I don't understand the term "power ground" but assuming you are talking about the electrical "System Ground" and also to the copper water pipe elsewhere in the home, I don't see this as a problem. The code requires you to connect the shield per Article 820 Section 820.100 NFPA70 2008 Edition. Now if you want to additionally bond to the water pipe somewhere else, albeit not necessary, be my guest as long as you have complied with the afore mentioned section of the code.
Greg- This is your specialty -What say you?
