Introduction to Health and Safety at Work

11.13.3 Interlocking 
guard
The advantages of interlocked guards are that they allow 
safe access to operate and maintain the machine without 
dismantling the safety devices. Their disadvantage stems 
from the constant need to ensure that they are operating 
correctly and designed to be fail safe. Maintenance and 
inspection procedures must be very strict.
This is a guard which is movable (or which has a 
movable part) whose movement is connected with the 
power or control system of the machine.
An interlocking guard must be so connected to the 
machine controls such that:
➤
until the guard is closed the interlock prevents the 
machinery from operating by interrupting the power 
medium
➤
either the guard remains locked closed until the risk 
of injury from the hazard has passed or opening the 
guard causes the hazard to be eliminated before 
access is possible.
A passenger lift or hoist is a good illustration of these 
principles: the lift will not move unless the doors are 
closed, and the doors remain closed and locked until the 
lift is stationary and in such a position that it is safe for 
the doors to open.
Special care is needed with systems which have 
stored energy. This might be the momentum of a heavy 
moving part, stored pressure in a hydraulic or pneumatic 
system, or even the simple fact of a part being able to 
move under gravity even though the power is discon-
nected. In these situations, dangerous movement may 
continue or be possible with the guard open, and these 
factors need to be considered in the overall design. 
Braking devices (to arrest movement when the guard is 
opened) or delay devices (to prevent the guard opening 
until the machinery is safe) may be needed. All interlocking 
systems must be designed to minimize the risk of failure-
to-danger and should not be easy to defeat (Figure 11.15).