Home Articles Downloads Forum Products Services EBME Expo Contact
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 2 1 2
Joined: Jun 2009
Posts: 796
Likes: 13
Philosopher
OP Offline
Philosopher
Joined: Jun 2009
Posts: 796
Likes: 13
Hi all,
Late last year Standards Australia released a new version of our "bible" AS3551. Some changes were made to bring it in line with 60601. The one I would like some advice on is "Addition of touch current testing to align with IEC 60601-1, Ed. 3.0."

AS3551 now states

Touch current shall be measured with the equipment powered from a supply with an earthed
neutral.
Touch current shall be measured between—
(a) a representative sample of accessible conductive parts of the equipment; and
(b) the earth pin of the fixed mains socket-outlet or multiple socket-outlet from which the
equipment is supplied.


The question here is in the interpretation of the phrase "a representative sample".

In practical terms, how do you guys operating under 60601 deal with this? Is it considered sufficient to take a sample of one or do you test to multiple points on the case. If so, how is this identified within the test data so that consistent retesting can be done? Any other relevant thoughts or ideas are most welcome

Thanks all, Dave

PS, feel free to PM me if you don't wish to "put it out there"

Last edited by DaveC in Oz; 09/10/13 8:21 AM. Reason: add PM reference

Thoughts and information provided on this forum are mine and mine alone and do not necessarily reflect the policy of NSW Health. They may also be complete bollocks!!
Joined: Feb 2004
Posts: 14,654
Likes: 60
Super Hero
Online Content
Super Hero
Joined: Feb 2004
Posts: 14,654
Likes: 60

Touch current? Formerly known as "Enclosure Leakage Current"? It's nice to hear that official bibles are trying to keep up with the (short-hand) terminology that many of us have been using for years.

Do you have the *current (official) definition of Touch Current for us, Dave? think

Yes ... "representative sample" is a bit slack. But as we know (and as a practical point) it is a waste of time trying to distinguish between Touch Current and Leakage Current measurements, as the only place(s) worth checking for Touch Current are from those points on the enclosure that are not intended to be protectively earthed. On many items of equipment, such points do not exist; but the carrying handle (if any) is as reasonable a place to check as any. smile

Regarding re-testing - what I have always done (and advocated) is:- if I actually get a Touch Current measurement worth mentioning, make a note of where I found it on the relevant document (bit of paper, computer record etc.) so it may be referred to next time. "Carrying handle", "screw at bottom-left of front panel", "missing paintwork on top case" ... etc., etc..

BTW: do you ever deal with (talk about) "Risk Current"? Which I reckon is Touch Current with the protective earth lifted ... for my money, that is (and always has been) the most valid (practical, useful) "leakage" test. Although known by another name (Enclosure Leakage under SFC), such a test was included for Class I equipment in Ye Olde HEI-95, only to be discontinued as "standards" (guidelines, whatever) evolved. So much, then for so-called progress. frown

* Oh yes; a pun!

Joined: Feb 2007
Posts: 289
Likes: 14
Master
Offline
Master
Joined: Feb 2007
Posts: 289
Likes: 14
One advantage of using a standard such as 62353 over 60601 is the former assumes the device under test already conforms to the latter! In other words why are you assuming there will be a fault with touch leakage as long as the equipment leakage conforms? Has the device really changed after it was manufactured and assuming it has been serviced to maintained in accordance with 60601 and the medical device directive?

The only time a touch leakage test makes sense is for permanently installed equipment, on parts connected to earth, to prove the item was installed correctly and no excessive touch leakage has occurred from incorrect wiring or, for parts that are not connected to the protective earth, again only really at installation.

Testing to 62353 would mean the normal equipment leakage measurements would take into account touch leakage by default. The only time you may need a touch leakage test is if the equipment leakage exceeded 0.5mA to prove the, up to 5mA leakage is not touchable. (But I have yet to find a product that has this allowable condition!)

Unless you are designing a medical product many of the 60601-1 tests are just not sensible for routine testing.

Performing excessive medical electrical tests may keep people in jobs but is highly unlikely to actually improve safety!

Going back to your question it seems the term 'representative' is wide open to an individuals interpretation, so if you want to waste lots of time and employ many to perform needless tests choose lots of 'representative' measuring points. Otherwise use 62353!

Joined: Feb 2004
Posts: 14,654
Likes: 60
Super Hero
Online Content
Super Hero
Joined: Feb 2004
Posts: 14,654
Likes: 60

Doesn't 62353 include the "touch current" (enclosure leakage current) test(s) then?

I thought that the "Direct Method" tests were identical to 60601.

Or has all that changed (again) since I last looked? think

Meanwhile (again, the last time I checked), volts are still volts, amps are still amps, and ohms are still ... yes, you've guessed it! smile

Joined: Feb 2007
Posts: 289
Likes: 14
Master
Offline
Master
Joined: Feb 2007
Posts: 289
Likes: 14
I have now had a chance to look at the ASNZS3551 standard and can add the following. Under section 6.3 it states that verification testing, using appropriate procedures, should consider the procedures recommended by the medical equipment manufacturer. Section 6.3.5 also allows for verification to be undertaken by the medical equipment supplier or 3rd party, who will issue a suitable certificate. This is also backed up by the test in Appendix B1 and B3 where the note 1 states that not all the tests listed are required.

In other words you should first look to the manufacturer or supplier for the recommended tests to perform, rather than just taking a shot gun approach of performing every test under the sun!

If no advice can be obtained from the manufacturer as to which points should be measured you have to use engineering common sense and decide if more than one measurement (the metal case) is rely needed. In my view only parts of a permanently installed device that are not connected to the protective earth are worth measuring (after the single test on the earthed part of the device). So for example an x-ray table base may be connected to earth but the metal accessory rails may be isolated from earth so a measurement on these is acceptable, although largely pointless as the device passed 60601-1 before it ever got near a hospital!

Joined: Feb 2004
Posts: 14,654
Likes: 60
Super Hero
Online Content
Super Hero
Joined: Feb 2004
Posts: 14,654
Likes: 60

Indeed. But that doesn't mean that it remains electrically safe after a couple of years of use in the hospital. whistle

I would have thought that the example you gave at the end there (accessible metal parts on an examination table) are in fact just the sort of place were a Touch Current "spot-check" might be actually worth doing! smile

It seems to me Mike that we are looking at this stuff from two different perspectives, you and I. Correct me if I'm wrong, but you seem to be looking at things from (what we might call) the manufacturer's point of view (and fixed equipment, at that), whilst I (and I dare say many others) tend to look at the "real world" situation (practical approach) of ensuring the electrical safety of the multitude of kit found (used and abused) in hospitals. frown

Manufacturer A might say "do it"; manufacturer B says "no need". Manufacturer C doesn't say anything at all. So where does that leave us? Left to our own devices, as usual (and in this case, literally).

Biomeds are trained in electrical safety matters. Why is that? Because it does matter; not to the n-th degree of arcane, esoteric, minutiae - but sufficient (in practical terms) to assure ourselves that the kit is electrically safe. So hopefully we can agree on the "engineering common sense" bit. smile

Joined: Jun 2007
Posts: 98
Adept
Offline
Adept
Joined: Jun 2007
Posts: 98
In my view the touch current is an important test for medical equipment and should be used on class 2 equipment with metal parts tested with reference to an electrical earth and for class 1 the test should be applied with a disconnected earth to the equipment under test and referenced to the electrical earth.


Barney
Joined: Feb 2004
Posts: 14,654
Likes: 60
Super Hero
Online Content
Super Hero
Joined: Feb 2004
Posts: 14,654
Likes: 60

Yes. And both were recommended by HEI-95. smile

That second one has traditionally been known as "Risk Current". It is one of my favourites (yes, how sad is that) because - in my mind at least - it relates to a "real world" fault scenario:-

The belt that you would get if the kit had lost its protective earth and therefore didn't blow the fuse(s) if or when an internal short circuit magically developed! whistle

Yes ... that's a "double-fault condition", but one that has been known to actually occur from time to time. So, once again, it can be seen that the integrity of the protective (grounding) conductor is the essential element of electrical safety. And in all but permanently installed (fixed) equipment, that means the mains power cable (which, as everyone knows, these days is almost always the detachable IEC-320 type).

Joined: Feb 2007
Posts: 289
Likes: 14
Master
Offline
Master
Joined: Feb 2007
Posts: 289
Likes: 14
When was the last time anyone found a touch leakage current above the allowable limit, after all other tests had passed?

I bet the answer, for items connected by a domestic plug, is never!

It is almost impossible for any device that conforms and has been maintained to the MDD or 60601-1 to have excessive touch leakage. That is why 62353 has no separate measurement for this, as it is taken into consideration in the equipment leakage test.

Now in the next edition of 62353 touch current measurements will be included, but essentially only for permanently installed equipment. This is to ensure that no excessive touch leakage has been created by incorrect or poor electrical installation (long coiled up power connections etc.).

I think medical service engineers need to stop thinking about 60601 as much and use 62353. Your not designing the equipment!

Joined: Feb 2004
Posts: 14,654
Likes: 60
Super Hero
Online Content
Super Hero
Joined: Feb 2004
Posts: 14,654
Likes: 60

Fair comment.* smile

What does 62353 say about the double-fault situation I described above? think

* Although I don't believe that techs do worry too much about this or that Standard - but rather just crack on with testing the kit!


If you don't inspect ... don't expect.
Page 1 of 2 1 2

Moderated by  DaveC in Oz, RoJo 

Link Copied to Clipboard
Who's Online Now
3 members (daisizhou, Geoff Hannis, Neoteny), 463 guests, and 522 robots.
Key: Admin, Global Mod, Mod
Newest Members
MDevins, EmsR16, Fergusben1, Guilherme Gomes, Salvatore
10,173 Registered Users
Forum Statistics
Forums25
Topics11,055
Posts73,708
Members10,173
Most Online5,980
Jan 29th, 2020
Powered by UBB.threads™ PHP Forum Software 7.7.5