This is very common all across Taiwan. Coming from the US, I was depressed with my first experiences with local household wiring. Seems all homes that are 10+ years old are only wired for two conductors. Businesses and new homes 3-5+ years old are now including a ground wire (that 3rd prong normally cut off on your power strips, microwaves, washing machines) and have 3 prong electrical outlets. That said, even in new homes that have 3 prong outlets, it very much depends on the skill (maybe attitude) of the electrician who installed them.
If you look in the electrical panel in a newer house, there will be a ground fault circuit interrupter (GFCI) which looks like a special circuit breaker with an on-off switch and a green (sometimes red) test button. If you have a GFCI in the electrical panel that controls the power to your garage door, this is the reason you get a electrical jolt instead of severe or fatal electric shock. The GFCI monitors the electricity that flows through the electrical panel. When it detects a small difference between the amount of current flowing and the current returning, it switches off the power to that circuit. It acts immediately, but some electricity always get through and you get a jolt. As soon as the circuit inbalance is corrected (you let go of the switch), it returns power to the circuit. So, after your jolt, the garage door continues to work.
A ground fault is any unintentional leakage of electricity to the ground (normally the ground we stand on). This is most commonly seen in old electrical appliances where a wire on the inside has become worn and is making metal to metal contact with a part of the frame or housing of the appliance. So when an old rice cooker is plugged in, it operates but you get a shock when you touch certain metal parts. Even in older homes without ground-fault circuits, I have seen old appliances/computers work decades past their designed lifespan an have gotten shocks of varying degrees. Small current shocks are easily mistaken for static and large elbow or gut jerking shocks are from direct contact with 110 volts of alternating current.
On to your problem:
Check your electrical panel first, see if you have a ground fault circuit. Turn off the power to your garage door, open the switch plate and see if you have 3 wires going into the back of the box and into the wall. The garage door motor assembly will certainly be 3 wire, but check to see if it has been connected. In a relative’s new house, even though all the outlets are 3 prong, I still found several that were not wired correctly and failed the ground-fault test. They failed because the electrician wired it backwards. I swapped the wires to the correct terminals on the outlet and the outlets passed.
Your switch is giving you shocks, so my guess is that the 3rd ground wire is connected directly to the outlet metal case. Sometimes this is a [green|red] screw inside the switch box and is connecting the ground wire from the electric motor to the metal case. If this was North America, everything would be fine because the whole house wiring needs to pass ground-fault standards by law. However, here in Taiwan, construction standards are thought about but not enforced. Each time you touch a part of the switch case, you are completing the circuit (so to speak) and providing the electrical path from the switch to the ground through your body.
Recommended solutions:
Your best solution is to hire a certified (read expensive) electrician, maybe someone has a friend or a relative who does quality work that will be a little cheaper. You need to insist on certified, but if the person that comes out to do the work cannot explain to you the problem in the first few minutes of looking inside the switch box, you will know whether or not they know what they are talking about.
My guess, is that the wiring installed inside the wall is 2 conductor wire, and the 3rd ground wire from the garage door motor is connected to the metal case of the switch.
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If by chance, the wire in the wall is 3 conductor, then you can use the ground-fault tester to verify that it is correctly connected the house wiring. If the tester returns true/yes, then simply connecting the 3rd wire on the motor to the 3rd wire from the wall will solve the problem. If the ground-fault tester returns false, it is telling you that the other end of the wire has not been connected correctly to the electrical panel. A false reading means, hire an electrician to correct it.
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If the wire in the wall is 2 conductor, then you can run an external, insulated wire out to ground. But, this will not pass a electrical inspection and again, this should be done by an electrician to ensure that the ground connection is done right and does not pose an electric hazard to others (children, animals, the rain …)
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If the wire in the wall is 2 conductor, you can simply tape up the end and leave it inside the switch box. This is a fire hazard and will not pass an inspection. The good part, you will not get shocked from touching the case, the bad part, if the garage is struck by lighting, or high winds or a typhoon brings down an overhead electric wire onto your garage, any electrical devices attached are toast and will burn up. High voltage electricity will follow the wiring back to the electrical panel and has a 50-50 chance if the ground-fault is not installed correctly (if this is a newer house), but usually will be grounded-out (stopped from propagating into the house) at the panel.
Testing AC circuits:
There seems to be some confusion on how to test AC wiring. The common testers I use are ground-fault: 3 prong connector with LED indicators for correct ground-fault wiring (returns correct or incorrect) depending on which lights come on, I have an AC/DC digital meter with a specialized section for AC testing. Touching the two test leads from the meter to two AC wires will display the voltage of the wires. Most electricians I have seen carry an AC current tester. This is C clamp style of meter that does not connect directly to wires, but simply is clamped over a insulated wire and will read the current flow though the wire itself. These multi-meters range in pricing but normally are not needed by most.
Please help.