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Leakage Current Measuring Instrument - IEC 60601-1 Requirement

M

MSouza

#1
I'm building a setup to measure leakage current described in IEC 60601. My problem is to find a multimeter to use to measure the voltage across the humam body model with these specs:

"The measuring instrument referred to as MD shall have an input impedance (Z) of 1MW and a frequency characteristic that is flat from DC to 1MHz."

It's possible to find a multimeter with these specs?

Tanks
MSouza
 
#3
Most test labs will use a multimeter for general purpose, and an oscilloscope if higher frequencies are suspected.

Multimeters vary greatly in frequency response so you need to check the specification. Typical ranges are from 20kHz for cheap to 100kHz for high end meters.

Many labs have a special wide band rms meter which is specified up to 1MHz or higher. But I found a close look at the specs on these meters is pretty bad. They were really designed for work in the dB region, where a 5 or 10% error is nothing much.

In truth, measurements above about 50kHz take some work to get accurate, not just the meter. The problems increase with frequency. Towards the 1MHz end, most ad-hoc laboratory set ups would probably have errors exceeding 50%.

But, it is not a real issue as it is rare to have a source of high frequency with significant current, enough to get close to the limits.
 
#6
The AD636 has a headline 1MHz bandwidth, but if you look closely the specs you find the 1% bandwidth is only 14-130kHz depending on signal size. I got caught out once with the AD637, with a similar problem that the headline spec is misleading.

The AD734 has a headline spec is 10MHz, which means at 1MHz it remains accurate, but it needs >100mV input, and has only 50k input impedance. So you will need a buffer amp with gain, I suggest LM7171.

And of course you will still need to take care with the design around front end at higher frequency.

Also note that for the "mains on applied part" and similar tests the voltmeter is at mains potential, so it needs to be isolated and should have CMRR >90dB. But as there are no higher frequency components in that test, using a standard battery operated multimeter should be OK.
 
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