Isolat. transformer: should the safety GND pin on the secondary side be grounded - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: February 10, 2025, 02:14:55 am
content: This seems like the most basic question, yet here I am, confused, unable to find a solid definitive answer.I'm wiring aHammond 169E isolation transformerto be used as a stepped variac isolation transformer for my test bench.Should the safety GND pin on the secondary side's 3-prong wall outlet be tied to the safety GND on the primary (and the chassis)??I read somewhere that it should be, for safety. But how is having the chassis of the isolated DUT grounded to earth any safer than not at all? If faulty wiring in the DUT causes high potential to touch the chassis, it won't trip the breaker (as far as I understand) since it can't round-trip through the safety GND to its source. And what about if the DUT has its own internal connection between neutral and GND (is that ever a thing)? Then connecting the safety grounds breaks the isolation.Bonus related question: If I want to add a switch so that the isolation transformer can be used in isolated and non-isolated mode, the switch needs to be placed between the 0V wire of the secondary and the neutral of the primary, right? Meaning when it's on the 0V of the secondary is referenced to ground, otherwise it floats. And then the switch should also break the connection between the primary and secondary safety GNDs, if they are not supposed to be connected in a proper isolation setup.Please enlighten meThanks!

author: indeterminate
author_rank: Regular Contributor
timestamp: February 10, 2025, 02:29:40 am
content: Readhttps://www.eevblog.com/forum/chat/do-you-guys-use-an-isolation-transformer-for-your-labworkbench/

author: cincin
author_rank: Regular Contributor
timestamp: February 10, 2025, 02:44:14 am
content: That's a long winded thread with the original question deleted.If you're pointing out that I should understand isolation transformers better before using one, then I agree. However, I'm not using mine to work on live chassis or anything high voltage, nor am I making any assumptions of safety when using one. I mostly use it with my oscilloscope even though I don't make non-GND-based measurements, the GND clip is always on ....... oh! there's the answer! duh,when I connect the oscilloscope probe GND clip to the DUT's chassis, I essentially tie the safety GND of the outlet on the secondary to the earth ground via the oscilloscope! (and that's even an important aspect of shielding it all)So I guess the safety GND pin of the secondary outlet might as well be connected to the primary safety GND at all times.Cheers!

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2025, 03:13:51 am
content: This (not-so) simple issue gets contentious. I use an iso-variac unit daily (BK 1655) and find it quite useful for a number of things. However, you do have to keep in mind exactly what your goals are in having an isolation transformer. Here's a few things to think about:1) Commercially available isolation transformers intended for service use (BK Precision, Sencore, etc) all will pass through the safety ground if their output sockets have the third prong. There are two-prong isolation transformers that don't have this issue. Medical and other-purpose isolation transformers vary as to whether the ground is passed through directly but in my experience most of them do.2) The original purpose of service isolation transformers was to mitigate the shock hazards in servicing equipment with a live chassis (one side of the line connected to the metal frame that the circuit was built on). These older types of equipment didn't have ground pins so the presentation of one in the output socket of an isolation transformer was irrelevant.3) Having your DUT chassis grounded and removing the ground reference from the line power (what isolation actually does) are two different things and are not directly related.4) If you leave the ground connected and a failure in your DUT connects one side of the line to ground, the grounded chassis will not be live but other parts of the circuit will float up to and sometimes beyond line potential. This results in a touch shock hazard at those points. Not connecting the ground pin mitigates this particular hazard--right up until you accidentally reintroduce a ground point by connecting an instrument (oscilloscope ground, soldering iron, etc) or accidentally push another device close to it. When I do want to interrupt the ground, I use a physical device (cheater plug) that interrupts ground, not having a switch in your transformer box. That way you don't lose track of whether you are grounded or not. I would do this for certain specific reasons, such as measuring the ground leakage current of a DUT.5) There is typically no reason to connect the isolated ouput "neutral" to the ground pin whether or not you have it connected to earth ground. However, some devices (like an APC Back-UPS) may complain and generate site wiring faults if the don't see the ground and neutral lines at the same potential.6) Keep in mind that if things go wrong in either #4 or #5, with the earth ground disconnected you can make errors or have faults in your DUT which will make the whole DUT, chassis and all, live to touch. That won't happen if you retain the earth ground.7) Even in configurations that do not fully mitigate shock hazards, isolation transformers still limit fault current and transients and that is a reasonable safety benefit, IMO.

author: fred001
author_rank: Newbie
timestamp: February 10, 2025, 03:19:20 am
content: Are you talking about the "isolation" transformers that have a power cord and outlet built in? It may be because of the safety certifications. The rules force the manufacturer to connect the grounds through the transformer, which basically defeats the isolation. The ground needs to be disconnected for full isolation. Rather than modify the isolation transformer or the scope power cord, I use a 2 wire cheater cord between the scope and the transformer

author: cincin
author_rank: Regular Contributor
timestamp: February 10, 2025, 03:24:12 am
content: Quote from: bdunham7 on February 10, 2025, 03:13:51 amThat's a very interesting aspect of it that I had not considered. Thanks!

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2025, 03:30:11 am
content: Quote from: cincin on February 10, 2025, 03:24:12 amI forgot to point out that one of the common arguments against using an isolation transformer is that this hazard still exists (either because you left the earth ground or reintroduced it with your scope) and an isolation transformer gives you a false sense of security. It also prevents a GFI device from tripping if you have one. However, a shock hazard from some points in the circuit is a lot different than the whole DUT being live.

author: cincin
author_rank: Regular Contributor
timestamp: February 10, 2025, 04:20:21 am
content: Quote from: bdunham7 on February 10, 2025, 03:30:11 amGood points.QuoteWould you mind quickly pointing out what type of failures could result in the entire isolated DUT being live to touch with the earth ground disconnected? I assume you mean with one hand, relative to earth ground, without any instruments making the ground connection. That seems counter-intuitive so I'm evidently missing something.I appreciate the help.

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2025, 04:48:41 am
content: Quote from: cincin on February 10, 2025, 04:20:21 amI wasn't excluding instruments making the ground connection, that's one common mistake that would cause this. Imagine you have a DUT where the issue is that it is occasionally tripping GFIs. The internal fault is a shorted component between the neutral wire and ground. You accidentally manage to ground some part of the DUT that has a large potential relative to the neutral, say by connecting an oscillsocope ground to the negative terminal of the primary capacitor on a simple SMPS. Your chassis is now energized.Your intuition is right--if you have no ground and you isolate the power, then no point in the DUT will be able to sustain any significant current to ground through you. It's when a ground sneaks back in there that you get burnt.Another scenario that could catch you is if you forget to unplug your DUT from the isolation transformer and you touch the wrong point with a grounded soldering iron. The bottom line is not to use the isolation transformer as a magic shield and to always carefully think through what you are doing.