Test: Measurement of Low Resistance - Electrical Engineering (EE) MCQ

Schering Bridge:

Schering bridge is a type of AC bridge that is used to measure the dissipation factor (dielectric loss) or phase angle and capacitance.

The circuit of the Schering bridge is shown below:

 

When the bridge is in the balanced condition, zero current passes through the detector, which shows that the potential across the detector is zero.

At balance condition:

Concept:

Schering bridge:

At bridge balance condition,

 

⇒ R₄Cx (1 + jωR₃C₃) = R₃C₁ (1 + jω Cx Rx)

By comparing both sides

Dissipation factor = ωCxRx = ωR₃C₃ = 2πf R₃C₃

Dissipation factor α frequency(f) --------(1)

Calculation:

Given:

Dissipation factor D at frequency 50Hz

Let Dissipation factor x at frequency 60Hz

From equation 1:
f1/f2 = D/z
x = 6/5 D

Dissipation factor:

The dissipation factor is defined as the value of the tendency of the dielectric material to absorb some of the energy when the AC signal is applied.

In a series RC circuit, δ refers to the angle between the series combination of R, C and the voltage across the capacitance C.

‘δ’ is also known as the loss angle

Therefore, the Dissipation factor is defined as the tangent of the loss angle.

Dissipation factor = tan δ

Schering bridge is used to measure the dissipation factor.

Schering Bridge:

Schering bridge is a type of AC bridge that is used to measure the dissipation factor (dielectric loss) or phase angle and capacitance.

The circuit of the Schering bridge is shown below:

 

When the bridge is in the balanced condition, zero current passes through the detector, which shows that the potential across the detector is zero.

At balance condition:
=

Schering Bridge:

Schering bridge is a type of AC bridge that is used to measure the dissipation factor (dielectric loss) or phase angle and capacitance.

The circuit of the Schering bridge is shown below:

 

When the bridge is in the balanced condition, zero current passes through the detector, which shows that the potential across the detector is zero.

At balance condition:

Concept:

1. The magnitude of impedances satisfy the relationship, Z₁Z₄ = Z₂Z₃

2. The phase angles of impedances satisfy the relationship, ∠θ₁ + ∠θ₄ = ∠θ₂ + ∠θ₃

These two are independent conditions for balance and both of them must be satisfied for the bridge to be balanced.

Calculation:

At bridge balance condition,

⇒ R₁C₃ (1 + jωR_xC_x) = R₂C_x (1 + jωR₁C₁)
⇒ R₁C₃ + jωR₁R_xC₃C_x = R₂C_x + jωR₁R₂C₁C_x
By equating the real parts on both the sides, we get

 
By equating the imaginary parts on both the sides, we get

Wagner's Earthing Device:

• The Wagner earthing device is used for removing the effect of earth capacitances.
• At high frequency, stray capacitance is induced between the bridge elements, ground, and between the arms of the bridge.
• This stray element causes an error in the measurement.
• It is a type of voltage divider circuit used to reduce the error which occurs because of stray capacitance.
• The Wagner Earth device provides high accuracy to the bridge.

Circuit diagram for Wagner’s earthing device:

 

Where,

Z₁ Z₂ Z₃ Z₄ is the impedance arm of the bridge.

Z5 and Z6 are the two variable impedances of the Wagner Earth Device

C₁, C₂ C₃, and C₄ are the stray capacitances of the bridge

D is the detector

a, b, c, d, and GND1 are nodes of the bridge

In Wagner’s earthing device is adjusted such that b, d, and GND1 have equal potential

At the balanced condition,

The capacitance C₁, C₂, C₃, C₄ all are eliminated from the bridge circuit along with the impedances Z₅ and Z₆.

As the current flowing through a low resistance circuit is low, the voltage drop across the terminals due to contact and lead resistances is negligible. Thermal e.m.f occurs in a circuit when its temperature increases due to high current flow.

Low resistance refers to resistance of the order of 1ῼ or less than that. Medium resistances range from above 1ῼ to a few kῼ. Any resistance value greater than a few kῼ is known as high resistance.