Actually all metal piping systems need to be bonded, but in old houses you never see metal sewer piping that was bonded. I have bonded a few of them when they had such stray voltage issues. Doesn't even need to be a metal tub. Last one I had an issue with was a non metallic tub/shower, with a metallic drain fitting, had some non metallic drain piping but eventually led to old metal drain piping. First time out I could find no problems, owner wasn't getting an obvious shock but complained of discomfort in her feet when using the shower. Second time out I got smarter and turned the shower on and took measurements. water in the drain pipe was the bridge over the gap that made voltage begin to appear. Bonded the metal drain assembly to the water piping and all the problems went away.
