Safety grade contactors are required in industrial applications to ensure that machines and systems can reliably and unpredictably switch to a safe state, typically in the event of operational errors or safety functional requirements. The first step in designing an industrial safety system is to determine whether the application requires a Safety Integrity Level (SIL) (as defined in IEC 62061, up to SIL 2) or a C-level Performance Level (PL) as specified in ISO 13849. Some applications require a higher level of security.
Consider the guiding principles in IEC 60947-4-1 and ISO 13849-1, where the former focuses on designing and testing low-voltage switches and control equipment, while the latter provides general principles for designing and integrating safety related parts of control systems, including hardware and software.
This article introduces how to use Siemens' 3RT2 safety level contactor with fail safe operation function to build a safety system, in order to achieve the optimal industrial safety solution. The article also reviewed how to achieve higher SIL and PL levels, as well as system integration issues such as contactor thermal management. Finally, it delved into how to customize safety solutions using redundant interconnects, surge suppressors, functional modules, and other accessories to achieve further application and safety optimization.
The 3RT2 contactor offers a choice between traditional and solid-state operation modes, with specifications ranging from S00 to S2, and a maximum power of up to 37 kW. For contactors that use solid-state operation mechanisms, an optional remaining life signal should be specified.
These contactors comply with the requirements of IEC 60947-4-1 AC-3e category and can be used in conjunction with high-efficiency IE3 or IE4 motors. They have multiple auxiliary output configurations, including normally open (NO) and normally closed (NC) contacts, which can be used to provide feedback on whether the main circuit is on or off. In addition, feedback can be used to trigger indicator lights, alarms, or other control devices. The example of 3RT2 contactor is as follows:
3RT20152AP611AA0-S00 specification, rated current 7 A, 3 kW/400 V, 3 poles, 220 VAC 50 Hz/240 VAC 60 Hz, auxiliary contact: 1 NO, with spring terminal
3RT20231AK60- S0 specification, rated current 9 A, 4 kW/400 V, 3 poles, 110 VAC 50 Hz/120 VAC 60 Hz, auxiliary contacts: 1 NO+1 NC, threaded terminal (Figure 1)
3RT20281AN20- S0 specification, rated current 38 A, 18.5 kW/400 V, 3 poles, 220 VAC, 50/60 Hz, auxiliary contacts: 1 NO+1 NC, threaded terminal
3RT20371_S2 specification, rated current 65 A, 30 kW/400 V, 3 poles, 24 VDC, with integrated variable resistor, auxiliary contacts: 1 NO+1 NC, threaded terminal
3RT20371SF30-S2 specification, with F-PCL-IN input, rated current 65 A, 30 kW/400 V, 3 poles, 83 VAC to 150 VAC/VDC, 50/60 Hz, with integrated variable resistor, auxiliary contact: 1 NC, threaded terminal
S0 specification contactor rated power of 4 kW picture
Figure 1: The rated power of this S0 specification contactor is 4 kW, with one NC and one NO auxiliary output. (Image source: Siemens)
Response time is crucial for safety
Understanding the impact of overall response time is crucial when designing industrial safety solutions. It consists of multiple parameters. When conducting risk assessment, response time is defined as the total time it takes to stop any hazardous movement when safety requirements arise. The factors that affect response time include:
The input response time of sensors in safety monitoring devices
The cycle time of the security plan
Delay time of communication protocol
Overtravel time caused by inertia of motor or actuator
The breaking time of the contactor
IEC 60947-4-1 specifies the breaking time for electromechanical contactors and motor starters. This standard specifies the requirements for safe breaking of current under various working conditions.
It also defines usage categories to classify load types and contactor operating conditions, such as AC-3e for high-efficiency motors. This standard includes procedures for testing the breaking time and other performance characteristics of contactors.
The breaking time of the contactor is a key parameter in the safety system. Its definition is the time from the removal of coil voltage to the disconnection of the main contact, including the disconnection delay time (OD) and the contact arcing time (AT).
For example, when using a contactor with switch performance as shown in Figure 2, the breaking time OD+AT is between 50 and 75 ms. When calculating the total response time, it is necessary to always consider the worst-case value, which is 75 ms in this example (Figure 2).