True or False: Shading rings MUST be used in AC relays and contactors.

Shading rings are indeed necessary components in AC relays and contactors because they provide a means to ensure that the electromagnetic coil operates effectively and reliably. The primary purpose of shading rings is to eliminate the potential for false tripping, which can happen due to the alternating nature of AC current. When current flows through the coil, it creates a magnetic field that can cause the armature to pull in and engage the contacts.

In AC systems, the magnetic field can change direction, leading to a brief moment where the magnetic field strength drops sufficiently, particularly in certain designs, which could potentially cause the armature to release and disengage the contacts. Shading rings create a delayed magnetic circuit that smooths out these fluctuations, ensuring that the armature remains energized during the full cycle of AC current. This enhances the stability and performance of the relay or contactor, preventing unintended disconnections.

The other options suggest scenarios where shading rings may not be necessary, which does not align with the context of their function within AC environments. Thus, the statement that shading rings MUST be used in AC relays and contactors is indeed true.