Is my Isolation Transformer not isolated? - Page 1
Forum section: electronics / beginners

author: AngraMelo
author_rank: Regular Contributor
timestamp: May 04, 2018, 11:58:09 pm
content: Hello everyone,I just bought an isolation transformer. It should be noted that I live in Brazil, isolation transformers are not a thing (meaning, no one has one, everybody just take the risk of working without one). Here in my city it is 220VAC with 2 phases, We do not have a neutral, just phase, phase and earth.On the primary it has 2 input voltages, 110V and 220V, and a ground wireOn the secondary it has 2 wires for 220V only.The seller claims it is a Galvanic Isolated transformer but I have my doubts.For the following experiments and measuraments I did NOT connect the earth wire from the primary to the outlet earth.I got a voltage from the secondary against the outlet earth, I connected a light bulb to those 2 connections and it did NOT light up. After, I measured using a Fluke 117 with a LoZ option and I got 0 volts AC on both wires of the secondary in relationship to earth outlet. Also...and this is dumb...I touched the secondary wires (one at a time) and I do not get a shock no matter what.Now the tricky situation, when measuring resistance from the primary to the secondary I always get a reading, it starts around 500K and goes up gradually. It does not matter which cables Im measuring, I always get a resistance reading. How in the world is that isolated?But if it was not isolated, the LoZ function would have accused a voltage right? Or the light bulb would have lit up, right?This is driving me nuts, I watched a ton of videos but I cant figure out what is going on.Please help me guys.I have a limited knowledge but I want to make sure I have the real deal.

author: helius
author_rank: Super Contributor
timestamp: May 05, 2018, 12:42:20 am
content: Quote from: AngraMelo on May 04, 2018, 11:58:09 pmEven with galvanic isolation, there is capacitance and mutual inductance between the windings. The mutual inductance is what makes it a transformer and is absolutely necessary for it to work, and the capacitance is unavoidable given the constraints of transformer assemblies (the primary is wound over the secondary unless you use a toroidal design). An ohms reading that starts high and increases is caused by the capacitance (just like when you connect DC across a capacitor, current flows briefly and then stops). It sounds like your transformer is correctly isolated.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 05, 2018, 01:08:28 am
content: If the Fluke 117 on Low-Z reads zero, it's as isolated as it needs to beSome photos please if possibleand welcome to the forum

author: AngraMelo
author_rank: Regular Contributor
timestamp: May 05, 2018, 01:14:24 am
content: I did not know that!thank you very much both of you!

author: Electro Detective
author_rank: Super Contributor
timestamp: May 05, 2018, 01:36:01 am
content: Also make sure the polarity is correct for input and outputYou do not want up to 440 volts appearing between both input and output 'Live' connectionsIf you get that, then either reverse the output wires, or the input wires, depending on what is connected to the chassis or transformer body. We need to see photos to make a proper judgement on this before you try thati.e. there should be ONLY 220 volt or zero to a few volts in any way you test the input and output connections with the meterBeware if you do these tests using Low-Z you will trip your GFCI or RCD, so watch out for that or use just the AC meter setting,but that will give you ghost voltages (perhaps 60v + 160v) , which may not be a problem as you are watching out for anything much higher than 220 voltsHope it made sense and probe carefully

author: AngraMelo
author_rank: Regular Contributor
timestamp: May 05, 2018, 03:34:22 am
content: Quote from: Electro Detective on May 05, 2018, 01:36:01 am

author: Brumby
author_rank: Unknown
timestamp: May 05, 2018, 03:40:57 am
content: In your situation, the earth connection is actingasthe neutral.

author: helius
author_rank: Super Contributor
timestamp: May 05, 2018, 04:30:18 am
content: Quote from: Brumby on May 05, 2018, 03:40:57 amThat ain't necessarily so. A neutral connection is a required return path, but the protective earth is only for safety and does not carry current under normal conditions. A split-phase feed with only the two phase wires is its own return path; much as a three-phase feed to a balanced load does not need a neutral wire. The earth connection provides protection in case of insulation faults, but isn't otherwise needed.However, what I think Electro Detective has homed in on is OP's statement thatQuote from: AngraMelo on May 04, 2018, 11:58:09 pmSo we know thatat the least, there are taps for 1:1 and 1:2 on the transformer. It's possible that it was wired such that the 220V enters the primary winding with fewer turns, being stepped up to 440V on some combination of taps. The transformer may even be of the "universal" type, which has 4 windings some of which may supply high voltage under some set of circumstances. It's a good idea to count the taps and see if they can be identified, and a schematic drawn, to ensure there are no surprises.

author: Brumby
author_rank: Unknown
timestamp: May 05, 2018, 05:18:12 am
content: Quote from: helius on May 05, 2018, 04:30:18 amWhile I agree that a power system doesn'tneedto be earth referenced to function - it does need to be earth referenced for safety ... on at least 2 counts. The most obvious is for the function of a protective earth. If the system isn't earth referenced somewhere, then a protective earth does nothing. Some might argue that if there is no earth reference, then nobody can get zapped - but herein lies the second reason for earth referencing. With no system defined earth reference, then ANY connected equipment that is earth referenced, causes the whole system to be earth referenced - at the potential of that connected equipment. If this connected equipment offsets the potential by 10kV, then the whole system is now 10kV above earth potential - and everybody's TV, fridge and toaster is now at 10kV in relation to anything earthed. A system defined earth reference will ensure that 10kV potential is shunted, limiting the risk to everybody else.The question, then, is what point in the supply systemisreferenced to ground? Is it one phase - or the other - or, like in much of the USA, a split phase centre point?Whether this point is actually a neutral or just a grounding point is a question that may affect how we view this transformer connection.Nevertheless, I do concede my one line statement was not well put.

author: Vtile
author_rank: Super Contributor
timestamp: May 05, 2018, 08:47:39 am
content: Lets not start to argue on the line distribution schema of the Brazil yet.. Lets just look the situation of the transformer first. The OP seems not to be a tool.Btw.. Brazil doesn't have a single line voltage level and I assume the electrical installation standards are as much varied between state to state.https://www.worldstandards.eu/electricity/plug-voltage-by-country/brazil/

author: Electro Detective
author_rank: Super Contributor
timestamp: May 05, 2018, 09:50:44 am
content: some photos of the unit internals would get this resolved 100% without assumptions and guesswork

author: ez24
author_rank: Super Contributor
timestamp: May 05, 2018, 07:25:54 pm
content: There is a good YouTube video on this. Does anyone remember it? I forgot

author: AngraMelo
author_rank: Regular Contributor
timestamp: May 05, 2018, 07:31:31 pm
content: Although taking the whole thing apart might seem a good idea, Im afraid I might not be able to put everything together and even if I could I would certainly loose my guarantee. If it is necessary to answer the question Im open to do so.To answer a few questions...I do not need a earth connection to have a current, using the 2 phases is enough, granted it is not as safe.The gentleman that said about states in brazil having different mains specifications, you are correct, every state is different, some are 110V with phase and neutral, some a 220V, some are 127V... crazy south america, I know...Do you guys have more questions about the transformer that could lead us to a more precise position about it being isolated or not?

author: Brumby
author_rank: Unknown
timestamp: May 06, 2018, 12:50:06 am
content: Quote from: helius on May 05, 2018, 12:42:20 amThis covers the 'worrying' observations you originally posted and explains what they mean.I'm inclined to agree - that based on what you have told us, the transformer you have is an isolated one.

author: Electro Detective
author_rank: Super Contributor
timestamp: May 06, 2018, 01:39:41 am
content: If you can't take it apart for a look, then going by your measurements earlier, regardless of what power arrangement is in your locality,and ignoring return paths, be they phase +phase, phase+N, phase+CT, phase+Earth, phase+E/N or whatever the earth and neutral relationships areand any other power company non standard practices/penny pinching shortcuts to make consumer and service personel lives complicated etcas long as you have two 'power' wires coming in to the transformer, and two wires out, with no connection/s to the transformer case or transformer body,and the tranformer box and YOU are both isolated with rubber mats and boots from the bench/table/floor/wall,ground/concrete/plumbing/swimming pool/EARTH itself!,then I would say it's a good bet (not a winning one) that you have electrical isolation from your mains transformer primary and output secondary,in other words a FLOATING electrical system isolated from any Earth return current/voltagei.e. you have your own isolated 'power station' limited only by the transformers output capacityBut then you have to think about the next device/s that will connect to this isolation transformer, and ensure it does not 'break' the isolation,as well as wiring faults, loose connections that initally test ok, and halfassed electrical assembly etcWhatever you do, gloves, eye glasses and rubber shoe/boot soles should always be part of the 'circuit'