Q: VS 510 Z shuts down after 15 minutes-cuts off power and is hard to reach set temperature. Any suggestions?
A: Does it give you any error messages on the display? What do you mean by "cuts off power" does the breaker trip? How do you get it to restart? Let me know as much as you can about the situation.
Q: Yes breaker trips after 15 minutes-therefore set temperature is difficult to reach-pr is only message that appears and even after priming pumps problem continues.
A: PR is a ritual the VS does on start up, it does not require you to prime the pump or take any action, just wait till it is done.
10 minutes is a long time for the breaker trip. Normally if you have a GFI related issue they will trip as soon as the troublesome component turns on, most of this stuff should have been on for 10 minutes or more at the 15 minute mark. I would have had you check the heater but that would happen at 3-5 minutes after power up.
My first suspect is the breaker. I am assuming the spa has worked on this electrical service successfully in the past? I put an amp meter on the wiring to the breaker and see if is drawing more than the rated amperage. If it is then we will have to look into why, but if it is not overdrawing see if this breaker is warmer than the breakers around it. Sometimes breakers will get a bad connection in them and warm up and trip early.
All this is predicated on your 10 minute trip. If the trip is immediately after priming (3-5 minutes or so) then we would have a different discussion.