It's worth noting that an isolation transformer is really there to protect equipment, not the user. The goal is to make sure that you can place your oscilloscope ground clip, for example, on any node inside a DUT (Device Under Test) without worrying that you will create a short through the ground clip.Yes. As soon as you clip on that scope ground clip, (or any other grounded test equipment lead), or even connect an aerial lead (as many aerial systems are grounded), that high voltage high stored energy circuit (D.U.T. - Device Under Test) on your bench that you thought was isolated has just become *FAR* more likely to kill you. The odds of it killing you before your grounded a node were pretty small, as you'd have to be fairly dumb and unlucky to get a high energy shock across your chest by touching it with both hands.
It's worth noting that an isolation transformer is really there to protect equipment, not the user. The goal is to make sure that you can place your oscilloscope ground clip, for example, on any node inside a DUT (Device Under Test) without worrying that you will create a short through the ground clip.
It's worth noting that an isolation transformer is really there to protect equipment, not the user. The goal is to make sure that you can place your oscilloscope ground clip, for example, on any node inside a DUT (Device Under Test) without worrying that you will create a short through the ground clip.
But not the second one! They both need to connect to the same point. Use the scope's A-B function instead!
Using an isolation transformer is such a bad idea that it should be banned from the planet and never discussed in forums!
An RCD ordinarily trips at 30 mA and this is a LOT. In the US, the GFCI trips at 15 mA and this can still do some damage. It all depends on the path. If it goes through the heart, you are probably screwed either way. That's why the "One Hand In Pocket" rule. It prevents current flowing through the heart.
Really, without a formal training course in electrical safety, beginners should stick to < 30V. I'm talking about a FORMAL COURSE, not a bunch of random musings on the Internet. Find a qualified instructor!
My main concern about an isolation transformer is that it can give you a false sense of safety.The same can be said about an RCD, which could also fail to activate, especially if you don't test it every time.
If you use one, you will be able to make one external connection without damaging anything or getting any indication that you have just made one point live that you would normally expect to be innocuous - such as the chassis. Then, when you move to another point in the circuit and forget what potential the chassis is at, you find out the hard way.As I mentioned previously, some kind of indicator to warn the user that one side of the secondary is earthed would help a lot.
An RCD can't protect you, because there isn't an independent fault path and if the chassis was properly earthed in the first place, you would likely have found your attempted connection blowing a fuse or some other dramatic indication.
Please understand - my responses are made with one very significant factor in mind....If he is really a beginner who is not able to deal with an element of "abstraction", there is only ONE ANSWER POSSIBLE....
... this question is posted in the beginners section - by someone who is clearly a beginner.
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.
A live wire and an earthed chassis is a much clearer scenario.
If he is really a beginner who is not able to deal with an element of "abstraction", there is only ONE ANSWER POSSIBLE....
You are not qualified to work with mains and high voltage related devices.....start with low voltage circuits (<50V), wait to have enough knowledge and experience before trying to deal with electricity and power electronics.
RCDs doesn't give any sort of protection in certain cases. Their working principle is to measure the current difference between "hot" and "neutral" leads (I'm not sure if there is models that also have separate protective earth sense, but it doesn't make the difference here ), so when you get shock with chest on neutral and hand in "hot" the RCD is happy considering that the appliance is working as intented (in case of electric chair it is true) and not do anything. They "only" sense a leakage to the ground wire (IF ANY!!!).
The request to ban "all isolation transformers" is a bit hastily written and silly, if someone do read what in example CENELEC HD60364-7-7704 or equivalent IEC parallel publication (60364 section 706) says about them.
PS. There is no magical blue pill to give you safety. The safety in this kind of work is a system consisting of many layers of protection, including the knowledge and proper training, mission-critical protection devices and gear, environmental considerations ... etc.
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. If one doesn't fully understand the risks, they shouldn't be working with mains voltage in the first place.
If an isolating transformer is used, the circuit must be earthed. The advantage is that you can choose what is earthed.That's an interesting statement. I thought one of the main advantages with an isolation transformer, is isolating everything from earth. One thing I know is that the IET doesn't recommend earthing anything connected to an isolation transformer, probably because it defeats the purpose of it being separated from earth.
RCDs doesn't give any sort of protection in certain cases. Their working principle is to measure the current difference between "hot" and "neutral" leads (I'm not sure if there is models that also have separate protective earth sense, but it doesn't make the difference here ), so when you get shock with chest on neutral and hand in "hot" the RCD is happy considering that the appliance is working as intented (in case of electric chair it is true) and not do anything. They "only" sense a leakage to the ground wire (IF ANY!!!).
I don't know anything about RCDs but I do know a little about the US style GFCI and this device measures the difference in current going out the hot wire and returning on the neutral wire. The ground wire is irrelevant to the operation.
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!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.
If one doesn't fully understand the risks, they shouldn't be working with mains voltage in the first place.That goes for any environment - whether RCD, isolation transformer or straight off the grid.
An RCD ordinarily trips at 30 mA and this is a LOT. In the US, the GFCI trips at 15 mA and this can still do some damage. It all depends on the path. If it goes through the heart, you are probably screwed either way. That's why the "One Hand In Pocket" rule. It prevents current flowing through the heart.
y choppy pattern which is very laggy. Perhaps I'm doing it wrong.
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.
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.
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 andThat'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.
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.
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.
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:
(http://electrical.theiet.org/wiring-matters/52/-images/generator.jpg)
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
Perhaps not in Australia, but isolation transformers are very common in the UK. Almost every bathroom has a small one powering the shaver socket.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!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.
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.)
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.... :-+
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 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...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.QuoteWell 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.
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 .QuoteThe 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.
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 .
Yes, same with an isolation transformer.QuoteThe 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.
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.
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.
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?
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?
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.
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.
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?
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)?
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).
Safety is not a game....It's often impossible to know the exact situation...
Perhaps not in Australia, but isolation transformers are very common in the UK. Almost every bathroom has a small one powering the shaver socket.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!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.
https://www.victorianplumbing.co.uk/white-dual-voltage-shaver-socket-shas?campaign=googlebase&gclid=EAIaIQobChMI8fq19Nam1gIVrrftCh3GIAuOEAYYASABEgLs4_D_BwE&gclsrc=aw.ds (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/ (https://www.hafele.co.uk/en/product/isolating-transformer-20-va-240-v-50-hz-input/000000f800004d5e00010023/)
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.
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.
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.Perhaps not in Australia, but isolation transformers are very common in the UK. Almost every bathroom has a small one powering the shaver socket.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!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.
https://www.victorianplumbing.co.uk/white-dual-voltage-shaver-socket-shas?campaign=googlebase&gclid=EAIaIQobChMI8fq19Nam1gIVrrftCh3GIAuOEAYYASABEgLs4_D_BwE&gclsrc=aw.ds (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/ (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.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.
Because an isolation transformer works on the same principle, irrespective or where it is installed, be it the bathroom or a technician's 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.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 just took issue with your statement that RCDs are easier, for the beginner to understand, than isolation transformers (https://www.eevblog.com/forum/beginners/rcd-or-isolation-transformer-what-is-best-to-use-for-safety/msg1302012/#msg1302012). 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.
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.I don't see how it's that complex.
Experienced technicians still have to think about that - and that's certainly not something I would expect from a beginner.
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.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.I don't see how it's that complex.
Experienced technicians still have to think about that - and that's certainly not something I would expect from a beginner.
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.
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.
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.
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.
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?
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.
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.
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.Both of your suggestions are well intended but I don't like either of them.
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.