Effects on equipment
There are several types of equipment that can be affected by electric and magnetic fields. However, the fields required to cause an effect are usually higher than those commonly encountered in the environment.
Credit cards and railway tickets
These have information encoded on a magnetic strip. This can be corrupted by magnetic fields above approximately 10,000 µT. Such fields would never occur at 50 Hz in public areas, but a problem can arise with static fields such as those from magnetic catches on handbags.
Car electronics
Some cars with electronic control systems have been found to be susceptible to interference from power-frequency magnetic fields above approximately 2,000 µT. Again, these fields are far higher than those produced by electricity infrastructure.
Car batteries
There are occasional reports of car batteries draining under overhead lines, but if this happens, it is just coincidence - there is no mechanism for the field levels under an overhead line to affect the chemistry of batteries or the drain on them from other equipment in the vehicle.
Quartz watches
Quartz watches with analogue dials use a small stepper motor to drive the hands. This stepper motor can be driven by a suitably oriented external power-frequency magnetic field of approximately 1000 µT or greater, causing the hands to rotate 100 or more times faster than normal. The effect is spectacular but has not been found to cause any damage to the watch.
Fluorescent tubes
A fluorescent tube works by an electric field inside the tube causing a discharge. This electric field can be generated normally, from applying a mains voltage across the tube, or from the electric field produced by a power line. So fluorescent tubes will produce a visible glow under a power line, though usually it is only visible after dark as it is much weaker than the light they normally produce.
The current through a fluorescent tube under a power line would be approximately 20 – 200 microamps (µA) depending on the field. This is much less than a person can normally perceive, so you can hold the tube yourself under the power line without it hurting. (For comparison, a 10 W tube at 230 V draws 40 mA, which is 200 times greater than 200 microamps).
You can sometimes make a fluorescent tube produce visible flickers by holding one end and rubbing your foot on a carpet to generate static electricity, though again, this needs to be done in a dark room.