Dielectric Strength of Triple Insulated Wire to comply with 2MOPP

Roland chung

Trusted Information Resource
#1
Hello all,

As we know, Annex L of IEC 60601-1:2005 is derived from Annex U of IEC 60950. After reading the Annex L closely, I found it is really difficult to find a triple insulated wire (TIW) to comply with 2MOPP since the test voltage of TIW for 2MOPP is too high.

For example, the max.measured voltage between HF transformer is 590 Vpeak. According to Table 6, the test voltage for 2MOPP is 4664 Vrms. So the test voltage for TIW is 9328 Vrms based on the L.3 (i.e. The test voltage is at least twice the appropriate voltage in Table 6 and Table 7).

Any thoughts?

Thanks and regards,
Roland
 
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Peter Selvey

Staff member
Super Moderator
#3
This is an issue (problem) at a couple of points where IEC 60601 has adopted IEC 60950 tests without careful consideration. For example, Clause 8.9.3.3 (cemented joints) requires test voltage to be increased by a factor of 1.6. And Annex L requires double.

For IEC 60950 with moderate starting point in test voltages, this is not as big an issue (e.g. 1kV/3kV basic/double).

However, for medical the standard values of 1.5 / 4kV (and higher with even switching peaks) factors of 1.6 and 2.0 put us in an impractical region.

The 1.6 factor has already caused problems with diagnostic ultrasounds, as they often rely on cemented joints at the probe. Endoscopic type probes, designed with space constraints, can pass 1.5kV but breakdown at 2.4kV. Already 1.5kV was overkill, but 2.4kV is getting crazy.

For Annex L, at high voltages there will be a large amount of corona discharge, especially for thin insulation like triple insulated wire. Corona discharge is not a failure, but it does cause heating. I would guess that at 9.2kVrms (peak around 13kV) the corona based heating would be enough to destroy the insulation.

Of course, one solution is to treat the insulation as 2MOOP, which brings everything back to IEC 60950 requirement. In such a case, it would be necessary to have 1MOPP somewhere else in the equipment.
 

Roland chung

Trusted Information Resource
#4
Thank you very much for your input.

Actually, the SMPS has been approved per iec 60601-1:1988/A2. And 60950 compliant TIW (6 kV) is fine for 2nd edition.

The SMPS is now applying for iec 60601-1:2005 approval. I was told that the TIW can not meet the requirements of Annex L based on the review of TIW certificate and data sheet.

The TIW supplier told us that there is no a TIW having electric strength higher than 8 kV in the market. Even the TIW is 60601-1 compliant, the test voltage for TIW is 8 kV only since the working voltage is on the basis of 250V.

It seems we have to degrade the SMPS to 2MOOP unless both primary and secondary windings are TIW.
 
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