Which statement best differentiates a relay from a contactor in aircraft electrical systems?

Which statement best differentiates a relay from a contactor in aircraft electrical systems?

• A. A relay is a low-current control device; a contactor handles high-current loads.
• B. A relay is used only in AC circuits; a contactor only in DC circuits.
• C. A relay stores energy; a contactor dissipates it as heat.
• D. A relay controls multiple circuits.

Answer: (B)

In aircraft electrical systems, you distinguish relay and contactor by what each is built to switch. A relay is a smaller, low-current device meant to control signaling and light to moderate loads in the control circuitry. A contactor, on the other hand, is a heavy-duty switch designed to handle high-current loads such as motors or main power feeders. This difference in current-carrying capability and duty is what sets them apart in practice. The idea that a relay is restricted to AC and a contactor to DC isn’t accurate—both types can be used with either current type, depending on the specific model and ratings. The key point is that contactors are chosen when you need to reliably switch large currents, while relays are chosen for control roles with smaller currents. Other statements don’t differentiate them as clearly. Saying a relay stores energy isn’t correct for how these devices operate, since any energy involved is momentary in the magnetic field, not stored for later release as a defining feature. And while both devices can be multi-pole, that capability isn’t what fundamentally separates a relay from a contactor. The practical distinction you should focus on is that relays handle low-current control tasks, and contactors handle high-current power switching.

In aircraft electrical systems, you distinguish relay and contactor by what each is built to switch. A relay is a smaller, low-current device meant to control signaling and light to moderate loads in the control circuitry. A contactor, on the other hand, is a heavy-duty switch designed to handle high-current loads such as motors or main power feeders. This difference in current-carrying capability and duty is what sets them apart in practice.

The idea that a relay is restricted to AC and a contactor to DC isn’t accurate—both types can be used with either current type, depending on the specific model and ratings. The key point is that contactors are chosen when you need to reliably switch large currents, while relays are chosen for control roles with smaller currents.

Other statements don’t differentiate them as clearly. Saying a relay stores energy isn’t correct for how these devices operate, since any energy involved is momentary in the magnetic field, not stored for later release as a defining feature. And while both devices can be multi-pole, that capability isn’t what fundamentally separates a relay from a contactor. The practical distinction you should focus on is that relays handle low-current control tasks, and contactors handle high-current power switching.