In the broadest terms there are two basic types of electrical interlocking systems:

Power interlocking - The power source of the hazard is directly interrupted due to the opening of the guard.

Control interlocking - The power source of the hazard is interrupted by the switching of a circuit which controls the power switching device.

The following text deals with the interlocking of electrical power supplies as this is by far the most common requirement, but the same basic principles can be applied to hydraulic and pneumatic systems.

POWER INTERLOCKING

The movement of the guard is interlocked with the direct switching of the power to the hazard. For equipment using low voltage and power most types of safety switches can be used for power interlocking. But because most industrial machinery uses a relatively high power three phase supply, we need specially designed power interlocking systems with the power interrupting switch capable of handling and breaking the load reliably.

The most practical method of power interlocking is a trapped key system such as the Prosafe system (see fig. 50). The power isolation switch is operated by a key which is trapped in position while the switch is in the on

position. When the key is turned the isolation switch contacts are locked open (isolating the power supply) and the key can be withdrawn. The guard door is locked closed and the only way to unlock it is by using the key from the isolator. When this key is turned to release the guard locking unit it is trapped in position and cannot be removed until the guard closed and locked again.

Therefore, it is impossible to open the guard without first isolating the power source and it also impossible to switch on the power without closing and locking the guard.

This type of system is extremely reliable and has the advantage of not requiring electrical wiring to the guard. The main disadvantage that because it requires the transfer of the key every time, it is not suitable if guard access is required frequently.

Whenever whole body access is required, the use of a personnel key is recommended (see fig. 51, 52 and 53). The Prosafe range is available in double key versions to meet this requirement.

FIG 50-51

The use of a personnel key ensures that the operator cannot be locked in the guarded area. The key can also be used for robot teach mode switches, inch mode controls etc.

By using double key locking units and keys with different codes together with a key exchange unit, complex systems can be formed. Besides ensuring that the power is isolated before access can be gained, it is also possible to use the system to enforce a pre-defined sequence of operation.

Required Features

Because the entire safety of this type of system depends on its mechanical operation, it is critical that the principles and materials used are suitable for the expected demand made on them.

If an isolation switch is part of the system, it should have positive mode operation and it should satisfy the requirements of the relevant parts of EN 60947.

The integrity and security of the system revolves around the fact that under certain conditions the keys are trapped in place, therefore two basic features need to be ensured:

1- THE LOCK CAN ONLY BE OPERATED BY THE DEDICATED KEY. This means that it should not be possible to “cheat” the lock by using screwdrivers etc., or defeat the mechanism by mistreating it in any straightforward manner. Where there is more than one lock on the same site it also means that the specifying of key codes must in itself prevent any possibility of spurious operation.

2 - IT IS NOT POSSIBLE TO OBTAIN THE KEY IN ANY WAY OTHER THAN THE INTENDED MANNER. This means that (for example) once the key is trapped, any excessive force applied to it will result in a broken key as opposed to a broken lock.

CONTROL INTERLOCKING

Control interlocking is the most commonly used method of interlocking. An interlock switch is attached to the guard to detect movement and open the switch contacts whenever the guard is not fully closed. The contacts are connected via a control circuit to the hazard source primary control element (contactor).

So first of all let us, by reference to fig. 49, look at the different types of devices suitable for control interlocking. The first major distinction is between devices without guard locking and with guard locking.

Safety Switches Without Guard Locking

These devices do not restrict access and the guard door can be opened at any time; but as soon as it is opened the safety switch isolates the power to the hazard via the contactor control circuit. If the hazard always ceases immediately, then the requirements are satisfied because the operator cannot reach the parts while they are dangerous.

If the hazard does not cease immediately, there is a possibility that an operator can reach it while it is “running down” and still dangerous. To avoid this unacceptable situation, there are three alternatives as illustrated in figures 56, 57 and 58. Note: The integrity of the braking device needs to be considered both in terms of fault resistance and wear characteristics.

If a precise calculation is required, the European standard EN 999 deals with the positioning of protective equipment with respect to approach speeds of parts of the human body. At present it does not specifically cover interlocked guard doors, but its principle for the positioning of other safety devices based on approach speed and machine stopping time can reasonably be extrapolated to cover interlocked guard doors without guard locking.

The next subdivision for interlocking devices without guard locking is their designation as mechanical actuation or noncontact actuation.