Safely modify a medical isolation transformer - Page 1
Forum section: electronics / beginners

author: jcc
author_rank: Newbie
timestamp: June 29, 2020, 02:32:47 am
content: Greetings,First post. I would like to use a medical isolation transformer to do bench work. I have a Tripp-Lite IS500HG. Modified it. Removed the neutral to ground connection at the secondary. Now, when the transformer is off, I have open circuit (the multimeter does not register ohms) between hot and ground, and between neutral and ground. For safety reasons the ground is connected to the chassis.When then transformer is operating, I get 84 volts between secondary hot and ground. Was expecting zero or microvolts. Is that normal? Thanks.

author: bdunham7
author_rank: Super Contributor
timestamp: June 29, 2020, 02:40:08 am
content: How are you measuring that voltage? You are likely measuring leakage current. And can you be more clear about "Removed the neutral to ground connection at the secondary", like with a picture or something?

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 29, 2020, 03:25:16 am
content: What did you disconnect? The manufacturers documentation says the secondary is floating with no connection to ground. Photos are worth a thousand words.Measuring 84 volts between hot and ground implies you’ve got a problem with your grounding. In the US, the neutral is bonded (connected) to ground at the breaker panel and no current should flow in the ground wire. You should measure the same voltage between hot & neutral as you measure between hot & ground.What do you measure between:1) Hot and neutral (should be ~120 vac)2) Hot and ground (should be ~120 vac)3) Neutral and ground (should be zero but a volt or so is okay)

author: Brumby
author_rank: Unknown
timestamp: June 29, 2020, 03:28:28 am
content: Quote from: WattsThat on June 29, 2020, 03:25:16 amIrrelevant to the Ops question. I draw your attention to this:Quote from: jcc on June 29, 2020, 02:32:47 am

author: Brumby
author_rank: Unknown
timestamp: June 29, 2020, 03:32:56 am
content: Quote from: jcc on June 29, 2020, 02:32:47 amThis is normal if you are measuring with a high input impedance meter. Such "ghost voltages" are rather high impedance sources. They are misleading and it is this reason why you will find a low impedance (Lo-Z) range on a number of meters, such as Dave's BM235 and 121GW. These ranges put a slight load on the circuit being measured which will pull down these ghost voltages - while voltages which have the ability to deliver some level of power will be only marginally affected.

author: jcc
author_rank: Newbie
timestamp: June 29, 2020, 03:34:52 am
content: Quote from: bdunham7 on June 29, 2020, 02:40:08 amThanks. I have attached a diagram.

author: bdunham7
author_rank: Super Contributor
timestamp: June 29, 2020, 04:06:32 am
content: Well, assuming your diagrams are correct, you have done the right thing. The voltage you are measuring is just the result of the capacitive coupling on the windings producing a small leakage current, typically a few tens of microamperes. You can estimate the current if you know the input impedance of your meter, or you can put a resistor across where you are measuring and calculate that way.Do you get the same voltage from either side of your secondary? I would expect a different voltage from the two different sides of the winding (neither is 'neutral' or 'hot' now) depending on the construction of your transformer.I don't know the exact original application for this transformer, but the diagrams you have seem to completely contradict the info from Tripp-Lite.https://www.tripplite.com/isolator-series-120v-500w-ul60601-1-medical-grade-isolation-transformer-4-hospital-grade-outlets~IS500HGIf one side is referenced to ground, it is no longer 'floating'. And if they advertise noise filtering, I'd expect a shield terminal to be grounded. Are you 100% sure that the ground you removed wasn't a shield ground and was definitely connected to that secondary? I know that sounds basic and I'm not trying to insult you, but something doesn't make sense. Anyway, measure your leakage current and make sure it is under 100uA or so and you are in business.Edit: I looked more closely at the diagram and it does show a grounded shield.

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 29, 2020, 04:23:51 am
content: Quote from: Brumby on June 29, 2020, 03:28:28 amIrrelevant? I draw your attention to the possibility of an open or high resistance ground. What is the point of troubleshooting a faulty output when the input is at fault? The OP never checked for a bonded neutral or an open ground.

author: james_s
author_rank: Super Contributor
timestamp: June 29, 2020, 04:51:02 am
content: Try connecting a small incandescent lamp between hot and ground, it should not glow at all. Then measure the voltage across the lamp, it should be very close to zero. A typical digital multimeter has such a high impedance that capacitive coupling can result in a significant voltage, but it should not deliver any appreciable amount of current.

author: bdunham7
author_rank: Super Contributor
timestamp: June 29, 2020, 05:00:57 am
content: Quote from: Monkeh on June 29, 2020, 04:53:59 amIt is capacitive coupling, yes, but not necessarily from the input. Capacitive coupling over the secondary to ground will also cause the secondary to have leakage current. The ground shields were originally intended to shield the secondary from certain types of noise and don't always help in reducing leakage current--in fact depending on how they are made, they can increase it.

author: Monkeh
author_rank: Super Contributor
timestamp: June 29, 2020, 05:02:42 am
content: Quote from: bdunham7 on June 29, 2020, 05:00:57 amTrue.Anyway, I just removed that post because he did actually have a point earlier - this is supposed to already be a shielded, floating secondary transformer. Perhaps the OP has just cut a shield connection..

author: jcc
author_rank: Newbie
timestamp: June 29, 2020, 05:13:17 am
content: Thank you, guys. I’m a beginner. Just starting to learn electronics. But have some electrical knowledge. At least some to be considered dangerous if you know what I mean. Have installed electrical main panels (ground and neutral bonded) and sub panels (ground and neutral separated).The transformer specs state that: “Your isolation transformer is UL60601-1 listed,1, Type B, Continuous Operation, Ordinary Equipment. The isolation transformer is Faraday-shielded and reduces the cumulative leakage currents of all connected equipment below 100 microamps.”I think this model do not have filtering. But have a board with a basic circuit. (see attachment) I think the purpose of that board is to ensure that if the secondary fails the mains are connected. So, the load does not lose power. I think that because the mains and the secondary are connected to that board. Because I do not want that I disconnected the cables that comes from the secondary.Because I’m starting, I do not have parts available to do some tests. Was thinking that maybe I can use a resistor (limit the current so I do not blow the multimeter fuse) (also must ensure that I do not exceed the resistor wattage). And see if the current exceeds the 100 microamps. Bus still even if it is “leakage” what I worry is about the potential. It’s above 48 and 60V. Could be a shock hazard. I know I must be careful. Because if I connect a grounded probe to one AC lead, I’m effectively grounding the secondary output. I understand that. What I want to avoid is blowing the scope or any other grounded test equipment.Performed a second test. I get 76 volts between one secondary ac leg and ground and 37v from the other lead to ground. I know that, maybe, some voltage is normal. I can live with 37. But 76 seems too high. Do now know if that is normal.Connected a fan, the laptop and they work. No problem. But if I connect an inexpensive outlet tester get the hot, ground reversed light combination. Also connected a somewhat more expensive tester. The one that test the ground impedance and get the bad ground light combination.Maybe I should get one of those 3 plug to 2 ungrounded adaptors. Or try removing the ground wire that ground the transformer (what separates the primary from the secondary). To be safe I will use a GFCI circuit, just in case. I know that even if I use a GFCI I’m only protected from shock hazard from the mains, not the secondary if I ground it by plugging a grounded test lead to one of the secondary legs.Again, thank you very much for your time and answers.

author: bdunham7
author_rank: Super Contributor
timestamp: June 29, 2020, 05:46:02 am
content: You'd have to take out the board, flip it over and let us see the back side and identify the components to be sure, but I think that is a filter board not some sort of fail-over system. That would be pretty odd. I suspect your device has been worked on. That jumper just shouldn't be there. And I can't tell for sure, but it looks like a bunch of MOVs are missing from the board--although some of the pads look unused as well.You haven't answered the question of what type (exactly--make, model) of meter you are using to measure the voltages. If your meter has a 10M or 1M input impedance, then the leakage currents are small enough.

author: jcc
author_rank: Newbie
timestamp: June 29, 2020, 06:21:35 am
content: Quote from: bdunham7 on June 29, 2020, 05:46:02 amA used Fluke 87-V. Not sure, but I think it is not a low impedance meter. Maybe that's the cause.

author: james_s
author_rank: Super Contributor
timestamp: June 29, 2020, 07:03:45 am
content: A Fluke 87, like almost all digital multimeters has a 10Meg input impedance. While typical in modern times, it is considered high impedance in a traditional sense.

author: richard.cs
author_rank: Super Contributor
timestamp: June 29, 2020, 08:37:14 am
content: Quote from: jcc on June 29, 2020, 05:13:17 amNormal, in that it adds up to nearly the full output voltage. It tells you that the dominant coupling to earth (which is probably capacitive) is about 1/3 of the way along the winding, which again is pretty typical.

author: bdunham7
author_rank: Super Contributor
timestamp: June 29, 2020, 02:00:57 pm
content: Quote from: jcc on June 29, 2020, 06:21:35 amYes, your leakagecurrentsare likely quite small, but it is not possible to say 100% for sure from measuring the voltage with a high impedance meter. The best way to measure leakage is with a resistor of approximately 10K--just large enough not to burn up if exposed to full line voltage--across the voltmeter. However, your 87V can measure the leakage current directly in the uA range, so you can try that. Just make sure to ONLY measure from either secondary terminal to ground whilst in the uA mode. If you make a mistake you'll pop a $5 fuse.

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2020, 05:14:01 pm
content: Quote from: jcc on June 29, 2020, 02:32:47 amCapacitive coupling to ground makes a voltage divider which your high impedance meter can detect. It is perfectly normal.Incidentally, this behavior with capacitive coupling allows one to discern which wire is hot and which is neutral by holding one lead of the multimeter in your hand and probing with the other, not that this is recommended for safety reasons.

author: jcc
author_rank: Newbie
timestamp: June 29, 2020, 05:48:52 pm
content: Thank you guys. Will do both tests. The one with the resistor and the one with the incandescent bulb. Again, thank you very much.PS: I have so much to learn. Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: June 29, 2020, 08:19:28 pm
content: Quote from: jcc on June 29, 2020, 05:48:52 pmLots of beginner questions and mains is an indication you lack the experience to work with mains safely.I hope you realise that an isolation transformer will not provide complete protection against electric shock.Do you also know it can actually increase the risk of shock, under certain conditions?It's possible to use an isolation transformer to measure mains voltages, but isn't the best and safest method, which is to use a differential probe. If you only need one channel, you can use two channels on your oscilloscope, set to differential mode.Another device which can protect against shock as an RCD or GFCI. It works by sensing the current imbalance between the phase (hot) and neutral conductor and cuts the power, if it exceeds a certain value.

author: EPTech
author_rank: Regular Contributor
timestamp: June 30, 2020, 09:03:56 am
content: Hi There,I agree with Zero999 on this one. When you disconnect ground, you actually have more risk of electrocuting yourself.On my own 3 phase 10kVA isolation transformer I have put one of the secondary taps and the ground on banana terminals. That gives me two options.1. Link them together. You get your own realistic mains, still isolated from the public mains.2. Unlink them to do scope measurements on PSU primary side or power circuits. As an extra, one can measure earth leakage current via the same terminals.Furthermore, the secondary is protected with its own automatic fuse (always) and RCD (when linked)Never isolate your measuring equipment, unless you know exactly what you are doing. Only ever isolate your device under test.

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2020, 10:27:53 am
content: Quote from: EPTech on June 30, 2020, 09:03:56 amIt depends on where you disconnect the earth? There's actually noting wrong with leaving the secondary of the isolation transformer floating. It's known as an IT earthing system. A permanent installation requires a warning device to indictate, the presence of an earth fault, which I don't belive is a requirement for a bench test, but check the regulations in your area.https://www.gses.com.au/getting-down-to-earth-earthing-explained/The problem with an isolation transormer is it renders any upstream RCD protection ineffective. This isn't an issue, whislt the circuit is floating, as you need to contact more than one point of the circuit to receive a shock, but it is when an earthed-referenced piece of test equipment is connected to it. Another issue is, if the isolating transformer is powering a high voltage circuit, which is not isolated, such as an autotransformer, the mains can float at a very high voltage, in the case of an earth fault. Not only does this present a greater shock risk, but also fire due to arcing.

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2020, 02:19:08 pm
content: Quote from: EPTech on June 30, 2020, 09:03:56 amAre you sure about that?

author: jcc
author_rank: Newbie
timestamp: June 30, 2020, 08:44:27 pm
content: Thank you. I have basic knowledge in electrical systems. I’m not an electrician. But know the difference between system ground (discharge static electricity, RF noise, etc.) and equipment ground (OCP).The modification I performed leave the isolation transformer with system and equipment ground in relation to the mains. But only have system ground in relation with the secondary output. The ground plug is only connected to earth, not the neutral on the secondary side.You are correct. If anybody is working with high voltage (above 48v) you must take extra precautions. Even if you are using a GFCI you can electrocute yourself. If you have rubber shoes and you touch the neutral and hot, and are unable to let go, you will die. GFCI only works with a ground fault. Not in a fault condition. Also, if you are bathing inside a 55-gallon drum. And someone throws an ungrounded electrical device connected to the mains. You will die. Even though it is connected to a GFCI. Because there is no ground fault (the drum is plastic).The only way you can get shock from a floating (ungrounded) secondary is if you touch both output leads (two points). But if you connect your oscilloscope, or any other grounded test device. You are effectively grounding the secondary. Now you can get shock if you touch the other ungrounded conductor. And you will die if you’re unable to let go in time.If one of the output leads from the secondary faults and touch ground. The system is grounded. You will get shock if you touch the ungrounded conductor and will die if unable to let go.Using an ungrounded isolation transformer should only be done to prevent damage to the test equipment. But you must take the same precautions as if working with a grounded system.

author: helius
author_rank: Super Contributor
timestamp: June 30, 2020, 08:57:30 pm
content: Quote from: jcc on June 30, 2020, 08:44:27 pmIf there is no path to ground, the water in the bucket will be at nearly equal potential throughout, making electrocution an impossibility if you do not grasp the live conductors in your hands.QuoteIn reality there is always capacitive or inductive coupling to the environment and AC passes through those. It is only from a DC perspective that the secondary is floating at all.QuoteIt is far better to use differential probes to prevent damage to test equipment. The use case for isolation transformers is, in fact, to provide safety to the technician when working on "live chassis" equipment that has no safety ground for the metalwork at all. Providing a PE terminal for the DUT is thus useless and potentially deadly because it leads to confusion about what is sane to plug into your isolation transformer.

author: jcc
author_rank: Newbie
timestamp: June 30, 2020, 08:59:28 pm
content: I’m not an electrician. Not 100% sure if you can leave the isolation transformer floating an be compliant with NEC. But a least it should have several labels or stickers that says: “No equipment ground”Also, not 100% sure, but if you have a GFCI on the primary side. And you touch two points on the secondary side, or even one point if the secondary is grounded, it will not trip the GFCI.

author: james_s
author_rank: Super Contributor
timestamp: July 01, 2020, 04:42:29 am
content: The NEC regulates the wiring built into your house, it doesn't apply to devices you plug into a receptacle.

author: jcc
author_rank: Newbie
timestamp: July 01, 2020, 05:37:48 pm
content: Quote from: helius on June 30, 2020, 08:57:30 pmI’m no expert. But electricity takes all paths. Not only the shortest path. Electricity goes to the source. Not ground. On a grounded system the electricity goes to ground because the source is connected to ground. The electrical device will create a voltage gradient in the water. In this case, because there is no ground connection (mains are a grounded system), the voltage gradient will follow the pattern of an electric dipole. It is more complex. Because there are multiple contacts points. But for simplification assume that what is thrown into the water is a cable with ends exposed.In the end, it depends on several factors if you get electrocuted. Skin resistance, solids in the water that increase conductance (pure distilled water does not conduct), voltage, current, etc.

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2020, 06:22:24 am
content: Yes, electricity takes all paths, it's just that most of it flows through the one of least resistance.A complete circuit is required for electricity to flow. In the case of the mains, the neutral side of it is connected to ground, so a person can receive a shock by touching the phase, if another part of their body is touching ground. If an isolation transformer, with a floating secondary is connected to the main, the person won't be shocked if the only touch one side of the secondary, because there isn't a complete circuit.Now you might ask, why is the mains earthed, if it increase the risk of a shock? The answer is it's done to prevent the mains from floating at a much higher voltage, with respect to earth, in case of a lightning strike, or a higher voltage conductor comming into contact with the mains.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 02, 2020, 06:12:12 pm
content: A very simple test I often do on my oscilloscope, just to see if it's still working and I've got the probe for the right channel is to touch the probe tip( 1:10, so 10M input) with a finger.This often measures 70Vac or higher, and significantly changes when I wave my other hand around, or go near some 230Vac wiring such as a desk light.This is all perfectly normal.70/1e7 = 7uA which is not much current.And yes, my oscilloscope is properly grounded.Just did a little test with my DMM (Wavetek Meterman 35XP)I connected one of the probes to the Earth lead of a plugged in extension cord, and held the tip of the other probe in my hand. By moving a bit I could measure upto 25Vac.Same with an Aneng AN8009.