Isolation transformer ground and safety questions - Page 1
Forum section: electronics / beginners

author: ez24
author_rank: Super Contributor
timestamp: May 13, 2015, 06:01:01 pm
content: HiHere is a transformer with a ground plug on both ends (I assume)http://www.ebay.com/itm/STANCOR-1000-VA-G0904-B-ELECTRIC-ISOLATION-TRANSFORMER-MODEL-GIS-1000-/381254152581?pt=LH_DefaultDomain_0&hash=item58c484b185My house does not have grounds ie I only have 2 prong wiringMy questions are : is the ground also on this also isolated? Since I only have 2 prong would I be getting thesafety benefit of this by having to use 3 to 2 prong adapters and NOT using either ground plugs on the transformer?Edit: I do not have one of these yet, so I cannot use a multimeter. If I had to guess, I think both ground wires are connected to each other via the frame of the transformer. But since I do not have one yet, I cannot be sure.thanks

author: Monkeh
author_rank: Super Contributor
timestamp: May 13, 2015, 08:26:31 pm
content: Ask yourself how it could be ground if it were isolated.

author: Zero999
author_rank: Super Contributor
timestamp: May 14, 2015, 07:52:15 am
content: That transformer has a metal case so should be earthed.You could get a toroidal transformer and put it in a plastic case so it doesn't need to be earthed.

author: tron9000
author_rank: Frequent Contributor
timestamp: May 14, 2015, 10:09:33 am
content: Shocks right, with an isolated transformer only the live an neutral are isolated from the grid (as it were). But if you were to connect yourself between the secondary - you would get a belt! You would EXPECT the transformer to have a an chassis or core earth.The ones I use have an earth straight through to the secondary side. As the secondary is totally isolated from the mains, accidentally shorting or touching live or neutral, whilst touching the earth, should not result in nasty consequences.Even though you don't have an earth in your house, the incoming mains from the outside will - or should do! Any metal surface like the kitchen sink or copper piping should have earth wires connect to it, they do here in Blighty by law. Not sure about the laws where you are, you will have to check.If in doubt about it but need it, get it isolation and earth bond tested (PAT) by a local electrician - or if in total doubt (I see the description is quiet limited) save your money and spare yourself some possible agro.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: May 14, 2015, 11:04:00 am
content: This subject is a can of worms! The trouble is, the term "isolation transformer" means different things to different people. And, there is a massive amount of misunderstanding out there!I will assume that we are talking about an electronics workshop:It is a mistake to regard an isolation transformer as a safety device. OK, you can touch either side of the secondary and not get a shock - assuming that it is truly floating, and hasn't become "earth-referenced" by something plugged into it. But,the isolation transformer is simply to allow the connection of grounded test equipment (e.g. a 'scope) to a Device Under Test. Normally, this is only required for limited aspects of maintenance (or design, perhaps) - such as when you need to look at the primary side of a switched-mode power supply, for example.The rules are:1. Only one DUT connected to the transformer output.2. The earth connection should not be carried across (though most commercially available units do, sadly).In the old days, TV sets and radios had a live chassis, so isolation transformers were an essential part of a maintenance workshop. Those days have largely gone, so unless you're working on vintage gear, isolation transformers are only occasionally useful (as mentioned above).It's easier, safer and cheaper to use an RCD most of the time.As mentioned, other uses of isolation transformers exist. For example, some use a transformer with a centre-tapped secondary, with the centre-tap grounded. This means that the magnitude of an electric shock will be reduced, as now the greatest potential between earth and a "live" conductor is now half the mains voltage. Of course, those yellow "site transformers" that you see on building sites work that way. But in an electronics workshop, an RCD is still easier, safer and cheaper here.Others violate Rule 1 by setting up an "earth-free environment". This is complex, and not worth the effort in most cases. I've never met such a setup directly, but have spoken to people who have - none of whom have managed to convince me of the merits.Another use of isolation transformers is to remove the leakage current of the devices supplied. I know of some OB vehicles that do that, for example. One side of the secondary is connected to earth, creating a live and neutral so that the RCDs on the vehicle can work, so on the face of it, why have the transformers? But tripping the RCD inside the venue is normally embarrassing!Finally, remember that generally, large transformers are a pain. They are incredibly heavy, they take valuable space, they generate heat, they are normally mechanically noisy, and they are something else to go wrong. So think carefully before getting one...Mine are here:http://www.markhennessy.co.uk/isolating/index.htm

author: tron9000
author_rank: Frequent Contributor
timestamp: May 14, 2015, 11:32:15 am
content: Nice project Mark.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: May 15, 2015, 08:44:33 pm
content: More tie wraps!maybe a GFCI or two???

author: madires
author_rank: Super Contributor
timestamp: May 15, 2015, 09:09:32 pm
content: Please watch:--

author: KenGaler
author_rank: Contributor
timestamp: May 16, 2015, 01:40:21 pm
content: Slightly off topic, I prefer the Hammond isolation transformers because they have a faraday shield. This prevents higher frequency noise from getting through in either direction.

author: ez24
author_rank: Super Contributor
timestamp: May 16, 2015, 06:22:06 pm
content: Thanks for Todd's video, I need to watch it several times to understand. He did answer my question about the ground (both sides grounded to the chassis). It was a Tripp Lite that I was bidding on when I started to think about the ground and I stop bidding.Funny when he started to talk about the ground plug on the extension cord, he seemed to get nervous or afraid to say anything about it. It seems to me that cutting the ground prong would do the same thing he did. I assume he was nervous because there must be some sort of "ground police" and they would arrest him, or YT would ban him for life. It might even be illegal for anyone to say so.OR maybe I can say - I only have a 2 wire extension cord. That way I do not have to mention the ground.Alright I am going to try and PM Todd and ask him if I can use a 2 wire extension cord.Also it was Dave's video that got me thinking in the first place. I have a power supply that does not work and I am afraid to probe it with my new Rigol. Also I have this memory in the back of my mind about 40 years ago a wire exploded on me. The more I think about it the more I think it was a scope ground lead, I just remember the shock of it and the details are blocked out of my memory. (and I had proper ground at that time). And it was the last time I even touched a scope.It seems that those that grew up with three wires cannot even understand or believe that a 2 wire system even exists let alone work.I am sorry that I have to talk about a "missing" ground, but my house does exist and I have been using the 2 wire system for 20 years and as far as I know no damage to anything. My kitchen was rewired and has proper electrical wiring so if I get a Tripp Lite then I can test it on a 3 and 2 wire systems. Actually I would like to do the same experiments Todd did to better understand it all.I will also watch every YT I can find in the hopes someone talks about a 2 wire system and did not get banned. I have cut the ground plug off of cords for years (I hope I do not get banned for saying this) when I could not find an adapter. If you have a two wire system without conduit - you do what you have to do.The only time I got shocked in my home was screwing in a screw that hit a 3 wire romax while my left hand was touching metal that the screw touched also. It was a bad shock (because I slumped into the metal and was sitting on wet ground) and this has left the fear of god in me. I have been shocked a few other times in my life and always on a 3 wire system (because 3 is more common). So I am trying to come up with a safe way for me and my scope to work on low voltage power supplies under power.thanks all

author: rs20
author_rank: Super Contributor
timestamp: May 16, 2015, 11:12:03 pm
content: Quote from: Shock on May 16, 2015, 11:02:07 pmI'm being super-pedantic here, but the RCD still protects (or at least activates) against touching live in the device with one hand and touching any other earthed device/water pipe, right? I'm just nitpicking one point, everything else you said is spot on.

author: madires
author_rank: Super Contributor
timestamp: May 17, 2015, 11:51:40 am
content: In most power distributions N (neutral) is earthed at the transformer station or pole. If your house got a two wire wiring, i.e. L and N, N has the potential of PE (earth) plus the voltage drop caused by the current through the N wire. In a 3 wire setup, i.e. L, N and PE, it's the same but PE provides a low impedance path to earth, since it doesn't pass any significant current, just a few mA from line filters, like in large SMPSUs, and leaky devices. Because N is earthed, L is also referenced to earth.An isolation transformer for an electronics lab provides floating mains. Actually you don't have any mains N or L anymore, just a simple AC output. It's like a 12V transformer. If the metal enclosure of the isolation transformer is connected to PE or not doesn't matter for the floating secondary. With a 2 wire mains you don't have PE anyway. With a 3 wire mains it becomes interesting. In case of a failure in the primary side, the earthed enclosure would protect you from an electrical shock and the RCD would be triggered (PE is an ideal path to earth) or the breaker. But it would also increase the risk, that a problem (loose wire) with the floating secondary could "earth" the secondary. The same goes for the secondary's socket, when it got a contact for PE too. A problem with the DUT (device unter test) or a bad wiring could earth the floating secondary. And you don't want this to happen. The isolation transformer I got is double isolated and has no PE, neither at the primary nor at the secondary, though it got a metal enclosure.If you have to use your scope please get differential probes or use the poor man's way (ch1 + ch2 inverted, and ground clips removed).BTW, another good video:

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: May 17, 2015, 03:09:12 pm
content: Quote from: madires on May 17, 2015, 11:51:40 amOf course, as soon as you connect test gear to the DUT, then it is no longer floating. But that is OK because the DUT is earthed-referenced at the point you wish to make your measurements with respect to.But this is whyonly oneDUT can be connected to an isolation transformer.It's all too easy to assume the isolation transformer gives you more protection than it actually does (http://en.wikipedia.org/wiki/Risk_compensation).

author: queennikki1972
author_rank: Regular Contributor
timestamp: August 21, 2019, 01:05:39 am
content: Quote from: madires on May 17, 2015, 11:51:40 amSo if I understand this correctly...I have a house built in 1947 with no PE (Earth Ground).I am purchasing a Isolation Transformer that has no chassis ground on the secondary side and when I use it on a two wire plug, (which has GFCI), I can have one DUT on the transformer?As for the Oscilloscope, I know you should not float your scope, however I am also plugged into GFCI with no PE, which does not protect my equipment and I should use either differential probes or use the poor man's way (ch1 + ch2 inverted, and ground clips removed)?Did I get that right?

author: madires
author_rank: Super Contributor
timestamp: August 21, 2019, 02:43:34 pm
content: Quote from: queennikki1972 on August 21, 2019, 01:05:39 amYes, the DUT would be floating as long as you don't create a connection between the DUT and PE or anything conducting to earth or the soil around/below your house.Quote from: queennikki1972 on August 21, 2019, 01:05:39 amYep, it's the recommended way to reduce the risk of an electric shock.

author: macboy
author_rank: Super Contributor
timestamp: August 23, 2019, 02:07:13 pm
content: Quote from: rs20 on May 16, 2015, 11:12:03 pmYou are correct. An RCD (a.k.a. GFI or GFCI) does not depend on a connection to earth/ground at all. It trips based on a difference in current between the live and neutral conductors. Any difference is assumed to be some leakage to earth (perhaps through a human body) and this trips the device. Fault current to earth absolutely does not need to pass through the earth connection of the protected outlet/socket/circuit. It can take any path to earth, and in the case of a dangerous fault, that path is unlikely to be back through the circuit's earth conductor.

author: pcmad
author_rank: Regular Contributor
timestamp: September 03, 2019, 01:32:38 am
content: you can get ground from the water pipes in your house

author: helius
author_rank: Super Contributor
timestamp: September 03, 2019, 01:48:17 am
content: Quote from: pcmad on September 03, 2019, 01:32:38 amIt's more correct to say that youmaybe able to getsome kind ofgrounded node on the main water service pipe. But not all water pipes are made of metal; some metal pipes may have no electrical connection to ground; if there is an electrical connection to ground, it may be of high impedance and therefore not trip the circuit breaker in case of a fault; and if there is a fault to a high-impedance water pipe, and the circuit breaker is not tripped, you have created an additional touch hazard throughout the building!When a low-impedance Protective Earth is not required, it may be reasonable to tie to the water service pipe, but knowing that is the case is not a job for newbies.

author: themadhippy
author_rank: Super Contributor
timestamp: September 03, 2019, 02:05:04 am
content: In the uk its not permitted to use water pipes as an earth electrode unless its a private water supply not connected to the public supply.

author: helius
author_rank: Super Contributor
timestamp: September 03, 2019, 10:19:51 pm
content: As an example of an application that allows water pipe connections, the customer unit for FTTH (fiber to the home) in this area includes a ground clip for the water service pipe. The entire unit is Class II: it is powered by 48 VDC from an isolated supply, and is all plastic-encased. Since it is Class II, there is no Protective Earth required; the ground clip is for noise suppression only and because the device would otherwise float at an undefined level to the equipotential zone of the building.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 04, 2019, 07:18:13 pm
content: People have died because Earthing got interupted after metal water pipes have been replaced by plastic pipes.