Force-guided relays are essential components in safety circuits, particularly in industrial automation and machinery. These relays are designed with mechanically linked contacts, ensuring a defined relationship between the normally open (NO) and normally closed (NC) contacts. This article delves into the functionality, failure modes, and advantages of force-guided relays, highlighting their importance in maintaining machine safety.
Traditional relays control circuits using electromagnetic coils. Applying voltage to the coil closes the NO contact, allowing current to flow and activating the controlled device. Conversely, removing voltage from the coil opens the NO contact, stopping the current flow. The NC contact behaves in the opposite manner: it is closed when the coil is OFF and open when the coil is ON. This operation is illustrated in states 1 and 2 in Table 1 (not included here), reflecting standard relay behavior.
The Significance of Force Guided Relays in Safety Applications
The use of NO contacts in power control circuits enhances safety. If the voltage to the coil is interrupted due to wire disconnection or other issues, the NO contacts open, stopping the machine. However, a potential hazard arises if a contact becomes welded.
Consider a scenario where the NO contact of relay K1 is welded shut (state 3 in Table 1, not shown). Even when the voltage to the coil is turned OFF, the welded NO contact remains closed. To address this, force-guided relays are designed with an NC contact in a monitoring circuit that operates in conjunction with the NO contact via a mechanical linkage. Even if the NO contact is welded, the NC contact should remain open.
Preventing Restart and Ensuring Machine Shutdown
In this state, a second relay, K2, operates normally. The machine can be stopped by opening the three NO contacts of K2. By connecting the NC contacts of K1 and K2 in series within the monitoring circuit, the machine cannot be restarted, even if the start switch S2 is engaged. This is because the NC contacts remain open due to the welded NO contact of K1, thereby preventing an unsafe restart. To resume operation, the faulty K1 force-guided relay must be replaced.
Using two force-guided relays allows the machine to be safely stopped, even if one of the NO contacts is welded together. This design prevents restarting and maintains the machine’s overall safety.
Addressing Failure Modes: Broken Plate Springs
Another critical safety consideration is the potential breakage of plate springs within the relay. Figures 3 and 4 (not included) illustrate the condition of force-guided and general-purpose relays with broken plate springs.
In a force-guided relay, the NO and NC contacts are physically separated by a wall. Thus, if a plate spring on one of the contacts breaks, it does not affect the other contact, minimizing the impact.
Why General-Purpose Relays Are Unsuitable for Safety-Critical Applications
General-purpose relays typically have a C-contact that integrates both NO and NC contacts. If a spring breaks in this configuration, it can cause both contacts to conduct, affecting neighboring systems. This could lead to the machine starting unexpectedly or failing to stop when commanded.
For this reason, general-purpose relays are unsuitable for safety-related components in control systems. Force-guided relays, with their mechanically linked contacts and robust design, provide a much safer alternative for critical safety functions.
In conclusion, force-guided relays are indispensable for ensuring safety in industrial machinery. Their design, which ensures a defined relationship between NO and NC contacts, prevents hazardous situations arising from contact welding or spring breakage. By understanding the principles behind force-guided relays, engineers and technicians can design safer and more reliable control systems.
