Some Basic Points to Remember in Thermal Overload Relay Maintenance

A thermal overload relay is a protective device which safeguards an electrical motor against thermal overloads, phase failure and overloads. It detects overloading of electrical motor and stops the current flow to the faulty electrical motor, thereby protecting it from overheating and electrical short-circuits. It relays regulated and constant current to the subsidiary circuit as required by the manufacturer, so that the overall efficiency of the system can be preserved. It also reduces the chances of overloading and short-circuits to a great extent. These relays are highly effective in preventing electrical faults.

Normally, a thermal overload relay has a pair of circuit breakers, a start button and a test button. The test button lets you check the condition of your thermal overload relays and the stop button lets you set the limit of the thermal overload currents. Normally, there is a control unit or a pair of relays, which switch the function between the primary and secondary circuits. This is done automatically once the overload is detected.

There are several causes for thermal overload relay malfunction. Overheating of any component causes the relays to start and regulate the incoming current. Some of these components are the heat sink, the thermal expansion heater and the condenser. When overloading takes place, they start conducting rapidly, which creates problems. It causes the breakers to open and the thermal current setting to zero.

The different types of thermal overload relay include the cover plate, contact surface, metal insulator, magnetized contacts and solid state contacts. The main difference among these types is that the cover plates are made of either metal or a combination of metals such as copper, aluminium, steel and brass. The contact surface is usually made of a metal such as aluminum or copper and rubber. On the other hand, the thermal expansion heater consists of two copper fins and a metal frame with contacts on the front and the back. The magnetized contacts are usually made of iron, aluminium or steel.

Both the cover plate and the contact surface of the thermal overload relay have their own design. The design of a contactor usually follows the natural arrangement of the components in a circuit. For instance, the metal contact is normally open at the bottom and the top. In the case of the thermal expansion heater, the contactor is normally open at the top and the bottom. If the thermal overload relay has two terminals, the contacts are normally open at both the top and bottom.

The operation of thermal overload relays can be explained using the electrical and mechanical principles. When an electrical current is applied across the thermal overload relay, the metallic contacts are heated above their normal operating temperature and begin to conduct. The resistance to this conductivity is equal to the metallic thickness. At the same time, the magnetic field on the device is altered by the alternating magnetic field on the switch contacts and changes the value of the magnetic field.

It is obvious that the process of switching off can be done only when the temperature of the contacts reaches the critical point. If the device operates under such condition, there will be either power overload relays electrical faults or magnetic fields that lead to malfunction. The problem can be detected using a continuity detector. If the malfunctioning has occurred due to thermal overload relays, then the circuit should be immediately switched off. This simple action will automatically prevent any further damage to the electrical appliances.

While performing the thermal overload relay maintenance, one should remember the following basic points: When you find that the device is working properly and is returning the desired temperature, then the main power cable should be disconnected and switched off. Then, the main power cable should be reconnected. If the device stops functioning, then the thermal switch should also be switched off.