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Lock‑out relays are typically installed on equipment that requires regular inspections, whether for maintenance or to maintain the cleanliness of machinery in food‑processing facilities. In the United States, the U.S. Department of Agriculture (USDA) mandates daily inspections of such equipment. The regulations require that the electrical source or control power be shut off from a central location that is secured with a key. The key is then locked in a separate box, preventing any personnel from starting or operating the machines during inspection.
All electrical machines are started and stopped by an input signal generated by a low‑voltage control source. This source is usually centralized for safety, allowing a series of emergency stop (e‑stop) switches to be placed along the process line so the entire line can be shut down in an emergency. A lock‑out relay is normally positioned in line—either before or after the e‑stop switch—so that power can be de‑energized from a single central point. The relay is powered by the same control source and is operated by a key‑lock switch. Inside the unit, the relay may contain up to 24 contact points, enabling a single key turn to lock out control power for multiple machines.
The voltage that operates the start/stop devices, the lock‑out relay, and the e‑stop is called the control voltage. It is typically much lower than the main power that drives motors and machines in an industrial setting. In most cases, the standard control voltage is 120 VAC. Although lower than the operational voltage of 480 VAC, 120 VAC can still pose a shock hazard in wet environments, such as food‑processing areas. For wet locations, lock‑out relay circuits usually employ a 24‑VDC power source. This low DC voltage reduces shock risk and can be easily safeguarded with a fast‑acting fuse that disconnects power in the event of a short to ground. The 24 VDC standard is widely adopted for lock‑out relay systems in wet environments.
The lock‑out relay is activated by a key switch that is managed by a single pair of keys. These keys are stored in a red lock‑out box that typically features two locks, ensuring that both parties are present when the box is opened to activate the relay. Once the relay is engaged and power is removed from the machinery, the keys are returned to the box and secured until the inspection is complete. After a successful inspection, the system is re‑energized and production can resume.
