Introduction to Health and Safety at Work

Figure 11.13
Adjustable guard for a rotating shaft, such 
as a pedestal drill.
Figure 11.14
Self-adjusting guard on a wood saw.

---

Introduction to Health and Safety at Work
176
11.14 
Other safety devices
11.14.1 Trip 
devices
A trip device does not physically keep people away but 
detects when a person approaches close to a danger 
point. It should be designed to stop the machine before 
injury occurs. A trip device depends on the ability of the 
machine to stop quickly and in some cases a brake may 
need to be fi tted. Trip devices can be:
➤
mechanical in the form of a bar or barrier
➤
electrical in the form of a trip switch on an actuator 
rod, wire or other mechanism
➤
photoelectric or other type of presence-sensing 
device
➤
pressure-sensitive 
mat.
They should be designed to be self-resetting so that the 
machine must be restarted using the normal procedure 
(Figure 11.16).
11.14.2 Two-handed control devices
These are devices which require the operator to have 
both hands in a safe place (the location of the controls) 
before the machine can be operated. They are an option 
on machinery that is otherwise very diffi cult to guard but 
they have the drawback that they only protect the opera-
tor’s hands. It is therefore essential that the design does 
not allow any other part of the operator’s body to enter 
the danger zone during operation. More signifi cantly, they 
give no protection to anyone other than the operator.
Where two-handed controls are used, the following 
principles must be followed:
➤
the controls should be so placed, separated and pro-
tected as to prevent spanning with one hand only, 
being operated with one hand and another part of the 
body, or being readily bridged
➤
it should not be possible to set the dangerous parts 
in motion unless the controls are operated within 
approximately 0.5 seconds of each other. Having set 
the dangerous parts in motion, it should not be pos-
sible to do so again until both controls have been 
returned to their off position
➤
movement of the dangerous parts should be arrested 
immediately or, where appropriate, arrested and 
reversed if one or both controls are released while 
there is still danger from movement of the parts
➤
the hand controls should be situated at such a dis-
tance from the danger point that, on releasing the 
controls, it is not possible for the operator to reach 
the danger point before the motion of the danger-
ous parts has been arrested or, where appropriate, 
arrested and reversed (Figure 11.17).