When an operator is required to enter a hazardous area of machinery and where fixed guards are not a practical option, interlocking guards are the next best protective device. Interlocking guards are defined as a guarding system which, when the hazard area is open, prevents the machinery from operating. Implicit in this definition are three important points that control the design and operation of an interlocking guard:
It must prevent movement of the dangerous parts of the machine when the hazard area is open.
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06 - IDipOSH 1B5 v2 160514 Page 15 of 30 ©British Safety Council It must not allow access to the hazardous area until the potential hazard has been made
safe.
It must not allow the machinery to operate until the guarding system is fully operational. Other factors of importance are:
If the interlock system should fail, it should fail in such a way that the system remains safe.
The interlocking system should be difficult to defeat.
The operation of an interlock may be electrical, mechanical, hydraulic or pneumatic. The choice is often dependent on the power medium (e.g. hydraulic) in use to operate the machine. In more complex machines, a combination of interlocks may be in place. Electrical Interlocks
The electrical interlock is used to ensure that the power to operate the machine is not available until the guard is in place. The position of the guard is detected by means of electrical limit switches. The two main types are described below.
• Normally-Open Limit Switch
The design of the switch is such that the spring will keep the contacts open until pressure is applied to the roller follower. (See the first of the two diagrams that follow. The second diagram shows how a normally-open switch can be used in conjunction with an interlocking guarding system.)
There are three serious defects in the use of normally-open limit switches:
− They are easy to defeat by simply holding down with the hand - or, more
permanently, with some adhesive tape.
− More seriously, if the spring should break, the switch would be left on, without any
external pressure, and the guarding would fail to danger, leaving the operator vulnerable to an unexpected operation of the machine while the guard is open.
− If the roller follower arm became bent or the bearing becomes still, the switch could
become jammed in the on position, producing the same danger as above. Normally-open limit switches are frequently used by manufacturers as a method of controlling interlocking guards. With the defects mentioned above, you can see they hardly satisfy basic safety criteria. You would be well advised to see they are designed out of new equipment before you take delivery.
Mains supply
SWITCH “ON” POSITION Spring
SWITCH “OFF” (OPEN) POSITION
Applied pressure Bearing Roller follower and arm
Normally-Open Electrical Limit Switch (Negative Mode)
Switch with a Guarding System
• Normally-Closed Limit Switch
In this switch, you can see that the spring is designed to close the contacts when there is no pressure on the roller follower, see diagram. The following diagram shows how the normally-closed switch can be used on an interlocking system.
Linear cam attached to guard
(a) Guard closed
Mains supply Roller follower (b) Guard open Mains supply Applied pressure
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06 - IDipOSH 1B5 v2 160514 Page 17 of 30 ©British Safety Council Normally-Closed Electrical Limit Switch (Positive Mode)
Normally-Closed Electrical Limit Switch on an Interlocked System
The design of the normally-closed switch is safer than that of the normally-open switch, in the sense that it is more difficult to defeat and, if the spring breaks, it will fail safe. There are, however, defects in the system:
− Wear, misalignment, or overrun of the linear cam on the guard will result in a fail-to- danger condition.
− If the guard is removed, the switch will become activated, creating another fail-to- danger condition.
With its improved safety design, the normally-closed switch can be used to replace the normally-open type.
Where limit switches have to be used, they can provide a safer system if each type is used in series.
Magnetic Switches
A common magnetic interlock uses magnetic forces to control and operate an electric switch. The following diagram illustrates this type of interlock.
'T'-shaped actuating magnet
E – Earth AB – common
A – contacts normally open
B – contacts normally closed 'T'-shaped recess
'T'-shaped magnet on rocker arm Switch rocker arm Retaining magnet Balance weight Electrical contacts
Magnetic Safety Switch
The electrical contacts in the unit are held open by the effects on them of the retaining magnet and the balance weight. When the external T-shaped magnet system is brought into contact with the internal magnet, a repulsive magnetic force is generated, causing the magnet to move away. This action causes the contacts to close and activates electric power.
This system is used on any machinery that has a lid or cover that is regularly being opened and closed. In general, the switch is robust and reliable, but there are two possible areas of weakness:
Although the design of the magnetic fields produced by magnets is complex, they can, with difficulty, be defeated. The switch can therefore be opened with the magnet removed.
There is a risk that the electric contacts, under adverse conditions, could fuse together, so the switch would fail to danger. It is not possible to predict how likely the chances are of
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06 - IDipOSH 1B5 v2 160514 Page 19 of 30 ©British Safety Council this failure occurring, but manufacturers of the units have recognised the possibility and their research has given them confidence to believe that such a fault could not occur.
Safety Switch in Operation
Key Systems
This type of interlock ensures that power to a machine is locked off if guards are open. Its operation is shown in the following diagram. A master key in the power-supply unit is turned to switch power off. This key may now be removed to operate the guard key access unit, which holds a number of keys necessary to open guards on the machine. When one or more guard keys are removed to unlock a guard, the master key is trapped in the access unit. Power remains locked off. When access to hazard areas is no longer required, guards are locked, and the keys are returned to the access unit. Not until all keys are securely in may the
Guard open
View from back of machine Guard closed
Counter balance weight
Magnetic switch closed
Magnetic switch open
trapped master key be removed. It is then available to operate the power supply to the machine.
Interlock Operation