Do all overload relay units have heaters?

The answer indicating that some overload relay units have electro magnetically operated contacts is indeed accurate, as it highlights a key aspect of overload relay design. While many traditional overload relays utilize heaters to sense current overloading by physical heating of the wire, there are modern overload relays that employ different technologies, including electromagnetic principles.

Electromagnetic overload relays function by using coils and contacts that operate based on the magnetic field generated by the current flowing through them. When the current exceeds the set point, the electromagnetic mechanism engages to open or close contacts, providing overload protection without relying on heat generation. This approach can offer advantages such as faster response times and reduced wear over time, which might not be present in heater-based units.

Not all overload relays are created equal; thus, some rely on heaters, some on electromagnetic principles, and others might use different methods altogether. This versatility demonstrates that the application and design of overload relays can vary based on the specific requirements of the electrical system in which they are utilized.