Leisure Bay LT LDR, 230v, LB102R by Balboa dated 6/10/99 with two pumps. O.K., I'm electrician called by regular customer for popped tub breaker. Pulled disconnect, breaker stays on. Opened controller, looked for H2O, loose wires and gnds, disconnected heater, still popped, disconnected pumps, ozone, all line fittings, still popped, pulled transformer, breaker stays on.... Tested transformer no bleed to casing.

First question, control board? Have I missed something? Second, does the EXCB54115 by SpaBabe cross ref to the LB102R.

Quick question....

1. The breaker is a GFCI right?
2. Did you remove BOTH electrical connections to the heater?

If you didn't remove both connections to the heater, then it'll still cause the gfi to trip if the heater is defective (cracked, leaky etc).

You've done everything correctly. If you're really getting infinity on a very high ohms setting to (case) ground at the input side of the transformer plug, then I would do a high ohms check from L1, L2, and Neutral to the case ground with the power input removed. If you get infinity - my guess would be a defective GFCI. One way to check would be to hook up two 115 volt test lights at the disconnect between L1 and neutral, and L2 and neutral, or, using a WIGGY (must be a electro-mechanical wiggy type tester to draw enough current from the breaker), check at the disconnect from L1 to N, L2 to N, then L1 to L2.
If the GFI trips, then the GFI's defective or installed incorrectly (I assume the latter would not be the case since this was an existing installation).

If that all checks out, then I'd pull the board and look for moisture or corrosion anywhere on it, front or back that could cause a continuity to ground anyplace.

Hope this helps and let me know what you find out.