UNI-T UT278A Clamp Earth Ground Testers Instruction Manual

UT276A+
UT2713A+
Clamp Earth Ground Testers
Operating Manual

Preface

Thank you for purchasing the clamp ground resistance tester. Before using this tester, in order to avoid possible electric shock or personal injury, please read this manual thoroughly, especially the safety part below.

In any situation, pay special attention to safety when using this tester.

• The tester is designed, produced, and inspected according to IEC61010 safety standards.

• When measuring, high-frequency signal generators such as mobile phones should not be used near the tester to avoid errors.

• Pay attention to the label text and symbols on the tester.

• Before use, make sure that the tester and accessories are in good condition.

• Before starting up, press the trigger once or twice to ensure that the jaws are closed properly.

• Do not measure in a flammable place. Sparks may cause an explosion.

• Do not press the trigger or clamp any wires when turning on the tester.

• After the tester is turned on normally, the “OL 0” symbol is displayed, and the measured object can be

• Do not place or store the tester for an extended period of time under high temperature, humidity, condensation, or direct sunlight.

• Before replacing the batteries, make sure the tester is turned off.

• When the low battery symbol “ ” is displayed, please replace the batteries in time to avoid errors.

• The jaw contact surfaces must be kept clean and cannot be wiped with corrosives or rough objects.

• When opening the trigger, avoid the impact of the clamp, especially the jaw joint.

• When measuring the resistance, it is normal for the clamp jaws to make a slight noise. Distinguish it from the beeps for alarming.

• Pay attention to the measuring range and use environment specified by this tester.

• Do not measure the current above the upper limit of the tester.

• The use, disassembly, calibration, and maintenance of this tester must be performed by authorized personnel.

• If it is dangerous to continue using the tester, it should be immediately stopped and sealed, and then handled by a qualified organization.

• When it comes to the safety warning sign “ “, users should strictly follow the contents of this manual for safe operation.

Brief Introduction

A clamp ground resistance tester, also known as a loop resistance tester, is used to test grounding resistance. The tester, designed with a new black luxury screen, displays resistance and current on the same screen and has data storage, data access, data upload, alarm, automatic shutdown, and other functions. It is beautiful and upscale and has a wide range, high resolution, convenient operation, accurate, reliable, stable performance, and strong anti- interference ability. With the shockproof, dustproof, and moisture-proof structure, the tester is commonly used for telecommunications, electricity, meteorology, computer rooms, oil fields, electromechanical installation and maintenance, and industrial enterprises that use electricity as industrial power or energy. The clamp ground resistance tester is controlled by the microprocessor and can accurately detect the ground resistance. It uses a fast filtering technique to minimize interference.

Model differentiation

Range and Accuracy

Technical Specifications

UT278A+ resistance: 0.01-1200 0
UT278A+ current: 0.00mA-20.0A
Measurement Method| Mutual induction
Resistance Resolution| 0.001 0
Current Resolution| 0.01 mA (UT278A+)
Jaw Size| 55mmx32mm
0 + A Synchronous Display| Displays resistance and current on the same screen (UT278A+)
Display Mode| 4-digit LCD display, black screen design
LCD Size| 46mmx29mm
Tester Size| L/W/H: 285mmx85mmx58mm
Measuring Time| 2 times/second
Data Storage| 300 groups, “MEM” storage indication, “FULL” symbol indicates that storage is full
Data Access| “MR” symbol indicates when looking up data
Overflow Display| “OL” symbol indicates over-range overflow
Interference Test| Automatically identifies interference signals, the “NOISE” symbol indicates when the interference current is large
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Alarm Function| Alarm when the measured value exceeds the alarm setting value
Battery Voltage| Real-time display of battery power, reminding timely charging when battery voltage is low
Automatic Shutdown| Turns off after 5 minutes
Power Consumption| 50mA Max
Weight| 1180g (including batteries)
Working Temperature and Humidity| -10°C – 40°C, below 80%rh
Storage Temperature and Humidity| 20°C – 60°C, below 70%rh
Insulation Resistance| Above 20M0 (500V between circuit and case)
Withstand Voltage| AC 3700V/RMS (between circuit and case)
External Magnetic Field| <40A/m
External Electric Field| <1V/m
Applicable Safety Regulations| IEC61010-1 (CAT III 300V, CAT IV 150V, Pollution degree 2); IEC61010-031

Structure of Tester

1. Clamp Jaw
2. Trigger
3. LCD
4. POWER Key
5. MODE Key
6. AL Key
7. MEM Key
8. SET Key
9. HOLD Key
10. CALIBRATION LOOP: 10
11. CALIBRATION LOOP: 100

LCD Display

1. Jaw opening symbol
2. Alarm symbol
3. Greater than symbol
4. DC/AC symbol
5. Data access symbol
6. Data memory symbol
7. Noise symbol
8. Data hold symbol
9. Battery symbol
10. Resistance unit
11. Resistance value
12. Current unit
13. Number of storage groups
14. Current value

Description of Special Symbols

1. Jaw opening symbol: As a jaw is in the open state, the symbol shows. At this point, the trigger may be artificially pressed, or the jaws have been seriously polluted, and can no longer continue to measure.
2. “Er” Boot error symbol: Trigger may be pressed or jaw is open when the tester is turned on.
3. “ Low battery symbol: When the battery voltage is lower than 5.0V, the symbol shows, and the measurement accuracy cannot be guaranteed at this point. Batteries should be replaced.
4. “OL 0” symbol Indicates that the measured resistance has exceeded the upper limit of the tester.
5. “L0.010” symbol Indicates that the measured resistance has exceeded the lower limit of the tester.
6. “OLA” symbol: Indicates that the measured current has exceeded the upper limit of the tester.
7. Alarm symbol: When the measured value is greater than the critical value of the alarm setting, the symbol flashes, and the tester beeps intermittently.
8. MEM Data storage symbol: Flashes when saving data.
9. MR Data access symbol: Displays when viewing data, and the number of stored data is displayed at the same time.
10. NOISE symbol: When there is a large interference current in the ground loop being tested, the symbol flashes, and the tester beeps. At this point, the measurement accuracy cannot be guaranteed.

Measuring Principle

The basic principle of the ground resistance measurement of the clamp ground resistance tester is to measure the loop resistance. See below. The jaw section of the tester consists of a voltage coil and a current coil. The voltage coil provides the excitation signal and induces a potential V on the circuit under test. Under the action of the potential V, a current I will be generated in the circuit under test. The tester measures V and I and uses the following formula to obtain the measured resistance R.

Operation Method

1. Turn On/Off
   ****| When turning on the tester, DO NOT press the trigger, open the jaws, or clamp any wire.
   ---|---
   Only press the trigger, open the jaws or clamp the measured wire after “OL 0” shows.
   Before starting up, press the trigger once or twice to ensure that the jaws are closed properly.
   When turning on the tester, keep it in the natural state of rest, do not flip it, and do not apply external force to the jaws, otherwise, the measurement accuracy cannot be guaranteed.

Press the “POWER” key to turn it on/off. When the tester is turned on, it automatically calibrates, displays “OL CT’, and enters the resistance measurement mode. If there is no normal self-calibration, “Er” shows, indicating that the boot error occurs. Common causes include the jaws are not closed properly, a wire is clamped during startup, etc.

The tester automatically turns off after 5 minutes. There will be 30 seconds of blinking display before automatic shutdown. Press the “POWER” key to delay the shutdown time by 5 minutes.

2.  **Battery Voltage Check**

If the LCD shows a low battery symbol “ it indicates that the battery power is low. Please replace the batteries to ensure measurement accuracy.

3.  **Resistance Test**

| When users think the ground value is abnormal, the calibration loop can be used to check whether the tester is normal. The calibration loop has two resistance values of 10 and 100.
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Press the trigger, open the jaws, damp the circuit under test, and read the resistance value.
Resistance and earth leakage can be measured simultaneously. Press the MODE” key to switch between resistance measurement mode and resistance + current measurement mode.
If ‘Kt 0″ shows, it indicates that the measured resistance exceeds the upper limit of the tester.
If ‘L0.01 0″ shows, It indicates that the measured resistance exceeds the lower limit of the tester.
Resistance + current mode:
The measured resistance is 0.510.
The number of storage groups is 1 and the Measured loop resistance is O.00mA.

4.  **Alarm Setting**

After starting up, short press “AL” key to turn on or off the alarm function, long press “SET” key to set the resistance alarm value, press “AL” key and “MEM” key to change the current value, short press “MODE” key to switch the alarm mode, and long press “SET” key to save and exit. When the measured resistance is greater than the critical value of the alarm setting and the alarm is turned on, ‘c) ” symbol flashes, and the tester beeps intermittently. The maximum alarm value of the ground resistance is 2000. As shown below.

5.  **Data Hold**

After the measurement is stable, short press the “HOLD” key to hold and save the current data. Short press the “HOLD” key again to exit the hold mode. As shown below:

6.  **Data Storage/Access/Deletion**

When the measurement is completed, short press the “HOLD” key to store data, and the “MEM” symbol flashes once. If the memory is full, the tester flashes to display the “MEM” symbol. Short press the ‘MEM’ key to enter data access and the “MR” symbol is displayed. Press the “AL” or °sun key to view the data corresponding to the group number with a step value of 1. Short press the “MEM” key again to exit. The left figure below shows that the “MEM” symbol flashes once when the data is stored, and the number of storage groups is 1. The right figure below shows the data access mode, and the number of access groups is 1.

In the data access state, long-press the “MEM” key, and then press the “POWER” key to delete the stored data.

Field Application

1. Multi-Point Grounding System
   As for the multi-point grounding system (such as electricity transmission tower grounding system, communication cable grounding systems, certain buildings, etc.), they are connected by overhead ground wires (shielding layer of communication cables) to form a grounding system. When the tester is used for measurement, its equivalent circuit is shown in the figure below.
   RO is the equivalent resistance of the grounding resistance of all other towers in parallel.

Although, from the strict grounding theory, RO is not the usual parallel value in the sense of electrical engineering (slightly higher than its IEC parallel output value) due to the existence of so-called “mutual resistance”. But, since the grounding hemisphere of each tower is much smaller than the distance between the towers, and, after all, the number of grounding points is large, RO is much smaller than RL. Therefore, it is reasonable to assume R0=0 from an engineering perspective. In this way, the measured resistance should be Rl.

2. Limited Point Grounding System For some towers, five towers are linked with each other through overhead ground wires. Besides, the grounding of some buildings is not an independent grounding grid, but several grounding bodies connected with each other through wires.
   Under such circumstances, regarding the above RO as 0 will yield a greater error on the measurement result. For the same reason mentioned above, the impact of the mutual resistance is ignored and the equivalent resistance of the grounding resistance paralleled is calculated in the usual sense. In this way, for the grounding system of N (N is small but greater than 2) grounding bodies, N equations can be listed:
   Where: R1, R2…… RN is grounding resistance of N grounding bodies.R1T, R2T……………… RNT is the resistance measured with the tester in different grounding branches.

It is a nonlinear equation with N unknown numbers and N equations. It indeed has a definite solution, but it is very difficult to solve the issue artificially, even impossible when N is large.

Users can use a PC to get the solution through the Limited-Point Grounding System Calculation software (optional) of our company.

In principle, in addition to ignoring the mutual resistance, this method does not have the measurement error caused by neglecting RO.

However, users need to pay attention to that: in response to the number of the grounding bodies mutually linked in the grounding system, it is necessary to measure the same number of testing values for calculation, not more or less. And the software would output the same number of grounding resistance values.

3. Single-Point Grounding System

From the measuring principle, the tester can only measure loop resistance, not single-point grounding. However, users can use a testing line and the earth electrode near the grounding system to artificially create a loop for testing. The following presented are two methods for single-point grounding measurement by the tester These methods can be applied to occasions beyond the reach of the traditional voltage-current testing method.

(1) Two-Point Method

As shown in the figure below, in the vicinity of the measured grounding body RA, find an independent grounding body of better grounding state RB (for example, a nearby water pipe or building). Connect RA and RB with a testing line.

The resistance value measured by the tester is the series value of the two grounding resistance and the resistance of the testing line.
RT = RA + RB + RL
Where: RT is the resistance value measured by the tester.
RL is the resistance value of the testing line.
RL can be measured with the tester by connecting the testing line end to end.
So, if the measurement value of the tester is less than the allowable value of the grounding resistance, the grounding resistance of the two grounding bodies is qualified.

(2) Three-Point Method
As shown in the figure below, in the vicinity of the measured grounding body RA, find two independent grounding bodies of better grounding state RB and RC.
First, connect RA and RB with a testing line. Use the tester to get the first reading Rl.

Second, connect RB and RC as shown in the figure below. Use the tester to get the second reading R2.
Third, connect RC and RA as shown in the figure below. Use the tester to get the third reading R3.

In the above three steps, the reading measured in each step is the series value of the two grounding resistance. In this way, the value of each grounding resistance can be easily calculated:

As: R1= RA + RB, R2 = RB + RC, R3 = RC + RA
So: RA = (R1 + R3 – R2) ÷ 2

This is the grounding resistance value of the grounding body RA. To facilitate the memory of the above formula, these three grounding bodies can be viewed as a triangle; then the measured resistance is equal to the sum of the resistance values of the adjacent edges minus the resistance value of the opposite side and then divided by 2.

The grounding resistance values of the other two grounding bodies are:

RB = R1 – RA

RC = R3 – RA

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