Home Articles Downloads Forum Products Services Seminar Contact
Previous Thread
Next Thread
Print Thread
Rate Thread
Practical approach to touch current testing #66052
Joined: Jun 2009
Posts: 738
Coffs Harbour, NSW, Australia
DaveC in Oz Offline OP
Philosopher
DaveC in Oz  Offline OP
Philosopher

Joined: Jun 2009
Posts: 738
Coffs Harbour, NSW, Australia
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 betweenó
(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; . 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!!
Recruitment / Jobs
Advertise your jobs with EBME
Re: Practical approach to touch current testing [Re: DaveC in Oz] #66056
Joined: Feb 2004
Posts: 13,574
the path less trodden
Geoff Hannis Offline
Super Hero
Geoff Hannis  Offline
Super Hero

Joined: Feb 2004
Posts: 13,574
the path less trodden

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!

Re: Practical approach to touch current testing [Re: DaveC in Oz] #66063
Joined: Feb 2007
Posts: 223
Yorkshire
MikeX Offline
Master
MikeX  Offline
Master

Joined: Feb 2007
Posts: 223
Yorkshire
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!

Re: Practical approach to touch current testing [Re: MikeX] #66065
Joined: Feb 2004
Posts: 13,574
the path less trodden
Geoff Hannis Offline
Super Hero
Geoff Hannis  Offline
Super Hero

Joined: Feb 2004
Posts: 13,574
the path less trodden

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

Re: Practical approach to touch current testing [Re: DaveC in Oz] #66074
Joined: Feb 2007
Posts: 223
Yorkshire
MikeX Offline
Master
MikeX  Offline
Master

Joined: Feb 2007
Posts: 223
Yorkshire
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!

Re: Practical approach to touch current testing [Re: MikeX] #66075
Joined: Feb 2004
Posts: 13,574
the path less trodden
Geoff Hannis Offline
Super Hero
Geoff Hannis  Offline
Super Hero

Joined: Feb 2004
Posts: 13,574
the path less trodden

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

Re: Practical approach to touch current testing [Re: DaveC in Oz] #66076
Joined: Jun 2007
Posts: 98
England
Barney Offline
Adept
Barney  Offline
Adept

Joined: Jun 2007
Posts: 98
England
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
Re: Practical approach to touch current testing [Re: Barney] #66077
Joined: Feb 2004
Posts: 13,574
the path less trodden
Geoff Hannis Offline
Super Hero
Geoff Hannis  Offline
Super Hero

Joined: Feb 2004
Posts: 13,574
the path less trodden

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).

Re: Practical approach to touch current testing [Re: DaveC in Oz] #66078
Joined: Feb 2007
Posts: 223
Yorkshire
MikeX Offline
Master
MikeX  Offline
Master

Joined: Feb 2007
Posts: 223
Yorkshire
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!

Re: Practical approach to touch current testing [Re: MikeX] #66079
Joined: Feb 2004
Posts: 13,574
the path less trodden
Geoff Hannis Offline
Super Hero
Geoff Hannis  Offline
Super Hero

Joined: Feb 2004
Posts: 13,574
the path less trodden

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!

Re: Practical approach to touch current testing [Re: Geoff Hannis] #66080
Joined: Feb 2007
Posts: 223
Yorkshire
MikeX Offline
Master
MikeX  Offline
Master

Joined: Feb 2007
Posts: 223
Yorkshire
Originally Posted By: Geoff Hannis

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


Well what does 60601 say about it as this is the only document that would, or should, even consider such a event?

If during the risk management assessment this fault was even considered likely then measures would be required to address it, such as making the device a class II device.

But we are going way off the original posters question now.

Re: Practical approach to touch current testing [Re: MikeX] #66081
Joined: Feb 2004
Posts: 13,574
the path less trodden
Geoff Hannis Offline
Super Hero
Geoff Hannis  Offline
Super Hero

Joined: Feb 2004
Posts: 13,574
the path less trodden

Of course it's likely ... I've encountered it myself a number of times over the years! whistle

But you're right about the thread (Dave was talking about AS-3551) ... I'm finding an old 62353 one as I write (so we can continue there, if need be). smile

Re: Practical approach to touch current testing [Re: Geoff Hannis] #66087
Joined: Jun 2009
Posts: 738
Coffs Harbour, NSW, Australia
DaveC in Oz Offline OP
Philosopher
DaveC in Oz  Offline OP
Philosopher

Joined: Jun 2009
Posts: 738
Coffs Harbour, NSW, Australia
Quote:
But we are going way off the original posters question now


nothing new there then once GH is involved cry


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!!
Re: Practical approach to touch current testing [Re: DaveC in Oz] #66088
Joined: Feb 2004
Posts: 13,574
the path less trodden
Geoff Hannis Offline
Super Hero
Geoff Hannis  Offline
Super Hero

Joined: Feb 2004
Posts: 13,574
the path less trodden

Are you saying you didn't get the answers you were looking for then, Dave? think

Re: Practical approach to touch current testing [Re: Geoff Hannis] #66098
Joined: Jun 2009
Posts: 738
Coffs Harbour, NSW, Australia
DaveC in Oz Offline OP
Philosopher
DaveC in Oz  Offline OP
Philosopher

Joined: Jun 2009
Posts: 738
Coffs Harbour, NSW, Australia
Interesting though the discussion on various standards is, it does not really fulfil the "practical approach" criteria of the original request. Guess I was hoping for a bit more input from those at the coal face.

Still, we live in hope.


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!!
Re: Practical approach to touch current testing [Re: DaveC in Oz] #66099
Joined: Feb 2004
Posts: 13,574
the path less trodden
Geoff Hannis Offline
Super Hero
Geoff Hannis  Offline
Super Hero

Joined: Feb 2004
Posts: 13,574
the path less trodden

Assuming that we are talking about routine EST, surely the "practical approach" is to ignore it altogether? As Mike has mentioned, if the kit sails through the other (current leakage) tests, how can there be a problem with "touch current" (enclosure leakage)?

Unless, as I say, you are looking at some other definition (that we are not aware of).

Unhelpful phraseology like "representative sample" can only be dealt with by making a note where (if, and when) you ever do find a "touch current" worth recording (that is, the reading that has traditionally been known as "the worse case"). And (again as Mike has said) it's up to the tech carrying out the test to decide how much time it's worth spending hunting here, there and everywhere. Just the carrying handle, the front panel, and (or) anywhere else that appears obvious, for Goodness Sake! smile

But I concede the point that your opening post should have warded off any discussion of (relative merits of) the various measurement methods available under IEC-62353, as it clearly states "touch current shall be measured with the equipment powered from a supply with an earthed neutral". That's why the conversation was moved to another thread. frown

Re: Practical approach to touch current testing [Re: Geoff Hannis] #66100
Joined: Jun 2009
Posts: 738
Coffs Harbour, NSW, Australia
DaveC in Oz Offline OP
Philosopher
DaveC in Oz  Offline OP
Philosopher

Joined: Jun 2009
Posts: 738
Coffs Harbour, NSW, Australia
Quote:
That's why the conversation was moved to another thread.


and I thank you for it.


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!!
Re: Practical approach to touch current testing [Re: DaveC in Oz] #66116
Joined: May 2008
Posts: 50
Forth Valley
G
Gordovan Offline
Scholar
Gordovan  Offline
Scholar
G

Joined: May 2008
Posts: 50
Forth Valley
To answer the original question, typically we would only take a measurement at one point, and that's if you can even find an un-earthed external conductor that is accessible (a deeply recessed screw, for example, could not really be classed as accessible).

If you particularly feel the need, and if your database system allows it, add a model-specific note detailing where measurements should be taken.


Nothing's Ever Simple
Re: Practical approach to touch current testing [Re: Gordovan] #73765
Joined: Feb 2004
Posts: 13,574
the path less trodden
Geoff Hannis Offline
Super Hero
Geoff Hannis  Offline
Super Hero

Joined: Feb 2004
Posts: 13,574
the path less trodden

Yes, "representative sample" is a poor choice of phrase (one good one should be enough) ... I wonder if it has been improved in any more recent editions of AS-3551? think

I have also followed your second remark, Gordon. I recall that we sometimes even used to mark the point (screw, etc.) - with an "arrow" from a felt-tip pen, for example - where a "good" reading could be found. Also good for repeatability of results, of course. smile


Moderated by  DaveC in Oz, KM, RoJo 

Who's Online Now
0 registered members (), 505 guests, and 25 spiders.
Key: Admin, Global Mod, Mod
Forum Statistics
Forums25
Topics10,290
Posts69,900
Members9,286
Most Online1,391
Mar 26th, 2018
Powered by UBB.threads™ PHP Forum Software 7.6.2