MOPPs for a BF part

Question -

Does a floating applied part need 1 or 2 MOPP separation from a protectively grounded part?

(It's all in one box, just one AP, plastic box with earthed metal panels. Let us assume mains is 250V and the working voltages are 50V).

Thanks in advance to anyone who can help.

See the rationale at Annex A Clause 8.

 

One, according to 8.5.2.1.

 

If the applied part has Uap inside it should have 2 MOPP @ Uap to prevent a path through the patient (or unintended paths in case of stimulators, electrosurgery etc where there are also intended electrical paths). This could involve having 2 MOPP @ Uap to earth, but there are other possible constructions.

Typically Uap is very low (e.g. 5Vdc max), and the Type F insulation requirements (1 MOPP @ 250V) exceed 2 MOPP @ 5Vdc. So, the 2 MOPP rarely appears in insulation diagrams. And in practice the 2 MOPP can appear in many ways, for example, from circuit to earth is 1 MOPP, from the circuit to patient is 1 MOPP (sensor insulation). Or, the circuit is constructed that the higher voltage is shielded from the patient (the 0V point is kept on the patient side of a sensor or transducer).

All these methods often exist in the real world thanks to common sense of designers, but rarely shown or investigated in test reports.

Checking Table 6 it seems you need a fairly high voltage (354Vp) in the applied part to make 2 MOPP in a secondary circuit >> 1 MOPP for mains part.

But in Table 12 it seems to be just 30Vrms/43Vdc. This reflects some of the crazy distances in IEC 60601-1 for MOPP at lower voltages. I have never had anyone explain why you need 3.4mm to protect against 0.1-17Vdc. I think it would be a work of art to get 0.1mA leakage across a 3.4mm internal barrier with even with a 17Vdc source.