RCD or Isolation Transformer?? What is best to use for safety - Page 1
Forum section: electronics / beginners

author: jastreb
author_rank: Contributor
timestamp: September 13, 2017, 06:07:16 am
content: HI everyone,I hope you can help me to understand where I should use each one. So I am a hobbyist and I would like to start playing with A?V receivers and TVs etc and pull apart and try and fix myself. I currently have a plasma TV and a denon amp I would like to start with.So these being potentially lethal, I have purchased one of those portable RCD devices such as thishttp://www.gasweld.com.au/toolex-safety-box-port-10amp-clipsal-4-outlet-583015?fee=23&fep=161616&utm_source=Toolex+Shopping+Feed&gclid=Cj0KCQjwi97NBRD1ARIsAPXVWWAvXagJ4KB__35Asggrsj4r7VPM2BNaG2dk-nlI7wahAUaibptlG5AaAoqUEALw_wcBBut I have been reading some more and what seems to be suggested is an isolation transformer. So what is better from a safety perspective. My understanding is that an RCD will not work in serial with the isolation transformer.Thanks for advance and please excuse my question if it seems obvious.

author: krystian
author_rank: Contributor
timestamp: September 13, 2017, 07:16:02 am
content: If you use isolation transformer it can still shock you and have enough current capability to kill you. The difference is, you have to touch two points in the circuit you are working on and they have to have different potentials. And if you do this, RCD won't help because voltage is not referenced to the ground. In my lab I would use RCDs with some small engaging current (3mA). But it is still only additional protection which may not help.I wouldn't recommend working on devices connected to line voltage without training.Working with line voltage is never safe.

author: Brumby
author_rank: Unknown
timestamp: September 13, 2017, 07:53:19 am
content: Anything that has mains power running into it can easily be lethal, so it is wise to be cautious. But, don't forget that even some battery operated equipment can have dangers as well.Having said that, the first step in safety is to know what you're sticking your nose into. Have someone show you or supervise you, if at all possible. Even if you don't have a circuit diagram, at least have a rough idea of how the power supply side of things works. If there is a nice heavy power transformer involved with separate input and output windings, then that offers some protection. If it's a SMPS, then get to know what to look for and what to keep away from. With live chassis - be VERY careful. In fact, I would recommend steering clear of these until you were much better versed.I would always recommend an RCD on any mains coming into the workshop - whether it's a main one in the meter box or a separate one like you have purchased. A separate one that is a little more sensitive than the one in the meter box would be preferable - so that any "oops" in the workshop trips your local one, before the household one (hopefully).An isolation transformer always sounds like a good idea - but you have to be particularly careful in how you use it. If you are not careful, you can actually end up with situations where the risk is greater.Using an RCD with an isolation transformer is not necessarily going to give you a safer environment. Putting the RCD before the isolation transformer is not going to give you any protection from the secondary, since there will be no fault path that it will be able to detect. Putting the RCD between the isolation and the device under test is not going to work, unless you establish a fault path avenue that will allow the RCD to function. The simplest way is to earth reference the secondary - which immediately removes the benefit from having an isolation transformer.If you have an isolation transformer, then it is probably safer to use it as a door stop ... and if you have a situation where you think it might be useful ... present it here before you try, so that we can give you some feedback.The one-hand rule is also a good one to learn.... When poking around mains and high voltage circuits, keep one hand in your pocket. While this may sound silly - it is not - and is based on a very significant risk if it is not followed: If you have two hands on a device with dangerous voltages around and one touches the live point and the other touches ground, then the fault current will pass through one arm, across the chest and down the other arm. It's that across the chest bit that is the real worry ... because your heart is right in the way......Care is essential, but knowledge is important in staying safe.

author: Assafl
author_rank: Frequent Contributor
timestamp: September 13, 2017, 08:36:06 am
content: David Aurora is a mushroom (fungi / mycology) expert who wrote one (if not the) best book on North American Mushroom and mushroom foraging. In addressing safety he set down rules (and I paraphrase from memory) that one should never forage until one is able to identify the lethal mushrooms (mainly Amanita Phalloides - the "death cap" and the "destroying angels" like Amanita Virosa). It is far more important to NOT PICK a bad Amanita than overlook a tasty one.Same is here. It is far more important to know what constitutes danger and what doesn't. And it isn't always a matter of voltage or current.A few examples:1. Main powered. Always dangerous. Understanding grounding and understanding if and where the mains ends is critical. In many power supply circuits the PCB is clearly marked. But in some it isn't. Personally I don't trust the markings. If I am not sure I measure. As an example, the mains side of a switch mode power supply may have a small 16V electrolytic. Isn't 16V safe? Nope. If it is used to stabilize voltage to the SMPS control chip it is 16V but it may float on 220V mains.Similarly, you may read that Magnetrons use 3V heaters. But the 3V is at a -2000VDC from ground since they use the heater pins as the cathode. (That is why the two heater pins use a big white insulator - it is a dead giveaway that something ain't kosher here).2. Capacitors. They can pack a punch. Obviously tiny pF ones meh. But the big electrolytic ones? They may have enough to make a high gauge wire disappear like a fuse. With a pop and exploding gasses. And that assumes the voltage on them is low. In a very high power Amplifier, the difference between the positive and negative rails may exceed 100V. That is dangerous. A particularly vile one is the ugly cap in a Microwave oven 2000v (or more) at 1.8uF. Calculate the coulombs and joules and compare to a defibrillator. Not quite there but not too far. Just one of the reasons microwave ovens are one of the most dangerous devices out there. Also the small value high voltage caps in SMPS. Nasty little things (if charged).Capacitors can also explode. I've had an exceptionally high quality electrolytic from Cornell Dubilier ooze dreck for days on my desk (it would bubble and foam as if angry). Tantalums capacitors can blast their heads off (especially if reverse biased) - and stink. If it hits you in the eye.... So how much voltage does your Ohmmeter give out when you reverse bias the Tantalum (in reverse - by mistake - of course)?3. Voltage doublers, sparking circuits, fluorescent supplies, etc. - usually will give a nasty kick. But if they get a capacitance charged - it may be more than nasty. Learn how to identify flybacks (rare today), doubler cap-diode chains, small isolated transformers, HV nipples, weird flexible HV cables, etc.4. Mixed isolation circuits. The nastiest I probably ever worked on was repairing my Brother In Laws treadmill. It did have an isolation transformer, but it also rectified the 220V AC into DC and used a capacitor to smooth the motor/generator voltage. It was very unclear what was mains and what was isolated on the PCB. I used an isolated channel oscilloscope (a Fluke 196 I keep handy for these types of circuits) and a handheld DMM to measure. It ended up being a faulty emergency break SCR that shorted. Washing machines, dryers, cheap espresso machines, etc all fall into the category of "partially isolated".5. SMPS, VFD and the like. Just don't until you have sufficient experience. The reason is that they are nasty to probe and can pack high energy. Not just shock hazard, but also venting capacitors and other flying crap. In any case I like to remove, figure out the controller, the topology, where the isolation line is - and only then probe. I do my homework: I rarely have to probe more than 2 points to verify what the problem is - and that homework leads to safety.Learn to identify the components, circuit topology and the safe way to probe - and only then start and work very slowly and thoughtfully. Mindfulness is key.Oh - and RCD and Isolation and Variacs - Nice tools. But if they ever trip or work - you've failed. I don't mind having one (and I do) - but they never tripped. If they do - thank them for your life and reconsider if this is a hobby for you.

author: technogeeky
author_rank: Frequent Contributor
timestamp: September 13, 2017, 03:34:26 pm
content: It's worth noting that an isolation transformer is really there to protectequipment, 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.

author: Ian.M
author_rank: Super Contributor
timestamp: September 13, 2017, 04:04:22 pm
content: Quote from: technogeeky on September 13, 2017, 03:34:26 pmYes. 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.If you want to retain the increased safety of a fully floating D.U.T. when using an isolation transformer, *ALL* your other test and measurement connections must be isolated or fully floating and double insulated so it doesn't introduce a ground to the D.U.T. If you mistakenly use another isolating transformer to float your normally grounded test equipment you've just increased the area and scale of risk considerably, as in that scenario, the grub screw in a knob you are adjusting on the test equipment can now kill you - one slip with the hand you are probing with while the other hand is on the knob will do it.Therefore the isolation must be between the D.U.T. and the test equipment - an isolated differential probe for the scope, and a signal isolating transformer on any signal source outputs (or if its a HF source, high voltage coupling capacitors in both signal and shield wires), USB isolators on PC connected debuggers, or logic analyser pods etc.Its also worth noting that its extremely unwise to just clip the probe ground to *ANY* node. If you clip it to a fast switching high voltage node (e.g. the collector or drain of a chopper transistor in a SMPSU, a CRT line output stage or similar), there is likely to be enough interwinding capacitance in the isolating transformer and capacitance to ground to cause problems ranging from damage to or destruction of the D.U.T. up to damage to test equipment and possible failure of the isolating transformer due to insulation breakdown.

author: rstofer
author_rank: Super Contributor
timestamp: September 13, 2017, 04:45:29 pm
content: Quote from: technogeeky on September 13, 2017, 03:34:26 pmBut 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!

author: technogeeky
author_rank: Frequent Contributor
timestamp: September 13, 2017, 04:59:42 pm
content: Quote from: rstofer on September 13, 2017, 04:45:29 pmI specifically mentioned this in the other thread.I suppose the real rule is: there is no case when you need to connect more than one ground clip except for shielding purposes, and in that case, all ground clips must be connected to the same potential.I assume it's possible to get small ground loops if you are probing 4 signals that are far apart on a PCB, but only one probe is grounded.edit:One case where the MATH A-B function isn't very good (or, put another way, where an analog oscilloscope is better than some digital scopes) for this is on the e.g. Rigol 1054z. Unfortunately every time I try this technique with mains AC I get a very choppy pattern which is very laggy. Perhaps I'm doing it wrong.

author: Zero999
author_rank: Super Contributor
timestamp: September 13, 2017, 10:01:00 pm
content: Both isolation transformers and RCDs have their place. I wouldn't agree with the statement about banning isolation transformers. The main advantages they have over an RCD, is that one has to touch the circuit in two places to get a shock and they're passive components, so there's less to fail. Yes it is true that connecting a piece of test equipment's earth to the secondary of an isolation transformer, defeats the safety benefits.One thing that can be done with isolation transformers, is to add a circuit to sense when the secondary is earthed and light a neon, that way the user is aware that isolation transformer is no longer protecting them. The circuit can be as simple as connecting one end of a neon lamp to both sides of the secondary, via some high voltage (>4kV impulse rated) 470k resistors and the other side to earth.

author: Brumby
author_rank: Unknown
timestamp: September 14, 2017, 04:33:34 am
content: My main concern about an isolation transformer is that it can give you a false sense of safety.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.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.I've been playing around with electronics since I was 8. Didn't get into mains until 10 years later ... and I still don't like isolation transformers because of the extra care required. I cannot endorse them for a beginner in any way, shape or form.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 14, 2017, 07:42:07 am
content: If messing with the Mains side of equipment power supplies is the most dangerous thing you do, you will live a long uneventful life.Every time you become a road user of any kind, you are subject to a much greater accident risk.Even something like cleaning your gutters is more dangerous, to say nothing of using any kind of power tool.Tube/valve equipment , certainly, in Oz, normally had a power supply consisting of a transformer, the secondary of which, provided heater voltage, &, most importantly, a high ac voltage which was then rectified to give the tube's required dc HT voltage.These secondaries were normally used with a two diode full wave rectifier, requiring a centre tapped secondary which provided, commonly, "285 volts a side", or for older equipment, "385 volts a side" for radios, & around "350 volts a side" for earlier TVs.Obviously, the complete secondary would be twice that, giving 570volts, 770volts, & 700volts respectively for the above examples.And they weren't little "pipsqueak" transformers, either, ----many were around the size of many isolation transformers.There were also quite unpleasant dc voltages available.People worked on this stuff with perfect peace of mind, as they knew " where not to stick their fingers".Beginners commonly made Mains operated stuff without mishap, as they learnt "first up" that they were working with equipment which contained nasty voltages.The traditional "ac/dc" design, such as the "All American Five", were hated, & dubbed "death radios" by Australian EEs & Techs, as they really do have the chassis connected to one side of the Mains .Active or Neutral,? How lucky do you feel?Modern equipment with SMPS is a totally different thing.The "guts" that performs the desired function is isolated from the Mains, just as with a transformer/rectifier linear supply.Unless there is a SMPS fault you should not be involved with any circuitry which has its "common" rail connected to the Mains., so can use your 'scope normally.I used an isolation transformer extensively when repairing the power supplies of Television Picture Monitors & Receivers, but if there was no power supply fault they were unnecessary.If you are going to get yourself so tangled up that you somehow zap yourself using an isolation transformer, I suggest you need to take a break.,

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2017, 09:09:01 am
content: Quote from: Brumby on September 14, 2017, 04:33:34 amThe same can be said about an RCD, which could also fail to activate, especially if you don't test it every time.QuoteAs I mentioned previously, some kind of indicator to warn the user that one side of the secondary is earthed would help a lot.I can see the potential dangers. The key is understanding the pros and cons of isolation transformers.By the way, isolation transformers are mandated in the UK for electric razor sockets, where they provide effective protection against electrical shock. I believe they were chosen because, they don't have contacts which can weld together like RCDs do, which makes them more reliable.

author: Vtile
author_rank: Super Contributor
timestamp: September 14, 2017, 10:53:23 am
content: 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!!!).RCDs are truely a good ones in normal use, but in repair style work their protective effect is questionable.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.

author: oldway
author_rank: Super Contributor
timestamp: September 14, 2017, 12:21:20 pm
content: As for security, there is a lot to say ... I unfortunately do not have time to develop the subject now.But the most important thing is that you have to know the dangers of electricity, understand the schematics to know what is dangerous and what is not and above all .... be very careful, never be distracted, not responding to his GSM while we repair or other thoughts or activity.The advantage of using an isolation transformer is that it is possible to choose the circuit that is earthed ... for example the negative rail on the primary side of a switched-mode power supply.If an isolating transformer is used, the circuit must be earthed. The advantage is that you can choose what is earthed.

author: Brumby
author_rank: Unknown
timestamp: September 14, 2017, 01:32:09 pm
content: Please understand - my responses are made with one very significant factor in mind....... 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.

author: madires
author_rank: Super Contributor
timestamp: September 14, 2017, 01:46:06 pm
content: Isolation transformer with built in variac, voltmeter and ammeter plus differential probes for your scope. A RCD is great for your work bench but not for repairing mains powered stuff. Relying on a single layer of safety is a bad idea.

author: oldway
author_rank: Super Contributor
timestamp: September 14, 2017, 02:01:06 pm
content: Quote from: Brumby on September 14, 2017, 01:32:09 pmIf 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.

author: Brumby
author_rank: Unknown
timestamp: September 14, 2017, 02:09:11 pm
content: I would certainly endorse that.Going into mains gear without a fundamental understanding and some experience is a bad idea.Adding an isolation transformer into the mix is complicating an already dangerous scenario.

author: Vtile
author_rank: Super Contributor
timestamp: September 14, 2017, 02:17:01 pm
content: For me the red-flag is always the black and white trashing of one method, while glorifying another. Indeed something like differential probe is easy way to get one piece sorted on the safety puzzle, but it only solves the ground referenced measuring instrument exposed conducting body issue (ie. in most scopes). It doesn't solve any other hazards.The trashing will eventually lead to situation where the parroting leads to self-regulated obsolescence of valid solution for certain safety issues.Oldways answer is good.Quote from: oldway on September 14, 2017, 02:01:06 pm

author: rstofer
author_rank: Super Contributor
timestamp: September 14, 2017, 04:53:34 pm
content: Quote from: Vtile on September 14, 2017, 10:53:23 amI 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.QuotePerhaps a bit hasty but it's not due to a lack of understanding the concept. It's the idea that these are discussed by professionals in an area of the forum related to beginners. It's true, we were all beginners once. But I would be willing to bet that every single one of us has gotten 'bit' a time or two. Come on, fess up!Surely you remember the hot chassis radios. They were great until you lost a plastic knob and you tried to set the volume in your bare feet on a concrete floor. This predates polarized plugs and receptacles. Ouch! That stuff is probably still around and it probably still doesn't have a polarized plug to match the modern receptacle.This was not the only time I got 'bit' but it was a learning experience. Note to self: Wear shoes! And get a knob!I would rather spend the time to come up with a plan to troubleshoot that didn't require isolation but I don't play in the part of the sand box where such a thing would come up. Clearly, others do!

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2017, 07:06:04 pm
content: Quote from: Brumby on September 14, 2017, 01:32:09 pmThe 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.Quote from: oldway on September 14, 2017, 12:21:20 pmThat'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.http://electrical.theiet.org/wiring-matters/52/shock-protection/index.cfmThe greatest issue I can is, is if a non-isolated, high voltage (real HV, above 1kV or so) step up converter, is connected to the secondary of an isolation transformer, there's a risk of insulation failure, if one side isn't earthed. The answer of course is not to do this. If one delves further into the safety standards, they'll discover this is also prohibited. Fortunately someone working with isolated switched mode power supplies won't do this anyway. The main hazards are things such as old valve powered equipment, neon sign transformers and microwave ovens.

author: oldway
author_rank: Super Contributor
timestamp: September 14, 2017, 09:02:33 pm
content: 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 andno 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.On the other hand, if you put yourself part of the circuit you choose to earth, you know what is under high potential and what is not.This allows safe measurements to be made on the primary circuits of the SMPS.Of course, there are voltages up to 400V related to earth, but you know this and you measure it with high voltage X100 probes...NEVER USE AN ISOLATION TRANSFORMER WITHOUT EARTHING THE SECONDARY CIRCUIT, THAT'S EXTREMELY DANGEROUS !!!!!

author: Brumby
author_rank: Unknown
timestamp: September 15, 2017, 12:18:53 am
content: Quote from: rstofer on September 14, 2017, 04:53:34 pmWhatVtilesays is true. RCDs and GFCIs operate on the same principle and calling them the same thing is fine by me.You have taken the reference to the "ground wire" too literally - which is why I tend to refer to the "fault current path". This can pass through the ground wire of a device, if you happen to bridge the live conductor and an earthed chassis - or not, if you happen to contact the live conductor and some earthed plumbing.The point is, if the current passing through an RCD or GFCI is balanced, theywillnottrip. If the current goes up your left arm and down your right - you could easily end up dead.

author: Brumby
author_rank: Unknown
timestamp: September 15, 2017, 12:27:24 am
content: Quote from: Hero999 on September 14, 2017, 07:06:04 pmI 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 isnota common sight ... even in EE circles!QuoteThat goes for any environment - whether RCD, isolation transformer or straight off the grid.

author: jastreb
author_rank: Contributor
timestamp: September 15, 2017, 05:36:24 am
content: QuoteI been reading up a lot about safety while working with mains. In regards to RCD my current understanding is that is is not just 30mA vs 15mA but also the time it takes it to detect that leakage. So while you may find that Australian RCDs may trip at 30mA they are faster at responding. I saw a nice graph somewhere which showed mA vs time and risks to human body at different levels.

author: jastreb
author_rank: Contributor
timestamp: September 15, 2017, 05:56:52 am
content: 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.

author: Assafl
author_rank: Frequent Contributor
timestamp: September 15, 2017, 06:13:09 am
content: 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).

author: Assafl
author_rank: Frequent Contributor
timestamp: September 15, 2017, 06:27:18 am
content: Quote from: jastreb on September 15, 2017, 05:56:52 amWhen 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).

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: September 15, 2017, 07:18:39 am
content: 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 thatearths 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 acaplessscope 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.

author: Brumby
author_rank: Unknown
timestamp: September 15, 2017, 07:41:23 am
content: Quote from: jastreb on September 15, 2017, 05:56:52 amYour 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.

author: Zero999
author_rank: Super Contributor
timestamp: September 15, 2017, 08:01:07 am
content: Quote from: oldway on September 14, 2017, 09:02:33 pmThat'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_deviceQuoteWell 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.QuoteNonsense. 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_.pdfAn 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.Quotehttp://electrical.theiet.org/wiring-matters/52/shock-protection/index.cfmQuote from: Brumby on September 15, 2017, 12:27:24 amPerhaps 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.dshttps://www.hafele.co.uk/en/product/isolating-transformer-20-va-240-v-50-hz-input/000000f800004d5e00010023/

author: Assafl
author_rank: Frequent Contributor
timestamp: September 15, 2017, 08:36:18 am
content: Quote from: IanMacdonald on September 15, 2017, 07:18:39 amLol. 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.

author: oldway
author_rank: Super Contributor
timestamp: September 15, 2017, 09:15:05 am
content: 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.

author: oldway
author_rank: Super Contributor
timestamp: September 15, 2017, 10:11:05 am
content: Of course, there are voltages up to 400V related to earth, but you know this and you measure it with high voltage X100 probes...QuoteI 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.

author: Zero999
author_rank: Super Contributor
timestamp: September 15, 2017, 10:51:43 am
content: Quote from: oldway on September 15, 2017, 09:15:05 amThe 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.QuoteIndeed 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.Quote from: oldway on September 15, 2017, 10:11:05 amYes, 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.

author: oldway
author_rank: Super Contributor
timestamp: September 15, 2017, 11:47:45 am
content: Quote from: Hero999 on September 15, 2017, 10:51:43 am

author: Zero999
author_rank: Super Contributor
timestamp: September 15, 2017, 12:29:30 pm
content: Quote from: oldway on September 15, 2017, 11:47:45 amIf 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.QuoteYes, same with an isolation transformer.

author: oldway
author_rank: Super Contributor
timestamp: September 15, 2017, 12:46:34 pm
content: 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.

author: madires
author_rank: Super Contributor
timestamp: September 15, 2017, 12:48:26 pm
content: 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.

author: madires
author_rank: Super Contributor
timestamp: September 15, 2017, 12:55:21 pm
content: Quote from: Hero999 on September 15, 2017, 12:29:30 pmThe important point is to not rely on a single layer of safety. Add a second or even third layer.

author: Assafl
author_rank: Frequent Contributor
timestamp: September 15, 2017, 01:05:49 pm
content: 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?

author: rstofer
author_rank: Super Contributor
timestamp: September 15, 2017, 01:12:46 pm
content: Quote from: jastreb on September 15, 2017, 05:56:52 amYou 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!

author: oldway
author_rank: Super Contributor
timestamp: September 15, 2017, 01:21:31 pm
content: 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.

author: rstofer
author_rank: Super Contributor
timestamp: September 15, 2017, 01:21:54 pm
content: Quote from: Assafl on September 15, 2017, 01:05:49 pmI 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...

author: Assafl
author_rank: Frequent Contributor
timestamp: September 15, 2017, 01:27:48 pm
content: 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.

author: madires
author_rank: Super Contributor
timestamp: September 15, 2017, 02:06:44 pm
content: Quote from: Assafl on September 15, 2017, 01:05:49 pmNo, 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.

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2017, 12:39:55 pm
content: Quote from: IanMacdonald on September 15, 2017, 07:18:39 amI'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

author: Shock
author_rank: Super Contributor
timestamp: September 16, 2017, 05:02:00 pm
content: 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.

author: Assafl
author_rank: Frequent Contributor
timestamp: September 16, 2017, 05:25:49 pm
content: Quote from: Shock on September 16, 2017, 05:02:00 pmFinally 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).

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2017, 06:10:20 pm
content: Quote from: oldway on September 15, 2017, 11:47:45 amIsn'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.

author: Shock
author_rank: Super Contributor
timestamp: September 17, 2017, 05:11:19 am
content: Quote from: Assafl on September 16, 2017, 05:25:49 pmThey 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.QuoteI 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.

author: Shock
author_rank: Super Contributor
timestamp: September 17, 2017, 05:35:58 am
content: Quote from: Hero999 on September 16, 2017, 06:10:20 pmI 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.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: September 17, 2017, 05:50:15 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: September 17, 2017, 06:57:00 am
content: Quote from: Hero999 on September 15, 2017, 08:01:07 amHow on earth is an "invisible" permanent installation like that evenremotelyapplicable 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.

author: oldway
author_rank: Super Contributor
timestamp: September 17, 2017, 07:03:37 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: September 17, 2017, 07:24:58 am
content: 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.

author: madires
author_rank: Super Contributor
timestamp: September 17, 2017, 10:34:42 am
content: Quote from: Brumby on September 17, 2017, 07:24:58 amYup! 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.

author: Brumby
author_rank: Unknown
timestamp: September 17, 2017, 10:49:44 am
content: Exactly.

author: Shock
author_rank: Super Contributor
timestamp: September 17, 2017, 11:46:00 am
content: Quote from: IanMacdonald on September 17, 2017, 05:50:15 amThe 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.

author: Vtile
author_rank: Super Contributor
timestamp: September 17, 2017, 01:14:46 pm
content: 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 )..

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2017, 01:33:14 pm
content: Quote from: Brumby on September 17, 2017, 06:57:00 amBecause 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.QuoteThe same is true for an RCD. It won't protect anyone from a shock between live and neutral.QuoteIn 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.

author: Brumby
author_rank: Unknown
timestamp: September 18, 2017, 11:14:10 am
content: Quote from: Hero999 on September 17, 2017, 01:33:14 pmYou are completely missing the point. Where it is installed and how it is configured is exactly the issue. It is not the principle - its theimplementation.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.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2017, 01:18:11 pm
content: Quote from: Brumby on September 18, 2017, 11:14:10 amI 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 thatRCDs 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.

author: Brumby
author_rank: Unknown
timestamp: September 18, 2017, 02:07:03 pm
content: Quote from: Hero999 on September 18, 2017, 01:18:11 pmTheoretically - 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 -unlessthe 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.

author: Shock
author_rank: Super Contributor
timestamp: September 18, 2017, 03:58:56 pm
content: Quote from: Brumby on September 18, 2017, 02:07:03 pmOr 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.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2017, 06:48:19 pm
content: Quote from: Brumby on September 18, 2017, 02:07:03 pmI don't see how it's that complex.Situation 1Isolation 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 2Clipping 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.

author: Brumby
author_rank: Unknown
timestamp: September 19, 2017, 02:32:01 am
content: 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.Quote from: Shock on September 18, 2017, 03:58:56 pmSeriously? 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.QuoteAbsolutely. I could not agree more.Quote from: Hero999 on September 18, 2017, 06:48:19 pmTo 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...QuoteLook at thisthrough 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.

author: Shock
author_rank: Super Contributor
timestamp: September 20, 2017, 06:28:04 am
content: Quote from: Brumby on September 19, 2017, 02:32:01 amThere 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.QuoteI 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.QuoteIf 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.

author: Brumby
author_rank: Unknown
timestamp: September 20, 2017, 07:35:55 am
content: Quote from: Shock on September 20, 2017, 06:28:04 amOf 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:QuoteI am not, never have and never will discourage beginners from learning -especiallywhen it comes to safety.My concern relates to the attempt to try and do this via a text-based forum which can be viewed byanyone. 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 whodoparticipate 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 properlyunderstandthe 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 saidand 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.

author: oldway
author_rank: Super Contributor
timestamp: September 20, 2017, 08:36:56 am
content: 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_classesTherefore, 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.

author: oldway
author_rank: Super Contributor
timestamp: September 20, 2017, 09:49:12 am
content: 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.

author: Shock
author_rank: Super Contributor
timestamp: September 20, 2017, 01:54:10 pm
content: 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 bumpedyour postOldway.

author: Cerebus
author_rank: Super Contributor
timestamp: September 20, 2017, 06:18:33 pm
content: Quote from: oldway on September 20, 2017, 09:49:12 amIf you're expecting this, then I presume you're expecting topaythem 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 forpaidprofessional 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.

author: oldway
author_rank: Super Contributor
timestamp: September 20, 2017, 06:58:00 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2017, 10:19:14 pm
content: Quote from: oldway on September 20, 2017, 06:58:00 pmBoth 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.

author: Brumby
author_rank: Unknown
timestamp: September 21, 2017, 01:52:15 am
content: I have to agree withHero999.The exercise of vetting such "authoritative" members alone is fraught with dangers on all sides.Who is competent (in the legal sense) to assess this? Aside from the various classes of licence, this is an international forum.Even presuming you could find someone suitable for that job, as soon as there is any sense of formal competence being offered - you are taking on the liability that comes with it. That, in itself, is a scary proposition, but this (a forum) is one of, if notthe, worst platforms to execute.Supposing you got this far ... you still have the fundamental problem of communication via a forum environment. This is something we already have big problems with - and a significant part of that is the unknown which is the person seeking advice. There is so much in this can of worms that it is impossible to even scratch the surface in a single post like this - but you could have someone who can safely measure a 10KV high energy power bus - or someone who could blow up a "D" cell. As it is, we have run around in circles trying to cover safety and functionality aspects ... this thread is a classic.The next step is the obvious one - actually communicating the necessary information. I won't go into details - you know how problematic this could be.The last point I will make is, I would argue, THE most critical consideration. When the advice has been given and the questioner embarks on the journey to act on that advice, they are going to attempt to perform a task without any physical monitoring of their work. A tradesperson with a licence, has gone through specific training that encompasses the whole field. They have had to pass tests including practical tasks.I'm not saying that we need to do that here - but I AM saying that those safeguards just don't exist through a forum. You can never say withabsolute certaintythat someone has followed the instruction correctly. Even photographs aren't adequate. You can't beat the mentor being in the same room as the student.I also agree on the ban being a bad idea.As messy as this thread (for example) may have become, it does send a message or two which includes functionality, safety and the diversity of members.

author: jastreb
author_rank: Contributor
timestamp: September 28, 2017, 05:17:17 am
content: Thank you all for your vigorous and passionate discussion. There is obviously a difference in opinion but none the less I have learned a lot. Nothing is full proof and what you use can be right in one situation but wrong in another. And I am still here alive and well so not planning on suing anybody. And I never would because I read somebodies opinion here. The risk and responsibility is with me and as I am a father of two young kids I will do my utmost best to stay safe so I can watch them grow and one diy perhaps ask the same questions as I do. (provided the computer games don't make everything else seem too boring and hard)Cheers everyone

author: Brumby
author_rank: Unknown
timestamp: September 28, 2017, 06:19:09 am
content: The differences in opinion are more about what is appropriate to mention in a "Beginner" board. We just don't know who is reading the discussion nor where their skill level lies.RCDs have their place - and so do isolation transformers. So long as the message gets across that neither are a magic solution, we can only hope that somebody understands this and applies some basic safety thinking - and asks for further advice, if needed.We all have our own experiences, preferences and stories to tell and, just like anywhere else on the internet, discussions that involve these can get a bit intense sometimes.Your response is the sort of thing that gives us some assurance that we have helped you - and not scared you off. You have survived.Welcome to the forum.

author: Ian.M
author_rank: Super Contributor
timestamp: September 28, 2017, 06:32:44 am
content: One thing that hasn't come up in this thread is a safe workbench and surroundings that minimises the risk of you grounding yourself (except for ESD prevention with >1Meg to ground). It wont keep you safe on its own but it is another layer of protection.If you use a metal desk or a bench with an metal frame where you can inadvertently come into contact with it and/or your flooring may have a high conductivity to ground (e.g. a concrete slab in a basement), you are at much higher risk of a serious shock, than if your bench is non-conductive (apart from ESD matting etc.), and the floor near the bench is insulated, e.g. with a rubber mat. Don't trust wooden floors or carpet as you don't know how close you are to grounded plumbing and dirt + a little moisture can make them conductive with very little warning.The same goes for any other grounded objects in your immediate vicinity - be particularly careful when working near older test gear with grounded metal cases.Stay safe.