T-H300

The T-H300 provides accurate automatic or manual fault location for cable sheath faults. It utilises either the DC resistance method or it can act as a bridge, in conjunction with an internal 12kV DC voltage source. The device can be used in automatic mode, providing accurate fault location based on only entering the cable length, or manual mode is also available. The equipment also allows the measurement of insulation impedance and, together with the separately available T-KF200, can provide fault pinpointing.

The main menu interface is divided into six sections: Automatic test, Manual test, Resistivity method, Insulation test, Fault pinpointing, and Connection.

The rear of the T-H300A features a cable and accessory bag. The working ground cable and the high voltage cable are 3.5m long and are fitted with heavy-duty crocodile clips.

For the DC resistance method, the faulty cable shield is connected to two healthy cores or healthy shields at the remote end. The benefit of this approach is that the shorting leads at the remote end are not part of the measurement circuit, so compromised connections will not adversely affect the measurement.

The DC resistance method calculates the fault distance by measuring the DC resistance from the measuring point to the fault point — initially along the faulty cable, then through a parallel healthy cable attached at the remote end to the far end of the faulty cable. The other healthy phase is used to connect the voltmeter, so that it measures the voltage across the length of the cable plus the section of the faulty cable between the fault point and the far end. This means there is no need to reconnect the voltmeter for the second part of the test.

With a cable of full length L: in the first step, current I1 flows from A to F and back through earth, giving U1 = U(AF) and R1 = R(AF). In the second step, current I2 flows from C to B to F and back through earth, giving U2 = U(BF) and R1 + R2 = R(L). The fault distance is then calculated as (R1 / (R1 + R2)) × L. The terminal short-circuit wire serves only as an extension for the voltage measurement and is not part of the measurement circuit itself, which eliminates these connecting wires from affecting the result.

The T-H300 does not use a traditional Murray loop approach to calculate the distance to the fault. Instead of comparing an external resistance to the resistances of the faulty core or sheath, it directly measures the resistances by applying a voltage source across the near end of the faulty cable and measuring the relevant voltages and currents — a modified bridge method.

Using the same two-step approach: in the first step, current I1 flows from A to F and back through earth, giving U1 = U(AF) and R1 = R(AF). In the second step, current I2 flows from C to B to F and back through earth, giving U2 = U(CF) and R2 = R(L) + R(BF). The fault distance is then calculated as (2 × R1 × L) / (R1 + R2) — an equation which emulates the traditional Murray Loop method.