Page 333 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Sodium vapour lamp

A sodium vapour lamp is a cold cathode lamp which gives higher luminous output than other lamps. The lamp
consists of long glass tube, two oxide coated electrodes are fixed at the ends. This tube contains sodium and
neon. To reduce the length of the lamp tube is made of ‘U’ shaped as shown in figure. This ‘U’ tube is arranged in
double walled glass tube, and vacuum is created between the glass walls.

Initial time sodium is in solid state and hence the lamp cannot be started direct as sodium vapour lamp. When
connected to supply it starts as a neon lamp giving reddish light. After 10-15 minutes’ sodium turns into vapour
and starts working as a sodium vapour lamp and giving yellow light. For starting discharge, high striking voltage
is essential and is given by an ignitor.

Mercury vapour lamps
It is a hot cathode discharge lamp. Luminous efficiency oflow Pressure mercury vapour lamp is low as Sodium
vapour lamp.So to increase efficiency, medium pressure and high pressure hot cathode mercury vapour lamp are
used

Types of high pressure mercury vapour lamp
1 MA Type (MV lamp with auxiliary electrode)
2 MAT type (MV lamp with tungsten filament)
3 MB Type (MV lamp with auxiliary electrode and Bayonet cap)
Ma type hpmv lamp

The two main electrodes are enclosed in discharge tube is made of borosilicate, contains mercury and small
amount of argon.Argon assist in the starting of discharging.To start discharge, there is an auxiliary electrode
is arranged close to the main electrode.The auxiliary electrode is connected to the lamp terminal through high
resistor to limits the current.This quartz tube is enclosed in another ordinary glass bulb whose internal surface
is coated with fluorescent powder.The space between tube and bulb is evacuated to reduce heat loss.The lamp
operates at high pressure. It has a screw cap and is connected to the main through the choke.
When switched ON, normal mains voltage is not sufficient to start the discharge between the main electrode but
it can start to flow between main and auxiliary electrode through argon gas. The argon discharge then warms up
the tube and vaporise the mercury. The discharge then take place in the mercury vapour.

The lamp takes 5 minute to start giving full output. Once the lamp is switched OFF,will not start again until the
pressure developed inside the tube falls back.It takes about 7 minutes to start again.The lamp should always be
hung vertically,otherwise the inner tube will be damaged.
The efficiency is 45 lm/watt for 400watts lamp

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CITS : Power - Electrician & Wireman - Lesson 54-59