EBME Forums Forums Maintenance Technical Assistance Class I electrical safety test on Class II equipment

Hi, all.
One silly question here.
Did any one test a ClassII equipment as ClassI equipment?
I tested an audiometer few days ago, put it on ClassI test because I did not find any class sign. The charger was burnt during the safety test. However, the charger was still magically working on audiometer after with smell.

I sent it back for inspection. Then now, the company said I burnt the charger and I should NOT test it on Class I, it is Class II. BUT, it did pass the protative earth test!!! (I put the enclosure connecter to the audiometer's outlet)

Can anyone give me some advice on this?
Using QA90 tester.

Class 2 devices can have a functional earth, i.e. a connection to the mains earth pin that's not used for protective purposes, under certain circumstances.

If it's class 2 then you probably accessed an exposed functional earth connection when you tested for earth-bond under class 1 - the earth-bond test current fused components in the functional earth path, or even parallel to it, given a high enough earth bond current, perhaps?

The audiometer enclosure is possibly connected to the charger ground that, in turn, is functionally connected to the earth pin on the charger.

Normally if there's no device mains classification I'd test class 1 as well. If the charger is class 2 and this isn't indicated by markings and on the power source I'd say the device failed according to the medical devices directive/manufacturing standards - if it is actually class 2 that is.

The supplier can't have it all ways and expect us to test appropriately without the correct markings being applied to the equipment at manufacture - what's the documentation, e.g. operating manual or service manual, tell you?

I'd ask to speak to the company's regulatory affairs person for clarification of the device classification or if they insist the device, that apparently isn't marked class 2, is class 2 then I'd contact the MHRA regulatory people for guidance.

OH, many thanks, R J Ling.
Thanks for kindly advice.

Hi Jerry and Mr Ling,
I agree with Mr Ling.
By the way, we used to have battery chargers for Laerdal 911 defibs. that had no markings to indicate class I or II They had an earth connection at the 13A plug but a fully insulated enclosure (plastic) with no external metal parts, no external earth point or equal potential earth pin. The earth connection went to the inside of the charger and was totally inaccessable. Accordingly, we had to treat them as Type B class II for the purpose of electrical safety testing with our BIO TEK 601 analyser.
I've always felt that manufacturers should be required to put a marked earth test point on their equipment. Our practice at the North Middlesex University Hospital MEMU is to mark the earth test point used with a green sticky dot for equipment that is class I and hasn't got a marked earth test point e.g. a lot of medical laboratory equipment falls into this category. This ensures that the same test point is always used when carrying out an electrical safety test for earth continuity.
I'd welcome other views on this.
Ian Rez Eng. Tech. MIET
Medical Equipment Management Unit (MEMU)
The North Middlesex University Hospital NHS Trust
Tel 020 8887 2582 or 2309
e mail: Ian.Rez@nmh.nhs.uk

Thanks for that Ian,

My view regarding testing for earth-bond is that we should test for the worst-case or "poorest" earth on equipment by "probing" for earth points rather than choose one for repetitive testing. Anything that's accessible, conductive, and not earthed may be considered as enclosure (accessible conductive non-earthed parts - unless inspection indicates otherwise) and enclosure leakage/insulation resistance measurements can be taken from these points independently of the earth if need be.

Whilst I agree your method of marking earths for testing purposes can ensure that degradation in the earth bond can be observed using repeatable and comparable testing over time it doesn't necessarily ensure that the earth bond to other accessible earths hasn't degraded. There's always a problem with automated tests that use a single point for insulation resistance/earth-bond/enclosure leakage tests is not adequate, even for routine tests, because operators are not encouraged to "look" for potential problems.

In my opinion this doesn't mean class 2 applies necessarily - markings must be provided to indicate that a mains device is class 2. If a device is internally powered at low voltage, e.g. a LSU, then it will take on the classification of the mains power source as far as I'm aware. If the power source had no class 2 markings, a conductive earth-pin, and no accessible earth on the housing then I'd always assume class 1 with inaccessible earth rather than an incorrectly marked class 2 PSU (that would fail to meet the requirements of MDD, incidentally, if it weren't marked appropriately).

Even if there's a non-conductive enclosure there's a requirement to give earth protection if the case/internal construction doesn't meet electrical and mechanical requirements for adequate seperation from mains (class 2 does hence no protective earth is required).

The protective earthed parts that are physically inaccessible because of the enclosure are considered "accessible" if the case and internal seperation from mains parts do not meet requirements. There is nothing that says the protective earth has to be provided externally on class 1 devices as far as I interpret it.

Either way I'd try to confirm the classification before releasing the device into use. I think I posted some stuff in another thread Acceptance Testing about the potential flaws in testing a class 1 device as class 2 but as you've already stated you've used this method for non-accessible earth devices only - but what if there were some accessible conductive enclosure or applied parts on the class 1 device that's tested as class 2?

The problem is that testing for class 2 on a class 1 device with inaccessible earth means that you still miss important safety tests that should be applied. So, personally speaking, I wouldn't recommend performing class 2 tests in lieu of class 1 under all but the most basic circumstances.

Earth leakage current is an important measurement in medical device EST that would tend not be measured under class 2 by safety testers following automated protocols, as far as I'm aware, Ian. Personally speaking, in the scenario you give, I'd be inclined to measure to class 1 and note "earth inaccessible" rather than test a class 1 device with a non-conductive case, to class 2 EST, and fail to establish if there's excessive earth leakage, for example.

Richard.

Ian, I’d say that your green dot procedure is a good (practical) one, worthy of universal adoption within the NHS. Why not take it further and put a green dot on all known Class I equipment, then we can we assume that no green dot indicates Class II? Doing the manufacturers’ job for them, no doubt, but it would save a lot of wasted time in debate later on, I would have thought.

Thanks for giving me the opportunity to get back into the debate Geoff,

Strange - I'd have thought the standard practice of manufacturers marking their devices as class 2 (not class 1) and a note in the service records to say that particular class 1 devices have "non-accessible earth" would suffice - saving the time and trouble of labelling with sticky-dots.

We shouldn't just decide to reclassify equipment for testing purposes because the tests may then no longer be appropriate for the IUT nor rely on sticky-dots to guide others' methods. In my view discussion in this matter is only necessary when there's disagreement as to whether those sticking on the dots are considered to be making sound decisions or not.

This is the problem with automated testing - individuals should be able to program testers for this scenario or, if testers aren't flexible enough to be programmed to do this, and have enough knowledege to know when to ignore insignificant "fails", e.g. when there's no protective earth accessible for testing purposes.

I guess that the lack of a dot implies that there's no earth therefore the device is tested to class 2 - dodgy practice if this is the case - especially if it's involving class 1 medical with no accessible earth but with applied parts, etc.

Or, in fact, with class 2 devices where you might need to do an earth leakage test because it has a functional earth? Automated testers set up inappropriately for class 2 tests would then miss tests (as well as earth bond) that are necessary for class 1.

I certainly wouldn't want to carry the can for testing devices inappropriately to class 2 with an automated tester because someone else has told me that if there's no green dot, hence no accessible earth, then I should test to class 2 - even if the device is class 1. What about the decision RE: functional earth leakage measurements on class 2 with functional earth? Do we use a red dot for that?

I wouldn't be inclined to use this sort of system because those doing the testing should be knowledgeable and skilled enough to at least identify the classification of devices from documentation, inspection and experience - the method of testing used shouldn't rely on one person (nobody's infallible) deciding which equipment has a sticky label or not and where it should go.

If there's doubt about safety testing any device then individuals should ask for guidance from documentation, colleagues, the manufacturer, MHRA or take the time to inspect the device more thoroughly in my opinion, i.e. obtain the knowledge and then share it perhaps.

Sticky labels are not infallible they can come off, be peeled off (when operators clean the equipment?) or be relocated incorrectly. Manufacturers markings are required to be clear and indelible. If they're not then the equipment shouldn't be accepted onto the inventory.

The docmentation on the worksheet should identify the classification so there shouldn't really be a need to identify class or where the earth should be tested because this should be indicated in the service record for future reference.

Labels are a greater waste of time and would indicate to me, if I were being asked to do the EST, that perhaps the individuals that doing the testing cannot be trusted. Also it's another example of the same old scenario of the automated tester influencing the test methods rather than the operator. e.g.

In Ian's example, reading in-between the lines, then class 2 testing is selected for the sake of obtaining a pass when there's a non-accessible earth on a class 1 device, for example, rather than producing a fail due to a lack of earth bond, when testing to class 1. In this case is the earth leakage, whether it be functional or protective (since there's apparently confusion over it's classification) be performed by the Biotek 601?

In my experience that's usually the reason for some individuals testing to class 2 rather than class 1. My knowledge and waffle RE: safety is applied to systems mainly - that's where the potential to put together a system, from a combination of devices, can become potentially risky to the patient. One green dot on a video stack to indicate where I should measure earth is not enough - there'd be dots all over the shop.

Anybody competent should be able to determine class1, class2, with or without accessible, functional or protective earth without the requirement for additional markings, i.e green dots. If not then, in my opinion, they shouldn't be involved in testing patient-connected devices and certainly not complex systems, such as video-endoscopy systems or multi-parameter, multi-module, monitoring systems used in theatre or for invasive procedures.

These involve multiple, interconnected, medical devices and may be used combined with a laser, ESU and other medical devices such as patient monitoring with invasive applied parts, etc. The sort of setups where poor safety testing can miss problems with inadequate seperation, excessive leakage currents and earthing problems.

Fotunately, most of the time, EST comes down to visual inspection and common sense - but the electrical measurements bit shouldn't just mean switching off our brains and relying on sticky-dots and an automated tester to do all the work.

Sorry there's been a confusion here. The "green dot" idea was to ensure that the same earth test point is used on Class I equipment to ensure that the same point is used each time when testing earth bonding to check that there hasn't been any degradation of earth continuity when you do an annual electrical safety test as part of a PPM process when earth bond impedance is recorded. Where do you put your safety tester's earth clip for a Class I all insulated item which only has an equal potential earth pin accessable which connects to the internal earthed metal chassis? (e.g. an HP defibrillator)
As I said, a lot of medical laboratory equipment does not have an external idendified earth connection, hence the green dot to identify which point you are connecting your earth clip each time you carry out the earth bond test on the equipment. I would of cause suggest that the earth clip is moved to another part of the earthed chassis for earth leakage current tests, incuding patient leakage.
I agree with Richard that the problem of using automated electrical safety analysers (or testers)is understanding exactly what is being tested and it's important to always carry out a very full visual inspection when testing equipment.
There are real practical problems of the amount of time that one can spend on electrical safety testing in a busy hospital Medical Equipment Management unit when the hospital has over 2000 items of equipment. I suppose to do this properly one would have to employ someone full time on this task, which is just not possible in our MEMU

Sorry, I hadn't finshed my reply - bloody laptop PC's!!!!

In my experience of nearly 12 years in our MEMU, there are very few items of equipment that fail an electrical safety test. The main problem that I've come across is the moulded 10A IEC lead to 13A plugtop where the earth impedance fails due to the poor connections at the IEC socket's earth connection over some time due to pulling this socket out of the equipment of flexing the lead by ward staff.
Most other problems are picked up at the visual inspection stage e.g. the plug hanging off the lead, the plug broken or the lead pulled out of the plug's cable clamp etc. Most of these problems being due to bad use by ward staff.
I recall that there was a survey carried out many years ago by the old MDA(I haven't got this to hand as I'm doing this at home!) which concluded that the main problem of medical equipment wasn't electrial safety but operational problems (e.g. IV pumps giving incorrect volumes etc. But that's another story....

Ian

Hi Ian,

Yes I gathered that the dot was being used as a means of identifying where the earth was to be measured and that it was probably a means of speeding up tests and avoiding confusion - depends what you're used to I suppose. I got the impression that if there's no dot, i.e. no earth, then the operator would test to class 2, irrespective.

However there's still the issue of whether the item can be identified as class 2 with a functional earth (no dot) or if it's class 1 with no accessible earth (i.e. no dot). In either case an earth leakage measurement needs to be performed and this is usually not achieved in a class 2 test, probe or no probe, unless it's selected as part of the test (or the safety analyser detects the conductive earth pin on a class 2 device - this indicates that a functional earth is likely to be present).

Admittedly most class 2 devices don't have a functional earth, i.e. conductive earth-pin, but it's frequent enough for most analysers to allow for this. By introducing a dot then the point I was making was it could cause more confusion than the markings that should already be on the IUT.

Earth leakage is usually measured via the earth-pin to ground via the safety analyser, not the probe (that's used for insulation resistance/enclosure leakage/earth continuity checks), neither is patient leakage current measured via this probe.

Another point I was trying to make is that I think the earth leakage measurement, whether it's functional or protective is quite an important one that may be missed if a class 2 test is performed on a class 1 device, as you indicated in your post previously.

Saying that even if excessive earth leakage has been missed on a class 1 device during acceptance checks for example, (because it's not got accessible earthed parts, thus it's tested to class 2), I suppose if there's no accessible earth on it then under SFC (earth O/C) it's unlikely that this will present excessive enclosure leakage on the device enclosure.

But it might inject excessive earth leakage into a medical system that's interconected via common mains distribution, under NC, eh?

We see new equipment coming into the workshop on a fairly regular basis where there's a failure to keep earth leakage and associated enclosure leakage within limits (or very little safety margin) - especially as part of more complex systems including IT that can have extras fitted by the operators.

We use the evidence from safety tests to justify to manufacturers that they should provide seperating transformers or replace components within systems that may compromise safety.

We can only do this by applying the correct tests to medical devices to establish a baseline for subsequent testing irrespective of whether they're going to install seperation, add a higher specification component, or alter the system in some way.