Static electricity in PC tower cover

Is it normal and safe to feel static electricity in the metal housing of my PC tower? This does
not happen all the time, but has happened. If unsafe, please suggest a way to ground the case.
Thank you.

Just to be sure we have the right diagnosis, with static electricity you get a small shock for a brief moment when you touch the case, then nothing. With a wiring fault, you will feel a continuous tingling as long as you keep touching the case.

Assuming we are dealing with static electricity, the good news is, your case IS grounded. It is you who are building up a charge, and it is safely grounded out whenever you touch the case. The puzzle is how you become charged. It could be carpets or clothing. It may be linked to extremely dry air in your house (moist air helps to allow small static charges to leak away before they become perceptible shocks).

Certain types of carpet fibers (nylon suggested above, also some polyolefins) can cause this. In clothing, cotton will NOT, but real wool, acrylic "wool", and many other synthetic materials like rayon will do it, too.