Number of MOPP for Type BF Applied Part (Medical Device Electronics)

#1
Hi,

I'm trying to understand the requirements for MOPP. The device I am considering is a battery (~3V) powered device with multiple patient connections(ecg electrodes) considered as one Type BF applied part.

We clearly need to meet the leakage current requirements of 8.4.2 a) and the expectation is that we would use a series impedance to limit the current that could flow to the patient connection. No current is intentionally supplied to the patient so this would only be to protect against an internal fault that applied a voltage to the patient connection circuitry.

So we would be single fault safe and this resistor would be one MOPP. What I cannot think is how to get the second MOPP?

This must be a very common scenario, what am I missing?
Thanks
Stephen

8.4.2 ACCESSIBLE PARTS and APPLIED PARTS
a) The currents from, to or between PATIENT CONNECTIONS shall not exceed the limits for PATIENT LEAKAGE CURRENT and PATIENT AUXILIARY CURRENT specified in Table 3 and Table 4 when measured as specified in 8.7.4.

8.5.1.1 General
ME EQUIPMENT shall have two MEANS OF PROTECTION to prevent APPLIED PARTS and other ACCESSIBLE PARTS from exceeding the limits specified in 8.4.
 
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#3
The MOP (MOOP and MOPP) and mainly written around electric shock from mains parts, and can be somewhat hard to read or not appropriate when considering shock from internal circuits.

In the case of an ECG, there is usually a concern for the right leg drive (RLD or any noise cancellation circuit). Usually, all measurement leads (LA, LL, RA etc) are high impedance op-amps so there is negligible leakage in normal use. But for example if one of the input protection diodes shorts to rail, the RLD will go opposite to rail. To protect this the RLD normally has a large resistor like 1M (e.g. 3V/1M = 3uAdc which << 50uA limit in SFC).

The question then is whether this is a 2 MOP system (1 MOP = diode; 1 MOP = resistor). But diode is just one of many possible faults, and difficult to characterise and control as an MOP. So it is better to think of the 1M resistor as a high integrity component (never fails, equivalent to 2 MOP). It's OK because the 1M resistor is rated to 100V but used at 3V, clearly never going to fail. Or to be extra safe you could break the 1M into 470K + 470k to make a real 2 MOP system.

And if you wanted to go crazy, measure spacing (creepage/clearance) around the resistor. But this is a point where 601-1 is fundamentally wrong, the 3.4mm for 2MOPP is way over the top. The value in the standard assumes overvoltage transients which can still occur between secondary circuits, but they can't exist at the board level inside the circuit.

Test labs don't normally go that deep and may not even consider the risk from a RLD circuit. Or maybe short the "RA input diode D4" but just report the result without figuring out what provides the protection.

If you have respiration (using higher frequency impedance measurement) again you need to look at the circuit, figure out where the shock could come from, what would be worst case fault, and what component keeps things under the limits. Often it is a low value capacitor in series with high frequency oscillator (e.g. 60kHz). Again you might treat the cap as a high integrity component or make two in series to create a 2 MOP system.

If there is any other applied part e.g. temp probe, they usually have plastic barrier which can be tested for 500V to make 2 MOPP.
 
#4
Hi Peter,

Thanks so much for your help! We are going to go with your advice and do two 470k on RLD to be extra safe as you recommend. However, we want to stay with our 0402 resistor size which is 50V not 100V. I think we can say it is clearly not going to fail in a battery powered device with 3.3V.

Please reply if you think this is not good...either the small footprint resistor or the 50V rating.

All the best,

Kevin
 
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