In the UK, commuter trains into London - the old Southern Region - operate at 750 V DC, and the London Underground operates at 600 V DC. In the rest of the country, most electric railways with overhead electrification operate at 25 kV AC, at the same frequency, 50 Hz, as the power system. In other countries, a variety of voltages are used, and some systems use a lower frequency, 16 2/3 Hz.
Exposures come from:
- electric fields produced by the overhead conductor - perhaps a few kV/m maximum on a station platform
- magnetic fields produced by the traction current flowing in the overhead condurctor (or the third rail) and returning in the running rails - perhaps a few microteslas.
- magnetic fields produced by conductors within the train carrying the current to the motors, and perhaps by equipment associated with the motors -sometimes as much as milliteslas close to the floor, but more usually, at waist height, tens of microteslas.
You wouldn't think of a normal petrol or diesel car as a source of EMFs but there are two ways it can happen:
- if the steel bands in the tyres have permanent magnetisation, the rotating tyre then leads to an alternating field - a microtesla or two at the location in the back seat closest to the tyre
- if the alternator and the battery are at opposite ends of the car, the wire connecting them can be a source of field, which is mainly DC but can have an AC component - a few microteslas at certain places in the car.