Allowing user access during normal operation is an enormous responsibility which, in the case of interlocking devices, lies with manufacturers of switchgear and controlgear. On the other hand, the purpose is to ensure a high level of safety for operators who must perform their tasks during the normal operation of the installation.
Standard IEC EN 62271- 200 requires provision for interlocks whenever there are operations that put safety at risk or even to simply make the switchgear and controlgear easier to use. These devices must also be sufficiently sturdy to withstand attempted incorrect operations without damage.
In the case of main circuits, racking-in and racking-out circuit-breakers, switch-disconnectors or contactors must only be possible when they are in the open position. Their operation must only be possible when they are in the connected, isolated, withdrawn, test or earthing positions, but never in intermediate positions.
Lastly, the interlock must prevent closing in the service position if these apparatuses are not connected to the auxiliary circuits associated with automatic opening or the apparatus itself.
Vice versa, the interlock must prevent the auxiliary circuits from disconnecting when the circuit-breaker is closed in the service position.
Make sure that disconnectors operate in the conditions for which they were designed. This means that the disconnector may only operate when the associated circuit-breaker or contactor is open.
Since the short-circuit making capacity of earthing switches is less than the admissible rated peak current value of the circuit, they should be interlocked with the associated disconnectors.