Principles, Standards and Implementation

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Protective Measures and Complementary Equipment

Output Devices

Safety Control Relays and Safety Contactors

Control Relays and Contactors are used to remove power from the actuator. Special features are added to control relays and contactors to provide the safety rating.

Mechanically linked normally closed contacts are used to feed back the status of the control relays and contactors to the logic device. The use of mechanically linked contacts helps ensure the safety function. To meet the requirements of mechanically linked contacts, the normally closed and the normally open contacts cannot be in the closed state at the same time. IEC 60947-5-1 defines the requirements for mechanically linked contacts. If the normally open contacts were to weld, the normally closed contacts remain open by at least 0.5 mm. Conversely, if the normally closed contacts were to weld, then the normally open contacts remain open. If the product meets this requirement, the symbol shown in Figure 100 is applied to the product.

Figure 100: Mechanically Linked Contact Symbol

Safety systems must only be started at specific locations. Standard rated control relays and contactors allow the armature to be depressed to close the normally open contacts. On safety rated devices, the armature is protected from manual override to mitigate unexpected startup.

On safety control relays, the normally closed contact is driven by the main spanner. Safety contactors use an adder deck to locate the mechanically linked contacts. If the contact block were to fall off the base, the mechanically linked contacts remain closed. The mechanically linked contacts are permanently affixed to the safety control relay or safety contactor.

On the larger contactors, an adder deck is insufficient to accurately reflect the status of the wider spanner. Mirrored contacts, shown in Figure 101 are located on either side of the contactor are used.

Figure 101: Mirrored Normally Closed Contacts

Dropout time of control relays or contactors play a role in the safety distance calculation. Often, a surge suppressor is placed across the coil to improve the life of the contacts driving the coil. For AC powered coils, the drop out time is not affected. For DC powered coils, the drop out time is increased. The increase is dependent on the type of suppression selected.

Control relays and contactors are designed to switch large loads, anywhere from 0.5 A to over 100 A. The safety system operates on low currents. The feedback signal generated by the safety system logic device can be on the order of a few milliamps to tens of milliamps, usually at 24V DC. The safety control relays and safety contactors use gold plated bifurcated contacts to reliably switch this small current.

Overload Protection

Overload protection for motors is required by electrical standards. Diagnostics provided by the overload protection device enhances not only equipment safety but operator safety as well. Technologies available today can detect fault conditions like an overload, phase loss, ground fault, stall, jam, under-load, current imbalance and over-temperature. Detecting and communicating abnormal conditions prior to tripping help to improve production up time and help prevent operators and maintenance people from unforeseen hazardous conditions

Figure 102 shows examples of overload protection devices. When dual contactors are used to ensure the switching off of a motor in Category 3, 4 or Control reliable solution, only one overload protection device is needed for each motor.

Figure 102: Contactor Overload Protection

Drives and Servos

Safety rated drives and servos can be used to prevent rotational energy from being delivered to achieve a safety stop as well as an emergency stop.

AC drives achieve the safety rating with redundant channels to remove power to the gate control circuitry. One channel is the Enable signal, a hardware signal that removes the input signal to the gate control circuitry. The second channel is positive guided relay that remove the power supply from the Gate control circuitry. The positive guided relay also provides a status signal back to the logic system. A block diagram of the implementation of safe off feature in the PowerFlex drive is shown in Figure 103.

This redundant approach allows the safety rated drive to be applied in emergency stop circuits without the need for a contactor.

Figure 103: Drive Safety Signals

Figure 104: Kinetix Drive Safety Signals

The Servo achieves a result in a manner similar to the AC drives. Figure 104 shows that redundant safety signals are used to achieve the safety function. One signal interrupts the drive to the Gate Control Circuitry. A second signal interrupts power to the power supply of the Gate control circuitry. Two positive guided relays are used to remove the signals and provide feedback to the safety logic device as well.