EBME Forums Forums Management Medical Equipment Management Electric Beds - PAT or Medical Safety Testing

Are there any parts that the patient can come into contact on a piece of Type B equipment that would not be considered to be applied parts then? If you consider that the patient could touch all parts of the equipment then everything could be applied part couldn't it?

Taking this further: Does anyone know how to differentiate between parts that are tested for enclosure-leakage and the applied parts associated patient-leakage on a Class "1B" item? I'm asking this question because patient leakage isn't done for applied parts on type 1B and if you consider all parts are potentially applied parts because the patient can touch them; which parts do you then test to for enclosure leakage?

If you say that the bed frame the patient touches is an applied part for example and applied parts are not tested on Class "1B" and enclosure leakage is not done because all parts are applied parts potentially is any leakage measurements from the enclosure/aplied parts, whatever, going to be done at all? Do you see what I mean?

Everything you guys are saying is there in the standard and you obviously know your standards but I am concerned about how I would go about doing a practical test using your interpretation of the definitions. I'd appreciate any comments - thanks.

Ged, read section 19d) and 19e) this explains how the tests should be done - my interpretation is that parts defined as enclosures are not likely to be earthed at all - that is the point of testing for enclosure leakage and is the opposite of what you've said.

Further; you say applied parts are not likely to be earthed - what about an earthed bed frame already defined previously as applied part? There are protectively earthed electrical beds all over the country aren't there?

RL -

Why don't you measure patient leakage current from a Type-B Applied Part ?

RL -

Our postings seem to have got out of sequence.

In reply to your last posting -

1) Class-1 equipment relies upon protective earthing for safety. Normally this involves earth-bonding of accessible conductive parts of the enclosure - hence earthing of conductive enclosures is the norm rather than the exception for Class-1 devices.
You will already have checked continuity of this earth-bonding as the first part of your safety test. If bonding is intact then enclosure leakage will be negligible, since your test load will be in parallel with the low-impedance bond to earth.
Section 19.1d) presumes integrity of this earth bond, hence it only specifies leakage measurement to any non-conductive (ie: non-bonded) areas of enclosure which may be present.

2) I said accessible parts on a Type-BF Applied Part wouldn't rely upon protective earth bonding; those on a Type-B AP may well do.

Dear Ged

My understanding of what constitutes an Aplied Part for a medical device is that it is supposed to be connected to the patient's body for monitoring, therapy or whatever. The frame of a bed definitely isn't an applied part. Its clearly the enclosure.

In fact if you redefine an Applied Part as anything that comes in contact with a patient you will be making no end of trouble for yourself to no purpose. I mean what about the hospital radio headphones for instance.
No doubt the standard itself could be clearer on this as on most things.

As regards electrically testing the beds the advice we SHOULD follow is that in the manufacturer's service manual.

I don't (yet) check beds, but I do check electric couches. What I personally do with them (as against what the manufacturer advises) is simply to make a thorough visual inspection of the cabling, motor and frame. Many problems come to light from such careful inspection.

If you run a Class 2, no Applied Parts test on a couch or bed and look at how many parameters are actualy tested you will see that its a rather pointless test to run anyway. You will naturally, get an extraordinarilly high pass rate (99.9999%). However a good visual check will often show a serious problem the electrical will give no hint of.

Marc

Ged,

Section A1.10 of 60601-1:1990;

If you look at the rationale for deciding whether part of a device is applied parts or enclosure then you will see what you are saying is inconsistent with the method of approaching testing of devices - I think you are confusing mechanical definitions of applied parts with electrical ones actually.

APPLIED PART:

"Applied part if the connection is essential for the normal use of the equipment, etc, etc....."

ENCLOSURE:

"Enclosure if contact is incidental to the functioning of the equipment, etc, etc......"

Considering a bed I know of no contact with users or patient that is essential for the bed’s normal operation, so I define the bed as enclosure. Non-earthed enclosure is tested for enclosure leakage, earthed items are tested separately for bond and earth-leakage. Patient leakage does not even come into it.

Just an aside: The mattress is actually an applied part in the mechanical sense isn't it? - Type B I think.

For Class1B I would test earth-bond, see what’s conductive and earthed and then any other accessible conductive parts are considered as enclosure. Then test for enclosure leakage of these non-earthed parts rather than testing for patient leakage via non-existent electrical applied parts. For class2 the accessible conductive parts should be tested for enclosure leakage as required.

In the standards the rationale actually states the procedure for assessing which are the applied parts in section A1.10. A Class 1B bed would not have applied parts according to the standard.

Just because you can measure patient leakage on a tester connected to what you think are "applied parts" does not mean that you're measuring patient leakage currents - it is more likely enclosure leakage as defined in the standard.

If you're measuring leakage via earthed parts you're only likely to see significant leakage under single fault conditions i.e. earth O/C (unless you have poor earth bond on Class1 equipment). Your description of testing class1 equipment is incomplete without testing under single fault conditions and testing enclosure leakage of non-earthed conductive parts. Hope you can see what I'm getting at.

There appear to be no maximum limits given for type B patient leakage with mains on applied part – is it, in fact, a valid test if you've got type B applied parts? Maybe your tester applies the patient leakage test inappropriately for 1B/2B devices? How do you apply the mains on applied part voltage on Class1B/2B applied parts if the applied part is actually earthed? The test must fail every time, in this case.

Sorry to harp on about it but I’m interested to hear your views and get a better understanding.

Thanks for the support Marcel - well put and brief - something I'm not good at!

Marcel –

Don’t shoot the messenger! I didn’t redefine anything – the definition of the term Applied Part comes from the 60601 Standard, from which the term itself originates. Whether or not it seems logical to you, or me, the quote I gave above suggests, to me at least, that the Standard would consider the bed frame an Applied Part.

I replied to Mark’s posting merely to agree that hospital beds (or any other electrical device in the Patient Environment) should be safety tested to medical device standards. On the way I queried RL’s interpretation of Type and got sucked into this debate; (Class 1 or 2 is the device; Type B, BF or CF is its Applied Part/s). In practical terms, for PM testing of in-service hospital beds , the debate whether the bed frame is an Enclosure or Applied Part is somewhat academic – the same leakage current limits apply (as RL observed). For some medical devices the distinction between Enclosure and Applied Part is more important however, which is why I queried RL’s definitions. That aside, Mark’s original question (as I understood it) was whether, when testing beds, to apply medical device leakage current limits or the usually higher limits programmed into basic PAT testers used for testing ‘normal’ electrical devices. I agreed with Mark that the former should apply to hospital beds – and in fact to any device (TV, fan, PlayStation, ventilator, bed, etc.) placed in the Patient Environment. This would apply to the headphones in your example, which should meet the electrical safety requirements for medical devices, rather than the safety requirements for the headphones you use with your hifi in your front room at home. One of the reasons that stricter leakage current limits are applied to medical (patient-applied) devices is because an ill patient may be less able to respond evasively to electrical shock than someone fit, well and unconstrained – this consideration applies whether the leakage current shock comes from radio headphones or an oximeter probe. It makes no sense to say that it is unsafe for leakage current from a probe attached to an earlobe to exceed 0.5mA, then to permit 1.4mA from a set of headphones placed upon the same ears. The bottom-line is that any electrical device in contact or contactable with the patient (or even with someone touching the patient) should meet the safety standards required of medical devices (see 60601-1-1). Having said that, this principle is routinely violated in hospital environments – which was Mark’s original concern.

(I agree with you that for in-service PM safety testing, most faults can be identified by good inspection without use of a tester device. However for Acceptance Testing the reverse is often the case. Also with regard to your example of a Class 2, no Applied Parts test – you could probably achieve a ‘pass’ without even plugging the device into your tester).

RL –

My point was to question your statement that Enclosures as defined wouldn’t normally be earthed at all – for Class 1 devices they normally would be. Your later point that Enclosure Leakage from earthed accessible parts on the Enclosure will be negligible under Normal Conditions was the point I was trying to make – apparently not very well. I agree that under single-fault ‘open-earth’ conditions the measured Enclosure Leakage would rise – for earth-bonded Accessible Parts you’d normally then expect it to approximate the Earth Leakage current you’d measured under no-fault conditions.

The Enclosure is regarded as the exterior surface of the equipment – as simple as that, (there’s a definition in the Standard if you wish). If I understand you correctly you appear to have used the term ‘Enclosure’ in a more restricted sense (which I believe, is the source of our confusion) - you appear to be restricting its usage to only those parts of the ‘box’ from which leakage currents are to be measured in the Section 19 tests you have referred to. But Section 19 isn’t intended to define Enclosure (that’s the job of Section 2), it merely describes how you should measure leakage from different parts of the Enclosure. If it doesn’t require you to measure from earthed parts of the Enclosure that doesn’t mean these parts aren’t part of the Enclosure.

“I think you are confusing mechanical definitions of Applied Parts with electrical ones ….”
I think this distinction is yours, not the Standard’s.

Examining your statement -
“For Class1B I would test earth-bond, see what’s conductive and earthed and then any other accessible conductive parts are considered as enclosure. Then test for enclosure leakage of these non-earthed parts ….”
I think you’re misusing the term ’Enclosure’ (see above).
That aside, using this method, in the event that you identify a non-earthed conductive Accessible Part – how do you immediately know whether it should be earthed or not? It may well pass your subsequent leakage current tests, but in the event of a single internal fault could it become hazardous live? – could a stray internal 230V conductor make contact with it, for example? (ie: Is it intentionally un-earthed because it is internally double-insulated from hazardous voltages? – or is it un-earthed because the bonding strap has become disconnected or, in error, was never connected on the production-line?). You appear to have assumed that the unearthed conductive parts you have found weren’t intended to be earthed; on the other hand they could be an indication of one of the very things you are testing for – a failed/missing bond. Further investigation is therefore required to confirm which is the case – it isn’t always obvious - before proceeding with the leakage current measurements you describe.

“.. procedure for assessing which are the applied parts in section A1.10”.
I don’t necessarily agree with your application of A1.10 either. (Firstly, if you classify the bed as 1B then I think the ‘B’ indicates that you have already acknowledged it has an Applied Part - see below). Applying A1.10 – if we say patient contact with bedframe is incidental to the functioning of the bed then the frame may be relegated to the status of Enclosure if contact can only be made by deliberate action on the part of the patient. Personally I would say contact with a bed frame is likely without deliberate action - in fact the condition of many patients is such that they are not capable of doing anything deliberately. You may well debate my reasoning here, but it appears consistent with the ‘table-top’ example quoted above (which comes from A2 of the Standard).

“Just because you can measure patient leakage on a tester …………”.
Couldn’t agree more – all your tester does is measure current; we put the label (‘Earth Leakage’, ‘Patient Leakage’, Auxiliary Patient Current’ etc) upon it, which requires a good understanding of the definitions of Enclosure, Applied Part, etc. – hence this discussion.

Mains-on-Applied Part is something different again. Personally I wouldn’t do this test on an in-service medical device - it is a ‘Type-test’ which potentially stresses the device much the same as dropping it on the floor to test its robustness might do – to be done by manufacturers on sample devices but not by maintenance departments. The test not only has no meaning for a Type-B Applied Part – which is why there are no limits specified for Type-B Parts in the Standard – but there then really is potential for damaging the device under test.

To go back to where we started – I queried your explanation of Type B, BF, CF allocations. I think your understanding is based upon old versions of the Standard. In the current version of 60601-1 the Type classifications have been redefined to be applicable only to Applied Parts. A device with no Applied Parts therefore can have no Type category, (conversely a device with more than one Applied Part could carry more than one Type categorisation if its different Applied Parts differ in their level/means of protection). My understanding is that under the old definitions it would have been possible to have classified Equipment without any Applied Parts as 1B – now the same device would presumably simply be Class 1, because it has no Applied Part to be categorised. I think much of the discussion we have had arises from this distinction.

I think that most of this debate arises from misunderstandings resulting from the fact that we aren’t using terms such as Class, Type, Applied Part and Enclosure in the same way. It shows the importance of consistent terminology. If you still think I’ve misinterpreted aspects of the Standard please let me know.

Ged,

Apparently the equipment enclosure you refer is actually applied parts by your own definition isn't it? The exterior surface of the equipment that can be touched by a patient! My question is whether the definition applied parts is valid for electrical testing purposes just because the type symbol indicates “B applied parts”? A lot of our syringe drivers indicate Class 2 “B applied parts” the applied parts being the syringe and line but we don't say that the driver has applied parts for the purposes of routine electrical testing.

Just because a patient can touch it during use I do not think enclosure becomes applied part unless it's essential for the equipment operation and forms a patient connection …….as defined in the rationale.

I don't think I've used enclosure to define non-earthed parts exclusively, except to discuss enclosure leakage and the fact that measuring leakage from enclosure, known to be to be earthed, is excessive. In any case from what you have said then it doesn't matter which parts I discuss – they're all enclosure. But for the purposes if this discussion what I'm discussing is electrical safety and the relevant tests from the parts that could, potentially, present a significant risk i.e. enclosure leakage from non-earthed parts e.g. parts that I've not already tested via earth-bond and earth leakage, etc, etc. I tend not to dwell on measuring the enclosure leakage from parts I've already determined to be earth-bonded. But I don't assume (as you do) that Class 1 enclosures are normally earthed – yes, they should be but I never make the assumption – I initially test to the enclosure, earthed or not.

What I have done is draw attention to the fact that if an accessible part on a Class 1 or Class 2 piece of equipment then it should be tested for enclosure leakage using a medical safety tester – precisely to look for single faults such as mains applied to them or high leakage indicating a potential problem. This partially answers the question you ask – how do you know whether it should be earthed or not? – well you don't know but at least if you test you're aware of a potential problem on Class 1 equipment. You could strip down equipment every safety test and start from scratch but this gets a bit tedious and you've already criticised type-testing re: patient leakage mains on applied parts haven't you? All you can do is follow a structured test that takes appropriate measurements.

You still have not explained how you differentiate between enclosure and applied parts? Especially when you consider that on type “B/BF/CF applied parts” any/all could be applied parts – does this mean that you don't have any enclosure to test to?

Yes; I do assume on any equipment, Class 1 or Class 2, B, BF or CF that there will be unearthed accessible conductive parts and that in some cases these parts are not intended to be earthed at all (Class 2 with exceptions in some cases on Class 1 equipment, even in the “new” standards BS EN 60601-1:1990) – this is based in practice. I don't go dismantling every item of equipment beyond initial acceptance on purchase, every time I see something slightly suspicious, I rely on testing to highlight a potential problem on acceptance but problems don't occur that often if the acceptance tests have been thorough. There are plenty of pieces of Class 1 kit out there that apparently meet the standards that have some non-earthed metalwork on them.

Just to clarify things – if your bed has applied parts does this mean you test to these applied parts via the applied parts connections on your medical tester?

Incidentally - I don't classify the equipment as any type – the manufacturer and test-house do. I just apply what I think are appropriate tests in an attempt to meet the requirement of the standards to ensure on-going electrical safety – there is plenty of new kit that still comes into the workshop that has type Class 1”B applied parts” or Class 2 “B applied parts” that will never come into contact with a patient under normal conditions even if I apply your interpretation of the standard (does this mean they are not classified correctly?).

The bottom line is that after reading the rationales and not relying on glossaries of terms, too much, I personally choose to measure enclosure leakage to accessible case parts rather than test such things as a syringe plunger actuator, for example, as an applied part.