EBME Forums Forums Maintenance Technical Assistance IEC leads control

Richard,
Well spotted. The leads from RS come with 13A fuses fitted as standard. In the majority of our leads we remove it and fit a 3A fuse. I believe that the IEC sockets are designed to operate at a max of 10A continuous (15A intermittent max), so if the medical equipment draws more than 10Amps continuous, it can't use an IEC socket.

The in line fuses protect the equipment, but the fuse in the 3 pin 13A plug protects the integrity of the supply in the event of a short circuit.

John, I think you hit the nail on the head there.

In a detachable lead, the fuse is there to Protect the lead and it is up to the equipment manufacturer to provide suitable protection for the equipment by means of the equipment fitted fuse and appropiate rating.

IMHO, its a time consuming affair to mark the leads or to try and police the use of leads on differen equipment.

By reducing the fuse rating in the lead you are provided an extra safe zone for your users to reduce further the risks of the age old problem of lead swapping.

Admittedly the policies are all decided at local level by means of trial and error, what works for one person may not work for another. Provided we can stand our ground on the choice we make then that should suffice.

John, we pretty much adopted the same policy at Haslar, by changing the fuse to a reduced level and marking the lead and equipment with seperate tested / due stickers so that if the lead was to become detached it would still have some identification of its suitability for use.

Rog Rocks - provided the cable and plug is manufactured to 1363/a then I cant see any problem of using long leads. The longest i have come across in general use has been the 3m type provided by RS, however there may be some 5m versions available. I wouldent choose to make up a long lead, but would choose a pre-fabricated off the shelf one as you will be guarenteed that it will pass earth continuity tests at the required levels.

One of the very first things I was taught when I started in this game is that the mains fuse protects the mains cable, not the equipment. The medical standards state the requirements for equipment fusing. Hence the requirement to fuse at, or below, the continuous current rating of the cable. Well worth making clear to all of those techs never having done EST, possibly having to pick it up as they go along.

Yes John, I don't see 13A rated, moulded, IEC leads around. The cables and moulded IEC connector are rated IEC at 10A maximum, as far as I can tell. The 13A rewireable IEC connectors that you see on kettles are not recommended for Medical Equipment - wasn't there a purge to remove these, on MDA advice, a good few years ago? I don't think it good practice to manufacture 13A rated IEC leads for general use. Moulded leads seem to be a better idea, as you say yourself.

However there are general concerns about the quality of the strain relief on the IEC connector from all manufacturers (usually sheath pulls away from strain relief, exposing the seperate, insulated, conductors) or the connector disintergrates around the holes where the male IEC connector pins enter the moulded IEC. Some individuals also worry about the quality of the connections inside the moulded plugs, from experience of problems, so they remove the moulded plugs and replace with rewireable plug tops.

Rog: The length of IECs is limited by the earth-bond resistance limit in practice - of course this is dictated by the cross-sectional area of the earth-conductor isn't it? I think the earth-bond limit for the detachable IEC itself is effectively 0.1 Ohm if the earth-bond from the appliance IEC connector earth terminal to accessible earthed parts of the instrument approaches the 0.1 Ohm limit speciified in the general standards for medical equipement i.e. 0.2 Ohm overall, when connected to the instrument, checking it as part of the system. Since leads can move around then I guess if you allow 0.2 Ohm earth-bond for the IEC lead then there is a possibility of particular instrument/IEC lead combinations equipment failing the earth bond test. Does anybody have any views on this? I suppose, in this case, if it fails then replace it.

Perhaps Alex has the right idea when he says that the policy at the hospital where he works is to retain the IEC lead to the equipment. Incidentally; what are people doing about inspection, testing, rating and on-going maintenance of multi-socket extension blocks? Does anybody actually suggest the users purchase or use them? Is the use of them actually acceptable in these days or just a necessity? Do we turn a blind-eye to this practice or what?

A longer lead necessitates the use of 13A rated cable (to increase earth-conductor cross-sectional area, to reduce electrical resistance) and appropriately rated (mechanically and electrically) rewireable IEC connector. I think the use of excessively long leads should be frowned upon. Rewirable 13A IEC connectors are not particularly good connectors, electrically or mechanically, in my opinion. I say bring the power-source i.e. sockets, to the equipment. Long leads can give problems due to damage from being run-over, twisting leading to conductors being exposed through the outer sheath and the possibility of self-heating of the cable (at higher loads) when coiled incorrectly. Again; there have been warnings from the MDA, I think, about the inappropriate use of coiled extensions, long cables and suchlike.

Is the writers from the tv series Casuality reading this ?
On saturdays show they couldn't use the defibulater because the lead was missing.

Spooky or what

RL's Question:- what are people doing about inspection, testing, rating and on-going maintenance of multi-socket extension blocks? Does anybody actually suggest the users purchase or use them? Is the use of them actually acceptable in these days or just a necessity? Do we turn a blind-eye to this practice or what?

This is our Trust Bio-engineering group's view:-

MULTIPLE PORTABLE SOCKET OUTLETS ON MEDICAL EQUIPMENT SYSTEMS

1. Advice on use of MPSOs

The advice we give to Trust staff on the use of mains extension leads with medical equipment is based on BS EN 60601-1-1:2001. The main requirements of this standard are:
· The use of MPSOs should be avoided as far as possible – priority should be given to the installation of additional fixed socket outlets
· MPSOs shall not be placed on the floor and shall be mounted to prevent ingress of fluids and mechanical damage
· Connection to a MPSO shall only be possible by use of a tool, or the multiple portable socket outlet shall be supplied by an isolation transformer.

2. Testing of MPSOs

Clinical Physics/EBME has responsibility for testing all MPSOs fixed to medical equipment systems. It was agreed that testing procedures would be written into local work instructions. These should include the following:
1. MPSOs should be tested annually in addition to the electrical safety tests of each individual connected equipment.
2. Visual inspection of mains plug, flexible cable and MPSO. Identify signs of damage, overheating, fluid ingress etc. Internal inspection of mains plug and MPSO cord security, polarity, connections and contacts, unauthorised modifications or daisy-chaining, switches, plug fuse size appropriate to cable etc.
3. Polarity and continuity test of live and neutral on each socket outlet. This is easily performed on the Rigel 266 with the use of a standard IEC lead. Otherwise a standard ohmmeter will have to be used.
4. Earth continuity (bonding) test between earth pin on mains plug and each socket outlet earth terminal and other accessible metal parts. This should be tested at 10A and the resistance should be less than 0.1 ohm .
5. Insulation resistance between earth and both live and neutral pins of the mains plug. The test should be performed with all switches on. The resistance should normally be greater than 20 Mohm .
6. Total medical equipment system earth leakage. The current should be less than 0.5 mA with normal and reversed mains connection with all units live. Test not applicable if isolation transformer is fitted.
7. If an isolation transformer is fitted to the MPSO, the following additional tests should be performed.
a) Transformer earth leakage (less than 0.5 mA) and enclosure leakage (less than 0.1 mA).
b) Insulation resistance on the secondary side of transformer should be greater than 20 Mohm with all switches on (primary side insulation resistance only is tested by 5 above). This will require construction of a special lead with a mains socket at each end to connect the MPSO to the safety tester. This should be clearly marked “Test Purposes Only”, each plug should be fitted with a 3A fuse and have the live and neutral leads connected together as a safety precaution against misuse.

Bill

This is an interesting topic, and discusses a problem that has not yet been finally (or, at least, universally) addressed and solved (as far as I'm aware). What's the present state of play, I wonder? Has anything changed?

OK, so we're agreed (?) that we "fuse to suit the cable". That's good, but which fuse do we use? 10 Amp?

Bill's post about multiple socket outlets on medical systems ("stacks" and the like, I presume) is also a good one. But what about trailing multiple socket outlets (Richard's point, I think)? In these days of "risk management", have we now all got rid of these? Especially from operating theatres and similar locations?

Note to Huw:- I feel another "EBME Best Biomed Practice" statement coming on - what news on the Wiki idea?

Well done Mr Geoff.

This is indeed an important yet unresolved issue in my department. Some time ago I wrote some findinds/observations on this topic and copied the information to the hospital's estates and EBME depatments. The estates department have taken action with their portable mains lead testing while my department languishes in doing nothing. Alas ignorance is bliss, but it will be interesting to see which department gets sued if a faulty lead electrocutes a patient.

As for the fuse rating of mains leads, when I did my electrical safety testing course, the then IEE recomendation was with either 3 or 13 ampere.

Barney

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.

Can we standardise on a 10 Amp fuse, then, do you reckon?

Meanwhile, did anyone miss Rojo's interesting suggestion about having different colour mains cables each year?

During my daily wander around the wards this afternoon, I encountered no less than five bits of kit "plugged in" to a mains outlet, but not being charged, due to the dreaded IEC mains cable not being fully home in the socket at the back of the equipment (NIBP/SpO2 "monitors", as you've probably already guessed).

Back in the good old days of HTM-8 we had fixed mains cables, which had to be longer than a specified minimum length. When did the IEC cables come in, I wonder, and why? Is it because of the "pull away" capability? If this is (was) indeed the reason for their adoption, how come I see so many secured to equipment by cable ties, P-clips etc.? (not that either is much use in preventing the problem mentioned above, of course ... they simply keep the tested cable with its designated piece of kit.)

I suspect that we have been lumbered with these detachable mains cables simply because their use makes it easy for manufacturers to sell equipment worldwide, and just supply a cable with the appropriate (hopefully) mains plug at the other end.

Meanwhile, I've noticed some new powered beds with non-detachable mains cables. So what's going on here?

The trouble with the old non-detachable mains cables was that they were not always easy to replace (... that is putting it rather mildly in some cases emerging from the mists of my memory)!

... but back to IEC cables:-


Seems that you’re not alone, Karl. Maybe we should start a campaign for a new standard. I suggest keeping the (now) ubiquitous IEC lead, but urging manufacturers to secure them by means of a retaining device needing tools to release (to change) the cable. The Graseby 500 volumetric infusion pump may be a good example. The WarmTouch bed warmer is another that comes to mind.

But, better yet, let’s hope that the next round of equipment re-designs will see the IEC cable secured within the equipment, accessible by removal of a cover plate or some other such (neat) arrangement.