HV Test of HF Surgical Unit

[Roshan khandare]

Hello,

We test the HF Surgical Unit . If suppose working voltage is 890Vrms Between live and applied part then as per the table 6 of 60601-1 we have applied test voltage 2 x (√2U+ 1 500) (approx : 6.3 KV). But EUT is not pass on 6.3 KV , so as per the particular standard 201.8.8.3
lf, during dielectric strength testing of solid insulation forming MOPP, a breakdown or flashover occurs at the Creepage Distance specified in
8.9 of the general standard and 201.8.5.1.2 of this standard, then the test shall be carried out on the components which provide MOPP, such as
transformers, relays, optocouplers or Creepage Distance on printed circuit boards.
So can we test on transformers, relays, optocouplers components? And how much KV we have applied.


Regards
Roshan

 

[Peter Selvey]

Yes, normal dielectric strength is for solid insulation only (usually some type of plastic), it's a kind of ageing test.

And most modern materials will be fine, recently I tested normal 300V PVC insulation at 10kVrms and it didn't break down.

But it is difficult to do in practice as you need to isolate out the plastic material and test. Even testing components may have trouble. For example, testing a transformer may still have cr/cl path for the windings around the edge of the plastic insulation. You really need to cut apart the transformer and test the plastic on its own. Or ask for a sample of the transformer bobbin without any winding and test that with foil on each side, making sure there is enough space. Same for relays etc.

I am surprised that IEC 60601-2-2 does not use any modified values. The ESU voltages that are output are highly regulated and only appear briefly. Therefore the insulation is not subject to long term stress or significant transient voltages. The test values in IEC 60601-1 are really designed around mains voltages that are long term with high transients.

However, the test should take into account the peak voltage, which may be around 6kV anyway. In my opinion, the test for the unit should be the same as the test for accessories, Vpeak (test) = Upeak +1000V. For example if the open circuit output voltage is measured at 6kVp, the test voltage peak should be 7000Vp, which can be 7000Vdc or 4950Vrms. I think the HF dielectric strength test should also be done, but for components (it would be difficult to test an assembled unit as the capacitive load would be too high).

Finally to note that measurement of peak and rms at high frequency is difficult to do accurately. If you not done any checking or validation of the measurement system at the frequency of interest, then assume an error of ±20%, and keep in mind that errors of ±50% are possible with HV probe/oscilloscope set up, even if "calibrated". The calibration is for dc/low frequency only and does not apply at high frequency. Plus there are common setting errors in the scope that can have a large impact on the measured values.