I think the latest medical electrical safety standard (BS EN 60601-1:2006) regarding medical systems states that 5mA earth leakage (NC) and 10mA earth leakage (SFC) are acceptable limits. The rationale states that the higher leakages do not present a significant risk above and beyond previous limits for earth leakage.

This encourages an increase in the number of connected devices, via MPSO, in my view but there is a lot more guidance and more explicit requirements when considering the use of an MPSO in the 3rd edition standard. Also the stuff published by NHS Estates and MHRA on appropriate use of trailing MPSOs, etc.

Acceptable limits for earth bond appear to have been pitched a bit higher for systems at a maximum of 0.4Ohm if MPSOs are used to interconnect devices producing medical systems. 62353 allows earth-bond up to 0.5Ohm as far as I'm aware - seems like acceptable earth bond limits are creeping-up slowly, generally speaking.

No doubt this current thinking on the use of MPSOs and relaxation of some EST limits is in line with the tendency for more devices to be interconnected as systems producing longer earth paths; plus acceptance of the fact that as long as additional earth bonding, non-conductive shielding, and mains separation, is used to limit patient leakages, under NC and SFC, then the other limits can be relaxed somewhat.

But we have to be careful - a few months ago I tested an endoscopy stack system where nearly every (well used) demo-stock IEC 606320 mains connector attached to each of the medical devices in the system exceeded 0.3Ohm (some >0.5-0.6Ohms).

It was the functional video connections (that can be removed/reconfigured anytime), in parallel with permanent, protective ones, that hid this fact until individual EST was performed on each device in the system (as it should). So beware.

Also, looking at MEIGN and references to group 2 locations, i.e. theatres using LIMS systems and suchlike, the recommendations from MHRA/IEE to install systems in the UK with 60601-1 compliant isolation/separating transformers - is apparently a means of imprvoving on safety aspects not necessarily covered the IEC standards for mains distribution in medical areas, perhaps.

The implication is that the connections of non-medical devices and MPSOs to medical devices are recognised in the latest manufacturing standards as is the requirement to improve the safety built into hospital distribution systems intended to connect to high risk areas.

This leads me to conclude that risks must be considered more acceptable as long as distribution & the medical systems that use MPSOs meet the requirements of the new 60601-1 standard, I suppose.

However this still means that selection and testing of appropriate MPSOs for use in medical systems becomes more important for those of us that must recommend their use in systems, re-configure existing systems or carry out acceptance tests and evaluations of medcial systems with MPSOs in them.

The fuse in the plug top protects the mains cable, the cross-sectional area of the mains cable determines the continuous current capacity of the conductors, the resistance of the earth conductor, which must be of the same or greater cross-sectional area, so this determines its maximum length.

It is possible to get 13A MPSOs with moulded plugs and appropriately rated cable but the concern is that if operators can access the sockets then it is possible to interconnect devices via the mains connections then this can compromise the safety of the medical electrical system.

For example under NC, if a combination of medical and non medical devices contributes to an earth leakage exceeding limits of 5mA, i.e. significant enclosure leakage currents over 10 times the acceptable enclosure leakage limit, and then the earth connection to the MPSO fails (SFC - Earth O/C) then patient and operator safety would be compromised.

This is why in addition to considerations relating to the MPSO inself consideration RE: non-conductive shielding, additional earthing or additional mains separation must be taken into account.

There are also more practical issues such as fluid ingress, higher risks relating to total loss of supply under fault conditions, robustness, fitness for purpose, etc, etc.

Fusing protects the mains lead fullstop. The mains BS 1362 fuse can continuously carry its rated value - hence the fuse rating should never exceed the mains cable, plug or IEC 60320 connector rating. 1A, 2A, 3A, 5A, 6A, 10A, 13A fuses are standard. Mains cables tend to be 10A rated, i.e. 1mm^2, on 10A rated IEC60320 connector terminated cables with 13A rated (and usually incorrectly fused at 13A, I might add) moulded plugs, up to a length of 3M.

Cable cross-sectional area of 0.75mm^2 - 6A rated, 1mm^2 - 10A rated, 1.25mm^2 - up to 13A rated. 2-pole fusing fitted to the Medical device looks after the device itself. So I say that 10A fuses probably give the most flexible use of detachable IEC leads rated at 10A (if you can ignore the fact that the time for a fuse to blow increases significantly when it passes a current in excess of its rating; as the value of the fuse increases).

The fuse in the mains plug should be rated at the lowest continuous rating of any component in the mains lead that is, obviously, also greater than the load current (usually a fuse of 1.5x non-inductive load current), i.e. maximum rating the same as Cable, Mains Plug, IEC Connector, or a lower maximum rating of fuse used, if a standard 1362 fuse does not exist at that limiting value (it should). The ratings are marked on all of the components if you look for them.