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SSC JE Electrical 2019 with solution SET-2
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____ Lamps are used to light up sports stadiums?

The most common type of floodlight is the metal-halide lamp, which emits a bright white light (typically 75-100 lumens/Watt). Sodium-vapor lamps are also commonly used for sporting events because they imitate daylight conditions, and colors appear natural.

They have a very high lumen-to-watt ratio (typically 80-140 lumens/Watt), making them a cost-effective choice when certain lux levels must be provided.

SSC JE Electrical 2019 with solution SET-2
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The names of the four materials have been given, select the one which has the least resistivity at 20°C?

The resistivity of iron at 20°C is 9.71 × 108

The resistivity of silver at 20°C is 1.59 × 108

The resistivity of Glass at 20°C is 1.10 × 109

The resistivity of nichrome at 20°C is 100 × 108

Hence silver is the least resistive material in the given option.

SSC JE Electrical 2019 with solution SET-2
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A/An is used in domestic mixers?

We use universal motors in mixer grinders. Universal motors can run both on a.c. and d.c.

The universal motor works on the same principle that DC series motor works. DC series motor has the characteristics of operating at high-speed when there is no load and operating at low-speed when the load is applied. It has high starting torque characteristics.

So it is used in mixers, where initially we put some load at starting.

SSC JE Electrical 2019 with solution SET-2
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If the number of poles in an 11 KV transmission line is 80, then how many disc insulators are required?

No of disc required for 11 kV transmission line = 1 disc

Since the transmission line is 3-phase, therefore, the number of discs required for a single-pole will be 3.

Total no. of poles = 80

No of disc = 3 × 80 = 240

SSC JE Electrical 2019 with solution SET-2
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In the two-wattmeter method of three-phase power measurement of a balanced load, if the reading of one meter is −200W, then the power factor of the load is

The reading of two wattmeters can be expressed as

W1 = VLILcos(30 + φ)

W2 = VLILcos(30 − φ)

(i) When PF is unity ( φ = 0°)

W1 = VLILcos30°

W2 = VLILcos30°

Both wattmeters read equal and positive reading i.e upscale reading

(ii) When PF is 0.5 (φ = 60°)

W1 = VLILcos90° = 0

W2 = VLILcos30°

Hence total power is measured by wattmeter W2 alone

(iii) When PF is less than 0.5 but greater than 0 i.e ( 90° > φ > 60°)

W1 = Negative

W2 = positive (since cos(−φ) = cosφ)

The wattmeter W2 reads positive (i.e.upscale) because for the given conditions (i.e. ( 90° > φ > 60°), the phase angle between voltage and current will be less than 90°. However, in wattmeter W1, the phase angle between voltage and current shall be more than 90° and hence the wattmeter gives negative (i.e. downscale) reading.