Here's a good article. http://www.hardhats.4ursafety.com/index.html

OSHA has regs, MSHA has regs. Probably a few other government agencies I've never heard of.

Basically there are three classes. There should be a label inside the hardhat that tells you what class it is.

Class A-Best head protection, but not for electricians.

Class B-Good head protection and electric shock protection.

Class C-General use. Protects against smacking into fixed objects, but not for things falling on you.