Principles, Standards and Implementation

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Protective Measures and Complementary Equipment

Trapped Key Interlocks

Trapped keys can perform control interlocking as well as power interlocking. With "control interlocking," an interlock device initiates a stop command to an intermediate device, which turns off a subsequent device to disconnect the energy from the actuator. With "power interlocking," the stop command directly interrupts the energy supply to the machine actuators.

Figure 77: Power Interlocking with Trapped Key System

The most practical method of power interlocking is a trapped key system (see Figure 77). The power isolation switch is operated by a key that 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 When the key is turned, the isolation switch contacts are locked open (isolating the power supply) and the key can be withdrawn.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 turned to release the guard locking unit, the key is trapped in position and cannot be removed until the guard is closed and locked again.

Therefore it is impossible to open the guard without first isolating the power source and it is 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 is 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. As shown in Figure 78, the "B" key is the personnel key. The "B" key is taken by the operator into the hazard area. The trapped key range is available in double, triple, and quad key versions for multiple access points. The use of a personnel key ensures that the operator cannot be locked in the guarded area. The key can also be taken into the cell and inserted into another switch to enable functions like robot teach and machine jog modes.

Figure 78: Full Body Access—Operator Takes "B" Key

In another example shown in Figure 79, Key "A" is rotated and removed from the power isolator. Power is then OFF. To gain access through guard doors Key "A" is inserted and rotated in the Key Exchange Unit. Both "B" Keys are then released for guard locks. Key "A" is trapped preventing power from being switched on. Two "C" Keys are released from the guard door locks for use in the next sequence step or as personnel keys.

Figure 79: Multiple Doors Are Accessible

Figures 80 shows another example of trapped key interlock applications by using both single and 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.

Figure 80: Defined Sequence of Events

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 IEC 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.