EBME Forums Forums Maintenance Spare Parts Class II EST

This foil wrapping technique is only required for type testing in accordance with 60601-1. The only test you need to do (62353) is the equipment leakage, assuming you can get to some exposed conductive part, and the applied parts leakage test.

Is it (in 60601-1)? Chapter and verse (or better still, a relevant quote), please!

Yes "equipment leakage" for Class II is included in 62353 ... it looks like (what we might call) "total equipment leakage" (1,2) to me. Easy enough to measure when there is a patient connection (like ECG leads connected together, for instance) ... but how do you pick up any leakage current (3) otherwise?

Leakage current generally has two components:- resistive and capacitive. Time, then, to bring out the foil! Or (as I have mentioned before), the old ESU return plate for the kit to sit on. What harm can it do (and how long does it take)? Just record "zero µA" if that's what you see (4).

As an aside, I believe that the general term "leakage current" is a bit of a poor choice. This current inherently finds its way from the live parts of any mains powered equipment to accessible metal casing or parts. In Class I equipment (that is, kit with a protective earth conductor), this "leakage current" flows harmlessly to earth.

So, one wonders, where does any similar current "leak" to in Class II equipment?

1) What Old-Timers might recognise as Patient Leakage and Enclosure Leakage combined.
2) The Rigel booklet mentions "touch current" (which is fine by me) and gives the limit as 100 µA.
3) Assuming a non-conductive casing.
4) But I'll bet you'll always get something.

Grab any old class II laptop power supply and connect an ammeter between earth (water pipe or mains socket) and the metal of the plug that has the DC going into the laptop. The current you measure is the equipment leakage current and will be about 80uA to 250uA. No expensive safety tester needed!

That's all very well for laptop "bricks" ... but what about Class II equipment with a built-in power supply, no power supply at all, no exposed metal ... or anything else we can get a probe on to?

OK ... some folk may say (in situations like that):- "where's the risk"? Those same folk may also consider a Class II Nebulizer Compressor (for example) to be beneath their dignity to bother about. But don't forget that kit like that often faces a hard life in patients' domestic dwellings!

In fact, in such cases, I think I would wrap my trusty ESU plate in a saline-dampened gauze before carrying out (and recording) my electrical safety tests! You know, just to be sure.

By the way (and back to Bricks), are we saying that we need not bother with the extra expense of a "medical grade" Brick, as long as we can demonstrate an Equipment Leakage of less than 100 µA ... in accordance with the quick and easy "Mike Test" as laid out in the above post?

And what about Class I bricks? Less than 500 µA in SFC?

What is a Class II laptop? most of the ones that I have seen are standard laptop running from what is essentially a UPS. When in use the UPS is unplugged from the mains so that the laptop supply comes from the battery. OK as long as a wireless network connection is used. Some laptops have have a Switch Mode Power Supply that is referenced to earth, and have a three core mains lead, so that it is not Class II. A SMPS using a two core mains lead will deliver the DC output that is floating at about half the mains voltage. The leakage current is low, as has been said about 100 microamps, but this will put the patient at increased risk if used during an interventional procedure. A tablet would be safe as it is normally run on its batteries, but my Ipad has just under 100 microamps when running on a genuine Apple power supply. It is not acceptable to simply warn users of the danger of plugging the tablet into the charger when using it in a medical location. Such warning are too easy to ignore.
BAM

So are you a "foil test" guy, when it comes to Class II testing, then, Brian?


Two "ifs and buts" there.

What about when the "brick" gets left plugged in (not unusual in my experience)?

Of course this refers to the electrical isolation class of 90% of all laptop power supply 'bricks'.
Almost all laptop power supplies are class II (and switch mode based), even those with 3 wires! The earth connection is for EMC purposes and has no effect on the isolation between the input (mains) and the output. Look at the marking on the PSU and you will see the double insulation symbol. In fact I have only ever seen one (very old) laptop PSU that was not class II (so the earth was connected to the output -ve connection).
Why will it put the patient at an increased risk? The interventional patient will be connected by CF applied parts. The laptop (PSU) will be medical rated, if used within the patient environment, so poses no more danger than the enclosure of other medical devices, which are allowed a touch leakage of 100uA. A non-medical PSU should not be used within the patient environment but still presents a low risk as the leakage is well below the single fault value of 500uA.
The only common place a non-medical device, such as an iPad or laptop, gets used while connected to the mains, is by a patient during haemodialysis. Again when normal dialysis is being performed (B type applied part) a leakage up to 500uA (SFC) is acceptable, since this could be expected if the patient touch the dialysis machine when the earth was broken and is in line with 60601-1. If CVC dialysis is being performed then only a haemodialysis machine with CF (applied part) rating should be used, so there is no chance of the touch leakage from a medical device flowing via the catheter, in the event the patient touches the machine, another medical device or their iPad!

Use the wrong machine for CVC dialysis and the patient is put at risk, from other electrical devices including medical ones!

Tingle!

Note the measuring circuit ... it looks kind of familiar. But where's the aluminium foil?


Yes, you're right ... for Class II. If you can get hold of a decent circuit diagram for a SMPS, you'll often find the "earth" (protective earth conductor of the mains cable) connected in-between filter capacitors across the mains input (that is, early on in the circuit - and in just the one place). So a Class II situation, but (confusingly, perhaps) with a three-prong mains input.

But occasionally you may instead see it connected straight across to the "zero volts" output, with other components on the output side also connected to it. A Class I situation, then.

If the "brick" only has two mains input connections (as shown in the link above, for example), then it has to be Class II (of course). Otherwise, one way of distinguishing between a Class I and Class II "brick" is by looking at the diagram (which you probably won't have). Another way is to see what your multimeter says (ohms) between the zero volts output connector and the earth prong on the input side.

If you're lucky (and as Mike says) you might also find a Class II "double box" symbol on the labelling ... but personally I wouldn't (don't) pay too much attention to that (or indeed its absence), preferring instead to trust my old multimeter.