Learn how thermal relays protect electrical devices from overheating by monitoring and controlling temperature to ensure safety and reliability. Thermal relays are a fundamental component in the field of electrical engineering, designed to protect motors and other electrical devices. Thermal relays are the perfect solution for providing protection to motors which provides the most precise tripping for the electric motor during single phasing and overload. This article discusses an overview of a thermal relay – working with applications. PSM – Plug Setting Multiplier (Current Setting Multiplier) What is PSM? 2). TSM – Time. Thermal Relay Definition: A thermal relay is defined as a device that uses the unequal expansion rates of metals in a bimetallic strip to detect overcurrent conditions. Without it, a failure caused by a damaged winding can quickly amount to several thousand euros in operating.