Electrical Apprenticeship Technology 2 (T2) Phase 4 Practice Exam

In an MBF lamp, where does discharge occur and how long does it take to reach full light output?

In ballast coil; 0.5 seconds In the discharge tube between main and secondary electrode through argon; 4-5 minutes In the outer jacket; 30 minutes In the phosphor layer; 1 hour

Correct Answer:

In the discharge tube between main and secondary electrode through argon; 4-5 minutes

Explanation:

Discharge happens inside the lamp’s arc tube, between the main and secondary electrodes, with the gas (including argon) providing the path for ionization. The ballast limits current as the arc forms. It doesn’t occur in the ballast, the outer jacket, or the phosphor layer. The phosphor only glows after ultraviolet light from the mercury discharge excites it, and reaching full brightness requires the arc to stabilize, mercury vapor pressure to reach operating conditions, and the phosphor to respond fully. In an MBF lamp, this warm-up takes several minutes, typically about four to five minutes, before full light output is achieved.

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