How to determine rated input power for the markings?



Within our organization we have some discussion wrt rated input power.
Approach A:
Sum up all the maximum power of the supplies that are in the MEE. This is about 200W.
Rationale: IEC60601-1 3ed Clause 4.11 last sentence: A supplier certification may be used in place of the above measurement as the basis for steady state current or input power specification.

Approach B:
Measure the input power (for 100/240Vac 50/60Hz) and either use single value or a range?
And include some realistic design margin (eg 5%) This is about 80W.
Rationale: IEC60601-1 3ed Clause 4.11: The steady state MEASURED input of the MEE or MES at RATED voltage.

The result in both approaches is imho too big 200W vs 80W.

I think that rated value on the marking should make sense and therefor e not be too much over rated?

What do you prefer?

Peter Selvey

Super Moderator
I think the last sentence in Clause 4.11 is intended as a get out clause for labs testing very large equipment (permanently installed, 3 phase, linear accelerators etc running at 50kVA type of equipment ...).

There is no upper limit on the rating. The standard allows a 10% tolerance so you can select a typical value (e.g. 75W), and it is no problem if the actual value is 80W.

That said, most manufacturers usually include a moderate margin, e.g. a device with a real input of 75W would normally use a 100W rating. The reason I guess is to allow for upgrades, new features etc that can be added to the secondary circuits without needing the marking to be updated.

I agree that it does make sense not to go too far with the overrating, but it is not as critical for small low power equipment (<150VA). In practice a user worried about overloading the circuit (causing a breaker to trip) would be looking out for equipment around 500VA or higher. Excessive overrating in high power equipment might cause a user to unnecessarily worry.

Note that a marking in W is unusual for low power equipment. Usually the power factor is less than 0.9 so a marking in VA is required. Smaller power supplies may not have power factor correction circuits, or they may not be effective at small loads.
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