Cable Fault Location on Cross-bonded Cable Systems

Cable systems of substantial length frequently apply cross-bonding as it allows to improve the current carrying capacity, and thus decrease power losses in the system. On the down side, this type of bonding presents a number of challenges to be tackled during cable fault location. This article describes a typical practice for testing cross-bonded systems.

What is cross-bonding

In cross-bonding, the circuit is made up of three (six, or nine) cross-connected minor sections, which form a major section. At the beginning of the first and at the end of the third minor section the shield is solidly grounded, with the second section isolated using shield ‘breaks”, and its sheath bonded to other phases. Due to such an arrangement, the total induced shield currents are reduced, which makes for an increased ability to carry useful current and higher safety to the cabling system.

Testing a cross-bonded cable system

The cable fault location procedure includes the following steps:

1. Learn about the cable system being tested, i.e. type of cable, insulation type, and total length of the circuit.

2. If needed, especially on new installations, run a sheath test to see if the integrity of the outer cable sheath is intact. For this test a test voltage is fed to the cable neutral. Current leakage resulting from the procedure indicates there is mechanical damage to the cable sheath.

VLF-60 Hipot

Note: In order to conduct this test on a cross-bonded system, the solid bonded link boxes need to be opened to remove the ground connections; any additional overvoltage protection devices should also be disconnected.

3. Run a preliminary VLF hipot test; this will make it possible to check the splices and terminations, to confirm a cable fault and identify the breakdown voltage.

4. Conduct cable fault pre-location (see below).

5. Pinpoint the fault with the acoustic method.

Cable fault pre-location on cross-bonded cable systems

One of the most popular approaches to fault pre-location is time domain reflectometry, which identifies cable events, like faults, splices, cable ends, based on the change in the characteristic impedance of the cable. In cross-bonded cable systems there are a large number of cross-bonding joints; predictably, the TDR signal is reflected off the first two or three joints and cannot go any further.

These influences can be eliminated by bridging the cross-connections with solid short-circuit jumpers that can be fixed directly.
If bringing cannot be applied, it is advisable to identify the cable section affected by the fault using a surge wave generator and a cable fault pinpointer.

Alternatively, while using TDR, attempt adjusting the pulse width and pulse voltage. In order to utilize this approach, it is important to understand that while a wider pulse width yields a stronger signal, the reflection at first-cross bonding points will be strong as well; at the same time, a too narrow pulse width means the signal will attenuate too fast. As for the pulse voltage, there is a direct connection between the amount of voltage applied and the quality of the TDR imprint: the higher the voltage pulse sent, the clearer the reflections in the longer cable sections.

Time Domain Reflectometer RIF-9

Alexei Tiatiushkin

Marketing manager

KharkovEnergoPribor Ltd.

marketing@keppowertesting.uk

http://www.kep.ua