Medical Grade Iso Transformer Mod - question (again) - Page 1
Forum section: electronics / testgear

author: psap
author_rank: Newbie
timestamp: December 07, 2021, 10:09:41 pm
content: I have searched and read and watched on this topic but they all seem to be a little different and I am new at this. Could you please take a look at my annotated pic and let me know if I am doing this correctly?The application is for safely testing vintage stereo amplifiers. I will also have a variac (soon) and will make the popular light bulb current limiter.The device in question is another cheap surplus hospital grade iso transformer.I have disconnected the coil secondary ground and all of the grounds to the outlets including the one to the outlet neutral (don't worry I will insulate all of those connectors). I have left the chassis ground from the incoming power cord.The outlets are still grounded to the chassis from their mechanical connection. So I still have continuity from the mains ground plug to the secondary outlet ground lug.So, have I done all that I can do and now still need to use a 3-2 plug adapter for the DUT? If I understand all the lessons, you don't want the DUT ground the same as the mains - right? which this still is - yes?

author: Gregg
author_rank: Super Contributor
timestamp: December 08, 2021, 12:47:29 am
content: Before you get too complacent thinking feeling safe about your personal safety as well as keeping your gear from letting out sparks and smoke; you need to really understand what an isolation transformer does and even more important, what it doesn’t do as well as how to wire everything appropriately. You probably will get a lot of safety cautions on this forum.A good and concise discussion is on the DigiKey website:https://www.digikey.com/en/articles/the-basics-of-isolation-transformers-and-how-to-select-and-use-themThe wire you labeled ‘coil secondary ground’ is a Faraday shield that should be grounded to the ‘mains ground’.The outlet ground wires may not be isolated from the outlet bracket that is bolted to the chassis; you can check with an ohm meter. Isolated ground outlets are available and are commonly used in medical facilities where a totally separate grounding system is installed. These often have a small green triangle on the face. If you want the ground outlet port grounded it is advisable to connect the appropriate wires back to the ‘mains ground’.The wire you designated ‘neutral ground’ is probably the one you want to disconnect for total isolation of the power output. Keep in mind that there will not be any distinction between line and neutral if this wire is removed. Again fully understand what you are doing.I would also suggest a two pole circuit breaker rated for somewhere near the max transformer rating on the power output; it is really handy to be able to easily switch the output with the transformer still powered.

author: Gregg
author_rank: Super Contributor
timestamp: December 08, 2021, 01:20:36 am
content: I added text to a picture of my medical grade isolation transformer; you might find some of it useful. I had to take out the stock receptacles and make an aluminum cover that fit inside to make it all work. There isn't a lot of spare room inside.

author: psap
author_rank: Newbie
timestamp: December 08, 2021, 04:48:32 pm
content: I appreciate the help. allow me to feedback to ensure my understanding please.1. reconnect the faraday shield to mains ground (the lug on the chassis).2. Leave the "neutral to ground" connections disconnected as shown.2. I did check the outlet ground with an ohm meter and they are grounded to the chassis. If I change the outlets to "isolated ground (orange) outlets", do I leave that ground lug unterminated? Isn't that the same result that I get in my pic by disconnecting the ground wires from the chassis? If they get wired back to mains ground then doesn't that defeat the purpose of the isolated ground outlet?Again, thanks for the help.

author: psap
author_rank: Newbie
timestamp: December 08, 2021, 05:10:54 pm
content: just realized my mistake when I walked away. regarding my number 2 below now amended.2. I did check the outlet ground with an ohm meter and they are grounded to the chassis. If I change the outlets to "isolated ground (orange) outlets", do I leave that ground lug unterminated?

author: Gregg
author_rank: Super Contributor
timestamp: December 09, 2021, 12:24:29 am
content: Quote--- Yes this may reduce some noise from and to the mains line.Quote--- Remove the wire entirely or route it to a mounted sheathed banana jack to enable easy jumper to ground if you ever find a need, don't leave bare ended wires flopping around. I haven't seen a true medical grade isolation transformer that has one of the output leads grounded; that would defeat most of the purpose of using an isolation transformer. On my similar transformer I connected each output line to a red sheathed 4mm banana jack and covered the openings with vinyl caps to keep pokey bits out. This allows for easy connection to a meter or something else.Quote--- This is very standard for North American wiring to have the U ground terminal of the socket connected the framework of the socket which in turn may be connected to a metal enclosure via the mounting screws and the center screw between the two parts of the duplex outlet will also connect to the framework providing a ground if a metal cover plate is used.Quote--- If you have to ask, you probably shouldn't. You have to think long and hard as to why, if ever, you would want the chassis of your DUT not grounded. If the actual need ever arises, there is always the option of using one of the two to three prong adapters -- but be aware that it is easy to forget. I would absolutely not install an isolated ground receptacle and leave it ungrounded. Note that on my own isolation transformer I connected the isolated ground for the receptacle to the green / yellow binding post adjacent to the green binding post with an easy to connect jumper bus. It is very easy to see and hard to forget. The green binding post is connected to mains ground and the transformer case.(Isolated ground receptacles are not all orange, they are available in many colors. Look for the green triangle on the face that indicates isolated ground. Medical grade isolated ground receptacles often are white or clear.)Quote--- No, the metal transformer enclosure is still connected to the outlet via the mounting screws; by all means don't isolate the transformer enclosure from mains ground.Quote--- A better question is: Why would you want or need your ground to be isolated? Probably not for whatever you will be doing with your isolation transformer. Medical installations install a totally separate ground grid to hopefully mitigate any possible transient currents that may be on the mains ground wiring due to a fault to ground somewhere in the system. For instance if you are working with RF and have installed a system of ground rods around your house to connect your isolated ground, it may help to use that ground ring isolated from the mains ground.Remember Murphy's Law is waiting in the wings to hand out Darwin awards.