Interpretation of cl. 8.9.1.12 requirements for secondary circuits

EMengineer

Involved In Discussions
Hi, I would like to ask about cl. 8.9.1.12, which states that where a secondary circuit is not earthed and is derived from a supply mains, the circuit shall be subjected to the requirements for primary circuits in Table 13 and Table 14. I wonder where this air clearance separation shall be applied? Let's say there is a device which primary circuit is separated by 2MOOP for mains voltage from the secondary circuit and this secondary is enclosed. Assume the device has a 12V output.
Should this output be separated by air clearance for the primary circuits from the operator? Or should it only be considered when secondary voltages are above 60 Vdc or 42.4 Vac?

I'm trying to imagine the risks when the secondary is not earthed or does not have attenuation means. If this is the case, then the secondary circuit cannot have contact with the conductive enclosure and other parts that the operator could touch because of the risk of transient voltage on the accessible parts?

In another scenario, if the secondary has attenuation means, it could be connected to the conductive enclosure and be reachable by the operator (as long as it does not exceed 60Vdc)?
 

Peter Selvey

Leader
Super Moderator
IEC 60601-1 is not well written for this situation. In the background, there is a risk if even low voltages like 12dc are connected to a patient in a special way, they can potentially cause harm. However in practice the "special way" is difficult to do and would rarely happen outside of applied parts like ECG electrodes or catheters, infusion pumps.

Unfortunately, IEC 60601-1 deems all voltages above 0.1V as dangerous, which is not practical.

There is an exclusion under 8.4.2c which can be used to allow connectors with <60Vdc, 42.4Vac to be accessible but it requires risk management and a warning for operators not to contact the patient at the same time as touching the connector. Note however that the risk is based on probability of harm, not the probability of contact which are orders of magnitude different.

Since the connector is directly accessible, by common sense this could also apply to circuits connected to the connector, i.e. all 2MOP secondary circuits <60Vdc and 42.4Vac, even if not accessible, using the same risk assessment.

Also in practice third party test labs don't look closely a secondary circuits. It's not feasible. Sometimes they do, but most apply common sense. In a broader sense, it is important to realize it is not just the third party test, but manufacturers are also obligated to have controls in design and production to ensure continued compliance, and these are an order of magnitude harder than the test. Plus, any change to the secondary circuit would trigger a re-assessment. All of this is nonsense, in general.

Again, there are special cases and it would be better all round if the standard identified the special cases and applied strict rules that are properly enforced, rather than applying broadly and everyone just ignores, including in the special cases where it is important.
 
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