Single Fault Testing for Secondary Circuits (60601-1 section 13.1)

JerryB8

Registered
Hello - This is my first time posting here, but I'd like to thank all of the contributors for the useful trove information on this forum.

We are designing a medical device that has a reasonably high power, low voltage (<60 VDC, slightly more than 100 VA) circuit outside of the fire enclosure. Looking at section 13.1.2, there is an out clause for single fault testing under this section if secondary circuits meet all the following conditions (second dash):
- flammability of v-1 or better...
- 60 VDC or less in normal condition and SFC...
- limited to 100 VA OR less than 6000 J in SFC
- Use wire insulation of PVC, TFE, etc...
Then, there are some tests to verify that the circuits draw less than 100 VA.

I have two questions:
1. The tests say nothing about the 6000 J requirement; is it permissible to have a secondary circuit that would normally draw more than 100 VA but has protection that activates fast enough so that the energy available would be less than 6000 J?
2. Is there any rationale for the wire insulation types listed here? This seems unnecessarily restrictive, and wondering if other (better) insulation types would be permitted without too much fuss.

Thanks in advance!
 

Peter Selvey

Leader
Super Moderator
For the 6000J requirement, yes but keep in mind that 6000J == 100W for 60s. So if you had a system that only required short term use e.g. a motor that pulled 200W for 10s, and then had a protection system that operated at a margin above that but at a margin below the 6000J limit e.g tripped at 5000J, then yes, you could claim to stay under 6000J in SFC. But I suspect that such a system would be rare in practice, mainly because the engineers would not want to do the math :)

As for the wiring material, what types did you have in mind?
 

JerryB8

Registered
Ah, I see, I slightly misread that clause. I see how this time restriction would apply now. This clause would still hold for circuits that continuously draw under 100 VA, however?

TPU seems to be quite common and often recommended for medical applications (among other types). Is there some precedent this is based on? Just unclear on the rationale behind this, and I wonder if this is based on some dated information?
 

Peter Selvey

Leader
Super Moderator
The 100VA is a SFC condition, so in practice there would be a margin between the normal use and the trip point of the protection (to prevent false triggers due to component tolerances), and margin again to the limit (to ensure a third party doesn't fail due to component and measurement system tolerances). So for example the circuit might practically run at 60VA, have protection that operates at 80VA, which ensures that in SFC the 100VA limit is not exceeded.

TPU is probably dated since the clause itself it most likely cut and pasted from an earlier versions of IEC 60950. It might be interesting to see if the latest standard for IT (IEC 62368) has an updated list. Personally I never heard of TPU, a quick search seems to indicate good mechanical properties but only average on the electrical front, with a very high dielectric constant and dissipation factor. Should still be OK for low voltage/low frequency applications.
 
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