The 24 volt transformer to the heat pump furnace burned out twice. It's a 240v step down transformer to 24v (40 VA). I notice that it burned out only when I was in t-pump mode for heating. When I started up the unit with the new transformer and put the furnace to run with the electric heater it runs fine. The input voltage measured to the transformer was 250v, is that significant? What should I check on the t-pump since it burns the transformer after 30 seconds of operation? I checked the wires and no cracks or short circuits are visible. How do I check the amperage on the DC side?

, there`s a possibility that you have a shorted contacter

As wmproop said the contactor could be bad. They are inexpensive & easy to replace. I'd also check or replace the wiring from the indoor unit to the outdoor. You should also install an inline 5 amp automotive fuse in the "common" wire off the transformer. Fuses are easier & cheaper to replace than transformers.