Hello Folks,
I’m hoping that the group can increase my understanding of touch current measurement techniques per IEC 60990. I’m working on the test set-up for measurement of touch current per IEC 60990, figures 9 & 10, single phase equipment connected line-neutral or line-line on a star IT supply system. The supply at my facility is 208 / 120 4-wire WYE. I believe that I need a 1:1, three phase isolation transformer to perform the tests correctly – namely, to incorporate the 1000 ohm neutral resistor and phase ground faults required. Can anyone recommend a good source for such a transformer? Figures 9 & 10 show a Delta – WYE transformer, but is it acceptable to use a transformer wired WYE – WYE ? Other questions regarding touch current measurement – the standard allows for the measurement of touch current to be made at any voltage within the rated range, then normalized to the max rated voltage plus tolerance by calculation. The standard also allows for frequency correction when the measurement is made with a supply frequency less than the max rated frequency. I believe that touch current is directly proportional to both frequency and voltage, such that: Touch current = (1/( Vtest / Vmax+tol))*Measured Touch Current; for a product rated 240 V but tested at 120 V, this simplifies to 2.2 times the measured touch current when supplied by 120 V; 1/(120/264) = 2.2. And Touch current = (1 / (Ftest / Frated)) *Measured Touch Current; for a product rated for 63 hz but tested at 60 hz, this simplifies to 1.05 times the measured touch current when supplied by 60 hz; 1/(60/63) = 1.05. Are these calculations correct? And what if the product is rated 100 – 240 V, 47 / 63 hz? Can I perform the test at 120 V 60 Hz and calculate the worst case touch current by applying both calculations? As always, your comments and shared experience will be greatly appreciated. Doug Massey Product Safety Engineer Advanced Compliance Solutions Ph. (770) 831-8048 FAX (770) 831-8598 Visit our web home at http://www.acstestlab.com <http://www.acstestlab.com/>
