The cable outer sheath fault tester is designed on the MURRAY bridge principle and can be used to locate breakdown points of various wires and cables as well as defective points that have not broken down but have a low insulation resistance value. The equipment uses a motorized voltage regulator and an R-type transformer to form a high-voltage constant-current source, with push-button control for raising and lowering the voltage. The no-load voltage is 15kV with a short-circuit current of 100mA; it adopts a high-sensitivity amplifier and a galvanometer for balance indication, and together with a proportional potentiometer forms a balanced bridge, with the whole arranged at high potential. The measuring cable is a specially designed twin-core high-voltage rubber cable, and a four-terminal resistance measurement method avoids the error introduced by lead resistance. The panel is at low potential overall. With one-key operation and an input of the cable length, the fault distance is calculated automatically. The high-voltage source and the bridge are integrated in one portable protective case, so the equipment is high in voltage, light in weight, easy to operate and safe to use.

The XHHG520 is positioned for cable outer-sheath (jacket) fault work — locating jacket breakdown and low-resistance defect points, and verifying jacket withstand performance, on high-voltage and extra-high-voltage cables. It is engineered for cable equipment maintenance teams at 10kV-500kV high-voltage and extra-high-voltage cable operating units, for extra-high-voltage cable engineering companies, and for power transmission & transformation engineering companies. In particular it serves the new-build acceptance and preventive testing of jacket systems on cross-bonding installations, where laid cables often fail the 10kV/1min outer-sheath withstand test because of mechanical damage during laying, earth filling and cover-plate work, and where long-standing hidden defects — termite bites, water ingress into grounding boxes, deterioration of existing defect points and moisture penetration in branched grounding wires — reduce the insulation resistance and are hard to pin down. Unlike the other instruments in the same category, which deal with the main insulation or with cable routing, this device targets the cable outer sheath: a MURRAY bridge measures the fault distance while a separate localization mode pinpoints the jacket fault on site.

Bridge section:

Withstand and localization section:

Bridge section:

Withstand and localization section:

Bridge function: the cable fault localization bridge method works as follows. The faulty phase and the non-faulty phase of the cable under test are short-circuited, the two arms of the bridge are connected to the faulty phase and the non-faulty phase respectively, and an adjustable resistor on one arm is adjusted so that the bridge is balanced; using the proportional relationship and the known cable length, the fault distance can be obtained. The bridge method is relatively simple and its accuracy meets the requirements of on-site engineering testing; for a two-phase short-circuit fault of a cable line it is very convenient to measure. Using the Murray bridge to locate a breakdown point is a classic approach, convenient and accurate. The basis of the bridge method is that the resistance of the conductor (or shielding layer) is uniform and proportional to the length. Let the distances from the two ends of the cable under test to the breakdown point be L1 and L2 and the total length of the cable be L, with corresponding conductor resistances R1 and R2; after balancing, L1 = P%·L, where P is the balance adjustment reading.

Localization function: the AC 220V mains supply is subjected to full-wave rectification to achieve an excellent conversion, transforming it into the high-power special signal required for identification; this signal is applied to the cable to be localized through the connecting line, and the “step voltage method” or the “current method” can be used to precisely pinpoint the cable fault. In the step voltage method, a test voltage is applied between the test point of the faulty cable and the ground so that a distribution electric field concentric with the entry point forms around the cable's entry point into the ground; there is no potential difference between any points of the same radius in this field, but there is a potential difference between any two points of different radii, and when the spacing between the two points is fixed, the closer the two points are to the center, the stronger the potential difference. By moving the two points A and B gradually toward the center, the potential difference becomes zero when the fault point is exactly midway between A and B; if the movement continues past the fault point, the polarity of the potential difference reverses, so moving back and forth allows the grounding point to be judged accurately. In the current method, a pulsed high-voltage signal is applied to the faulty cable so that the fault point discharges; the currents before and after the fault point then have opposite polarities while the current at the fault point is the smallest. A receiving clamp is used to judge the direction of the current before and after the fault point: clamping the receiving clamp on the cable, the receiving box indicates a direction, and if the movement continues past the fault point the current polarity reverses, so moving back and forth allows the grounding point to be judged accurately.

Withstand function: switch the function key to the withstand function to output 0-15kV for a withstand test on the cable.

Bridge section main steps: prepare by judging the faulty phase and connecting correctly (confirm the cable breakdown state with a multimeter, megger or other withstand equipment and record the insulation resistance to ground or the breakdown residual voltage of each core; record the length, model and cross-section of the cable under test and patrol along the cable route, short-circuit the faulty cable and the outgoing terminals of the auxiliary cable at the far end and leave a person to watch at the far end to avoid high-voltage injury; connect the grounding terminal of the instrument reliably to the grounding body at the site, with the measuring head end red clamp on the core of the faulty cable and the measuring end black clamp on the core of the auxiliary cable). For measurement: connect the power to AC 220V, select bridge function as the mode, switch on the sensitivity switch and turn it to maximum, and the battery indicator comes on; zero the bridge by rotating the zeroing knob so the galvanometer indicates zero; raise the voltage (switch on the power, the zero-position indicator comes on — if not, press the voltage-reduction button until it does — then press the start button and the high-voltage indicator comes on); press the voltage-boost button and watch the voltmeter and ammeter until the ammeter exceeds 20mA (if the current is unstable, continue raising the voltage and hold it for a while to form a stable arc or conductive region so that the current remains stable during testing); then adjust the balance knob so the galvanometer deflects noticeably and indicates zero and record the dial reading P1. For reverse-connection calibration: lower the voltage, discharge, swap the measuring clamps and repeat the steps to obtain another reading P2, which should give P1 + P2 = 100; the fault point position is X = 2 × L × P1%, and particular attention should be paid to the “2” in the formula, because the auxiliary cable doubles the cable involved in the calculation.

Localization — step voltage method (for pinpointing outer-sheath faults of directly buried cables): the main unit transmits a signal in “localization mode” to ensure a continuous pulse current output; install the A-frame and connect it to the receiver with the A-frame connecting line; set the receiver position switch to “I” (weaker signal) or “II” (stronger signal), zeroing the microammeter with the zeroing knob in position “I” (in “II” position the zeroing knob has no effect); along the cable route, directly above the cable, insert the A-frame probes into the ground and observe the magnitude and direction of the ammeter, using the adjustment knob to set a suitable pointer swing; keep the adjustment knob and the front-to-back order of the A-frame probes unchanged and keep moving the A-frame forward above the cable, first finding the section where the polarity changes and then the point where the pulsating signal is “0” or reversed, which is the fault point; when finished, set the receiver position switch to “0”, switch off the high-voltage pulse signal and discharge the cable and capacitor.

Localization — current method (for pinpointing outer-sheath faults of cables laid in tunnels): the main unit transmits a signal in localization mode to ensure a continuous pulse current output; connect the current receiving clamp to the receiver; set the receiver position switch to “I” (or “II” when the current is strong) and zero the microammeter with the zeroing knob; within the rough measuring range, clamp the current receiving clamp on the cable and observe the magnitude and direction of the ammeter, using the adjustment knob to set a suitable pointer swing; keep the adjustment knob and the direction of the receiving clamp unchanged and keep moving the clamp forward, first finding the section where the polarity changes and then the point where the pulsating signal is “0”, which is the fault point; when finished, set the receiver position switch to “0”, switch off the high-voltage pulse signal and discharge the cable and capacitor.

Withstand mode: insert the red high-voltage line into the “high-voltage output” terminal with the other end connected to the metal sheath or armoring and the “ground” reliably connected (never connect wiring while the unit is on); after wiring is complete, select the withstand function as the mode and connect the AC 220V supply and switch on; confirm the on-site cable condition and that the wiring is correct, and before starting check whether the voltage adjustment has been turned counter-clockwise to zero — when the start button lamp is lit it indicates the zero position has been reached, and the unit cannot be started if it has not been zeroed; turn the voltage adjustment clockwise and watch the voltmeter and ammeter to select a suitable output voltage; after the test, turn the voltage adjustment counter-clockwise to the zero position, press the stop button so the instrument stops working, and then switch off; only after discharging the faulty cable with high voltage may the test wiring be removed.

Warranty: within one year from the date of purchase, free repair is provided for product quality problems, with lifetime maintenance and technical service.

It is designed on the MURRAY bridge principle and is used to locate breakdown points of various wires and cables as well as defective points with a low insulation resistance but no breakdown. Its output of 15kV and below fully meets the cable outer-sheath withstand test requirements of GB50150-2006, including acceptance and preventive tests of outer sheaths suitable for cross-bonding systems, and it can also perform 5kV-15kV, 1-minute DC withstand tests on HDPE and PVC cable outer sheaths.

The no-load voltage is ≥15kV, the short-circuit current is ≥100mA (5~40mA recommended when balancing), the locating ratio accuracy is ±(0.2%·L±1) meter, the weight is 35kg, and the working power supply is AC 220V (±10%), 50Hz±1Hz with an 8.4V built-in battery.

The input voltage is 220V (±10%), 50Hz (±2Hz); the output voltage is 0~15kV (square wave) and adjustable; the output current is 0~200mA; the output capacity is 2kVA; and the frequency adjustment ranges from a minimum of 0.2Hz to a maximum of 5Hz (adjustable).

It uses the bridge (Murray) method to obtain the fault distance, and for on-site pinpointing it uses the step voltage method for directly buried cables (with the A-frame) or the current method for cables laid in tunnels (with the receiving clamp), following the polarity change and then the point where the pulsating signal is zero.

It has over-current, over-voltage and over-heat automatic protection, zero-position start protection, and a unique self-discharge design (once the high-voltage output is cut off, its own high voltage becomes zero). It uses dual pointer meters for current and voltage, and allows one-key switching between localization mode and withstand mode, with no blind area in the test range.