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Triple insulated wire for transformer isolation
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Triple insulated wire for transformer isolation
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  Post Number #1  
Old 17th April 2018, 10:58 AM

Total Posts: 3
Please Help! Triple insulated wire for transformer isolation

This is my first post here but I was hoping for some advice.
I am redesigning a power supple for a piece of medical equipment, which is subject to 60601 Pt 1 and Pt 2.2. The transformer is subject to a dielectric stress test of 3kV AC 50Hz.
I am considering using triple insulated wire as this makes construction far easier, and leads to a smaller, more efficient transformer.
The rating of the wire is 700V, which is below the operating voltage of the transformer primary.
The breakdown of the triple insulated wire is said to be 'approx. 700V', however there is no certificating for this. I was considering using the triple insulated wire for primary and secondary, for extra safety.
My question is, is the passing of the dielectric stress test a matter of just passing the test, or must thee material used for the insulation be certified to some recognised standard?
Thanks in advance

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  Post Number #2  
Old 18th April 2018, 08:15 PM
Peter Selvey

Total Posts: 899
Re: Triple insulated wire for transformer isolation

To answer the last question directly, no it is not sufficient just to pass the dielectric strength. Components should be used within ratings unless there is a justification in the risk management tile (see IEC 60601-1, Clause 4.8).

Separately, there are some unclear points in the description.

It seems you are considering IEC 60601-2-2? These days a typical construction has mains circuit --> intermediate circuit --> HF output.

It is not clear if the transformer is between mains:intermediate or intermediate :HF output. There are different requirements, it could be worth to check.

Next, it is mentioned the rated voltage is 700V, but the actual working voltage is higher, and it has to be tested for 3kV dielectric strength. Those numbers don't seem to add up, as 3kV dielectric is normally associated with mains voltages up to 150Vrms only, so if the working voltage is more than 700V (peak or rms) it would imply more than 3kV is needed. Anyway, first it would be good to know what the actual working voltage is (both peak and rms), and again the location in the circuit to see if 3kV is the correct figure.
Thanks to Peter Selvey for your informative Post and/or Attachment!
  Post Number #3  
Old 19th April 2018, 05:44 AM

Total Posts: 3
Re: Triple insulated wire for transformer isolation

Hi Peter,
Thanks for your reply, I will try and answer your points.

The transformer is mains to intermediate. Actually the stress test is 3.8kV RMS. The rated voltage of the wire is 700V. The actual voltage across the transformer primary is approximately 550V peak, though the insulation will not be subject to this voltage as the windings are single layer across the bobbin width.
I was concerned about the stress test, primary to secondary. The wire has a typical breakdown of 7kV, though there does not seem to be a rating to a standard. I know TIW is used widely so I thought I must be missing something. I was considering using TIW for both primary and secondary.
Alternatively I could use tape and margins/ sleeving for the secondary.

On the face of it the TIW for both seems the better option however.
Thanks again
  Post Number #4  
Old 20th April 2018, 12:07 AM
Peter Selvey

Total Posts: 899
Re: Triple insulated wire for transformer isolation

It sounds like it should be fine. It easy easy to get confused between ratings dielectric strength (stress tests).

Dielectric strength is like an ageing test, so the values are quite different to the actual rated voltage of the material. It's a bit like leaving a bit of plastic in the sun, it slowly degrades over long time as the radiation picks off the odd errant electron causing a molecule here an there to rearrange. An electric field does the same thing, the insulation slowly degrades over time. But we don't have 40 years to test at 250Vrms, so we bump up the voltage by a large factor and run for a short time (e.g. 4kV/60s), and using exponential relation between volts/time we can predict that the insulation will last at least 40 years at 250Vrms.

But that does not mean the insulation is rated at 4kV, the rating still stays at 250Vrms.

If your insulation is rated at 700V, it should have no problem to pass the 3.8kV.

The TIW ratings might also be a bit fuzzy as there is a separate aspect which is not really covered by standards or test agencies - high frequency dielectric strength. A material might pass 4kV/60s/50Hz but still be hopeless at say 100kHz. The TIW manufacturers know this and generally use ETFE which is good for high frequency but it's not clearly what the limits. It is likely that even ETFE would fail at 4kV/60s/100kHz. But for the high frequency side it should not be an ageing test, probably +100% is enough (e.g. rating 700Vrms, test at 1400Vrms). Anyway the area the TIW manufacturers might be reluctant to make to a dielectric strength claim on the same page as claiming OK for use at high frequency.

  Post Number #5  
Old 20th April 2018, 04:43 AM

Total Posts: 3
Re: Triple insulated wire for transformer isolation

Hi Peter
Thank you that is very useful information.
It ties in with my feeling that the stress test is a voltage far in excess of the working the insulation will see. The transformers are 100% tested, as is the finished equipment.
I had read about the dielectric heating effect when higher frequencies are used. The wire I intend to use is ETFE, which has a very low loss factor, as you mention.
Best regards

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