Every time we have a thunderstorm my outside GFCI receptable for my pond pump trips. It appears to happen when there is a strong flash from lightning. I think that it is more than possibly the unit getting wet from rain and shorting out. Works fine during normal heavy rain showers. Any thoughts as to the problem. I also have a sensor light that is connected to a transformer plugged into the same outlet. Appreciate any help or ideas.
It is also the only receptable on that line from the breaker box.
:confused:

What make is the GFCI Receptacle?? If it is a hubble, There was a product safety bulletin late last year regarding them. Also, do you have an in-use cover on the receptacle?

Sparky 3000,
I looked at the GFCI and all I can see is a "SD" oe "HD" on the plate. I guess I may have to take it out of the box to tell for sure. The box has a cover that is hinged from the top. It is solid but lay on the wires where they plug in so the sides are open I guess to any blowing rain. What is a "In Use Cover"? Thank you for all your help.:)

Lightning carries with an enormous amount of electrical energy. I'm sure you know that. Please consider the GFCI circuit itself. If it is in any way susceptible to an induced burst of electrical energy, that alone could cause a .004 through .006 amp current to cause the GFCI receptacle to trip.

So my questions would go to the installation of the circuit. Are the branch conductors buried the proper distance in the ground. Are you using the correct wiring between the Main Panel and the receptacle.

Can you rig a temporary shield, say a metal bucket over the top of the receptacle to shunt the effects of the lighting away from the receptacle and to earth ground. Is the ground rod for the residence installed properly and the grounding conductor is in good condition, no cuts or frayed wire.

Also, just out of curiosity, does anything else in the home or outside fail? Is it only this branch circuit and only when lighting occurs.

It may also be something relating to the pump itself that only shows up in wet weather. The following advice has some inherit risk, but consider giving the pump (not the outlet, cover it with bucket as suggested to be sure) a good spraying from your water hose to soak it as would occur in the rain storm. If the GFCI holds, you can better presume you don't have a current problem past the outlet. Then consider spraying the outlet to a similar degree, you want to see if water is getting in there and shorting the terminals. Don't go nuts, with enough water you can certainly short the thing out, your goal here is to confirm that the outlet is not shorting from incidental humidity and moisture. A plastic cover on one of my outside lights tended to gather moisture and occasionally trip the GFCI until I widened the opening at the bottom, you may need to do that same. And if you do this test please take care not to become the path to ground yourself.

Check for ants bringing sand or dirt into the box, which causes the GFCI to trip when it gets wet. Simply remove the outlet cover and clean the box, after turning the power off. Check every box on that circuit. Ensure outside covers seal properly. If necessary, apply Gardner Bender (GB) Duct Seal, which some call "dumb dumb," to keep ants and moisture out. It's available at Home Depot.

Thanks everyone for all the fine input. The line is a separate line from the breaker box to the outside outlet and the only thing on it. Nothing else in the home trips during a storm. I am not sure about the depth of the wires in the ground. What should it be? I will be careful to dig down from the box to see how far it goes into the ground. I do know that they buried some extra wire in the ground in case we ever wanted to move the box to another location. I will try all of your suggestions and let you know how it comes out. Thanks again!:) :)