UNI-T UT276A clamp earth groud testers User Manual

UT276A/278A
Operating Manual
lamp Earth Groud Testers

Safety Instructions

Thank you for purchasing the Clamp-on Ground Resistance from our company and for the full use of the product, please:
– – -Read the user manual carefully.
– – -observe the operation notes listed in the manual.

• Under no circumstances shall fail to pay attention to the safety of using the clamp meter.
• Pay attention to the stipulated measurement scope and operation environment.
• The instrument shall apply to ground resistance testing instead of the current of power frequency power wire, or it will affect the clamp’s magnetic property and the accuracy of ground resistance testing finally.
• Pay attention to the labeled words on the surface board and backboard.
• Before starting it, pull the trigger once or twice to confirm the jaw of the clamp can close or open freely.
• When starting it, don’t pull the trigger and clamp any wires.
• Clamp the objects being tested only after displaying the sign ·OL o” after starting the instrument.
• The contact surface of the clamp jaw shall keep clean and refrained from cleaning by corrosive and coarse matters.
• Refrain from any shock on the clamp meter, especially the junction surface of the clamp jaw.
• It is strongly recommended to use our company’s explosion-proof Clamp-on Ground Resistance tester on dangerous occasions.
• The explosion-proof product is prohibited to dismantle and replace the battery in dangerous places.
• The clamp meter shall sound sporadically with a slight “hum” in measuring the resistance, which is normal and shall be distinctive from the warning sound “beep–beep–beep–“.
• Do not exceed the upper limits of the clamp meter in measuring the wire’s current.
• Take out the battery if do not use it for a long time.
• Dismantling, calibration, and maintenance of the clamp meter shall be conducted by the authorized qualified personnel.
• If any danger might be resulted in the continuation use by the inner reason of the meter, stop using it immediately and pack it to the authorized qualified institution to address the problem.
• The contents labeled with ” * ” in the user manual shall be only applicable to UT278A.

Product Overview

The series of clamp-on ground resistance testers represent a significant breakthrough of the traditional ground resistance measurement techniques and apply widely to grounding resistance measurement in electricity, telecommunication, meteorology, oilfield, architecture, and industrial electric equipment.
In measuring the ground system with loop, the series of clamp-on ground resistance tester don’t need to disconnect the ground down lead and any auxiliary electrode, safe and fast, easy to use.
The series of clamp-on ground resistance tester can measure the grounding problems with the traditional methods and apply to occasions traditional ways can’t measure as the tester measure the composite value of the grounding body resistance and grounding down lead resistance.
UT276A, 278A clamp-on ground resistance tester is a medium and high-end Product in the UT270 clamp-on ground series, its uniqueness is:
No long time waiting for start-up, enter into testing immediately.
The only clamp-on ground resistance meter with an RS232 port uploads the stored data to the PC.
A breakthrough in the relay self-checking method, adopting the advanced arithmetic and digital IC processing techniques.
Solving the bulkiness of old products, and meeting the portable meter property.
Add the sound-light alarm function with “beep–beep–beep–”
Add the recognition indication function of interference signal with “beep–beep-­ beep–“.
Measurement scope has increased to 0.010-12000.
With 99-group storage data, lower energy consumption, and with a working current of less than 50mA in the maximum start-up.

Unpack and Check

Unpack and take out the instrument, please check carefully if the following attachment is complete or intact if any lack or damage were found, please contact your supplier.

Product Specification

1. Series Types
   T ypes| Measuring resistance| Measuring current| Storing data| RS232 port| Warning function
   ---|---|---|---|---|---
   UT276A| Yes| No| Yes| Yes| Yes
   UT278A| Yes| Yes| Yes| Yes| Yes
2. Range and Accuracy
   Measurement mode| Measurement Scope| Resolution| Accuracy
   ---|---|---|---
   Resistance| 0.010 Ω-0.099 Ω| 0.001Ω| ±(1%+0.01Ω)
   0.10Ω0-0.99Ω| 0.01Ω| ± (1%+0.01Ω)
   1.0Ω-49.9Ω| 0.1Ω| ± (1%+0.1 Ω)
   50.0Ω-99.5Ω| 0.5Ω| ± (1.5%+0.5 Ω )
   100Ω -199 Ω| 1Ω| ± (2%+ I Ω )
   200 Ω -395 Ω| 5Ω| ± (5%+5Ω )
   40Ω-590Ω| 10Ω| ± (10%-1-10Ω )
   600 Ω -880 Ω| 20Ω| ± (20%+20Ω )
   900Ω -1200 Ω| 30Ω| ± (25%-1-30Ω )
   *Current| 0. 00ma-9. 95mA| 0.05mA| ± (2.5%-F lmA)
   10. 0mA-99. 0mA| 0. lmA| ± (2.5%-F5mA)
   100mA-300mA| lmA| ± (2. 5%+10mA)
   0.30A-2.99A| 0.01A| ± (2.5%-1-0. 1A)
   3.0A-9.9A| 0. 1A| ± (2.5%-FO.3A)
   10.0A-30. 0A| 0. 1A| ± (2.5%-1-0.5A)

Technical Specification

on-off status.
Scope of alarm critical value setting| Resistance : 1-199Ω ; current : 1-499mA
Power| 6VDC (4 AA dry alkaline batteries )
Working temperature and humidity| 0°C-55°C; 10%RH-90%RH
LCD| 4bit LCD digit display, dimension ( 47mm x 28.5mm)
Clamp jaw dimension| 65mm x 32mm
---|---
Clamp jaw stretch size| 28mm
Clamp surface mass| 1160g (inclusive of battery)
Clamp surface dimension| Length, width, and height 285mm x85mmx 56mm
Protection rate| Double insulation
Structural characteristic| Clamp CT
Shift| Full automatic shift
Outside magnetic field and electric field| <40A/m; <1V/m
Single measurement time| 0.5s
Resistance measurement frequency| >1KHz
Frequency of the current being measured| 50/60Hz (automatic)

Clamp Meter Structure

1. Long clamp head: 65x32mm

2. Trigger: control the closure and opening of the clamp jaw

3. Function keys
     key: (described as key POWER) power on and off/exit
   SAVE key: data inquiry/wipe data AL alarm function key: alarm function on/off
   Left/right arrow key: resistance/current mode conversion

4. HOLD key: hold/unhold display/store

5. LCD

6. RS232 port

Liquid Crystal Display

Liquid Crystal Display

Instruction for Special Symbol

1. clamp jaw opens symbol. It shall display when the clamp jaw is open. It indicates that it might be an artificial pull-off of the trigger, or the clamp jaw has been too dirty to measure, requiring clamp jaw cleaning.
2.  “Er” : power-on error symbol. It might be caused by the dirtiness of the clamp jaw, please clean the clamp jaw surface.
3. battery low voltage symbol. When the battery voltage is lower than 5.3\/, the symbol would appear and replace the battery in time or the measurement accuracy couldn’t be guaranteed.
4. “OL Q” symbol, means the resistance being measured exceeds the upper limit of the clamp meter.
5. “L0.010” symbol, means the resistance being measured exceeds the lower limit of the clamp meter.
6.  “OLA” symbol, means the current being measured exceeds the upper limit of the clamp meter.
7. Alarm symbol, when the measured value exceeds the alarm critical value, it will flicker and the meter will sound sporadic “beep–beep–beep–“
8. [MEM] full stored data symbol, when the stored data has reached 99 groups and can’t store data anymore, the symbol] MEMl shall flicker.
9. MRI Data inquiry symbol, shall appear in inquiring data and indicate the No. Of stored data.
10. [NOISE]Symbol, when there is quite a big interference current to the earth loop being measured, the symbol shall flicker and the meter sounds sporadic “beep–beep– eep–“, the measurement accuracy of the time can’t be guaranteed.

Display Demonstration

(1 ), as the clamp jaw is open, it can’t measure.
(It might be caused by the dirtiness of the clamp jaw, please clean the clamp jaw surface.)| (2), power-on error indication Er(Error) (It might be caused by the dirtiness
of the clamp jaw, please clean the clamp jaw surface.)
---|---
|
(3), the resistance of the loop being measured <0.01Ω| (4), the resistance of the loop being measured is 5.1Ω
|
(5), • resistance of the loop being measured is 2.1Ω,
• hold the present measurement value: 2.1Ω,
• store automatically as the No.08 group data.| (6) • inquiry the No.26group stored data
• resistance of the loop being measured is: 0.028 Ω
|
(7). • start alarm function and the current being measured exceed the set
alarm critical value
• battery low voltage display and the measurement accuracy can’t be guaranteed.
• current being measured is: 8.40A
• hold the present display value
• store the present hold value as the No 37 group data.| (8), • inquiry No 08 group stored data
• resistance being measured is: 30.0Ω
• the data is measured under a strong interference signal
|

Operation Methods

1, Power-on
A\ Notes:

• When starting it, don’t pull the trigger, open the clamp jaw and clamp any wires.
• pull the trigger, open the clamp jaw and clamp wires only after power-on and display the ·Olo”
• Before power-on, pull the trigger once or twice to confirm the clamp jaw can close or open freely.

When power-on, keep the clamp meter static, no turning and no force on the clamp jaw, or the measurement accuracy can’t be guaranteed.
Press the key POWER, check the LCD automatically and symbols shall appear as shown in picture1. Meanwhile, the meter calibrates automatically, after power- on, it shall display “OL Q” and enter automatically into the resistance measurement mode, as shown in picture2. If automatic calibration under improper power-on, the meter will display the symbol “Er” indicating an error occurs in power-on, as shown in picture3.

The power-on error might be the result of the bad closure of the clamp jaw or the dirt on the clamp jaw surface, please clean it.
If after the power-on self-check, it appears a big resistance instead of “Oro”, as shown in picture 4. It might be a result of the bad closure of the clamp jaw or of the dirt on the metal surface, please clean it.

If the result is correct in detection by the test ring, it represents that the clamp meter has a big error just in measuring high resistance (>100Ω), and remains original accuracy in measuring low resistance, the user can use it with assurance.

2, Power-off
Press key POWER to power off after power on.
After working for 5 minutes, the LCD shall flicker continuously for about 30 seconds and then power off to reduce battery consumption. Press key POWER to delay power -off, the clamp meter shall continue working.
Under the\HOLD] status, press key/HOLD/to exit the status and press key POWER to power off.
Under setting alarm critical value status, it is required to press key POWER or key AL 3 seconds to exit the status and press key POWER to power off.
3, Resistance Measurement
If display “OLΩ ” after power-on self-check, you can measure the resistance.
Pull the trigger and open the clamp jaw, clamp the loop being measured to read the resistance value.

Users are required to test by random test ring as shown in picture 5.
The displayed value should be the same as the standard value on the test ring ( 5.10), which is the value under temperature 20C.it is normal if there is a decimal difference between them, for instance, the standard value of the test ring is 5.10; the displayed value is 5.00 or 5.20.
Display “OLΩ ” means the resistance being measured exceeds the upper limit of the clamp meter, as shown in picture 2.

Display “L0.01 Ω” means the resistance being measured exceeds the lower limit of clamp meter, as shown in picture6.![UNI T UT276A clamp earth groud testers

• 18](https://manuals.plus/wp-content/uploads/2022/07/UNI-T-UT276A-clamp- earth-groud-testers-18.png)

Flicker display symbol with sporadic “beep–beep–beep–” it means the resistance being measured exceeds the resistance alarm critical value.
Under the\HOLD/ status, press key[HOLD]to exit the status and press key POWER to power off.
Under the\ MR’ status, press key SAVE to exit the status and continue to measure.
Under setting alarm critical value status, it is required to press key POWER or key AL 3 seconds to exit the status and continue to measure.
*Under the current test mode, press key Ω covert to the resistance test mode.

Current measurement

Notes:
the meter is mainly used for the ground resistance tests and the current measurement shall be only applicable to the current test on the ground wire but the power line affects the magnetic property of the clamp head and finally affect the ground resistance test accuracy.
After the power-on self-check, the clamp meter shall enter into the resistance measurement mode. After display the “OLΩ”, press key A, clamp meter enters into current measurement mode, displaying “AC 0.00mA”, as shown in picture 7.
Pull the trigger, open the clamp jaw, and clamp on the wire to read the current value.
“OLA” on the clamp meter, it means the current being measured exceeds the upper limit of the clamp meter, as shown in picture 8.
Flicker display symbol with sporadic “beep–beep–beep–“, means the current being measured exceeds the current alarm critical value.

Under the] HOLD] status, press key HOLD to exit the status and continue to measure.
Under the MR  status, press the key SAVE to exit the status and continue to measure.
Under setting alarm critical value status, it is required to press key POWER or key AL 3 seconds to exit the status and continue to measure.
Under the resistance test mode, press key A covert to the current test mode.
5, Data hold/unlock/store
Under the test mode, press key HOLD to hold the present value, and when displaying the symbol HOLD, the hold value could be set as a group of data and numbered automatically orderly, and stored. Press key HOLD to cancel a hold, the symbol I HOLDI would disappear and continue to measure. Operate as the above steps; you can store 99 groups of data. If the memory is full, the symbol [ MEM] will flicker.
As shown in picture 9, hold the measured resistance 0.0160, and store it as the No 01 group data.
As shown in picture 10, hold the measured current 278mA and store it as the No 99 group data, when the memory is full, the symbol MEMI will flicker. 

Under the data inquiry mode, press key SAVE to exit the data inquiry and hold, and store the data.
Under setting alarm critical value status, it is required to press key POWER or key AL 3 seconds to exit the status and continue to hold and store data.
The stored data would not lose when power on is after power off the meter.

6. Data Inquiry
Press key SAVE to enter into the stored data inquiry mode and display the No 01 group stored data, as shown in picture 11. Then press the right arrow key, inquiry
upward stored data or press the left arrow key to inquiry downward all stored data.
If no stored data, it will display as shown in picture 12.

Under setting alarm critical value status, it is required to press key POWER to exit the status and press key SAVE to enter into the stored data inquiry mode.
7, Alarm Function Setting
Under the test mode, press key AL to open or close the alarm function.
Under the test mode, press the key AL for 3 seconds to enter into the alarm critical value setting function, and the highest digit number shall flicker; set the highest digit as shown in pictures 13 and 14. Press key AL to convert the number on high digit and low digit. When the present digit number is flickering, press the right and left arrow key to change the number “0,1, ···9” after setting all numbers, press key AL 3 seconds to confirm the set alarm critical value. It means it is able to operate the alarm function and then back to the measurement mode automatically. If the load is bigger than the alarm critical value, the meter shall display the alarm symbol with sporadic sounds “beep–beep–beep–“.
During the set process, press key POWER to exit the alarm critical value setting function and back to the measurement status, without changing the previous set value.
Under the data inquiry mode, press the key SAVE to exit the mode and to set the alarm critical value.

8 Alarm Critical Value Inquiry
Under the test mode, press key AL 3 seconds to inquire the alarm critical value, and the highest digit number shall flicker and each time inquired is the previous set value. Press key AL 3 seconds or key POWER to exit and check, back to measurement status. As shown in picture 15, the previous set resistance alarm critical value is 20Ω.

9. Data Wipe
Under the data inquiry mode, press key SAVE+POWER, and automatically wipe all stored data, after it, the meter shall display as shown in picture 12, and all wiped data shall not able to recover.
10 Data Uploading to Computer
Connect the meter host with a computer with the RS233 communication wire deployed with the meter and the meter shall power on and operate software, select history check, and read, store and print the history data. The history data could be saved as Txt text or in Excel format.

Measurement Principles

1. Resistance Measurement Principle
   The basic principle of the clamp-on ground resistance meter in measuring the ground resistance is measuring the loop resistance (detailed in appendix).
   Refer to the following picture; the clamp jaw of the clamp is composed of a voltage coil and current coil. The meter measures E and I, and know the measured resistance R by the following formula. R=E/1

2. current Measurement Principle
   The basic principle of the UT278A clamp-on ground resistance meter is the same with that of current transformer measurement.

The AC current I of the measured wire, passing through the current magnetic ring and current coil on the clamp jaw, produce the induction current I1, which could be measured by the clamp meter, and know the measured current I by the following formula: I=n·I Of which: n is the transformer ratio coefficient between secondary and primary coil.

Ground Resistance Measurement Method

1. Multi-point Ground System
   Multi-point ground systems (ex. Electricity Transmission System tower grounding, communication cables grounding system, some buildings, etc.) are connected by the overhead ground wires and form into the ground system. As shown in the following picture, when measuring by the clamp meter, the equivalent circuit shall be shown as follows: Of which, R1 is the estimated ground resistance and RO is the equivalent resistance of all other towers’ ground resistance in parallel.
   From the aspect of the strict ground theory, as the existence of the so-called “mutual resistance”, RO, in general, electrical engineering meaning, is not the parallel value (slightly bigger than that of general electrical engineering meaning), but the ground hemisphere of each tower is much smaller than the distance between towers and the great number of ground points, RO is much smaller than R 1. Thus, it is reasonable to speculate that RO=O from the engineering aspect, and the resistance we measured should be R1.
   After many comparison tests between tests under different environments, occasions, and traditional ways, the above speculation is fully reasonable.

2. Limited Point Ground System
   The circumstance is quite common, for instance, some towers are connected with each other by the overhead ground wire, and some buildings’ ground is not an independent ground net but several ground bodies connected with each other by wires.
   Under this circumstance, regarding RO in the above picture as O shall result in great errors.
   Owing to the same reason, we neglect the impact of mutual resistance and calculate the equivalent resistance of ground resistance in parallel with the general meaning. Thus, for a ground system of N C N is small, but bigger than

2. ground bodies, we can list out the N formula: Of which, R1, R2, ······.RN is the ground resistance of N ground bodies we need to solve.
   R 1 T, R2T, ··· ···RNT are respectively the resistance measured on each ground branch circuit by clamp meter.
   These is nonlinear equations with N unknown and N equations. It has a certain solution, but it is difficult to solve by manual operations and even impossible when N is quite big.
   Purchasing the limited point ground system calculating program software from our company, users shall be able to solve it by using an office computer or laptop.
   Principally, except for the neglect of mutual resistance, the method would not result in any measurement error brought by neglecting RO.
   Users need to pay attention that: the number of connected ground bodies in your ground system shall be exactly equal to that of the measurement value for program calculation and the program shall output the same number of ground resistance values.

3. Single Point Ground System
   From the test principle, the UNIT series clamp meter can only be used to measure loop resistance but the single-point ground. However, users can make a loop to test by using a test line and ground polarity near the ground system. Two methods of measuring single point ground by clamp meter are introduced in the following and the method could be applicable to occasions that the traditional voltage-current method fails to measure.

4. Two -point method
   As shown in the following picture, find an independent ground body RB with good ground near the ground body RA being measured (ex. adjacent metal running water pipe, metal fire hydrant, building, etc.) and connect RA with RB by a test line. As resistance measured by the clamp meter is the parallel value of 2 ground resistance and test line resistance,
   RT=RA+RB+RL.
   Of which, RT is the resistance measured by the clamp meter and RL is the resistance of the test line.
   Resistance RL could be measured by clamp meter by connecting the test line end to end. If the measurement value of the clamp meter is smaller than the allowable value of ground resistance, the ground resistance of the two ground bodies shall be qualified.

5. Three-point method
   As shown in the following picture, find two independent ground bodies RB and RC near the ground body RA being measured. If no existing independent ground body, make 2 auxiliary ground polarities with a 3-5m distance between them and about 0.7m depth each.

Step1, connect RA and RA with a test line, as shown in the following picture, and read the first data R1 by clamp meter.

Step2, connect RB and RC with a test line, as shown in the following picture, and read the second data R2 by clamp meter.Step3, connect RC and RA with a test line, as shown in the following picture, and read the third data R3 by clamp meter. Of the above three steps, the reading of each step is the parallel value of two ground resistance, so each ground resistance value can be easily calculated:
As: R1 =RA+RB R2=RB+RC R3=RC+RA
So: RA= (R1+R3-R2) :2
This is the ground resistance value of ground body RA. To memorize the above formula easily, the 3 ground bodies could be regarded as a triangle and the resistance being measured equals the sum of adjacent lines’ resistance minus the resistance of the opposite line, and then divided by2.
The ground resistance values of the other two reference ground bodies are: RB=R1-RA RC=R3-RA

On-site Application

Application of Power System

1. Measurement of the power line tower ground resistance
   Generally, the power line tower ground form into a multi-point ground system and clamp on the ground down lead by UNIT series clamp meter, the ground resistance value of the branch circuit could be measured.

2. Measurement of transformer neutral point ground resistance
   In measuring the transformer neutral point ground resistance, if data fluctuates greatly, power off and test.
   There are two circumstances for the transformer neutral point ground: form into the multi-point ground system if repetitive ground exists; single point ground measurement if no repetitive ground.
   In measurement, if the clamp meter display “L 0.010″°, it is possible that more than 2 ground down leads of the same tower or transformer are connected underground. It is needed to maintain only one ground-down lead, and unfasten others.

3. Application of power plant and substation
   UNIT series clamp meter can test the loop contact and connect condition and can measure the connection between equipment in the station with the ground net by a test line. The ground resistance could be measured as the single- point ground.

Application of Telecommunication System

1. Ground resistance measurement of the floor computer room
   The computer room of the telecommunication system is always established on the upper floor of the building, difficult to measure by tramegger. While it is quite easy for the UNIT series clamp meter to test, just connecting a test line with a fire hydrant and the ground polarity being measured (each computer room is equipped with a fire hydrant) and measuring the test line with the clamp meter.
   Clamp resistance value=computer room ground resistance +test line resistance +fire hydrant ground resistance If the fire hydrant ground resistance is quite low, the computer room ground resistance clamp resistance value- test line resistance

2. measurement of the computer room and launching tower ground resistance
   Generally, the computer room and launching tower ground form into a two-point ground system, as shown in the following picture. If the measurement value of the clamp meter is lower than the allowable value of ground resistance, the ground resistance of the computer room and launching tower shall be qualified. If the measurement value of the clamp meter is higher than the allowable value, please measure it as the single-point ground.

3. Application of the building lightning protection ground system
   If the ground polarities of the building are independent, the ground resistance measurement of each ground polarity shall be referred to in the following picture:

4. Application of the gas station ground system
   Under the explosive gas atmosphere, equipment or infrastructure such as gas stations, oil fields, oil chutes,s, etc. shall be equipped with the explosion prevention product.
   In accordance with the JJF2-2003 Test Code for Static Protective Ground Device, gas stations shall measure ground resistance and connection resistance of the following infrastructure. The equipment used in measurement shall comply with the requirements in GB3836-2000 Electrical Apparatus for Explosive Gas Atmospheres.

explosion-proof mark being ExiaII BT3. The clamp can apply to a flammable and explosive atmosphere.

1. Oil tank and loading point ground resistance measurement
   As shown in the above picture, in the gas station ground system, oil tank ground polarity A is connected with oil-filling, and the loading point ground polarity C is an independent ground polarity. Find another independent ground polarity as auxiliary ground polarity B (ex. running water pipe), measure R1, R2, and R3 in accordance with the three-point method by clamp meter.
   We can calculate that:
   Oil tank ground resistance: RA= (R1+R2-R3):2
   Loading point ground resistance: RC=R2-RA
   Auxiliary polarity ground resistance: RB=R1-RA
   Notes: in measuring R 1, there should be no leading wire between BC and AC and by analogy to R2 and R3.

2. Measurement of the oil-filling ground resistance
   As shown in the above picture, find a ground polarity independent from the oil-filling machine, for instance, loading point polarity. Connect them with the test line and read RT with the clamp meter. We can calculate that:
   Oil-filling machine ground resistance: R=RT-RC
   Of which, RT is the resistance value measured by the clamp meter, and RC is the loading point ground resistance.

3. Measurement of the oil-filling machine oil hose connection resistance

Connect the oil stopper with the oil-filling machine by a test line and measure the reading RT by clamp meter, then calculate:
Oil-filling machine oil hose connection resistance: R=RT-RL
Of which, RT is the resistance value measured by the clamp meter, and RC is the test line resistance.

Notes about Ground Resistance Measurement

1. If users find a great difference after conducting a comparative test between the series product and the traditional voltage/current method, please pay attention to the following notes:
   (1 ). Find if tripping is using the traditional voltage and current method (if disconnecting the ground body is measured from the ground system). If no tripping, the measured ground resistance value is the parallel value of ground resistance of all ground bodies.
   It is meaningless to measure the parallel value of ground resistance of all ground bodies, as our purpose of measuring ground resistance is to compare it with an allowable value stipulated by relevant standards, judging whether the ground resistance is qualified.
   For instance: the ground resistance allowable value stipulated in the GB50061-97 Code for Design of 66kV or Under Overhead Electrical Power Transmission Line is designed for Each Tower. It is clearly stated in the standard that: the ground resistance of each tower refers to the resistance value measured after disconnecting the electric connection of the ground wire from the ground body. If the ground body has not been disconnected electric connection of the ground wire from the ground body, the ground resistance measured shall be the ground resistance of multi-towers in parallel.
   The stipulation is clearly stated.
   As mentioned above, the measured result of the series clamp meter is the ground resistance of each branch circuit, under the condition of good conductivity of the ground wire; it is the ground resistance of a single ground body.
   It is evident that under the circumstance, it is no comparability between the measurement results tested by the traditional voltage current method and the series clamp meter. As the object being measured is different, the evident difference between measurement results is reasonable.
   (2). The ground resistance value measured by the series clamp meter is the composite resistance of the ground branch circuit, it includes the contact resistance, lead resistance, and ground body resistance of the branch circuit to the public ground wire. The value measured by the traditional voltage current method in tripping is only the ground body resistance.
   Clearly, the former measurement value is greater than the last one, and the difference reflects the amount of contact resistance between the branch circuit and public ground wire.
   It is required to specify that ground resistance stipulated in the national standard concludes the ground lead resistance. Noun terms in the DL/T621-1997 AC Electric Equipment Ground states that: the sum of ground resistance of polarity or natural polarity and ground wire resistance is called the ground resistance of ground equipment.
   The stipulation is equally stated clearly as in term of lightning prevention, the wire resistance and ground body ground resistance is equivalent.

2. Selection of the Measurement Point
   In some ground systems, as shown in the following picture, you should select a correct measurement point of measure or get different measurement results.

In measuring at point A, the branch circuit measured has not formed into a loop and the clamp meter displays “OLΩ”, and replaces the measurement point.
In measuring at point B, the branch circuit being measured is a loop formed by metal conduct and clamp meter display “L 0.01Ω” or the resistance value of metal loop is just a few hundredths, quite small, and should replace the measurement point.
In measuring at point C, the branch circuit being measured is its ground resistance value of it.

Appendix: loop Resistance Analysis
Loop resistance includes ground resistance value of point A to the ground, the resistance of ground down lead metal conduct, the resistance of metal over head wires, connection resistance between the ground down lead and metal over head wires (contact resistance), the composite resistance value of point B to the ground, namely the parallel value. The ground net A and B have not been connected underground, requiring different ground nets.

If the parallel value of ground A and B loop measured by the 2-point method is 5.0Ω: RA+RB+R overhead wire +R ground down lead=5.00, as over head wires and ground down lead are metal conduct with low resistance, which can be neglected, the real ground resistance value of the ground net A, B in parallel to ground shall definitely <2.5Ω, then we can judge if the real ground resistance value of the ground net A, B in parallel is qualified. If the real ground resistance value of the ground net A, and B in serial is smaller than the required value of engineer standard, the ground net A, and Bis are all qualified.
The loop resistance test requires that the ground net A and B are different and disconnected from each other underground. If the ground net A and B are connected underground, the metal loop resistance value measured by the meter shall be very small, about a few hundredths Ω, not the ground resistance. Thus, the meter is easy to detect the metal loop connection resistance value and equal potential resistance value and is applicable to detect the connection and welding between metal loops.
If there are no overhead wires between the ground net A and B, namely no metal connection on the ground, the ground net A and B is single point ground and shall be tested as the single point ground.

The manual information is subject to changes without prior notice!

LRNI-TFEIND TECHNOLOGY (CHINA) CO., LTD.
No6, Gong Ye Bei 1st Road,
Songshan Lake National High-Tech Industrial
Development Zone, Dongguan City,
Guangdong Province, China
Tel: (86-769) 8572 3888
http://www.uni-trend.com
P/N:110401104578X
DATE:2018.06.26
REV.3

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