Insulation from MAINS of the PE of a CLASS I PS used as FE


Involved In Discussions
My design uses a medical grade, CLASS I type-BF compliant AC/DC converter (power brick), thus 2xMOPP between MAINS and SECONDARY output. The APPLIED PART will be also isolated by a 2xMOPP barrier. The PROTECTIVE EARTH of the PS will be used as FUNCTIONAL EARTH as we believe it will be less challenging meeting EMC requirements.

The ENCLOSURES will be plastic, with the exception of a metal adapter that may be attached to mount the ME EQUIPMENT to a support (e.g. a pole), but the nuts that fix the adapter will be embedded in plastic, giving the 2xMOP insulation.

8.6.9 states
The insulation between internal screens, including internal wiring connected to them and ACCESSIBLE PARTS, shall provide two MEANS OF PROTECTION.

Other than the aforementioned insulation by the plastic ENCLOSURE that will guarantee the 2xMOP, is there any need, for instance, for the insulation between the FE itself (which is a PE by construction) and the MAINS lines? Would the plug be considered ACCESSIBLE PART in NORMAL USE? I think not, but I am not a hundred percent sure.

Peter Selvey

Super Moderator
8.6.9 is not applicable as the power supply is Class I.

Electrical devices connected to secondary output of a power supply are in principle Class II but many of the key requirements don't apply because there is no mains connection. So for example 7.2.6 which requires the double box symbol on Class II devices looks applicable but if you look closer the title of the clause is "connection to the supply mains" and therefore is totally N/A for secondary devices. Clause 8.11.5 for fuses again looks to require a fuse but the clue again is is in the title ("mains fuses"). Finally in 8.6.9 where it mentions "power supply cord" which in turn is defined as cord for connection to the supply mains. So again N/A for secondary powered devices.

This makes sense as well, there is no safety reason to have 2 MOP between a functional earth derived from a Class I power supply from accessible parts in a device connected to the secondary. It's not practical or reasonable.
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