Protective impedance as MOOP/MOPP - Requirements?

Benjamin Weber

Trusted Information Resource
#1
Hi all,

in IEC 60601-1, cl. 3.60 impedances are listed as one possibility for means of protection (together with insulation, clearances...). But neither is the term defined, nor more details are given regarding the corresponding requirements of such a protective impedance.

The specific case:
A patient impedance is measured by applying a patient auxilliary current (10µA_eff) and measuring the voltage drop over two electrodes, one of which is at system ground, the second is the current source. This patient auxiliary current is generated by a circuit which is supplied by 12Vdc. The current is then limited by the output of this circuit (3,5Veff) and a resistor (330kOhm). This resistor shall be used as means of protection in the case the corresponding supply circuit delivers a higher output voltage, worst case 12V. This would limit the patient auxialiary current to 36µA. But what are the requirements for this resistor?

Creepage distances and air clearances are quite obvious around the resistor and the circuit parts before (supply) and after (patient). But what about dieelectric strength? If the resistor as rated for working voltages up to 50V, do I have to check any additional/higher voltages to see if the resistor will still be OK? I know that IEC 62368-1 has very clear requirements (Annex G.10). But are these applicable here? Or are there any component safety standards for protective impedances?

I really appreciate any help!
 
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Peter Selvey

Staff member
Super Moderator
#2
Although it's nowhere specified, the requirements for cr/cl and dielectric strength are really intended to be applied between "isolated circuits", and not within those circuits, i.e. not around normal SMD resistors, ICs, capacitors, diodes and so on. The requirements are largely driven by mains supply issues (transients, surges etc) which can still reach secondary circuits, but they usually raise the whole "circuit" up in potential rather than individual components. This is because the impedances between isolated circuits are usually much greater than the impedances within circuits. So a 1000V transient in the mains could still appear as 250V transient in a secondary circuit, but all the parts are raised to this potential, so we only need to worry about the isolation between that whole circuit and the patient (for example).

However, there are many cases where safety is reliant on components within a circuit. Patient auxiliary current is just one example. A circuit protecting a mains heater against overtemperature (which is still an electric shock issue due to the potential to damage MOP insulation) may include sensors, resistors, op-amps etc which if assessed for cr/cl/D.S. would obviously fail. SMPS overcurrent/overvotage protection is another example. According to 8.1a these components can be shorted if they do not meet cr/cl/DS, and any control related components as well, resulting ultimately in MOP insulation damage, and eventually exceeding leakage in 8.7.

Obviously, this cannot be the case. So upshot is that we turn a blind eye to cr/cl/DS for these kind of components. Nevertheless, the parts should be used well within ratings and failure of the part should be considered single fault condition.

It's not unusual for a manufacturer to claim that modern electronic components such as an SMD resistor to have high integrity characteristics as the failure rates are often tiny (e.g. <one per billion parts per year) . However this has a legal problem: despite the low failure rates, the part manufacturers usually state not to use the parts in this way. The reason is that they don't guarantee low failure rates, it's more of a economic byproduct. If for example they move production from country A to country B and experience a short period of high than usual failure rates, you can't blame the part manufacturer if that ends up killing a patient. This is very different to say a Y2 cap that is designed, tested and production monitored to have high integrity characteristics.

Upshot: the 330kΩ is good for normal condition (10µA limit), but it should be assessed what happens if this 330kΩ is shorted, or any other components in the circuit such as ESD diodes, op-amp input etc. However this assessment should be only be one component at a time, assuming the parts are used well within specifications, and the limits for single fault condition should apply when a component is shorted.
 

Benjamin Weber

Trusted Information Resource
#3
Hey @Peter Selvey ,
Thank your very much for the detailled answer!

Here are my thoughts:
In my oppinion we could say, that there are two circuits: The supply circuit generating the 3,5V voltage and the output circuit delivering the current to the patient. And the barrier is the resistor. (OK, a resistor is by definition no isolation.)

So I think that the creepages/clearances should be applied to separate those circuits to ensure, that the 12V cannot "bypass" the limiting resistor, e.g. by pollution.

If the 330kOhm resistor is shortened, the 3,5V would go directly through the patient via electrode 1 and back through electrode 2. Not good ;-( The single resistor could be replaced by two 115kOhm resistors. In SFC of one single resistor shortened, the patient auxiliary current would be 30µA which is below the SFC limit of 50µA. (Though cl. 8.7.2, third dash says, that patient leakage and auxiliary currents are not measured in SFC when on single part of double insulation is shortened.)

But still: I have the feeling, that it is not sufficient to require the resistors beeing used within their standard ratings. Somehow I am missing the safety-relevant technical specification (such as dielectric strength for silod insulation). Or would the 115kOhm nominal resistance in conjunction with nominal working voltage of at least 12V be enough?

High-integrity could by applied, but I agree to your thoughts on that.
 

Peter Selvey

Staff member
Super Moderator
#4
I think that creepage and dielectric strength are always needed, it's just that reasonable values in the <15V region are so small that they are pointless to consider as a limiting factor. It's a bit like worrying about the melting point of the plastic net on my table tennis table (it just happens to be what I'm looking at at the moment :) ). Yes, there is a temperature limit, but the room is never going to get anywhere that temperature so not really parameter worth discussing.

The stresses are squared and even exponentially related to voltage, so a resistor rated at 50V and used at 5V is not 1/10 stressed but <<0.01. So a resistor that might have a failure rate of 0.001/year at 75°C, 50V (max ratings) would have a failure rate too small to measure at 5V, 45°C.

Clearance is purely related to overvoltage transients which don't exist inside a secondary circuit.

It always good to test for consistency as well. Patient auxiliary currents are not actually that bad, you can get some minor recoverable tissue damage from them, and for most electrodes the larger surface area means you really need >10mA (not 10µA) to have anything significant. The 10µA limit is based on 1mm² area. Anyway, this is not life and death stuff.

In contrast, a dialysis system has about 10 different ways to kill a patient. Inside the system, safety is reliant on many SMD resistors and similar parts in control and protection circuits. It would be impractical to apply cr/cl/DS limits to all these parts. And there are more frequent examples of electronic protection in SMPS, heaters, motors, mechanical movement etc which protect against high severity harm and rely on SMD and electronic parts. Even protection in lithium-ion packs is critically dependent on small electronic circuits that would not pass 601-1 cr/cl/DS.

So, in reality it's normally overlooked, in most cases without being aware of it. Probably too much, because there are cases where a part is being pushed close to ratings e.g. a transistor running at max voltage/temp/power should be considered shorted in normal condition similar concept to 8.1a. But this is very rare. Most parts are used well within ratings, so the assumption of being subject single fault assessment (strictly faults in a single part only, not multiple parts) is reasonable.
 
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