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In a loop test performed for localizing the earth fault in cable, fault resistance
Affects the balance condition
Affects the sensitivity of bridge
Affect the value of cable resistance
All of above
Affects the sensitivity of bridge
In a Murray loop test used for localizing earth faults in cables, the fault resistance exists in series with the battery or the galvanometer branch. While it does not affect the bridge balance equation (which depends only on the ratio arms and cable resistance), a high fault resistance significantly reduces the sensitivity of the null detector, making the balance point difficult to determine accurately.
In a Murray loop test used for localizing earth faults in cables, the fault resistance exists in series with the battery or the galvanometer branch. While it does not affect the bridge balance equation (which depends only on the ratio arms and cable resistance), a high fault resistance significantly reduces the sensitivity of the null detector, making the balance point difficult to determine accurately.
RbтАЛRaтАЛтАЛ=l2тАЛl1тАЛтАЛ тАФ Bridge balance condition for Murray loop
S=╬ФR╬ФIgтАЛтАЛ тАФ Sensitivity definition representing detector current change per unit resistance change
The Murray loop test employs a Wheatstone bridge configuration where the faulted cable section forms part of the bridge circuit. The balance condition is given by RbтАЛRaтАЛтАЛ=l2тАЛl1тАЛтАЛ. High fault resistance increases the impedance of the galvanometer path, which limits the current flow through the detector, thereby diminishing the deflection per unit change in bridge resistance (sensitivity).
The Murray loop test is primarily used for finding low-resistance earth faults.
The bridge balance equation is independent of the fault resistance value RfтАЛ.
A very high RfтАЛ makes the bridge 'dead', as the galvanometer receives insufficient current to detect the null point.
For high resistance faults, techniques like the 'Varley loop test' or impulse current methods are preferred.
Simple circuit arrangement
Accurate for low-resistance faults
Ineffective for high-resistance (insulation) faults
Sensitivity decreases as fault resistance increases
Localizing earth faults in underground power cables
Short-circuit fault localization in transmission lines
Option A is incorrect because fault resistance does not shift the balance point balance condition RbтАЛRaтАЛтАЛ=l2тАЛl1тАЛтАЛ.
Option C is incorrect as the cable physical resistance R remains constant regardless of the fault resistance.
Standard procedure requires the use of a sensitive null detector (galvanometer) to mitigate sensitivity issues.
B is correct тАФ The fault resistance increases the total series resistance in the galvanometer/battery loop, reducing the current flow and decreasing the bridge sensitivity.
Always remember that in bridge circuits, fault or contact resistance in the detector or supply branch affects sensitivity, whereas contact resistance in the ratio arms directly impacts accuracy.