Many electricity companies, including National Grid, have the facilities to perform some maintenance on circuits that are still live. This avoids the need to switch circuits out to do maintenance. The picture shows one technique, with access to the conductors from a bosun's chair suspended below a helicopter. Other techniques include working direct from a helicopter, access from the pylon using an insulated ladder, and access from a "cherry picker" with an insulated arm.
Once the worker is on the live conductors, they are potentially exposed to high EMFs, because they are very close to the conductors. They normally wear conducting suits, which screen them against the electric field. To ensure they do not receive exposures above the relevant limits -ICNIRP in the UK - National Grid imposes a limit on the current in the conductors while live work is in progress. The size of that limit is calculated from numerical dosimetry. National Grid funded a set of calculations specific to live-line work. The abstract is reproduced below.
Phys Med Biol. 2002 Apr 7;47(7):995-1012.
Magnetic field exposures for UK live-line workers.
Department of Electrical & Computer Engineering. University of Victoria, BC, Canada.