Leakage Current AC-DC discrimination per cl. 8.7.3 b)

Benjamin Weber

Trusted Information Resource
#1
According to subclause 8.7.3 b) the AC leakage current limits apply to frequency "not less than 0,1 Hz".

Is this a strict value for AC-DC discrimination? Or could this be interpreted as "it could be a higher frequency, but never less than 0,1 Hz"?

If it was a strict value, how could this be applied a everyday-routine? Usually the measurement equipment has internal filters for determination of AC and Dc components. But the cut-off frequency is never as low as 0,1 Hz! Most DMMs or similar have some Hz (even the famous Rohde & Schwarz URE-3 has 8 Hz or so). Thus it could not be used for standard-compliant measurements.

Does anybody have some competent thoughts on this?
 
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Peter Selvey

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#2
Interesting question!

I think it's usually not a practical issue as the source of leakage is either dc (for example, patient auxiliary currents from ECG circuits), with the next lowest frequency being mains (50, 60Hz), followed by switching power supplies (>30kHz) and special sensing (usually >50kHz); there is nothing I can think of that would be more than dc but less than 50Hz.

It's also not critical for safety. The zone of high risk currents (ones that mess up the heart rhythm) is from about 20Hz to 100Hz. The standard uses 0.1Hz to 1kHz for this critical zone (most strict ac limits) but that's more about keeping it simple rather than reflecting the true risk.

The limit of 0.1Hz is probably more to do with dc tissue necrosis which has the special limit of 10µAdc even for Type B/BF equipment. The different limits for ac and dc means it is necessary to distinguish between "ac" and "dc". But again it's not a critical point as (a) an actual ac source in the 0.1Hz zone would be rare, and (b) the special limit of 10µAdc is based on small contact area (1mm²) and long duration contact (hours), which is also rare. If either of these is not true, the 100µA limit would be find for both ac and dc.

From a laboratory perspective (which should be blind to the backstory), if it is suspected that there is something in this very low frequency range, the dc function on a DMM will respond to frequencies less than 10Hz (typically 30Hz is the cut-off frequency used). You won't get a stable reading but it lets you know there is a low frequency ac component. I would then use either the trend/graph function on 6.5 digit DMM or an oscilloscope to investigate the parameters of the waveform and determine the frequency of the waveform and if the ac or dc limits should apply.
 
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