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Contact resistance is measure by which method
Double bridge
Kelvin bridge
Both
None of above
Both
Contact resistance is a low-resistance measurement often in the range of micro-ohms (╬╝╬й), which requires a four-terminal measurement technique to eliminate the resistance of the test leads. Both the Kelvin double bridge and its predecessor, the Kelvin bridge, are specialized instruments designed specifically for measuring these low-resistance values accurately.
Contact resistance is a low-resistance measurement often in the range of micro-ohms (╬╝╬й), which requires a four-terminal measurement technique to eliminate the resistance of the test leads. Both the Kelvin double bridge and its predecessor, the Kelvin bridge, are specialized instruments designed specifically for measuring these low-resistance values accurately.
R=IVтАЛ тАФ Ohm's Law governing the basic resistance calculation
RxтАЛ=PтЛЕSQтАЛ+P+Q+rrqтАЛ тАФ The balance equation for a Kelvin Double Bridge where r is the resistance of the link
The principle relies on the four-terminal configuration where current is injected through a pair of high-current leads, and the voltage drop across the resistance is sensed by a separate pair of potential leads. This eliminates the effect of lead and contact resistance in the measuring circuit because the voltage sensing path draws negligible current, ensuring the measurement is solely of the DUT (device under test).
Contact resistance measurement is critical for circuit breakers, busbar joints, and isolators.
The Kelvin Bridge is a modified version of the Wheatstone bridge designed for low-value resistors.
Four-terminal sensing is mandatory to avoid the errors caused by test lead resistance.
The 'Double Bridge' is simply another common name for the Kelvin Bridge circuit configuration.
Eliminates errors due to lead resistance.
High accuracy for values below 1 ╬й.
Requires four separate connections to the terminal.
More complex setup than a standard Wheatstone bridge.
Circuit breaker contact resistance testing (CRM).
Measurement of transformer winding resistance.
Testing of heavy-duty switchgear joints.
In field testing, the instrument used is often referred to as a 'Micro-ohmmeter' which utilizes the Kelvin principle.
Option A and B are functionally the same device, which is why C is the correct answer.
C is correct тАФ Kelvin bridge and Double bridge are two names for the same configuration used for low resistance measurement.
Always remember that Wheatstone bridges are for medium resistance, while Kelvin bridges are for low resistance; high resistance requires Megger testing.