| Measuring circuit and equipment involved for the measurement of tanδ with Schering Bridge |
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For the measurement of active power loss in the dielectric on applying ac voltage, this bridge circuit was developed by H. Schering in 1919. Since then it is used widely for HV measurements and non-destructive testing for quality assessment of the factory finish and working state of dielectrics in different equipment. |
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Fig 35.1 Circuit of the Schering bridge |
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Test object |
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Standard capacitor, loss free |
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Balancing elements |
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Fixed resistor |
N |
Null indicator |
S |
Screening |
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The portions of the circuit shown under screening are all at low voltage. Only the test object Cx and the standard capacitor C2 are at high voltage. C2 must be an active power loss free (R2 = ∞) capacitor. The bridge is equipped with over voltage protective devices. By adjusting resistor  and the capacitor  the bridge can be balanced. When the bridge is balanced, the following relation holds good, considering  and  to be in parallel. |
or
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On comparing the real and imaginary parts of the equation, we obtain: |
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The other required quantities are given as; |
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For 50 Hz, if  is chosen to be equal to  and since 
It is directly measured by varying  and putting its value in Farads.
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Guard-ring capacitor
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| This is a measuring cell especially designed according to VDE specifications (German) for measuring dielectric properties of liquid dielectrics. The provision of guard-ring eliminates the effect of stray capacitances. The details of the Guardring capacitor are as follows. |
A. Principal data
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| Cylindrical capacitor conforming to VDE 0370 with guard-ring, used as cell for the measurement on liquid insulating materials. |
Spacing between electrodes |
2 mm |
No-load capacitance  , with air/vacuum |
about 60 pF |
Max. measuring voltage |
2000 V = 10,000 V/cm (field) |
Necessary amount of liquid required |
about 40 cm3 |
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The measuring cell may be heated up to 150 0C and can be evacuated for carrying out measurements in an inert gas. |
Material: (high-tension and measuring electrodes, guard-ring)
Stainless steel. |
Dimensions |
240 mm dia. x 220 mm high |
Total net weight |
about 10 kg. |
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Figure 35.2 |
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Direct measurement of insulation Resistance at room temperature by a Megaohm meter
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The Insulation Resistance  can be measured directly with the help of a measuring instrument known as MΩ meter and the specific insulation resistance of the oil sample is calculated as follows,
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Figure 35.3
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 - resistance in 
 - capacitance in pF of the Guardring capacitor with air/vacuum.
 - absolute permittivity in vacuum =8.854 x 10-12F/m.
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