sides) would be in a position to travel down that path.

For those that have not - find someone who can walk you through it.

[oldway]

There is a fundamental design error in the question posed by jastreb: the rcd was never designed to provide protection or safety to persons under the conditions encountered in an electronics workshop.

RCD is only a secondary and redundant protection, it is never a primary safety.

Primary safety is ensured by the fact that the appliance supplied by the electricity network meets the standards of Class I or Class II.
https://en.wikipedia.org/wiki/Appliance_classes

Therefore, either all the conductive parts are earthed and no conductive part under tension is accessible (class I) or no conductive part is accessible (double insulation) or the insulation is reinforced (HighPot tested at 4.5 KV) (Class II).

In an electronic workshop, we work on circuits (SMPS for example) that no longer correspond to these two standards, so, in any case, we are no longer in legal conditions of safety.

This is why an electronic workshop is not accessible to people who are not qualified and not aware of the dangers of electricity.

Before working with dangerous voltages (> 50V), a beginner must first have the proper training ... a simple topic on a forum is not enough, it must begin with the follow of a qualified person.

That's why I find that a topic like this is not only unnecessary, but also dangerous as a beginner might believe that just follow a few simple tips to be safe, while that's not true.

[oldway]

Most electronic engineers and technicians are not qualified to answer questions about electrical safety.
It is a subject that is addressed rather to the field of electricity and power electronics.

Some electronic forums forbid topics related to applications that put participants safety at risk.

I had opened a topic asking to make a totally separate subject for power electronics and electricity (with safety advices) but it seems that my proposal did not interest Dave.

Answers to security questions should be made by people who prove their qualification and in a non-anonymous way so that they can be held responsible for the erroneous and dangerous advices they give and even be suited in the courts for their consequences.

[Shock]

I think we have already established that there is more to safety than two electrical safety devices (one of which Brumby is telling people to use as a door stop, LOL) so I don't feel like their is a lack of warning already conveyed here or in other threads.

As for the rest I've bumped your post Oldway.

[Cerebus]

Answers to security questions should be made by people who prove their qualification and in a non-anonymous way so that they can be held responsible for the erroneous and dangerous advices they give and even be suited in the courts for their consequences.

If you're expecting this, then I presume you're expecting to pay them for this advice. Because if you're asking people to expose themselves to some liability then they are going to need professional liability insurance, and all the professional liability insurance policies I have ever seen only cover the insured for paid professional work.

This is a forum, it's not an engineering consultancy. I think it's an error to try and suggest that it ought to somehow become in some way more like the latter. If you want reliable advice, from qualified professionals, who take formal responsibility for giving bad advice, you make a contract with them and pay for it.

[oldway]

My idea is not that the real name should be published on the internet but that a new "special" account could be created where it is necessary to identify and justify its security skills. Only Dave would have this information related to the identity of the account holder.
These avatars would be identified as safety experts ...

For technical subjects, wrong advices may be allowed but not for a subject such as safety.
It is essential to give a certain reliability to the advices because the stake is the human life.

Another option is simply to ban such subjects.

[Zero999]

My idea is not that the real name should be published on the internet but that a new "special" account could be created where it is necessary to identify and justify its security skills. Only Dave would have this information related to the identity of the account holder.
These avatars would be identified as safety experts ...

For technical subjects, wrong advices may be allowed but not for a subject such as safety.
It is essential to give a certain reliability to the advices because the stake is the human life.

Another option is simply to ban such subjects.

Both of your suggestions are well intended but I don't like either of them.

How does one go about proving their credentials? Quite often electrical safety qualifications are very specific. For example, my neighbour is a domestic electrician. He could provide good advice on UK household wiring but not industrial installations or domestic appliances. Safety laws/standards vary from from one jurisdiction to another. It can also be challenging deciding which standard is relevant, especially in the absence of all of the information. What happens if two safety experts give conflicting advice?

As far as banning it is concerned: that's more workable but I think this place already has enough rules, as it is. It's better to have an open discussion. The person asking the question, is always free to, and should, do some independent research.