EBME Forums Forums Management Medical Equipment Management EN60601-1:2006 Earth leakage current of multiple socket -outlet

Reading through the new standard I was suprised to read under the Medical Electrical Systems sub clause 16.6.2 that earth leakage current of a multiple socket outlet shall not exceed 5mA.

5mA seems a bit high and seems to contradict 16.6.1 which states a maximum of 500uA touch current with the earth interrupted.

Hi,

On the face of it there appears to be a contradiction but my interpretation of the standard is that touch currents of ME devices and systems within the patient environment must be reduced or limited, to levels that are acceptable, by one or more of the three methods given in the rationale (16.6, p303). Actually I think this is reasonable and achievable despite what seem to be relatively high combined earth leakage currents, of up to 5mA (NC) or 10mA (SFC) in total, compared to the limits that we're used to.

If a seperating transformer is fitted to the ME system then earth leakage current, hence touch current from accessible earthed parts in particular, is reduced under NC and SFC. If additional earthing is fitted then under SFC (earth O/C) the earth leakage current, hence touch current under SFC, is diverted via the additional alternative path to earth. If a non-conductive enclosure is fitted to accessible parts of the system the operator and patient is shielded from the parts providing touch current that would flow otherwise if touched.

5mA is the "new" acceptable maximum earth leakage current for all non-permanently installed medical devices & systems (that may include non-medical devices) under NC and 10mA under SFC it seems (see 8.7.3d). This is not an issue if touch current, under NC and SFC, is limited to acceptable values in my opinion. For ME systems it appears that the earth leakage is only considered under NC since there's a requirement to use one or more of the methods above, prevent connection/disconnection of other devices, via mains, to the MSO, etc, etc.

In practice, for single ME devices rather than ME systems, I doubt that earth leakage current would ever exceed the maximum allowable ME touch current from accessible parts under NC or SFC since, considering the way Class 1 equipment is currently manufactured, in particular, with accessible parts earthed, it's likely that under certain SF conditions touch current would be equal to earth leakage current thus fail the required ME device safety tests, as you imply Kevin.

Thanks Mr R J,
I feel it will be interesting trying to apply this part of the new standard when commissioning new devices.

In the definitions section 3.129 Touch Current is defined as "..leakage flowing from the enclosure.." as we know,but the note below clarifies that "..the term has been changed to align with IEC 60950-1 and to reflect the fact that the measurement now applies also to parts that are normally PROTECTIVELY EARTHED."

If we are to treat enclosure leakage and earth leakage currents as "touch current", then I think 500uA SFC is the maximum acceptable earth leakage in most cases,as opposed to the 10mA value.

However,my interpretation is that maximum 10mA earth leakage SFC will apply if there are no exposed earthed (protective or functional) parts on the enclosure of the device and should therefore be applied to those devices(often dc adaptors) that are class 1 but are encapsulated in an ABS case.

Any of the three methods described in the rationale would achieve acceptable touch currents despite the 10mA (SFC) in my opinion. Generally speaking the standard could be a reflection on 1). Practical ME systems and 2).how class 1 equipment is now tending to be manufactured, i.e. without accessible earthed parts, perhaps?

Kevin, I do not have access to the said standard but I think you need to look at this as the
total amount of earth-leakage current that can be allowed on a system earth feeder (as opposed to a piece of medical equipment and it's touch-current potential) if say you connect a multiple-outlet socket-block extension to a utility outlet in a medical area.

This would mean that if for example you have one (or even more) of these blocks in the ICU around a very critical patient, due simply to a lack of available wall socket outlets, the potential is there for connecting say two times six pieces of equipment. Assuming a worse-case scenario in which each one has an earth-leakage current of
the maximum allowed (500 micro-amps), that would mean between two blocks at least five milliamps
could in fact legitimately flow down that earth wire.

The reason for the limit would be that current flowing down the earth wire will inevitably
produce a voltage drop and this in itself can be a potential safety hazard if the combined amount
from say the whole ward area becomes large enough. In areas like Cath Labs it is very important to make sure that the voltage measured between any two outlet earthing-conductors in the same area is not more than typically twenty-five millivolts. Assuming a one-ohm resistance in the earth cable this would equate to a combined 25 milliamps of earth-leakage current. In a busy (many items of medical equipment) ICU total earth leakage currents can mount up.

In practice of course, reaching five milli-amps in a handful of outlets from a block would be
unusual since leakage currents are generally quite low these days. But if one looks at the fact that the outlet connected to your multiple block is also part of a spur that will have other 13-amp outlets on it, the combined earth leakage currents do start to mount up just for that multiple socket earth-line feeder.

Connecting multiple devices by either functional connections or mains, e.g. an MSO, we are obliged to consider the combination of devices as a medical system if there is at least one medical device connected. The standard makes that quite clear. The requirement is to limit earth leakage from any medical device or a medical system to 5mA under NC (this applies to systems manufactured according to BS EN 60601-1:2006).

This "new" limit for earth leakage has been aligned to the IT/telecoms safety standard - thus BS EN 60601-1 now appears to recognise the changing nature of medical devices, i.e. that they tend to consist of a combination of medical and non-medical devices these days with the potential for significant functional and protective earth leakage currents.

5mA appears to be an acceptable earth leakage as long as the manufacturer, i.e. anyone who puts together the system, can limit the touch current on accessible parts of the system to the required values under NC and SFC. I commented earlier on how this can be achieved in practice, in my experience, and this is mentioned in the rationale given in the BS EN 60601-1:2006 standard; as far as I interpret it.

It is a fact that system earth leakages have tended to increase as multiple devices such as video printers, DVD recorders and suchlike are being combined into PC-based medical systems for example - these are not necessarily manufactured to medical device standards. It's my view that we may see more completely enclosed Class 1 devices with non-conductive cases/non-accessible earths and Class 2 devices with functional earths as alternatives in devices manufactured that will satisfy the requirements of this new standard.

In the past earth leakage has only been limited to values less than or equal to the limits for enclosure leakage (in previous medical electrical safety standards) since accessible Class 1 enclosures have tended to be protectively earthed and due to the requirement for testing under SFC. Now, as far as I can see, manufacturers have more leeway to design medical systems, that can have higher values of earth-leakage and functional earth currents which allow a greater number and variety of non-medical devices to be included in medical systems with touch currents that can still be limited to acceptable values using a variety of methods.

BS EN 60601-1:2006 actually clarifies some concerns I've had about medical systems (particularly inconsistencies in testing of enclosure leakage current) and that the earth leakage limit has tended to be a limiting factor in systems (unecessarily, in my opinion, if the design actually limited enclosure leakage under SFC). However there has to be a realistic limit to earth leakage as Nick suggests.

An important aspect is that manufacturers and responsible users need to avoid inappropriate use of MSOs fitted to medical systems (or provide an "inaccessible without a tool" MSO under some circumstances, to limit the potential of mains disconnection, interconnection within a system by unauthorised users).

Others need to be aware that connecting more than one device, including a medical device, to an MSO may, in fact, constitute "manufacture" of a medical system that may not meet safety requirements. Medical systems should be designed to avoid connection, disconnection, or reconfiguration of functional, or mains connections in particular, by indivduals who are not authorised to do so. Otherwise we do end up in a situation that Nick has described - as is actually the case in many hospitals as far as I'm aware.

Thanks Nick and RJ,

The touch(enclosure and exposed earth leakage) current remaining below 500uA SFC was my main concern.

My interpretation is that this is the case for individual devices or medical systems(via isolation transformer if necessary) with exposed metal parts(not patient connections).

However,it makes sense for a higher level of earth leakage to be acceptable for those devices enclosed in a non-conductive case as the risk is reduced.