Determining the location of cable insulation damage by the contour method

The cable circuit method (Murray's method) is an application of a single bridge circuit (Fig. 1). For determining the location of the damage — breakdown between the core and the armor or ground, the ends of bb 'of the working and damaged cable cores are short-circuited. A galvanometer and resistance boxes R and rA are connected to the other two ends of aa'.

Resistances, as in the case of a record bridge, can be replaced by a calibrated wire, which is divided into two parts with a movable contact. The clamp to which the resistance boxes are connected is connected to ground through the battery of cells.

Rice. 1. Scheme for determining the location of cable insulation damage by the contour method.

Let us denote by r the resistance of one core of the cable, the resistance of the cable section between the point of failure and the end a'— to rx. In the zero position of the galvanometer needle, as for a bridge, you can write:

Where

Having determined rx, knowing the specific resistance of the material of the cable cores ρ and the cross section of the cable cores S, by the formula

it is possible to determine the distance Lx from the end of the cable a 'to the point of insulation failure.

If the cross-section of the cable along the entire length is the same, then in the formula for determining rx, you can replace their expressions in terms of length, cross-section and resistance instead of rx and r, then we get

where

The latter formula usually determines the distance from the end of the cable to the location of the fault.

To check the result of the measurement, it is recommended to make a second similar measurement by exchanging the ends of the cable a and a '. In this case, the distance Ly is determined by the formula

where R' and r'A are the resistance values ​​of the bridge arms in the second measurement.

If measured correctly, Eq

The resistance of the connecting wire bb ', the transition resistances at the points bb' and the resistance of the wires connecting the ends of the cable to the resistance boxes must be small; otherwise, significant measurement errors are inevitable.

To determine the location of cable insulation failure by the contour method, you can use a UMV-type bridge or a KM-type cable bridge.