Principles, Standards and Implementation

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Protective Measures and Complementary Equipment

Guard Locking Switches

In some applications, locking the guard closed or delaying the opening of the guard is required. Devices suitable for this requirement are called guard locking interlock switches. They are suited to machines with run down characteristics but they can also provide a significant increase of protection level for most types of machines.

For most types of guard locking interlock switches, the unlocking action is conditional on the receipt of some form of electrical signal, for example an electrical voltage to energize a lock release solenoid. This principle of conditional release makes the solenoid controlled guard locking switch a very useful and adaptable device. Whereas with most devices the safety function is achieved by stopping the machine, guard locking switches also prevent access to the machine and prevent restart of the machine whenever the lock is released. Therefore these devices can perform two separate but inter-related safety functions: prevention of access and prevention of dangerous movement. This means that these switches are fundamentally important in the field of machinery safety. The following text describes some typical application based reasons why guard locking interlock switches are commonly used:

Protection of machine and people: In many situations tool or work piece damage can be caused or significant process disruption incurred if a machine is stopped suddenly at the wrong point in its operating sequence. A typical example of this would be the opening of an interlocked guard door of an automated machine tool in mid cycle. This situation can be avoided by using a solenoid controlled guard locking switch. If access through the guard door is required a lock release request signal is sent to the machine controller which will then wait for a properly sequenced stop before sending the release signal to the guard locking switch.

Figure 62: Simplified Basic Solenoid Guard Locking Switch Scheme

Figure 62 shows a very simplified schematic view of the principle. In practice, the start, stop and lock release functions of the push switches shown would typically be achieved by inputs and outputs of the machine’s PLC. The PLC would accept a lock release request input at any point in the machine cycle but would only action a release command at the end of that cycle. The release command would be the equivalent of pressing the stop and lock release push switches.

When the lock is released and the guard door is opened, the switch contacts open causing the isolation of power to the hazard.

This type of approach can be further developed by using a key operated switch or button as the lock release request. In this way it can be possible to control not only when the guard can be opened but also who can open it.

Figure 63: Timed Delay Controlled Solenoid Guard Locking Switch Scheme

Protection against machine run down: On many machines, removal of power to the motor or actuator will not necessarily cause a reliable and immediate stopping of the dangerous motion. This situation can be addressed by using a solenoid controlled guard locking switch with its release conditional on implementation of some form of delay that ensures that all dangerous motion has stopped before the lock is released.

Timed delay: The simplest method is to use a timed delay function configured so that the switch will not release the guard until the contactor is OFF and a preset time interval has elapsed. This is shown in Figure 63. The timed delay function can be provided by a Safety PLC or a dedicated controller. It is important that it is safety rated because failure that causes a shorter time delay than specified could result in exposure to dangerous moving parts.

The timed delay interval should be set at least to the worst case stopping time of the machine. This stopping time must be predictable, reliable and not dependant on braking methods that may degrade with use.

Stopped motion confirmation: It is also possible to make the lock release conditional on the confirmation that motion has stopped. The advantages with this approach are that even if the machine takes longer than expected to stop the lock will never be released too early. It also provides better efficiency than a timed delay because the lock is released as soon as the motion has stopped without having to always wait for the worst case stopping time. An example of this approach is shown in Figure 64.

Figure 64: Simplified Stopped Motion Controlled Solenoid Guard Locking Switch Scheme

This stopped motion monitoring function must be safety rated and is usually achieved by one of the following methods:

Proximity sensors or shaft encoders combined with a dedicated controller or safety PLC.

Back EMF detection using a dedicated control unit.

Future generations of variable speed drives and motion control systems will also provide this functionality as safety rated.

Slow speed safety: For some types of machinery it may be necessary to have access to some moving parts in order to perform certain tasks such as maintenance, setting, feeding or threading. This type of activity is only considered if adequate safety can be provided by other measures. Typically these other measures will take the form of at least both of the following:

a) Access is only allowed under conditions of a safe slow speed

b) Any person with access to the moving parts must have personal local control for stopping, or prevention of starting, of the motion. The local control must override any other control signals.

This should be taken as a minimum. Whether this is acceptable or not will depend on risk assessment and relevant safety standards and regulations. However where it is found to be acceptable this type of safety functionality is often implemented using a solenoid controlled guard locking interlock switch in combination with a slow speed monitoring unit and a three position enabling device.

The safe slow speed monitoring unit constantly checks the speed of the moving parts via its input sensors and will only allow the sending of the lock release signal when the speed is not greater than its preset threshold value. After the lock has been released the slow speed unit continues to monitor the speed. If its preset threshold is exceeded while access is allowed, power to the motor will be switched off immediately. Also the safe slow speed can only continue while the enabling switch is held in the middle position (see Figure 70 for more information). It is clear that the guard locking switch, the safe slow speed unit and the enabling device must be connected to some form of safety rated logic solver in order implement the required functionality for both safety and production. In its most simple form this can simply be the way that the units are hardwired together, typically switchable via a manual mode selector switch. This switch is often key operated to restrict the safe slow speed access mode to authorized people. Greater operating efficiency and flexibility can be gained by using a configurable or programmable device for the logic solving function. This could be anything from modular configurable relay through to a Safety PLC.

This type of safe slow speed functionality is often required on complex integrated machinery systems where the equipment is divided into different operating zones each with different and interdependent operating modes. In these types of applications a Safety PLC or a dedicated configurable control unit such as the MSR57 is often a more suitable solution than individual relays and control units.

Most guard locking switches are adaptations of tongue interlocks. A solenoid is added to the interlock. The solenoid locks the actuator in place. There are two types of solenoid locking:

	1.	Power-to-unlock
	2.	Power-to-lock

Power-to-unlock devices require power to the solenoid to unlock the actuator. As long as power is applied to the solenoid, the door can be opened. With power removed from the actuator, the guard locks as soon as it is closed.

During a power loss, the gate remains closed and locked. If the guard locking device is used in full body access applications, a method of escape must be provided in case someone becomes locked in the hazard area. This is accomplished by providing a rotating lever, a pushbutton, or mechanical methods, as shown in Figure 65.

Figure 65: Escape Methods for Guard Locking

The power-to-lock requires power to the solenoid to lock the guard. A risk assessment must consider the potential hazardous situations that may arise if power is lost and the gate becomes unlocked while the machine is running down.

An important criterion when selecting guard locking interlocks is the holding force. How much force is required to hold the guard locked? When the door is manually operated, holding force can be minimal. Depending on where the guard locking switch is installed, operating leverage may suggest higher holding forces. Motorized doors may require higher holding forces.

Figure 66: Inline and Offset Solenoid

Another important criterion for the selection process involves the relationship of the solenoid and the actuator. Two relationships exist: inline and offset, as shown in Figure 66. The solenoid is in the same axis as the actuator contacts or the solenoid is offset from the actuator contacts. The offset arrangement provides separate contacts that provide status of the solenoid.

The inline arrangement does not provide separate contacts for the solenoid. The inline arrangement is a little easier to apply. The offset arrangement provides more information on the operation of the switch. With the offset arrangement, the machine designer must ensure the solenoid status is monitored by the safety system. Selection of either arrangement is based on user preference.

A second type of guard locking device is manually operated and the guard can be opened at any time. A handle or knob that releases the guard lock also opens the control circuit contacts.

On a device such as the bolt switch, a time delay is imposed. The bolt which locks the guard in place operates the contacts and is withdrawn by turning the operating knob. The first few turns open the contacts but the locking bolt is not fully retracted until the knob is turned many more times (taking up to 20 seconds). These devices are simple to apply and they are extremely rugged and reliable. The time delay bolt switch is suitable mainly for sliding guards.

The stopping time of the hazard must be predictable and it must not be possible for the bolt to be withdrawn before the hazard has ceased. It must only be possible to extend the bolt into its locked position when the guard is fully closed. This means that it will be necessary to add stops to restrict the travel of the guard door, as shown in Figure 67.

Figure 67: Sliding Bolt Interlock