'Touch Current' test - Seeking clarity about clause 8.7.4.6

[WillPaul]

Hi All,

I really hope that someone can help me. I can see that someone has posted a very similar question to this before, however I didn't fully understand the response.

To simplify the problem, lets say that I have battery which is to be fitted to a medical power tool before every operation, like a DIY drill. At the moment the two contacts can be touch simultaneously, therefore failing the 'touch current' test - 8.7.4.6:

"Internally powered me equipment is to be investigated for touch current but only between parts of the enclosure, not between the enclosure and earth unless 8.7.4.6.c) applies".

How do I get around this problem, or am I misreading the standards?

Thanks in advance for all the help

Will

[20110131]

60601-1:2005 3rd edition sub-clause 8.7.4.6, or 2nd edition, sub-clause 19.4g Subject matter being: "enclosure leakage"
The leakage is defined as:
Enclosure Leakage Current: Current flowing from the enclosure or from parts accessible through an external conductive connection to another part of the enclosure.
Referenced @
http://www.conformity.com/artman/publish/printer_176.shtmlp
Your device is battery powered. I would advise against some-how applying mains potential to either battery terminal, with or without a battery inserted. You are attempting to determin the maximum flow of current, from live part(s) within device to the surface of you properly insulated, (likely plastic), device.
approach to understand the test: You are trying to determine the maximum leakage current, which could flow through the enclosure. The enclosure likely is a high resistance, (likely plastic, and over 10Giga-ohm surface resistance, at any points), cover. The very high resistance of the cover dictates the maximum possible leakage current. The directions indicate that this can be tested from any and all conductive contact points within the device. It is likely that testing from the battery +/- terminals would work well. With a Giga-Ohm meter you could measure the resistance and calculate the maximum leakage current. Example: [Max possible applied voltage]/[measured Giga-ohms]=[Max leakage current]
Max possible applied voltage = As described in parts of the standard, would be AC mains, (110VAC/220VAC)(Alternately DCV=ACV-peak value, being 166/311VDC)
Max leakage current = Regulatory requirements. If 10uA, then a surface resistance of [311/0.00001 ohms] or 31.1Mega-Ohms must be met. Over 1oGohms is likely, unless you have a metal case with poor isolation.

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The resistance example is a method of understanding the current flow situation only
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I hope this helps, and does not confuse you more

[WillPaul]

Thanks for the reply.

As far as I am aware 60601 does not classify the touch current as the current between live and the enclosure, it just states that between two points of an enclosure (for internally powered devices), in which case this could be between a DC 1.2v battery and its enclosure or either end of an AA battery, that the touch current can not be exceeded. How is it the case, how are batteries used?

If I am missing a trick here, please can you direct me to the clause/statement that either exempts dc tool/floating voltages/internally powered devices etc, or just explains how to get around this problem.

Many thanks,

Will

[20110131]

You said it best! The standard seems to almost assume Mains voltage is an input, and comments how the device should handle it as being a maximum imput. The battery terminals would have to be an excluded. I did not see 8.7.4.6c I am sure this is covered in the "c" part.
The resistance comment was a way of describing the test DC impedence using I=V/R or R=V/I, and so on.
The testing can be interesting at times. The wrong probe or wrong voltage in the wrong place can be explosive.

[Peter Selvey]

Sorry for slow reply here:

Both the 2nd and 3rd edition have "get out of jail for free" cards for this case.

In the 2nd edition it is under Clause 16 e) 5).

In the 3rd edition it is under Clause 8.4.2 c).

Batteries are one of the specific examples called out.