M
MDDPeter
Hi All,
We are embarking on a new design and we were wondering to what degree single fault conditions on electronic PCB components must be considered in the design. All PCB mounted components are likely to be surface mounted.
In particular we are referring to the circuits that might be patient contacting where the 10uA current limit applies with a 50uA limit under single fault conditions.
For a Class II medical device, battery powered, non-mains connected, non-earth connected and fully enclosed in a plastic enclosure (with the exception of patient contacts), what do people regard as being adequate single fault risk analysis (ISO 14971 also ties into this).
Two-lead passive components (caps and resistors)
For example, if a patient contacting probe is supplied by a 3V DC battery and current limited through two resistors before returning to 0V. One resistor is 10K, the second 330K? would a single fault short circuit in one of the resistors need to be considered as a potential fault? Note that if the 330K resistor fails short, the current might exceed the 10uA limit. The likelihood of a surface mount resistor going short circuit in electronics is extremely low.
Simple semi-conductors (e.g. MOSFETs)
Similar to the above, if you introduce a MOSFET to switch the current on and off. Would you consider a MOSFET going short circuit as a potential single fault? Again, MOSFETs are very reliable components if they are operated within their bounds.
Integrated circuits (e.g. logic gates and MUXs)
Again, based on the above, if you include some type of multiplexer or other IC, would you consider this device failing as a short circuit to 0V as a failure mode? If it were to go short, there is a risk that the 330K resistor might be shorted and thus current higher than 10uA might flow.
Programmable code executing semi-conductors (e.g. micro-controllers)
If we now tie the input to an analogue input on a micro-controller, there is a risk of failure whereby the micro fails creating a short to ground, again bypassing the 330K resistor. Would that be a single failure mode that would need to be mitigated against?
The IEC 60601-1 standard and ISO 14971 risk analysis standard are a little vague on this and allow for ?expert opinion?.
I guess part of the question is, where do you stop with your single fault risk analysis?
We are embarking on a new design and we were wondering to what degree single fault conditions on electronic PCB components must be considered in the design. All PCB mounted components are likely to be surface mounted.
In particular we are referring to the circuits that might be patient contacting where the 10uA current limit applies with a 50uA limit under single fault conditions.
For a Class II medical device, battery powered, non-mains connected, non-earth connected and fully enclosed in a plastic enclosure (with the exception of patient contacts), what do people regard as being adequate single fault risk analysis (ISO 14971 also ties into this).
Two-lead passive components (caps and resistors)
For example, if a patient contacting probe is supplied by a 3V DC battery and current limited through two resistors before returning to 0V. One resistor is 10K, the second 330K? would a single fault short circuit in one of the resistors need to be considered as a potential fault? Note that if the 330K resistor fails short, the current might exceed the 10uA limit. The likelihood of a surface mount resistor going short circuit in electronics is extremely low.
Simple semi-conductors (e.g. MOSFETs)
Similar to the above, if you introduce a MOSFET to switch the current on and off. Would you consider a MOSFET going short circuit as a potential single fault? Again, MOSFETs are very reliable components if they are operated within their bounds.
Integrated circuits (e.g. logic gates and MUXs)
Again, based on the above, if you include some type of multiplexer or other IC, would you consider this device failing as a short circuit to 0V as a failure mode? If it were to go short, there is a risk that the 330K resistor might be shorted and thus current higher than 10uA might flow.
Programmable code executing semi-conductors (e.g. micro-controllers)
If we now tie the input to an analogue input on a micro-controller, there is a risk of failure whereby the micro fails creating a short to ground, again bypassing the 330K resistor. Would that be a single failure mode that would need to be mitigated against?
The IEC 60601-1 standard and ISO 14971 risk analysis standard are a little vague on this and allow for ?expert opinion?.
I guess part of the question is, where do you stop with your single fault risk analysis?