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Cable Fault Detection Methods

 

1  Low Voltage Pulse Method (LVPM)

Transmit a pulse signal into a cable, the pulse propagates along the cable, and is reflected back at fault point.

Lx = V·T/2

V is the pulse propagation velocity that is a constant related to the cable type. T is the time that the Pulse to and from the fault point. Lx is the distance (Length) between testing point and fault point.

LVPM is applicable to open circuit, short circuit and low impedance fault.        

 

                          

2  Impulse Voltage Flashover Method (IVFM)

Regarding high impedance fault, it is difficult to identify the reflected wave when using LVPM.

A sphere gap is used in IVFM to generate impulse high voltage, to detect high impedance fault. When sphere gap discharges, the fault point of test cable is breakdown. Then the flashover pulse is reflected to and fro at test point and fault point. Please refer principle diagram as follows:

As there is self inductance for Inductance L, so at the beginning, it will appear open circuit reflection because of choked flow effect of L. After certain time, it will appear short circuit reflection with current increasing. However, all circuits make a L-C discharge process by capacitor C and inductance L. Therefore, the waveform is nearly damped cosine curve superposed with fast pulse multiple reflection wave. See as follows. From the interval of reflection wave, fault distance can be calculated.

Fault distance   Lx = V·T/2

T+ΔT≥T, including ΔT is discharge delay time.

 

                                  

 

 

3  Waveform Comparison Method (WCM)

Waveform Comparison Method is special LVPM in practice. It gets both waves of normal cable and fault cable by LVPM, then compares them to detect puzzling fault.

 

 

 

Cable Fault Type and Test Method (for reference)

No.

Fault Type

How to determine the fault type

Test Mode

Locating method

1

Low impedance

Using megger to measure:0

Using Multimeter to measure: <20

LVPM

Audio Frequency (Alternative)

Potential Difference Method

2

Disconnection

Conductor incontinuity

Potential Difference Method

3

High impedance

Using megger to measure: >0 or using megger to measure: 0

Using Multimeter to measure:

 200Ω

IVFM

4

Flashover

Using megger to measure: The insulation is normal

Withstand voltage test: Fail

 

 

 

Other Information of the Cable Fault Detection Methods

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