Isolation Transformer Question - Page 1
Forum section: electronics / testgear

author: mart1n
author_rank: Contributor
timestamp: February 25, 2020, 09:15:11 pm
content: I recently purchased a benchtop isolation transformer from Jameco Electronics.Product URL:https://www.jameco.com/z/GRP-1200-Benchtop-Transformer-Isolation-120VAC-7A-60Hz-450VA_2214336.htmlDatasheet & schematic:https://www.jameco.com/Jameco/Products/ProdDS/2214336.pdfSpecifications:Product type: Power transformerPrimary side voltage: 110 VACPrimary side current: 14 AmpsPrimary side frequency: 50 to 60 HzSecondary side voltage: 110 VACSecondary side current: 7 AmpsSize: 8.86"L x 6.10"W x 4.33"HPrimaryDirect fuse: 110 VAC, 14ADirect AC #1: 110 VAC, 60 Hz, 900 VADirect AC #2: 110 VAC. 60 Hz, 900 VAInput: 110 VAC, 60 Hz, 1800 VASecondaryIsolated fuse: 110 VAC, 7AIsolated AC #1: 110 VAC, 60 Hz, 450VAIsolated AC #2: 110 VAC, 60 Hz, 450VAMy Question:Even though the schematic in the datasheet doesn't show this, all mains earth pins (third, middle pin on a 3 prong plug) on all outputs (isolated & non-isolated) are connected to the mains earth pin on the power cord (see FYI for details).I was not expecting this. Is this standard / common for a isolation transformer?and if so, why?If the goal is to have a "floating" DUT, why not also float the earth pin?I did verify that the isolated output(s) hot/neutral are not connected to non-isolated and/or power plug hot/neutral.FYI:The earth pin on the power cord is connected to the chassis, via a ground lug. All of the output's earth pins are connected together via a single unshielded taut wire. Then a shielded cable runs from the output earth pins to the chassis grounding lug.Thanks!

author: nctnico
author_rank: Super Contributor
timestamp: February 25, 2020, 09:32:48 pm
content: Return it and buy a differential probe. The ones from MicSig are very affordable and it is much safer compared to floating a DUT. Once you clip the ground of any piece of test equipment to the DUT (or if the DUT touches something which is grounded) it will be grounded again but you GFI won't trip if you touch a live wire. This has been discussed many times already. Just an isolation transformer alone isn't enough. Your entire work-area and set of equipment needs to be designed to work on floating equipment. You'll need safety training as well in order not to create your own death-trap. Oh, and note that the isolation transformer you bought doesn't have any safety rating at all!

author: Electro Detective
author_rank: Super Contributor
timestamp: February 25, 2020, 10:00:18 pm
content: If I'm understanding the issue correctly, and CONFIRMED beyond any doubt there is no sneaky connection of earth/ground wire or metal chassis to transformer secondary output sockets supplying Live 1 and Live 2then all you need is a two prong cable going to the secondary output (Live 1 and Live 2)to isolate the DUT/s from the chassis and input earth/ground plug, and whatever is going on at the main 120-0-120 switchboardThe chassis connection 'should' trip a GFCI if there's a fault at the input side, but should not on the secondaryi.e. with a labelled two prong adapter cable you don't have to mess or mod the Jameco box,or think about someone having a drama with it in the futureas suggested abovea differential probe would be nice too,and easier on the back

author: mart1n
author_rank: Contributor
timestamp: February 25, 2020, 11:21:14 pm
content: When I first created this post, I didn't notice this link on the product page, but there is a link to a photo of the inside. You can see what I mentioned in the original post'sFYIsection.Photo inside the chassis:https://www.jameco.com/Jameco/Products/ProdDS/2214336Drawing.pdfI appreciate the advice guys and I'll work on getting a differential probe, but can anyone comment on why an isolation transformer would be designed this way? Is this a common setup?Thanks!

author: trobbins
author_rank: Frequent Contributor
timestamp: February 25, 2020, 11:56:03 pm
content: It is likely a part of any countries electrical codes for equipment that require or use a protective earth in to the equipment, and then distributes that AC supply to other electrical equipment, and has been for decades.As advised, you should not be removing the protective earth connection through to any electrical equipment you are powering unless you are an electrician, or are competent to know the electrical regulations and safety hazards (which shouldn't be inferred from internet forums or blogs or other random information).Another possible solution for you is to use an isolated scope or meter that has suitable ratings for attaching to hazardous voltages. You could dodge that up by battery powering a cheap ebay scope module and placing the equipment in an insulated box so that you can't touch the controls or probe leads and have no other comms connections except for wireless types.

author: masterx81
author_rank: Frequent Contributor
timestamp: February 26, 2020, 12:02:35 am
content: Sorry if i "invade" this topic, but if i've well undersood, the only safe way to use an isolation transformer is to have the ground of the DUT disconnected, and also the ground of the scope removed. In this case, as the circuit is always floating there is no risk. The only risk is touching both cables out of the transformer, but in this case also an RCD on live mains not work. But in case of a fault with the scope, with ground disconnected, i don't think as it's safe anymore. Maybe powering also the scope via a different isolation transformer will do the trick. All is floating, nothing referenced, no risks.

author: Helix70
author_rank: Unknown
timestamp: February 26, 2020, 12:10:34 am
content: The point of the isolation transformer is that the active and neutral are no longer referenced to earth (as in an MEN system). It is not to float earth.The earth still connects to a real earth on both sides, but there is no electrical path from active or neutral to earth on the secondary of the isolation transformer.Be careful though, as some equipment bonds the common ground to earth internally, so the reference will still be with respect to earth, even if the primary of the transformer or switch mode power supply are not.That said, there are isolation transformers that float earth, seehttps://www.tortech.com.au/product/isolation/isolation-transformer/for an example.