- The connection diagram for Varley loop test for both earth fault and short circuit fault is shown in the Figure.
- This test provides for the measurement of total loop resistance.
- The ratio arms P and Q are fixed, balancing of bridge is done by adjusting variable resistance S which is placed in series with the one end of the cable lies inside ground in this test.

**Switch
K1 connected to position 2**

- The variable resistance S is varied till the bridge is balanced.
- Let the variable resistance is S1 for balancing of bridge.

P
( X + S ) = RQ

(
P / Q ) = R / ( X + S )

The
bridge is balanced at value of S = S1

(
P / Q ) = R / ( X + S1 )

( P
+ Q ) / Q = ( R + X + S1 ) / ( X + S1 )

Therefore
( X + S1 ) = Q ( R + X + S1 ) / ( P + Q )

X
= { Q ( R + X + S1 ) / ( P + Q ) } – S1

= { Q ( R + X + S1 ) } – S ( P + Q ) / ( P + Q )

= { QR + QX + QS1 – SP – QS1 } / ( P + Q )

= { Q ( R + X ) – PS1 } / ( P + Q ) ………….. (
1 )

**Switch
K1 connected to position 1**

- The switch K1 is connected to position 1 for earth fault or short circuit in the cable.
- The bridge is again balanced for new value of S = S2.

At
balance condition

PS2
= Q ( R + X )

(
P / Q ) = ( R + X ) / S2

( R
+ X ) = PS2 / Q ……….. ( 2 )

From
equation ( 1 ) and ( 2 )

X =
PS2 – PS1 / ( P + Q )

= P ( S2 – S1 ) / ( P + Q )

As
the value of P, Q, S1 and S2 are known, the value of X can be easily
calculated.

Loop
resistance = R + X = PS2 / Q

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