NOT the usual Isolation Transformer question - Page 1
Forum section: electronics / testgear

author: watchmaker
author_rank: Unknown
timestamp: March 06, 2025, 01:05:08 pm
content: How do you verify the condition of a bench isolation transformer before you put it into service?I can guess:Resistance across each windingResistance between windingsVoltage between Iso and passthrough outlets (live to neutral, etc)Voltages between PE and live and neutral sides of iso outletWhat are allowable voltages between iso and PE? (I get 1V)Any other tests?Thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: March 06, 2025, 01:17:00 pm
content: 1 KV Hi-Pot insulation test between primary and secondary, (both terminals of each winding connected together), and also from each to core or screen if accessible.Leakage current from iso side (either terminal) to PE. Start off by measuring voltage across a 10K resistor, and calculating the current to avoid blowing up your DMM! Once you are sure its within your current range limits, you can measure direct.

author: nctnico
author_rank: Super Contributor
timestamp: March 06, 2025, 03:48:25 pm
content: Quote from: watchmaker on March 06, 2025, 01:05:08 pmI don't. I use differential probes to measure mains. Much safer as the GFI protection stays in place and dangerous voltages are limited to a well defined area. You have to realise that an insulation transformer is just one item on a list with a million items to work safely with mains; an insulation transformer is part of an environment. On a regular bench it is super easy to connect the DUT to ground and thus making it unsafe. Worst, you don't see it is unsafe and the GFI won't protect you.

author: Fungus
author_rank: Super Contributor
timestamp: March 06, 2025, 04:02:17 pm
content: Quote from: watchmaker on March 06, 2025, 01:05:08 pmInsulation tester.The real question is: Why would you want to?No short term convenience is worth the extra dangers that will constantly be present in that environment.

author: bdunham7
author_rank: Super Contributor
timestamp: March 06, 2025, 04:06:04 pm
content: Quote from: watchmaker on March 06, 2025, 01:05:08 pmYou should be thinking and measuring leakage in terms of current, not voltage. There is always going to be some leakage due to capacitive effects, but it should be microamperes and not problematic.

author: floobydust
author_rank: Super Contributor
timestamp: March 06, 2025, 04:38:41 pm
content: With low dollar chinese or korean products that have no safety agency approvals, I'll go through a basic checklist.First is the grounding.Mistakes like a lug mounted to a painted surface, no star lockwasher or double-nut and then the issue it must handle a branch (15A) circuit fault, no super skinny wire. Usually connection to the chassis is at the transformer (mounting) screws which is OK.Relative to the Tenma 72-6670 isolation transformer, they are notorious for having shit, unsafe grounding. Many threads posted about it on other forums.An extra issue here is the secondary-side outlets, they can have the ground terminals connected- but may or may not be tied to the main chassis ground ;fI think they confused "isolation" as for all (line, neutral, ground) verses the only usual line and neutral floating. (It's very rare I lift the PE ground on equipment, only to eliminate ground loops in a pinch). One Tenma guy added a ground-lift switch to it.But I would make sure the (isolated and non-isolated) output outlets are PE grounded.Second is the incoming mains hot/line must (always) go the the fuse first. Then to the on/off switch etc. Common manufacturing mistake is to swap line and neutral, which if you think about it means full mains (15A) available inside the appliance to all components even when switched off...Third, see if the fuse is a reasonable rating, not some oversized value that will not blow until there is a bonfire.Fourth are the basic continuity tests for grounding and pri-sec isolation.You can measure capacitance between primary and secondary to get an idea of the transformer's interwinding stray capacitance. This will show up as a ghost leakage current that can make things not seem 100% isolated. Measuring 1VAC is very low leakage, almost too low? Was that between the output and PE ground?

author: watchmaker
author_rank: Unknown
timestamp: March 06, 2025, 05:15:15 pm
content: THANK YOU. I do think we need a checklist such as yours. Who knows what they are getting.OK. I will verify the Iso to PE voltage. There I just checked with the Fluke 45 at the outlet and PE. What do I know?BTW, this is a Tema 72-6670. Before I reflowed some solder joints the cross voltage was higher.I will recheck my measurement this PM.

author: watchmaker
author_rank: Unknown
timestamp: March 06, 2025, 07:03:36 pm
content: Quote from: floobydust on March 06, 2025, 04:38:41 pmOK. I just rechecked.1 Vac Iso neutral to PE; 4.5 Vac Iso Hot to PE (pin on xformer, all PE pins (Iso,direct and chassis) are connected and resistance to my external PE is less than 1 Ohms.) Open outputs, straight to Fluke 45.

author: floobydust
author_rank: Super Contributor
timestamp: March 06, 2025, 07:41:09 pm
content: To get that low VAC reading, the transformer has low stray (pri-sec) capacitance- which is good. It might have an electrostatic shield or split-bobbin construction to achieve that.Point is things look isolated and if you ever are in doubt an ohmmeter reading from the secondary to anything will confirm if a shock hazard exists.

author: watchmaker
author_rank: Unknown
timestamp: March 06, 2025, 07:55:27 pm
content: THANK YOU!