Are multi-faults needed? Effects of a Short Circuit

Roland chung

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#1
The principle said short circuit of the spacing which less than basic insulation is considered as normal condition. However, if the short circuit (regarded as normal condition) causes equipment to loss the functions obviously (detectable), whether multi-faults specified in 60601-1_3rd edition are required in this case?
 
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Peter Selvey

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Super Moderator
#2
I've written a rather lengthy (too long!) series of articles on SFC and protection system design: http://www.medteq.info/med/Protection_Systems

In summary it is recommended to shift the focus from "faults" to "protection against hazardous situations".

I believe the shorting of a spacing less than basic was intended to be used specifically with respect to direct "means of protection" against electric shock, i.e. basic and supplementary insulation.

It's use comes up in rare cases where it is necessary to prove to the manufacturer that their insulation design is not really good.

It does not fit to general (electronic) protection systems as these systems are not designed with internal spacing meeting basic and would be impossible to do so, and unnecessary.
 

Roland chung

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#3
Peter, your article is really too long but is very good. It does make sense.

Actually, the standard does not exclude the general (electronic) protection systems. If short circuit of internal spacing less than BASIC could fail the protection systems and the equipment also operates normally (undetectable), it could be a hazard.

For my case, I'm sorry I did not explain more clearly. In secondary side of equipment, we employ a high frequency transformer (30 kHz) to drive a handpiece. Because the spacing between input and output of high frequency transformer is far less than BASIC (reference voltage: 150Vrms). The shorting of such spacing is therefore regarded as normal condition per standard. After short circuit, the equipment could not remain functional and the leakage current from handpiece to earth was about 0.4mA. I am confused that whether this case can be considered as SFC? In SFC, the leakage limit is 0.5mA and therefore we can pass the standard.
 
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Peter Selvey

Leader
Super Moderator
#4
Yes, in this case it is clear the spacing is specifically with regards to electric shock, so it can be shorted as normal condition and the equipment fails the standard.

I assume that the 0.4mA was measured with the MD filter in place, meaning the "raw" leakage was about 12mA @ 30kHz. So this would fail also the 10mA leakage limit.

From experience the actual measured values are not all that stable for these systems. One reason is that the output is usually a tuned circuit with feedback and load dependence, so the voltage source is not stable. The other is that at 30kHz stray paths become significant.

One design solution for such systems is to "capacitively earth" one electrode of the transducer (the side that is closest to the patient). This capacitance then limits the leakage at 30kHz (while still providing some isolation at 60Hz).

But in reality there are a couple of reasons why this is not a significant risk:

1) the spacings in IEC 60601-1 are based on several worst case assumptions, which are unlikely to apply in a secondary circuit with regulation. In a typical ultrasound transducer design there are at least two transformers between mains and the output; pollution degree 2 applies and the spacings on a PCB with low tracking index. I would recommend looking at making sure the spacings are at least basic using IEC 60664-1 values, rather than IEC 60601-1.

2) At 30kHz, the risk of shock dramatically reduces, even more than what the MD filter compensates for. If the contact area is large (>>1mm2) also the 10mA limit is not a concern.

(by the way, I agree the standard does not exclude electronic protection, but in practice it is treated this way. If we did not, all switch mode power supplies would have to be withdrawn from the market!!).
 

Roland chung

Trusted Information Resource
#5
I agree with you. Similar equipment already exists in the market for years and no electric shock incidents caused by the shorting of HF transformer have been reported. Anyway, it is always a hard job to convince the test house. Murphy's law said if anything can go wrong, it will.

Regarding the switching power supply, the electronic protection actually bypassed when do the overload test. So I would think the standard does not exclude electronic protection. One German expert from TUV PS told me that it is typically improbable that within the expected service life (12-15 years) more than 2 independent failures will occur which are not detectable based on functional safety basic. Therefore only 2 independent failures are needed (one example for SMPS, failure of electronic protection + overload) to be simulated at the same time.
 

Peter Selvey

Leader
Super Moderator
#6
Re electronic protection: I worked for TUV PS for 7 years and this was a sticking point. Multiple faults evolved from high risk devices, where it makes sense, but when applied to electrical safety it suffers gross overestimation of probability associated with multiple events (see the section on Probability in the above protection system article).

There are a number of holes in the theory which can be picked at ranging from the need, practicality, consistency and impact to regular production.

In my experience it's also not that widely implemented, even within TUV PS. I've heard of the test being done in Munich, but nowhere else.
 

Roland chung

Trusted Information Resource
#7
Yes, you are absolutely right. There are a lot of particular points from TUV PS in Munich. I know some experts in Munich really have many experience and the points from them seem reasonable.

As I know, the point for SMPS mentioned above had been written into WG 14 recommendation (IEC/TR 62296). I think you must know that the convener of WG 14 is from TUV PS Munich.
 

Roland chung

Trusted Information Resource
#8
With respect to the probability, it seems no consistent rule to determine how is suitable. So different manufacturers can make out different probability for similar risk and similar equipment. That said, the process can be subjective. And I have never received the rejection from the auditor of Notified Body.
 
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