Underground Cable Tester

HP-W10 underground cable tester is based on the MURRAY bridge principle. It is used for locating a variety of wire and cable breakdown point or no breakdown but low insulation resistance value defect point, and high-voltage cable protection sheath faults. It can also be used for cable factory to test cable defects.（Requires supporting cable fault tester host and high voltage source）

The device using SMPS to constitute the high-voltage constant current power,no-load voltage is -15000V,short current is 30mA. Using high sensitivity amplifier and galvanometer to indicate balance, with proportional potentiometer to constitute a balanced bridge and placed at high potential. The testing wire is specially designed dual-core high voltage rubber cable.

Use four-terminal resistance measurement to avoid the error arising from lead resistance,the test wire is reliably grounded through a braided shield,the operation button on the panel is at earth potential, operate the bridge through the insulation bar.high voltage constant-current power and bridge are integrated in a portable industrial cabinet. Therefore, the device has high voltage, light weight, easy to operate and safe to use.

HM-K10 underground cable tester have three function:

1)DC withstａnd test: the device can output 0 ~ 15kV DC voltage, can be used in cable dc voltage withstａnd test.

2)burn-through fault point: burn-through or reduce the high resistance and flashover fault point resistance.

3)Fault pre-locating:using bridge principle to pre-locate,four-terminal resistance measurement method to avoids the error arising from lead resistance.

Meanwhile, the tester is particularly suitable for:

1)Installed cable high resistance breakdown point, in particular for the linear HV breakdown point which is difficult to burn to low resistance, such as cable middle connector linear high resistance breakdown.

2)Flashover breakdown point, after breakdown the constant current power can maintain the arc, a stable current through the bridge, the bridge has enough sensitivity.

3) Not yet breakdown but low resistance fault, for example, megger shows cable resistance is low, but under operating voltage the insulation defects can not be breakdown.

1) Test voltage: 0 ~ 15kV, negative highvoltage, continuously adjustable

2) The maximum current: 30mA

3) The test ratio accuracy: ± (0.2% • L ± 1) meters

4) Output voltage of outer sheath burn through: 0 ~ 15kV, continuously adjustable

5) Output current of outer sheath burn through: 30mA

6) Power supply: AC220V ± 10%

7) Anti-interference ability: > 100VAC

8) Weight: ≤ 12kg

9) Dimensions: L478mm × W360mm × H316m

1.Proper use of the measure clamp

When pre-location,the red and black clamp shoul connect respectively with the scale potentiometer and the galvanometer,Not direct short circuit.

Attantions when ues this device to do voltage withstａnd test

2.When use this device to do voltage withstａnd test,please note that it didnot have the function of over-current trip,so should observe the reading of the voltage and current meter to judge insulation state. Attantions when connection:two clamp output high voltage in same time,should connect with cable core in same time,metal shielding or other core should connect with ground of the device.

3.How to make current stable

The bridge tester is balanced by a steady current.Before boost,the sensitivity level should be in the "off" position,short circuit the bridge tester,to prevent damage galvanameter amplifier board by impulse current.there frequently have creepage at high resistance breakdown point when begain to boost voltage, which will make the current to fluctuat,please keep the max current for a few minites, the current will go to stable. For some flashover fault,it will take more time to make the current to stable .Fault point discharge frequently, when form electric arc, the current will be stable.Use impulse power supply and bridge tester rise voltage same time ,that will inprove breakdown power ,shorten the time of making current stable.

4.Sensitivity selection of bridge tester

.The greater the current passing through the bridge, the higher the sensitivity.

.The greater the resistance of the cable conductor, the higher the sensitivity of the bridge.The sensitivity of thin and long cables is higher, and that of thick and short cables is lower.For cable with big cross section and short length , shoult increse current ,to select higher sensitivity.

5.Matrixing when anxiliary core have different cross section

The cable with different cross sections can be used as the auxiliary cable. In calculation, the length of the auxiliary fault cable should convert to the length of the fault cable.

If the cross section of fault cable is Sx,ａnd the cross section of the auxiliary cable is S, so X = P1‰×（1+Sx/S）×L

Which can be understａnd like that:more thin of the auxililary cable is ,more bigger the resistance is, that equal to Longer fault cable.

6.Location for Cable in coil

The bridge tester also can be use to locate defect point in cable factory test.usage like this:connect the clamp to two end of the test cable, don't have to use a low resistance short line and auxiliary cable, calculate formula is X=L× P1‰

7.Can the steel bａnd serve as the bridge arm?

The accurary location of bridge tester depends on the uniform resistance of conductor.Cable factory does not necessarily equip copper and steel strip for joint,the copper strip has little contact resistance and has little effect on locating accuracy.use steel strip should be careful, may cause large error, so should avoid using steel bａnd to locate .

8.Location for aerial cable

aerial cables are usually single core, with only insulation layer. The defects found in the inundate and voltage withstａnd test can also be located. The only different is the ground pole is water.The instrument can be connected to the ground point of cistern , or the copper strip can be placed in water as the grounding pole.

9.Multipoint defect location

No necessary to distinguish the defect point is low resistance point or breakdown point in this case.Although Theoretically, the positioning ratio means the barycenter composed of multiple leakage currents, the bridge tester method cannot locate multiple fault points.On the running cable, the fault over-voltage surge can occasionally cause the cable to breakdown in turn, resulting in multi-point breakdown,but in fact,these multi-point have different situation,as the dc voltage rises, the weakest point first breakdown,the position calculated on a scale is very close to the point where most of the current flows.Get rid of that point and find the next one.In practice there is little chance that two or more points will flow through a larger current at the same time,therefore, there is no need to worry about the bridge tester difficult to locate multi-point breakdown.The low resistance points without breakdown are mostly converted to breakdown points as the voltage increases.Special low resistance points, such as poor insulation of bulk materials, have a positioning ratio of around 50%, which is worthy of vigilance.

10.Interphase breakdown location

The difference with above is that the current should flow back to the bridge tester through another core.so, another core( interphase breakdown) shoul connect to the ground end of bridge tester.It might actually be that interphase breakdown and the shield breakdown coexistence, can try connect other core and the shield to ground end, the result is mostly: the breakdown and the phase and the shield breakdown is the same point.

11.treatment of no good insulated auxiliary.

It is similar in article 10, all phases and shielding may have been broken and auxiliary cable phase could not be found.treatment as follow:Use multimeter to select one phase insulation resistance larger as the auxiliary cable, and the reason is similar to the multi-point breakdown.If all burned into a whole, judge gold attribute short circuit , can only useother cables by parallel laying ,not yet, can only put temporary auxiliary cable.

12.Single core cable insulation defect location

The single-core cable is usually 35kV and above high voltage cable. The positioning connection as below.The biggest difference with multi-core cable is that the possibility of external interference influencing bridge balance is increased.Short metal sheaths at M, X, N, and Y points are effective, refer to section 14.High voltage cable spacing, should be equipped with large length short connection.

Pic 6 singal core cable connection diagram

13.high voltage cable sheath defect point location

High voltage bridge test is the effective way to locate HV cable defect point,connection as below.

Pic 7 sheath cable fault connection diagram

14.Interference type and elimination method

For accurate location of large-section cable, high sensitivity galvanometer is required. This instrument eliminates the interference of high-voltage source on the bridge galvanometer and greatly attenuates the external interference signal.However, some disturbance may affect the bridge balance.power frequency interference for Single - core cable location.Near the fault cable, there are usually other lines running, through the power frequency high current.the larger the area of the fault cable core and auxiliary cable, the larger the magnetic field induced interference.The multi-core cable does not affect the balance due to the small included area and the shielding effect of the metal protective layer.But locating the high voltage cable may cause too much interference to be balanced.Taking the location of defect point of high voltage cable sheath as an example, the improvement method is as follows: connect core of the fault phase and the auxiliary phase to ground end, or the cores are short-connected with each other at both ends, forming the reverse phase magnetic field, have obvious effect.

If the galvanometer still cannot be set to zero after the battery is replaced, the amplification module may be damaged. The amplification module should be replaced.

If canot be regulated to zero, the possible situation is:

1）No short circuit at the far end or wrong cable clamping

2）Bad connection for clamp，the two nippers of a clamp respectively introduce the current and potential signals,bad contact on either side will affect the balance.It is shown as the minimum deflection when the potentiometer is rotated to the start or end point, but cannot be balanced.check clamp, if the pole for bad contaction should wear off oxide layer of the pole .

3) Measuring cable break should be maintained.

4) sensitivity is too high, it is difficult to find the balance point, the sensitivity level should be adjusted according to the need , make the galvanometer pointer within + / - 25 grid.

5) The fault is in the terminal, the proportion that can be balanced should be less than 1‰ or greater than 999‰, which is beyond the range of the proportional potentiometer. In the process of balancing, the pointer goes to zero when the proportion reaches 1‰ or 999‰, but cannot be balanced.exchange the test clamp and adjust the proportional potentiometer to the other end, the pointer will go to zero and still cannot be balanced.When the wire core breaks at the fault point, it has a similar performance: because the wire break point has a much higher resistance than the wire core, it forms the representation of the bridge balance point at the end.