Thank you for your question regarding the measurement of chassis grounds. It is our pleasure to help.
There should be less than 1-volt (0.5-volts preferred) between the chassis of your attendance system and the earth ground if it is properly connected. If you were to measure one-leg of a single-phase 220-volt supply (hot wire) to the earth ground, you should read between 115 to 120 volts. If you were to measure across both legs of the hot wires, you would read the 220-volts.
If you or your employees can “feel” a tingle on your attendance system, this is indicative of a very serious electrical problem that can be life threatening. Turn the unit off ASAP and call an electrician!
Assuming the circuit-breakers are not tripping, it is likely that your attendance system or some other electrical system up-stream from it, has either a short, or an illegal neutral-to-ground connection.
In either case, you should turn off your attendance system immediately and have a licensed electrician look into it right away.
Here is some additional information that relates to your question:
We hope you find this information useful. If you should have any further questions, please do not hesitate to contact us again in the future.
The Engineering Team at E&S Grounding Solutions
Photo credit: http://www.flickr.com/photos/25035545@N04/3503195989/sizes/z/in/photostream/