How prevent discharging static electricity when touching metal case of sensitive circuit?
The problem may not be that the metal box has a voltage potential with respect to ground  It may be that you have picked up a static charge (from the air, petting the cat, or rubbing your shoes on the carpet, etc.). If so, the best bet is to discharge yourself first, by slapping a nearby metal water faucet, etc. or other grounded appliance, before touching your sensitive electronics.  Or using a high impedance ground strap on your wrist, standing on a grounded static discharge mat, and etc.