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

[jastreb]

Just want to say thank you to all of you for taking the time to respond to my question which seems to have lead to a lively discussion. I would have to say you are not very encouraging for a beginner and that's a good thing when you think about it as we are talking about possible life or death here.

At same time though, I think that you should appreciate that I am not a cowboy and posting on here shows that I am keen to learn and find out more before I start playing with mains. As a little feeedback from a noob on here on eevblog, if I may is that rather than just telling somebody to stay away and do formal training etc, perhaps more direct answer would be handy for me. The topic quickly became a discussion amongst yourselves. Its human to do so as we don't all always agree on things.

Thank you all once again for sharing your knowledge.

[Assafl]

The ability of a circuit to destroy living tissue is a matter of Energy more than anything. Hence defibrillators (which - given correct timing - can reanimate a dead person) are measured in joules. So when you say that the tripping time is important - yes indeed. Power * time = Energy. Now power is V^2/R - so the lower your body resistance and the higher the voltage - well the quicker you'll join the choir invisible. But then again, the quicker the RCD may (or may not) trip.

Defibrillators are precision devices that cost a pretty penny. And still used only when the last resort. An RCD is a plasticky device that comes from China. It is sort of precision (or, using hope as a strategy - hopefully precision). It is not bad to have one on the circuit (I do) but I hope it never has to work.

As for lifting ground (which is what an isolation transformer does) - the only time I've ever had to do it - is when an old oven stored for a while has to be returned to duty. Sometimes the heaters absorb humidity and trip the GFCI. Lifting the ground enables them to heat up and release the water vapor (as an alternative to replacing the heating elements). It is dangerous thing (no kids around) but still the GFCI should trip if something bad happens. An isolation transformer will do the same - with the exception that if there is a fault to the oven body - you will be electrocuted and the GFCI will not help (the loop will close on the secondary side).

As a safety device, isolation transformers are used grounded as stated above. Since it is grounded - you'd need a real reason to use an isolation transformer - for example, in a defibrillator to prevent the joules from leaking to earth (you want them between the two paddles), and to help prevent a shock to the defibrillator operator.

As a ground lifting device, and only as a last resort, electricians will first define the test that will narrow down the fault, then quarantine the offending device under test with tape and signs, hook up the probes, set all the controls - and only then run the appropriate tests from the grounded test equipment. Nothing downstream from the isolation transformer should ever be touched. I've been part of such tests on military SONAR equipment that had leakage we couldn't isolate. As an EE I sat from afar (with the schematics) looking with disgust at non grounded system sitting in the lab. The senior technicians managed the tests. How I hate that sort of crap (but sometimes you have to).

The only more terrifying EE stuff was when I studied power electronics and we went to a base station. They took us inside the caged area to look at HV power distribution. I wanted to cover myself in foil and lie on the floor as far away from any capacitor transformer or ceramic stack (it was safe - the HV was way above us). That was the last I ever considered working on transmission lines (albeit it is amazing to be as powerful as a god).

[Assafl]

At same time though, I think that you should appreciate that I am not a cowboy and posting on here shows that I am keen to learn and find out more before I start playing with mains. As a little feeedback from a noob on here on eevblog, if I may is that rather than just telling somebody to stay away and do formal training etc, perhaps more direct answer would be handy for me. The topic quickly became a discussion amongst yourselves. Its human to do so as we don't all always agree on things.

When I was 14 my father agreed to me servicing our ITT television set. It was a CRT model and therefore had a flyback and doubler and anode nipple and the like. And TVs back then were not isolated. By that time I had etched my own PCBs, designed my own circuits and built (and serviced) low voltage electronics for about 3 years. Being an EE he verified I could identify the risks on the TV chassis and sat back watching that I was being careful and diligent.

I agree with you. It is your decision what you feel comfortable doing. Everyone here sat - at one point - their first time looking at exposed 220V (or worse - 380V 3~ or even worse). Nobody is ready for the thought that it can literally, kill them. bright shiny copper pad, a jolly brown Nichicon capacitor or a deductive nipple on the back of the CRT (that we all popped out and immediately shorted to the chassis).

I wonder about the proselytizing nature of the answers. Perhaps the issue is the question: the real question that should be asked isn't about people opinions - but their experience. You'll soon note most have GFCI and very few have isolation transformers (if at all).

[IanMacdonald]

Firstly I'd say that there is seldom any reason NOT to have an RCD (GFI for you Yanks) on the bench feed, and since they are inexpensive and may save your life.. get one if the building doesn't already have them.

The main point to understand about work on opened high voltage equipment is that earths are a danger to you. When the equipment case is closed the earth provides safety, but when it is open any earth in its vicinity does the reverse. To receive a shock calls for two connections, and the most likely way this will arise is through touching a live connection whilst resting your other hand on the earthed case of an item of test equipment.  Or, whilst holding an earthed test lead.

To minimize this risk the one-hand approach should be used as much as possible. Avoid resting your other hand on metal. If a certain action requires two hands, consider cutting the power whilst it is done.

Isolating transformers are a huge improvement in safety when used correctly, particularly on SMPS work. Your main enemy here is the earthed outers of scope probes, and I really wish that the manufacturers would rethink this arrangement. On which topic, never use a capless scope probe on PSU work. Unless, that is, you insulate the exposed metal ring close to the tip. Seen a good few blowups through this ring grounding-down a live connection.

Beware of 'isolating transformers' sold for building site use, which are not actually isolating transformers at all, but change a 240v supply into a 120v-0-120v supply. If one of these finds its way onto the testbench it can be extremely hazardous since the potential difference between the supply live and the '-120v' output of the transformer is 360v. (Seen an accident where this scenario left a guy with 'welding flash' to his eyes and an impressive hole in a PCB.)

Oh, and never trust equipment on/off switches. Pull out the plug when soldering or whatever, and place the plug where you can see it. The worst case here is a double pole switch on which the live side is welded shut and the neutral side operating. In this case the equipment will seem to switch off but will still be live. 

[Brumby]

At same time though, I think that you should appreciate that I am not a cowboy and posting on here shows that I am keen to learn and find out more before I start playing with mains. As a little feeedback from a noob on here on eevblog, if I may is that rather than just telling somebody to stay away and do formal training etc, perhaps more direct answer would be handy for me.

Your comment is understood - but you must also appreciate that we have no idea of your experience or skill.  The tendency will always be to favour safety.

You should also understand that we are not a consensus - but a group of diverse people with collective experience.  We often have opinions that can differ between members - and sometimes those differences are the result of culture, language or perhaps the ability of someone to communicate their thoughts in a way that is accurately received - by a diverse audience.


If you can take in all the viewpoints presented here, appreciate what each person is saying and condense it down to something appropriate to your situation, then that will be the best result.


We are just a bunch of individuals offering help from out personal understanding and experience.... on the internet.  It's never going to be perfect and often, it's not even close - but we try.

[Zero999]

An isolation transformer without an earthing of the circuit that it supplies is intrinsically dangerous because if there is an insulation fault putting a part of the circuit to the earth, you are not not warned of it and
no protection can protect you, nor RCD (30mA differential switch) nor circuit breaker.

Furthermore, you do not know which part of the circuit is at a high potential in relation to the earth ... it is extremely dangerous.

That's what an insulating monitoring device is for. It's designed to be connected to the secondary of an isolating transformer and provide a warning, if the secondary develops an earth fault. I recommend getting one or even making one for use on a test bench.
https://en.wikipedia.org/wiki/Insulation_monitoring_device

Of course, there are voltages up to 400V related to earth, but you know this and you measure it with high voltage X100 probes...

Well that shouldn't happen on a single phase supply, unless the isolation transformer is connected to something which boosts the voltage and shares the neutral.

NEVER USE AN ISOLATION TRANSFORMER WITHOUT EARTHING THE SECONDARY CIRCUIT, THAT'S EXTREMELY DANGEROUS !!!!!

Nonsense. This is routinely done in hospitals and industry, in situations where an RCD would be unreliable, due to nuisance tripping. It's known as an IT earthing system. As mentioned previously an insulation monitoring device is used to warn of any faults.
http://www.schneider-electric.pl/documents/designers/top-downloads/090197c6800bcbde_.pdf

An alternative is not earthing anything connected to the secondary of the isolating transformer. All exposed metal parts are bonded together with what's known as earth free equipotential bonding, to prevent the user from being exposed to dangerous potentials. This isn't possible with a home set-up though so it's off-topic.

Electrical separation for the supply to one item of current-using equipment

Shock protection, via electrical separation, for a supply to one item of current using equipment, is provided when fault protection utilizes simple separation of the circuit from the main circuit and all of its associated circuits and Earth. Basic protection is provided by insulation or barriers.

When the basic protection fails in this type of installation the fault protection is afforded by simple separation from the main circuit because it has no fault path to Earth and therefore no electric shock. The downside of this is that with the loss of basic protection the fault is generally not cleared and goes undetected until there is a second fault, which may prove to be hazardous.

A generator commonly utilizes this type of simple separation, which is isolated from the main installation and not connected to Earth or any other earthed circuit. It is vital when using this type of protection method not to allow any exposed conductive parts of the equipment to be connected to the protective conductor or exposed conductive parts of other circuits. Another very common example of this type of protection is a shaver socket. See below for diagram of a generator supplying one item of current using equipment:

Diagram of a generator supplying one item of current using equipment

Diagram of a generator supplying one item of current using equipment
Electrical separation may be used for more than one item of current using equipment; however, the risks associated with this are greatly increased. Extra measures are required to ensure that the installation is safe. These measures include the requirement of the installation to be under the supervision of skilled or instructed persons in order to ensure that no changes are made that could lead to a dangerous scenario. Warning notices must also be present to control the connection of protective bonding conductors as these must not be connected to Earth.

The protective bonding conductors associated with the
electrical installation in this location

MUST NOT BE CONNECTED TO EARTH


Equipment having exposed conductive parts connected to
earth must not be brought into this location

http://electrical.theiet.org/wiring-matters/52/shock-protection/index.cfm

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/

[Assafl]

Beware of 'isolating transformers' sold for building site use, which are not actually isolating transformers at all, but change a 240v supply into a 120v-0-120v supply. If one of these finds its way onto the testbench it can be extremely hazardous since the potential difference between the supply live and the '-120v' output of the transformer is 360v. (Seen an accident where this scenario left a guy with 'welding flash' to his eyes and an impressive hole in a PCB.)

Lol. But dangerous.

Reminds me of Chinese auto transformers. Many get the wiring wrong directly from Shenzhen. When used as a down converter (from 220 to 110) you end up with the 110v referenced to the 220v phase (not the neutral).

It will work until you plug in an earthed 110v device with some MOV or TVS or NTC protecting against surges to ground. It will see 220v and explode.

Always measure their output with a DMM to ensure it is is  110v and 0v from the source neutral (or ground) and NOT 220v and 110v from neutral.

Just goes to show that even the simplest of circuits: the lowly transformer can be a tricky bastard.

[oldway]

The situation that is discussed here is totally different .... we are talking about the safety of a work bench where measurements / repairs are made on devices powered by the electrical network.

In electricity, primary safety is the physical protection against contact with a live conductive element.

Here, this primary protection does not exist or has been removed.

Therefore, the rules that can be applied to medical or other equipment where such primary protection exists can't be applied in the present situation.

The use of an electronic device to signal an earthing fault of secondary of an isolation transformer is still a dangerous mistake because the primary safety can never depend on an electronic device which can be faulty.

I read a lot of dangerous and erroneus statements, be careful, there are people who does not know nothing about safety here.

[oldway]

Of course, there are voltages up to 400V related to earth, but you know this and you measure it with high voltage X100 probes...

Well that shouldn't happen on a single phase supply, unless the isolation transformer is connected to something which boosts the voltage and shares the neutral.

I did not understood....PFC output voltage is indeed between 390 and 400V....
When negative rail of the primary side of a SMPS with PFC is earthed, we have indeed a PFC voltage of up to +400Vdc related to earth.

[Zero999]

The situation that is discussed here is totally different .... we are talking about the safety of a work bench where measurements / repairs are made on devices powered by the electrical network.

In electricity, primary safety is the physical protection against contact with a live conductive element.

Here, this primary protection does not exist or has been removed.

The protection is only removed, when the user connects the secondary to earth. Until then, the user is protected against shock from a single point contact with the circuit, because there's no return path for the current.

The use of an electronic device to signal an earthing fault of secondary of an isolation transformer is still a dangerous mistake because the primary safety can never depend on an electronic device which can be faulty.

Indeed the same could be said about any piece of safety equipment, such as an RCD or GFCI. The fact that the earth fault detector may fail, doesn't make it dangerous or bad in any way. The user just needs to routinely test it before use, to ensure it works.

Of course, there are voltages up to 400V related to earth, but you know this and you measure it with high voltage X100 probes...

Well that shouldn't happen on a single phase supply, unless the isolation transformer is connected to something which boosts the voltage and shares the neutral.

I did not understood....PFC output voltage is indeed between 390 and 400V....
When negative rail of the primary side of a SMPS with PFC is earthed, we have indeed a PFC voltage of up to +400Vdc related to earth.

Yes, that is true. As the PFC circuit is not isolated from the input, so will have around 400V across it, which isn't really any more dangerous than 230VAC. The only extra hazard is the capacitors can remain charged, after the mains power has been removed.

[oldway]

The situation that is discussed here is totally different .... we are talking about the safety of a work bench where measurements / repairs are made on devices powered by the electrical network.

In electricity, primary safety is the physical protection against contact with a live conductive element.

Here, this primary protection does not exist or has been removed.

The protection is only removed, when the user connects the secondary to earth. Until then, the user is protected against shock from a single point contact with the circuit, because there's no return path for the current.
Safety is not a game....What is dangerous is not to know what is the exact situation....There may be an insulation fault, perhaps, you earth a circuit with the alligator clip of the probe, or by an auxiliary power supply...What is dangerous is not high voltage, it is not to be sure 100% where there is a lethal voltage. And without earthing the secondary, you can't be sure of nothing...So, you have to earth the circuit you will use as reference ground BEFORE powering the transformer, use only one earth clip, the other probes must be used without it .

The use of an electronic device to signal an earthing fault of secondary of an isolation transformer is still a dangerous mistake because the primary safety can never depend on an electronic device which can be faulty.

Indeed the same could be said about any piece of safety equipment, such as an RCD or GFCI. The fact that the earth fault detector may fail, doesn't make it dangerous or bad in any way. The user just needs to routinely test it before use, to ensure it works. One more mistake ! RCD or GFCI ARE NOT PRIMARY SAFETY PROTECTIONS AND CAN'T BE USED FOR THIS PURPOSE ! Primary safety is the case of your device that protect you against contact with hot live conductive wires or elements....Obviously, that protection does not exist nomore when you open the cover or case of your SMPS to measure voltages on the PCB.

[Zero999]

The situation that is discussed here is totally different .... we are talking about the safety of a work bench where measurements / repairs are made on devices powered by the electrical network.

In electricity, primary safety is the physical protection against contact with a live conductive element.

Here, this primary protection does not exist or has been removed.

The protection is only removed, when the user connects the secondary to earth. Until then, the user is protected against shock from a single point contact with the circuit, because there's no return path for the current.
Safety is not a game....What is dangerous is not to know what is the exact situation....There may be an insulation fault, perhaps, you earth a circuit with the alligator clip of the probe, or by an auxiliary power supply...What is dangerous is not high voltage, it is not to be sure 100% where there is a lethal voltage. And without earthing the secondary, you can't be sure of nothing...So, you have to earth the circuit you will use as reference ground BEFORE powering the transformer, use only one earth clip, the other probes must be used without it .

If the secondary is not earthed, one knows the exactly where the lethal voltages are and the insulation monitoring device will tell one, if there's an earth fault. As with any piece of test/safety equipment, the insulation monitoring device needs to be tested regularly.

Once the secondary is earthed, all of the protection offered by the isolation transformer is lost. This is why many people here don't agree with using isolation transformers in the first place. They believe an RCD/GFCI is safer, because it will disconnect the circuit, in the case of an earth leakage. The worst thing with an isolation transformer, with the secondary earthed, is any RCD protection upstream will be lost.

This is why I advocate using an isolation transformer, along with an insulation monitoring device. One gets the benefit of being warned when the power source is earthed or not. It will let one know if there's an insulation fault.

The use of an electronic device to signal an earthing fault of secondary of an isolation transformer is still a dangerous mistake because the primary safety can never depend on an electronic device which can be faulty.

Indeed the same could be said about any piece of safety equipment, such as an RCD or GFCI. The fact that the earth fault detector may fail, doesn't make it dangerous or bad in any way. The user just needs to routinely test it before use, to ensure it works. One more mistake ! RCD or GFCI ARE NOT PRIMARY SAFETY PROTECTIONS AND CAN'T BE USED FOR THIS PURPOSE ! Primary safety is the case of your device that protect you against contact with hot live conductive wires or elements....Obviously, that protection does not exist nomore when you open the cover or case of your SMPS to measure voltages on the PCB.

Yes, same with an isolation transformer.

[oldway]

From the moment you remove the primary security and the device no longer meets the Class I or Class II standards, there is no longer any security other than that which you yourself will establish.

This is why a repair shop for mains powered appliances is only accessible to qualified persons.

I am confortable to work even with dc drive of 5000A 550Vdc but:

- I want to have control of the situation, I refuse any unknown situation .... a not earthed circuit is indeterminate, therefore dangerous.

- I do not get distracted ....

- I have someone close who can intervene if necessary

- I do not trust anything, neither any person ....

- I check everything at least 2 times.

- I do not do anything I do not know the consequences if something goes wrong

- I use only good quality tools and instruments

- I use protective/security equipment, but without exaggerating.

[madires]

If you earth/ground the isolation transformer's secondary in any way you'll will allow current of the secondary to flow to earth/ground if the loop is closed by the EE/hobbyist. You can do that to choose a specific grounding point for using standard scope probes, actually the first (and only) ground clip determines the grounding point. But that defeats the benefit of the isolation, i.e. no current path from the secondary to earth/ground. In that case you should also wear rubber gloves to prevent touching any voltage potential, so that you won't close the loop to ground. A better approach is to use differential probes which allow you to keep the DUT floating. It's also more practical for checking several signals without having to change the complete ground setup each time you need to change the reference (grounding) point for the next signal.

[madires]

Once the secondary is earthed, all of the protection offered by the isolation transformer is lost. This is why many people here don't agree with using isolation transformers in the first place. They believe an RCD/GFCI is safer, because it will disconnect the circuit, in the case of an earth leakage. The worst thing with an isolation transformer, with the secondary earthed, is any RCD protection upstream will be lost.

This is why I advocate using an isolation transformer, along with an insulation monitoring device. One gets the benefit of being warned when the power source is earthed or not. It will let one know if there's an insulation fault.

The important point is to not rely on a single layer of safety. Add a second or even third layer.

[Assafl]

Rubber gloves, isolation transformer, earth monitoring device, linesman suit etc.

No wonder our hobby is dying. I know many many technicians. Few have access to such a setup. None have died or even experienced electrical shock. Neither do I.

Is that really how you guys got started with your own lab? An isolation transformer before your first DMM? Before your first differential probe? WTF?

[rstofer]

Just want to say thank you to all of you for taking the time to respond to my question which seems to have lead to a lively discussion. I would have to say you are not very encouraging for a beginner and that's a good thing when you think about it as we are talking about possible life or death here.

At same time though, I think that you should appreciate that I am not a cowboy and posting on here shows that I am keen to learn and find out more before I start playing with mains. As a little feeedback from a noob on here on eevblog, if I may is that rather than just telling somebody to stay away and do formal training etc, perhaps more direct answer would be handy for me. The topic quickly became a discussion amongst yourselves. Its human to do so as we don't all always agree on things.

Thank you all once again for sharing your knowledge.

You can see from the variety of responses a genuine concern about safety.  Our safety, and yours!
One idea to take away is to always ask yourself, "How is what I'm about to do going to hurt me?" and "How am I going to prevent it?".  This applies to all facets of life.

There is no general agreement on procedures.  This is due to the fact that one process doesn't cover all situations.  If it did, we would all be quoting from the same hymnal.  What is common among the responses is the need to think through what you are about to do.  And it needs to be thought through all the way back to the panelboard where the neutral and ground are tied together.

I don't want to work with elevated voltages any more.  I've done my time and now, deeply into retirement, I play with low voltage stuff.  I like it over in this corner of the sandbox.

Have fun!

[oldway]

I never said that a differential probe is not a safe and good solution....But it is very expensive, too expensive for multichannels probing.
I work often with power electronics and I don(t have such probes.
But, again, for your safety, you can't trust an insulation monitoring device even if you test it  regularly.

There no situation more dangerous than to think you are safe when you are not....

RCD/GFCI : You know when it will trip ? Without isolation transformer, only if you hold a conductive earthed element with one hand and you touch a high voltage circuit with your other hand..

Earth current passing from your hand to the floor will not be high enough unless your workshop is in the badroom, the floor is wet and you have bare feet....

With isolation transformer, even if you earth the negative rail of the SMPS, chassis and other metallic parts are no more earthed, then no more danger of electric chock between these parts and the circuit.
Of course, between the rail +400V and earth, (your scope for exemple), that's dangerous....

I prefer using an isolation transformer.

[rstofer]

Rubber gloves, isolation transformer, earth monitoring device, linesman suit etc.

No wonder our hobby is dying. I know many many technicians. Few have access to such a setup. None have died or even experienced electrical shock. Neither do I.

Is that really how you guys got started with your own lab? An isolation transformer before your first DMM? Before your first differential probe? WTF?

I don't have an isolation transformer and I don't have a differential probe either.  Yet, somehow, I've managed to play with a variety of equipment for over 60 years and I'm still here.

If I get into something where I can't make a ground referenced test, I might think about A-B.  If that isn't sufficient, I would simply buy the differential probe.  There's a store.  You give them money, they give you a probe.  Pretty simple!  Sure, they're expensive but they are the right way to do the job.

I am very comfortable with the idea that all exposed non-current carrying metallic surfaces are grounded.  That's the way it is in the electrical world and I'm pretty happy with the idea.  The fact that the BNC connectors on my scope are grounded means I won't get bit plugging in a probe.  Comforting...

[Assafl]

Another tip I have for the OP is to work on stuff he hasn't the service manual for (or at the very least schematics). At least for the first few repairs/mods.

It is far easier to be safe when you don't have to second guess what a component does.

[madires]

Rubber gloves, isolation transformer, earth monitoring device, linesman suit etc.

No wonder our hobby is dying. I know many many technicians. Few have access to such a setup. None have died or even experienced electrical shock. Neither do I.

Is that really how you guys got started with your own lab? An isolation transformer before your first DMM? Before your first differential probe? WTF?

No, we've started with battery powered circuits. And we've checked mains powered stuff only when disconnected from mains or if it had isolation built in, i.e. a standard transformer. But when you're starting to poke around in live mains powered SMPSUs you should get an isolation transformer.

[Zero999]

Beware of 'isolating transformers' sold for building site use, which are not actually isolating transformers at all, but change a 240v supply into a 120v-0-120v supply. If one of these finds its way onto the testbench it can be extremely hazardous since the potential difference between the supply live and the '-120v' output of the transformer is 360v. (Seen an accident where this scenario left a guy with 'welding flash' to his eyes and an impressive hole in a PCB.)

I've never encountered that before. Building site transformers output 110VAC. Single phase units are centre tapped to earth, giving 55VAC per split phase, and three phase units give 63.5VAC to earthed neutral. They predate RCDs and are based on the fact that 55V to 63.5V to earth is less dangerous than 230V. I don't like them because they're big and bulky. I think more people have injured themselves as a result of improper manual handling, than electric shock. I'd rather use 230V, with an RCD. Heck, if I was working in a very damp environment, with a 110VAC supply, I'd still want to use an RCD.

The 230V output units, such as the one linked below, have the secondary winding fully isolated from earth. I have used one before and tested it to ensure the secondary is unearthed. In fact one of the reasons why it was purchased, was to test an RCD on in some equipment that was already running off a supply protected by an RCD. A jumper was connected to the secondary, linking it to earth, it was marked with a warning indicating that it would void any upstream RCD protection and it was kept in a secure area, to stop anyone, other than a technician from using it.
http://uk.rs-online.com/web/p/products/4368805/?grossPrice=Y&cm_mmc=UK-PLA-DS3A-_-google-_-PLA_UK_EN_Power_Supplies_And_Transformers-_-Transformers-_-PRODUCT+GROUP&matchtype=&gclid=EAIaIQobChMIhcSgpNmp1gIVQ77tCh3pIghQEAQYASABEgIOvvD_BwE&gclsrc=aw.ds

[Shock]

For personal safety my bench has RCD protection covering all outlets, mainly to prevent chassis live situations on test equipment. Lighting is not on the RCD circuit so I can see after a trip occurs. For my test outlet I have an isolation transformer, variac and dimbulb tester. I can isolate the DUT and it's chassis and reduce the voltage potential to negligible levels. The dimbulb tester works like a reactive current limiter and reduces the chance of sudden failures which covers burning and exploding components which otherwise requires ninja like reflexes to prevent.

I also have a HV multimeter probe, HV active differential oscilloscope probe, capacitor discharge lead, megger/insulation tester, battery powered oscilloscope. Having a power meter, volt/ammeter or current clamp on your test outlet is handy as you don't have to monitor mains current constantly through a multimeter which is to me not an ideal situation.

Obviously you wouldn't just rush out and buy all this immediately when starting out, but they have other benefits aside from personal safety. It's something to work on anyway as your hobby (especially if repair is your thing) expands. I paid next to nothing aside from the variac as it's hard to get a decent variac here.

Isolation transformers do have a learning curve but just like an RCD you should confirm it's operation and specifications. 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.

[Assafl]

For personal safety my bench has RCD protection covering all outlets, mainly to prevent chassis live situations on test equipment. Lighting is not on the RCD circuit so I can see after a trip occurs. For my test outlet I have an isolation transformer, variac and dimbulb tester. I can isolate the DUT and it's chassis and reduce the voltage potential to negligible levels. The dimbulb tester works like a reactive current limiter and reduces the chance of sudden failures which covers burning and exploding components which otherwise requires ninja like reflexes to prevent.

I also have a HV multimeter probe, HV active differential oscilloscope probe, capacitor discharge lead, megger/insulation tester, battery powered oscilloscope. Having a power meter, volt/ammeter or current clamp on your test outlet is handy as you don't have to monitor mains current constantly through a multimeter which is to me not an ideal situation.

Obviously you wouldn't just rush out and buy all this immediately when starting out, but they have other benefits aside from personal safety. It's something to work on anyway as your hobby (especially if repair is your thing) expands. I paid next to nothing aside from the variac as it's hard to get a decent variac here.

Isolation transformers do have a learning curve but just like an RCD you should confirm it's operation and specifications. 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.

Finally an interesting post.

1. 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)?

2. 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).

BTW - For the wattage meter I just made an adaptor that has a socket and a plug and uses shrouded bananas to connects to the Gossen so it isn't too bad. I also did a 1 or 10 turn handy split for current clamps I put on thingyverse (https://www.thingiverse.com/thing:570766).   
 

[Zero999]

Safety is not a game....What is dangerous is not to know what is the exact situation....There may be an insulation fault, perhaps, you earth a circuit with the alligator clip of the probe, or by an auxiliary power supply...What is dangerous is not high voltage, it is not to be sure 100% where there is a lethal voltage.

Isn't that often the case anyway, irrespective of whether an RCD or isolation transformer is used, earthed or not?

It's often impossible to know the exact situation. The status of the device being tested is often unknown. It might be safe or not. There might not be a schematic and if it does have a one, it could be incorrectly wired, or have a fault. It's a good idea to carry about some preliminary safety checks: insulation between live & neutral and earth and continuity of the earth to exposed metallic parts, but there's a limit to what can be done before applying power.

Whether I'm working on mains powered equipment, with an RCD or isolation transformer, unearthed or not. I make the assumption that all conductors connected to the mains are potentially hazardous and keep one hand in my pocket, at all times.