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Figure 3 - The TE Connectivity SCHRACK SR2M force guided relay (source: TE Connectivity SCHRACK)
Force guided relays
The most viable and least complex way to reliably and safely switch circuits is by using a force guided relay (FGR). These relays are covered by the IEC 61810-3 standard highlighted. A force guided relay uses a construction method that ensures normally-open (make/NO) contacts cannot assume the same state as normally-closed (break/NC) contacts. Should contact welding or another potential failure occur within the relay, the forced movement of contacts maintains a specified minimum distance of 0.5 mm from the failed contacts. Figure 1 illustrates a close-up view of the construction method of a set of forced guided relay contacts.
TE Connectivity’s SCHRACK portfolio of force guided relays are used in safety- related applications. Examples include safety interlocks, emergency stop switches, and robotic control systems. Outside of industrial use cases, FGRs appear in various control systems, including elevator door controls, railway signalling, and medical diagnostic equipment.
The SCHRACK portfolio features a variety
of fl exible design options such as multiple contact confi gurations, different contact
plating options, and a variety of pin layouts to achieve higher voltage ratings. Standard and sensitive coil options offer a longer service life expectation and an improved heat dissipation profi le. Figure 2 highlights the SCHRACK FGR portfolio, ranging from the 2-pole SR2M series to the 7-pole SRL7 family. An example of an FGR is the SCHRACK SR2M - see Figure 3. This 2-pole relay complies with EN61810-3 (formally EN50205) and has reinforced insulation between poles. Switching contacts are rated at 250 VAC, with a maximum of 400 VAC and a rated switching current of 6A. The contact material is silver nickel (AgNi). The contact arrangement is 1 Form A (SPST- NO) and 1 Form B (SPST-NC). The relay coil operating voltage is model-dependent and accommodates all the popular nominal voltages from 5 VDC to 110 VDC. It is available in either a plug-in or soldered pins format. Typical applications for the SR2M include emergency shut-off, hydraulic press control, elevator and escalator control, and safety monitoring. Another FGR example is the four-pole SCHRACK SR4D/M. This relay, see Figure 4, is available with either 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 a silver tin oxide (AgSnO2), and the rated current per contact is 8 A.
FGRs for space-constrained industrial applications Most industrial production assets are controlled by DIN-rail mounted process logic controllers (PLCs) housed inside a control cabinet. As machinery becomes more complex and equipment density grows, factory fl oor space is at a premium. Typically, engineers are limited to a single control cabinet per production asset, placing constraints on the control equipment.
with either a 24 VDC coil or a 24 VAC, 115 VAC, or 230 VAC coil.
Keeping industrial electrical systems operating safely Compliance with applicable safety standards is paramount for any human safety
Figure 5 - The SCHRACK XT DIN-rail mount- ed force guided relay (source TE Connectivity SCHRACK)
across all Figure 4 -
TE Connectivity’s SCHRACK SR4 D/M force guided relay (source: TE Connec- tivity SCHRACK)
Modern industrial controllers need to offer a high density of features, squeezing more capability into a given space. This approach, however, also requires heat dissipation to be kept to a minimum.
An example of a DIN-rail force guided relay is the compact SCHRACK XT - see Figure 5. The SCHRACK XT is available in single pole or two-pole variants and rated currents of 16 A (1P) or 8 A (2P). The FGR relay can be ordered
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aspects of our daily lives. This is particularly important for industrial processes where machinery and process equipment present potential safety hazards. Functional safety standards stipulate the overall framework for assessing and determining risks, and for relays, the IEC 61810-3 specification dictates the use of force guided relays for safety control applications.
Force guided relays from TE Connectivity’s SCHRACK portfolio are essential for any safety-related application to protect lives.
www.mouser.com Components in Electronics April 2022 15
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