Rewire hospital-grade isolation transformer for use in DUT testing - Page 1
Forum section: electronics / testgear

author: puterboy
author_rank: Regular Contributor
timestamp: September 17, 2020, 03:54:03 am
content: I bought a used Tripp-Lite IS500HG 500W hospital-grade isolation transformer.I would like to convert it for electronics testing to float the hot and neutral when probing the 120V mains side of circuits with my scope...Currently, the secondary outlet grounds, the secondary neutral, the chassis, and the input ground are all bonded together.Specifically:The secondary/output side looks as follows:Secondary BLACK wire --> hot side of output outletsSecondary WHITE wire --> neutral side of output outletsSecondary GREEN wire --> output outlet ground, output outlet neutral, input plug ground, chassisTo convert this I know I need to unbond the secondary neutral from the common (and input) ground but I am puzzled by what this secondary GREEN wire is doing. It does not seem to be connected to any of the transformer windings or the core though it does emerge from the secondary side of the large transformer.So which of these mods is correct:1. Secondary GREEN wire --> output outlet ground, input plug ground, chassis (all bonded)Unbond secondary neutral from the above common ground2. Secondary GREEN wire floatingOutput outlet ground, input plug ground chassis = bondedUnbond secondary neutral from the above common ground3. Other?

author: Circlotron
author_rank: Super Contributor
timestamp: September 17, 2020, 05:27:27 am
content: The green wire might be an electrostatic shield between primary and secondary. If that's the case, I'd leave it grounded.

author: Shock
author_rank: Super Contributor
timestamp: September 17, 2020, 11:38:36 am
content: All test equipment should be RCD protected if possible and as such their chassis or ground/earth wiring should be left unmodified to ensure the RCDs proper operation.A bench isolation transformers secondary (for electronics testing) should be floating and not connected to ground/earth. The output ground/earth wire however is optional depending on how you will be using it. By using a ground lift plug directly on the output it's possible to exclude, include or move the ground/earth (in the DUT) as desired.The output of an isolation transformer (or any device connected to it) will not be protected by an RCD. Both isolation transformers and RCDs do not circumvent the need to follow safe bench practices. It's best to use the one handed rule and treat all circuits as live until they are fully disconnected and deenergized.When using an isolation transformer test only one device at a time and be mindful connecting test equipment leads, moving or reintroducing the ground/earth (i.e. with your test equipment) if you make a mistake you can create fault current paths through you or your equipment.

author: dan46n2
author_rank: Contributor
timestamp: September 18, 2020, 07:53:14 am
content: Might want to check out this video, same transformer...

author: dietert1
author_rank: Super Contributor
timestamp: September 18, 2020, 12:04:51 pm
content: Before you modify the wiring, you need to check what is the green secondary wire. It may be a center tap of the secondary. Then in order to do scope measurements on switch mode power supplies you need to disconnect it from the output socket PE and leave the output socket PE connected to input PE. If the green wire is some kind of shield with no galvanic connection to the secondary, you can leave it like it is.When testing a switch mode power supply, it may be a good idea to include a light bulb as a resettable fuse, as mentioned in another thread here.https://www.eevblog.com/forum/projects/diy-build-of-an-isolation-transformer/msg3230454/#msg3230454Regards, Dieter