Emergency lighting circuits often use _____ so the lamps in such circuits can be lighted instantly after the failure of the main power supply?

Emergency lighting circuits often use _____ so the lamps in such circuits can be lighted instantly after the failure of the main power supply?

• A. Magnetic relays/ control relays
• B. Solid-state relays
• C. Thermistors
• D. Mechanical timers

Answer: (A)

Instant transfer of the lighting load from the mains to the emergency source is needed to ensure no delay in illumination when power fails. Magnetic relays provide that immediate, full-contact switching by using an electromagnetic coil to move mechanical contacts. When the main power is present, the relay holds the contacts in a position that keeps the lamps on the normal supply; when power is lost, the coil is de-energized and the spring- or magnetically held contacts snap to the emergency circuit, connecting the lamps to the backup power right away. This simple, robust method handles the lamp current reliably and without deliberate delay. Solid-state relays could switch quickly too, but they can introduce leakage current and higher cost and complexity, which isn’t ideal for standard emergency lighting arrangements. Thermistors are used for inrush control, not for switching power to light loads. Mechanical timers would add delay, defeating the requirement for instant illumination.

Instant transfer of the lighting load from the mains to the emergency source is needed to ensure no delay in illumination when power fails. Magnetic relays provide that immediate, full-contact switching by using an electromagnetic coil to move mechanical contacts. When the main power is present, the relay holds the contacts in a position that keeps the lamps on the normal supply; when power is lost, the coil is de-energized and the spring- or magnetically held contacts snap to the emergency circuit, connecting the lamps to the backup power right away. This simple, robust method handles the lamp current reliably and without deliberate delay.

Solid-state relays could switch quickly too, but they can introduce leakage current and higher cost and complexity, which isn’t ideal for standard emergency lighting arrangements. Thermistors are used for inrush control, not for switching power to light loads. Mechanical timers would add delay, defeating the requirement for instant illumination.