Main Power Problems... Circuit Breaker Trips Digital Air
 
Electric Heater Troubleshooting,
Self-Contained/Portable Spa 
GFCI Breaker Trips 
This problem can drive you crazy.  Heater elements, due to the nature of their construction and the environment in which they operate, will have a mind of their own.

The thing that you must remember with heater elements is, never trust them.  Particularly if you haven't been that observant of your water quality.  I've seen new heater elements go bad in as little as 6 months... all due to non- maintenance of the water's ph!  

The culprit:  micro size cracks (that you can't see) or breaks in the elements outer sheath.  This is especially true where the element's sheath meets (and bonds) with the stainless steel attachment "through-bolts".  

Inside a heater element is a relatively simple design.  A heater wire, that is, a wire made from metals that create resistance to electricity, surrounded by a powdered ceramic, encapsulated in an extremely strong metal tube.  You folks with acetylene torches, try to melt one of these things sometime.  It's not as easy as it seems.

But, as I've said many times here, acid water knows virtually no boundaries.  

A heater element can work fine one day, and trip a GFCI the next...  without any predictability.  It all depends on the severity of the breach of the outer casing of the element itself.

Example Elements
 
This Brett heater element has holes big enough to crawl through. But... this Balboa Control System flow-thru style heater element has been compromised just enough to trip the breaker when it "feels" like it.  Note!  This is typical acid water damage.
Same flow-through element type as the Balboa system (above right).  This shows where to look for corrosion.  If you've got anything other than shiny ceramic or steel, then your element is a canadate for replacement! This is a continuation of the above left image.  Notice the backing plate of the element shows NO EVIDENCE of corrosion or failure, (one reason why these things can be so difficult).  The business end of this element however,  is toasted!
All Spa Installations

There are several ways to check the heater element integrity, and ensure that the element is defective.

1.  GFCI trips constantly 
 

A.  Simply turn the spa's thermostat all the way down.  (If the water temperature is near freezing, then skip this step).  Turn the spa power back on.  Wait a minute or so to see if the spa will run normally without tripping the breaker.  
(1) 115V Spa.  Be sure that the pump is running on low speed.  
(2) 230V Spa.  Be sure that the main pump is running, (high or low speed).
Turn up the thermostat to activate the heater.  If the GFCI trips, then continue with step B.

B.  With the power to the spa disconnected, remove both power wires to the element, and wrap the wire lugs with electrical tape to ensure that they don't contact anything.  Re-energize the power to the spa, with the thermostat turned up enough to activate the heater.  If the GFCI does not trip, then continue to step C.

C.  With the power to the spa disconnected, use an ohm meter to check the resistance from either one of the element leads to the heater casing or a known good ground.  If you measure anything less than 5 meg, (five million ohms), then your heater element is defective.

Realistically, you could use any of these steps alone to pretty much determine if the element is defective.  I've presented all three to provide you with "proof positive" that the element has failed.

2. GFCI trips when it feels like it. 

This one's a little tougher.  However, there is a rather fast way to determine the heater element's integrity if you have an ohm meter.  The spa MUST have water in it to make this check.
 

A.  With the power to the spa disconnected, and with (both!) power wires to the element removed, use an ohm meter to check the resistance from either one of the element leads to the heater casing or a known good ground.  If you measure anything less than 5 meg, (five million ohms), then your heater element is defective.  This is where having a good digital meter comes in handy.  There may not be enough conductance to ground to trip the GFCI, but there will typically be enough to show an undetectable crack in the outer sheath or ceramic insulation to register on a digital meter.

B.  Without the benefit of a good meter, with the power to the spa disconnected, the best thing to do is disconnect the power leads from the element, re-apply power, and let it run for a day or two to see if the GFCI doesn't trip.