“Choosing the right relay for an industrial application requires careful thought. Common considerations for selecting these products include checking the voltage and current required, as well as the maximum space available for the relay, as well as paying attention to the separation distances of any particular component. Other more complex considerations include whether there are safety standards for that specific application. This article will discuss the requirements and challenges that design engineers face when developing industrial and commercial applications and selecting the right relay.
By: Mouser Electronics Mark Patrick
Choosing the right relay for an industrial application requires careful thought. Common considerations for selecting these products include checking the voltage and current required, as well as the maximum space available for the relay, as well as paying attention to the separation distances of any particular component. Other more complex considerations include whether there are safety standards for that specific application. This article will discuss the requirements and challenges that design engineers face when developing industrial and commercial applications and selecting the right relay.
During the relay selection process, engineers need to carefully review two technical aspects. First, this article will focus on how inrush current spikes can severely damage relay contacts so they can no longer function properly. Such failures highlight the need to comply with safety standards for equipment such as elevators, industrial cutters and railway signals. The second topic concerns space-constrained environments, such as industrial control cabinets, which need to match enough available space to install the required number of relays.
Choose Relays for Electrically Demanding Environments
The industrial manufacturing landscape has undergone rapid transformation over the past decade. Focused on improving overall operational efficiency, industry trends such as Industry 4.0 and the Industrial Internet of Things (IIoT) are driving the demand for industrial automation equipment and systems.
Industrial systems need an efficient way to control high power rails to equipment from a low voltage interface, and electromagnetic relays provide an efficient way to achieve such actions, and often multiple sets of contacts can be opened or closed with a single signal. While this humble relay can provide a simple and low-cost solution, in some cases the controlled equipment may need to meet stringent functional safety requirements. For example, a hydraulic press operator must be 100% sure to only operate when instructed to do so. Functional safety standards such as IEC 61508 and its derivatives for industrial equipment specify the Safety Integrity Level (SIL) required to protect equipment users and prevent equipment damage. Another safety standard, IEC 61810-3 (previously known as EN 50205), explicitly applies to relays for safety and self-monitoring control applications.
Coping with inrush current
Inrush current can cause damage to any electrical switchgear such as switches, contactors and relays. Inrush current occurs when the power supply voltage applied to the load is too high, causing excessive current flow. Inductive loads such as motors and transformers, as well as capacitive loads such as energy storage devices, are particularly prone to high inrush currents, whose magnitudes can be up to forty times higher than the average steady-state current. In relays, a sudden inrush current can create a momentary arc between the relay contacts, causing them to be welded together. The relay may appear to still be operating as expected, but its ability to turn off power to the load has been compromised.
There are several ways to reduce inrush current. These include slowly increasing the voltage applied to the load or using a zero-crossing switch (ZCS) such as a solid state relay (SSR). ZCS works by detecting when the AC mains voltage goes to zero at positive and negative peaks (zero crossings), and the current is also zero. Once a zero-crossing occurs, the relay can turn the load on or off.
Unfortunately, the nature of highly inductive loads is that the maximum current point can lag significantly behind the voltage peak, misaligning the timing of zero voltage and zero current. Therefore, zero-crossing switching and solid state relay (ZCS/SSR) methods can result in switching large currents. Additionally, ZCS/SSR is known to be a potential source of disruptive and harmful electromagnetic interference (EMI) in currently used switching methods, and compliance with ZCS/SSR functional safety requires sophisticated monitoring of semiconductor circuits.
force guide relay
The most feasible and easiest way to switch circuits reliably and safely is to use force-guided relays (FGRs), which are required to comply with the previously highlighted IEC 61810-3 standard. The construction method used in force guided relays ensures that normally open (make/NO) contacts cannot assume the same state as normally closed (break/NC) contacts. In the event of contact welding or other potential faults within the relay, forced movement of the contacts maintains a specified minimum distance of 0.5mm from the faulty contact.
Figure 1: A close-up photo of an FGR contact from TE Connectivity.
Figure 1 is a close-up photograph of a set of force-guided relay contact construction methods.
TE Connectivity’s portfolio of SCHRACK force guided relays can be used in safety related applications including safety interlocks, emergency stop switches and robotic control systems. In addition to industrial applications, FGR can also be used in various control systems, including elevator door control, railway signaling systems, and medical diagnostic equipment.
The SCHRACK product family offers flexible design options such as multiple contact configurations, different contact plating options, and various pin layouts for higher voltage ratings. Standard and Sensitive Coil options provide longer life and a more optimized and improved thermal profile.
Figure 2 highlights the SCHRACK FGR product family, which ranges from the 2-pole SR2M series to the 7-pole SRL7 series.
Figure 2: TE Connectivity’s SCHRACK portfolio of force-guided relays. (Source: TE Connectivity SCHRACK)
A typical example of a force-guided relay is the SCHRACK SR2M. This 2-pole relay complies with EN61810-3 (previously known as EN50205) with reinforced insulation between the poles. The rated voltage of the switch contacts is 250VAC, the maximum is 400VAC, and the rated switching current is 6A. The contact material used was silver nickel (AgNi). The contact arrangement is 1 Form A (SPST-NO) and 1 Form B (SPST-NC). The operating voltage of the relay coil is model dependent and can cover all popular nominal voltages from 5VDC to 110VDC. It can be supplied in plug-in or solder pin format. Typical applications for the SR2M include emergency shutdown, hydraulic machine control, elevator and escalator control, and security monitoring.
Another example of a force-guided relay is the four-pole SCHRACK SR4D/M, which is available with 3 Form A and 1 Form B contacts 3 NO + 1 NC or 2 Form A + 2 Form B contacts 2 NO + 2 NC. The contact material is silver tin oxide (AgSnO2), and each contact is rated at 8A.
FGR for space-constrained industrial applications
Most industrial production equipment is controlled by a DIN rail mounted Process Logic Controller (PLC) in the control cabinet. As machinery becomes more complex and the functional density of equipment increases, factory floor space becomes at a premium. Typically, engineers are limited to one control cabinet per production facility, which imposes strict restrictions on the use of control equipment. Modern industrial controllers need to provide a high density of functions and need to do more in a given space, but this approach also needs to keep heat dissipation to a minimum.
An example of a DIN rail mounted force guided relay is the compact SCHRACK XT. SCHRACK XT is available in unipolar or bipolar models with current ratings of 16A (1P) or 8A (2P). Force Directed Relays can be ordered with 24VDC coils or 24VAC, 115VAC or 230VAC coils.
Keep industrial electrical systems operating safely
For all aspects of our daily lives, compliance with applicable safety standards is critical to ensuring the safety of any person, especially industrial processes where there are potential safety hazards for machinery and process equipment. Functional safety standards provide an overall framework for assessing and determining risk, and for relays, the IEC 61810-3 specification specifies the use of force-guided relays for safety control applications.
To protect lives, force-guided relays in TE Connectivity’s SCHRACK product line are essential for any safety-related application.