Alternatives to Metal Foil for touch current measurement of enclosures made of insulating material.

[Kenny03]

Hello,

first time posting here and excited to be a part of this community, so we've got two parts (a power adapter and an applied part) in our system with enclosures made of insulating material. So, as part of production line safety tests, we wrap these parts in aluminum foil to measure touch current for the power adapter and patient leakage current for the applied part.

So I got a request from the production team asking "Is there a better way to do this?". We'd prefer not to use aluminum foil. is it possible to simplify this to use a metal plate or box? Would appreciate any tips and recommendations here.

 

[Peter Selvey]

As part of design tests (type tests) it's good to do the foil method, but not in production. Production tests should be tailored to suit the design. For example, the secondary circuit of the power adaptor should pass the touch current test for operator. If so, foil around the adaptor is a waste of time, since it will obviously comply as well being one step removed. So you could do the test direct to the secondary rather than to the foil, which is much easier to implement and far more reliable, and should yield consistent results that can be analysed (e.g. if leakage changes >10%, you can tell something is changed and ask adaptor manufacturer what is going on). Or the adaptor might be fully certified with it's own production tests monitored by a third party, and this includes secondary touch current, if so you can skip this in your own production line tests.

Patient leakage can also be simplified based on the design, and it can even be eliminated if it is blindingly obvious by design that it can never fail.

There are always special cases for example a medical ultrasound probe might rely on a sealed joint to protect against internal voltages/currents reaching the patient. In this case, it's reasonable to do patient leakage current tests with the probe immersed in 0.9% saline. This is better than using foil since it's hard to ensure the foil always contacts all the critical points.

 

[Kenny03]

Hi peter,

thanks for the reply.

Ah yes, i see your rationale, as touch current limits are intended to protect against the fault case where the secondary side hot line is shorted to chassis. And so if we directly connect to the secondary side hot line for our touch current measurement and we pass, then we certainly will pass with the foil wrapped adapter touch current. Have i got that right?

As for the applied part, yeah i saw that alternative method mentioned in the standard but that really only works for seal/waterproof parts and unfortunately, ours is not. It seems like we're stuck with the foil wrapped option in this case.