Why my Toroid Isolation Transformer has 112.3v to soil

Question

I'm looking for a brand of Toroid isolation transformer with electrostatic shielding. There may be one brand which has this. Unless there is just none in the whole world? Is it not possible to put electrostatic shielding in any Toroid isolation transformer?

Original message (findings: It's a semi isolation transformer)

I bought a Hammond Isolation Transformer (dual 234v/117v primary and secondary) for isolating test projects from ground. But when I measured it from one of the output to the soil, it measures 112.3 volts. The other lead is near zero. I thought it was supposed to be isolated to ground.

Here's the model.

https://www.hammfg.com/electronics/transformers/power/1182.pdf

I chose the m117 model or 500va.

Here's the primary connection

Here's the secondary connection

Note all the 4 windings have no connection to each other initially (tested using continuity meter). Even after connecting them right. There is still no connection between primary and secondary (any of the wires), when not powered.

It's not an autotransformer which has connection between one of the leads of the primary and secondary. Unless a toroid autotransformer has no connections between the primary and secondary too?

So why does one of the leads reads 112.3 volts to the soil? Isn't it an isolation transformer? But the primary and secondary windings are supposed to be separate which is what makes isolation transformer, isn't it. What do you think?

An autotransformer only has one winding. The Hammond 1182 toroid has 4 windings. So how could it still be an autotransformer??

"it measures 112.3 volts". But did you try to measure the resulting current? I bet it will be less than 100uA — Ale..chenski, Dec 07 '18 at 07:40
If I accidentally touch the lead and ground of the above mentioned semi isolation transformer (?). Would I get a shock or not? — Jtl, Dec 07 '18 at 08:09
There is no "fully isolated" power sources. In this case there must be some DC coupling between primary and secondary, because it is possible that some high-frequency components of AC could be rectified over parasitic junctions and charge the secondary wires into kV area, which will be really dangerous. So some level of coupling is necessary for safety. And no, 100 uA won't give you any shock, although some tickling sensation is possible if you touch floating winding with your underarm. — Ale..chenski, Dec 07 '18 at 08:18
"If I accidentally touch the lead..." The Hammond probably sold these transformens to millions of people over nearly 100 years, and no one yet complained about any shock. Why don't you try to measure the actual current, and tell us? — Ale..chenski, Dec 07 '18 at 08:24
I don't have any loads. will look for 120v loads. And it's raining now. — Jtl, Dec 07 '18 at 08:25
You don't need any "loads", just stick you DMM leads (in AC current mode) between the "semi-isolated" lead and ground. — Ale..chenski, Dec 07 '18 at 08:32
when connected directly, it reads 58.6 uA. see https://imagizer.imageshack.com/v2/800x600q90/924/nMQSfH.jpg When not using the 1182 toroid, and connecting one of the 240v leads directly. it measures 0.1A. see https://imagizer.imageshack.com/v2/800x600q90/921/t3YjhS.jpg So what can you conclude? It's a semi isolation transformer? btw.. why didn't Hammond state it's "semi isolation transformer"? — Jtl, Dec 07 '18 at 08:53
"When not using the 1182 toroid, and connecting **one** of the 240v leads directly" Why would you do such thing? What if your lead was accidentally on "hot wire"? You would fry your DMM (or fuse inside), or even kill yourself... Please stop doing your own experiments. — Ale..chenski, Dec 07 '18 at 10:58
What I meant in the sentence was simply putting the ammeter directly between the hot wire and the soil, to get the amperage. Of course I won't put myself between the hot wire and soil because it's called electrocution :) Anyway. I wonder how you know the amp measures below 100uA. I guess true toroid autotransformer has certain schematic? Can anyone share a pure toroid autotransformer schematic? Thanks. — Jtl, Dec 07 '18 at 11:01
@Jtl Putting an ammeter across a mains supply (hot wire to the soil) is a bad idea: the ammeter could explode violently, damaging your eyes, or worse. — Andrew Morton, Dec 07 '18 at 15:07
Why would it explode? The ammeter has 10A maximum rating. Are you saying the main supply to soil can flow more than 10A? — Jtl, Dec 07 '18 at 20:45
@Jtl Yes. There could be a surge of several hundred amps for a few milliseconds before the circuit breaker trips or something else interrupts the current. It is even possible to use the ground as one of the mains wires: [Single-wire earth return](https://en.wikipedia.org/wiki/Single-wire_earth_return). A video: [Stock Multimeter Explosion](https://www.youtube.com/watch?v=i9jpwGTy66g). — Andrew Morton, Dec 08 '18 at 08:41

Ale..chenski · Answer 1 · 2018-12-07T07:48:00.257

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First, the 1182 series are not truly "isolation transformers". At least not on the linked datasheet. They are regular transformers with electrically unconnected secondary winding. While the secondary winding is not connected, the area of windings overlap is huge along the toroidal core, and thus there is capacitive coupling, since the whole thing operates on AC.

My take on the difference in readings is that by design, the windings are made in multiple layers, so one end is closer to primary winding, and the capacitive coupling is higher, while the other end is likely on the surface of the transformer, and has higher distance from primary winging.

The value of capacitive coupling could be 1 nF - 2 nF, which is about 3 MOhms, so a high-impedance DMM will see some AC voltage between floating leads and earth ground. And the effective leakage will be about 30 uA, which will be easily grounded once you connect it to your workbench ground.

True "line isolation transformers" usually include a single-layer winding (for toroidal core, and a non-shorted copper foil in rectangular bobbins, as Jeroen3 commented) with only one end exposed, which is connected to enclosure ground. In this case the cross-talk between primary coil and secondary coils is minimized, see Section "Line Transformers"

from main Hammond transformer catalog.

edited Dec 07 '18 at 07:48

answered Dec 07 '18 at 07:15

Ale..chenski

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So if the only connection to the primary is via capacity coupling, do you still call it an isolation transformer? Even in ordinary full fledged isolation transformer, there is still capacitive coupling between the primary and secondary. – Jtl Dec 07 '18 at 07:40
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@jtl Yes it still isolates. It just has poor common mode. True safety isolation transformers only have capacitive coupling to earth due to a metal foil between primary and secondary winding. – Jeroen3 Dec 07 '18 at 07:44
I plan to buy this Hammond 240 series https://www.hammfg.com/electronics/transformers/line/240 Is it full fledge isolation transformer and I won't expect to measure any voltage from any one of the output to the soil? – Jtl Dec 07 '18 at 07:47
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@Jtl, the 240 model is of toroidal type, so windings still go over each other a lot. Good isolation transformer has coils on opposite sides of magnetic core, where the coupling is much less. Unless you demand coupling data from manufacturer, you shouldn't expect anything. Why a 100 uA leakage is bothering you? – Ale..chenski Dec 07 '18 at 07:53
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@jtl no. Still a toroidal. I have not seen those as isolation transformer. You need a [normal iron core transformer with foil](https://www.hammfg.com/electronics/transformers/line/169), or [two separate coils](https://en.wikipedia.org/wiki/Transformer#/media/File:Transformer_winding_formats.jpg). – Jeroen3 Dec 07 '18 at 07:54
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@Jtl, note the marketing bullet on 169 model quoted by Jeroen3 which says: "Electrostatic shield between primary & secondary". This feature is not listed in 240 series. – Ale..chenski Dec 07 '18 at 07:58
I also owned a traditional Hammond isolation transformer, but it is shell type where the windings share the middle legs. Do you know where I can find isolation transformer that uses the core type (where it is at both ends)? I can't find these. I know there are more core loss but need better isolation. – Jtl Dec 07 '18 at 08:01
I bought this month 2 months ago https://www.hammfg.com/electronics/transformers/line/172.pdf It has so strong magnetic field.. so I bought the Toroid above. So if I got the 240 model, I'd still get shock if I or my instrument (accidentally) touch live to ground? btw.. help me find tranditional one with two separate coils because I can't find these anywhere – Jtl Dec 07 '18 at 08:07
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@Jtl, it is getting more and more as "X-Y" problem. You are not telling why "strong magnetic field" is of any importance in "test projects", nor why 100 uA (easly grounded off) is of any concern. – Ale..chenski Dec 07 '18 at 08:11
test projects involve magnetic field sensors, so need least interference. The toroid has 20 times less magnetic field than the shell type. So 100 uA can't shock me? I know shocking would require 8mA. So the toroid I bought is really a semi isolation transformer? – Jtl Dec 07 '18 at 08:14
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@Jtl, if you have special requirements as low magnetic interference, then encapsulate the entire transformer into a thick custom-made magnetic enclosure made of transformer steel or Nu Metal. No one will do it for you. – Ale..chenski Dec 07 '18 at 08:27
I'm looking for a brand of Toroid isolation transformer with electrostatic shielding. There may be one brand which has this. Unless there is just none in the whole world? Is it not possible to put electrostatic shielding in any Toroid isolation transformer? – Jtl Dec 07 '18 at 14:57
Please tell me Ale. I plan to get the same 1182 toroidal series but 1000va (my existing is only 300va). This is to be used on 2 refrigerators that don't have any grounding (our house is so old and just 2 wire). Now can the 1182 series isolates against soil level ground (in case hot wire touches refrigerator chassis). Is it really isolated from primary soil level ground.. or does the poor capacitive coupling which doesn't eliminate common mode still able to protect from ground like what isolation transformers are supposed to do? Please let me know before I buy the 1000va 1182 toroids. Thanks. – Jtl Dec 08 '18 at 06:50

Curd · Answer 2 · 2018-12-07T08:56:23.447

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Why do you assume it is not an autotransformer?
The 3rd bullet point of the pdf you linked contradicts what you stated:

Note: Units are designed to have all windings engaged (either series or parallel connected) or connected as an autotransformer.

In an autotransformer primary and secondary are connected.

EDIT: I think Ale..chenski's answer is right.

Now I understand the sentence quoted above, that primary and secondary are isolated but the manufacturer assumes/suggests that the transfmormer will be connected by the user in such a way, that all windings are engaged or as autotransformer.

edited Dec 07 '18 at 08:56

answered Dec 07 '18 at 06:41

Curd

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see https://imageshack.com/a/img923/6457/a4BM5X.gif An autotransformer only has one winding. The Hammond 1182 toroid has 4 windings. So how could it still be an autotransformer?? – Jtl Dec 07 '18 at 06:51
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Maybe the diagram shows those 4 windings without the connection between them. – Curd Dec 07 '18 at 06:56
Yes. But what does the phase "Note: Units are designed to have all windings engaged (either series or parallel connected) or connected as an autotransformer." mean? How could you have not any series or parallel connection and still have outputs? Any illustration? – Jtl Dec 07 '18 at 07:04
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`Hi-Pot test of 4,000VAC RMS between primary & secondary` is not possible on a autotransformer. – Jeroen3 Dec 07 '18 at 07:41
So the 1182 is a semi isolation transformer? Would one get a shock if one accidentally touch one of the secondary output and ground? – Jtl Dec 07 '18 at 08:16
@Jeroen3: Theoretically the Hi-Pot test could be done **before** the connection of primary and seconadary is made. But then, such a test wouldn't make much sense, if it is connected as autotransformer afterwards. Also not building the transformer from the beginning as autotransformer foils the advantages of an autotransformer (lower cost). Therfore this is in deed a good point.suggesting that it is not a autotransformer. On the other hand the statement I quoted is quite unambibuous. – Curd Dec 07 '18 at 08:50
The quote means that the maximum voltage between the multiple secondaries is limited. They should be connected. – Jeroen3 Dec 07 '18 at 09:06
@Jeroen3: Yes, exactly. They **should** be connected. That's how I understand it **now** too. – Curd Dec 07 '18 at 09:08
But how do you connect it as autotransformer without engaging the windings as parallel or series? This is list of all ways to connect it. No autotransformer mode mentioned. http://hammondmfg.com/pdf/1182_Insert.pdf – Jtl Dec 07 '18 at 11:15
@Jtl: it is mentioned; ust not shown in a diagramm. – Curd Dec 07 '18 at 13:05
Jeroen, I want to buy the 500va Hammond 240 Toroidal transformer this https://www.hammfg.com/electronics/transformers/line/240.pdf to isolate my Refrigerator so in case hot touches the chassis. No one would get shock. So even if the 240 is not a bonafide isolation transformer. It is sufficient even if it has no electrostatic shielding? I can't find any 240v-240v shell or core type isolation transformers and the core loss of such can add so much heat and not to mention billing lost. Thanks. – Jtl Dec 08 '18 at 04:03
Curd. I just realized this could be how it can be connected as autotransformer.. by not using the secondary and only tapping the middle at primary and one of the end right?? see https://imagizer.imageshack.com/v2/640x480q90/923/Oa3m54.jpg – Jtl Dec 08 '18 at 12:56

Why my Toroid Isolation Transformer has 112.3v to soil

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