I went to a house and at the cold waterpipe the ground was arcing against the water pipe.
I checked the neutral at the tap,meter socket and panel all were tight and made sure the neutrals and grounds were tight inside the panel. I tightened the ground connection and it stopped what could be the cause?

The problem was a loose connection at the water pipe.

The 'cause' is required by the NEC.

The bonding of multiple services to a common underground water pipe results in current being carried on the water pipes.

Thanks I just wanted to make sure I wasn't overlooking anything. I ran into this problem once before and it was a losse neutral at the taps along with alittle loose on the water pipe.

I have seen grounds carry current even with the mains to the house open!
Sometimes your ground connection is a path for current should one of your neighbours say have a bad or open neutral somewhere in their service..
A.D

Sandro:
A posibility also could be a 'bad' neutral connection on the utility side. Had that quite a few times, both OH & BUD.

John

Just came back from job plumber left and told homeowner to call electrician, we found loose neutral at service head, cut back. and cleaned, new bug, and we hope the plumber will return.

Bad neutral at a neighboring property could be putting current onto the water pipe and back out your client's neutral.

Check for a bad water heater element or another high resistance ground fault. A high resistance ground fault can cause significant current flow and power consumption, without tripping a breaker.

After you tighten the ground connection, it is worth putting a clamp meter on the GEC and flipping breakers to see if some particular circuit is causing the current flow.

-Jon