5. Technology and Implementation

5.1 Technological aspects of protective measures

• For a very long time, safety systems consisted solely of electromechanical devices. They included measuring components that involved three categories of phenomena:

  • induction-based devices: disc relays, which operate on a principle similar to that of energy meters, and bell relays;

  • electromagnetic actuators: plunger-core relays or toggle-arm relays;

  • Electrodynamic actuators: moving-frame relays.

  Certain mechanical parts, particularly the contacts, were fragile and had to be handled with care. Furthermore, since the moving parts were at rest when there were no faults,...