AC reference circuits measurement

[KTChiam]

How can I safely measure AC referenced (Neutral) DC voltage? The measured line has possibility of shorted to Live AC as well. I know some transducers can provide isolation, but is there any NI devices that could do the same? NI 6218 seems possible, but there is a limit of 60V to Earth (which may not be suitable if the measurement line is shorted to Live).

[johnsold]

If the line being measured is subject to faults to line voltage, I would strongly recommend isolated transducers. Do you really want to risk your DAQ device, your computer, and your life that the isolation in a USB device is good enough and that no one miswired anything?

This is a case where the consequences of failure are too high to take inexpensive shortcuts.

What is the magnitude of the DC voltage you want to measure? How fast do you need to make measurements? Do you need to be able to make measurements with the line at the AC mains voltage (fault condition)?  Is your power line frequency 50 Hz or 60 Hz?

Lynn

[KTChiam]

Hi Lynn,

Thanks for your feedbacks. The DC voltage is usually stable voltages eg 3.3V, 5V etc, although there is possiiblity that we also measure waveforms. The fault condition is not expected during design, but it could occur if the unit under test is not assembled properly. The line condition could be 50 or 60 Hz.

[johnsold]

Thanks for the details. 

With that information I will add one more reason to use suitable transducers: The operators (even you!) may tend to think of 3.3 V and 5 V lines as "safe" when they could be at 120 VAC + 3.3 VDC. The use of proper isolation equipment will tend to reinforce that these measurements are NOT SAFE.

Lynn

[KTChiam · Accepted solution]

Hi Lynn,

Thanks again. 

[Henrik_Volkers]

I had a similar problem in the past (home appliance controller PCB) and used a isolation transformer on my bench , however for end of line test on life wire I recommend something like this:

http://download.dewetron.com/_media//files/products/signal/daqp/brochures/english/lowres/splitted/de...

Spoiler

I'm not involved with that company, I just worked with their modules

Life line voltage can carry nasty high voltage spikes!