Safety Isolation Transformer Question - Page 1
Forum section: electronics / projects

author: Bryan
author_rank: Frequent Contributor
timestamp: February 23, 2017, 08:53:13 pm
content: Ok, have been watching videos and reading some threads on isolation transformers and I am pretty sure (I think) I have a good understanding. On the secondary side of the isolation transformer it is a no-no to connect/use a ground connection from the primary side to the secondary side as somewhere down the mains line the neutral and ground are connected putting the operator or DUT at risk.My question is what is the best practise to ground the enclosure for the isolation transformer. I assume ok to ground the to protect the user should something break free inside the enclosure and make contact inside. BUT could one put themselves or the DUT at risk should they accidently touch the isolation transformer enclosure and DUT/hands or body under the right/wrong conditions.

author: nctnico
author_rank: Super Contributor
timestamp: February 23, 2017, 09:03:36 pm
content: You wrote you have read the various threads and you still want to use it for measuring mains powered devices?

author: Benta
author_rank: Super Contributor
timestamp: February 23, 2017, 09:06:36 pm
content: The idea of an isolation transformer is to have a completely floating secondary that has no voltage reference to anything, except what you decide.The enclosure should be potential-free for your protection, and as such earthed, of course.This reduces your question to: "what happens if my DUT comes into contact with the plumbing, or the isotransformer case?"Answer: the same.

author: Bryan
author_rank: Frequent Contributor
timestamp: February 23, 2017, 09:07:12 pm
content: Quote from: nctnico on February 23, 2017, 09:03:36 pmWell umm yes, isn't that what the DUT is usually powered from, otherwise why would one use a isolation transformer.

author: nctnico
author_rank: Super Contributor
timestamp: February 23, 2017, 09:18:01 pm
content: Quote from: Bryan on February 23, 2017, 09:07:12 pmJust read the previous threads. In a nutshell: get a good A-brand ground fault interruptor (GFI) and one of more differential probes with the proper CAT rating.With an isolation transformer you need to isolate all equipment to make it really safe. Do it half-assed and you will create a death trap because there can be lethal voltages anywhere. Without an isolation transformer you keep the danger zone limited to the device itself and whatever you connect to it the wrong way (through a false ground path) will trip the GFI or blow the fuse.

author: Benta
author_rank: Super Contributor
timestamp: February 23, 2017, 10:05:16 pm
content: QuoteCorrect.The short answer to isolation transformers is: "use your brain." (and if you don't know what you are doing: stay away)But the original question was how theenclosureshould be connected. I've already answered that: earthed.Cheers.

author: Bryan
author_rank: Frequent Contributor
timestamp: February 23, 2017, 11:09:51 pm
content: QuoteYes, would agree 100%, but just don't have access or the pocketbook for diff probes. Thanks for the advice though, more concerned about blowing up a scope than myself as I am usually very careful around high voltages.