OBJECTIVE:-To measure earth resistance using fall of potential method.
APPARATUS REQUIRED:-Earth electrode, tester, two earth stake (inner stake and outer stake)
THEORY:-The 3 point method fall of potential ground resistance test requires complete isolation from the power utility. Not just power isolation, but also removal of any neutral or other such ground connections extending outside the grounding system. This test is the most suitable test for large grounding systems and is also suitable for small electrodes.
→ Calibrate a single-phase energy meter by phantom loading at different power factor by Phase shifting transformer
Induction type of energy meters are universally used for measurement of energy in domestic and industrial a.c. circuits. Induction type of meters possesses lower friction and higher torque/weight ratio. Also they are inexpensive and accurate, and retain their accuracy over a wide range of loads and temperature conditions. There are four main parts of the operating mechanism:
- Driving system
- Moving system
- Braking system and
- Registering system.
→ Measure the power in 3-phase star connected load by two-wattmeter method at Different values of load power factor.
Power in a 3-phase three wire system, with balanced or unbalanced load can be measured by using two watt-meters . The load may be star or delta connected. The current coils of the two watt-meters are connected in any of the two lines and the pressure coils are connected between these lines and the third line, as shown in Fig.Let eRN, eYN and eBN be the voltages across the three phases of the load and iR, iY and iB the currents flowing in the three lines. Total instantaneous power in the load = eRN* iR + eYN* iY + eBN* iB. Instantaneous current through the current coil of wattmeter W1 = iR
For a calibration of wattmeter, a circuit similar to fig. may be used. However, result is considerable consumption of power especially when the wattmeter has a large power rating. The current coil of wattmeter is supplied from a low voltage supply and a series rheostat is inserted to adjust the value of the current. The Potential circuit is supplied from the power supply. A voltage ratio box is used to step down the voltage fro the potentiometer to read. This type of arrangement is known as “Phantom loading”. The Voltage V, Current I are measured in turn with the PM employing a double pole throw (DPDT) Switch. The true power is then VI and wattmeter reading may be compared with this value.