UNI-T UT256B Operating Manual

I. Overview

UT256B is a safe and reliable AC/DC digital clamp meter in 3 5/6 digit with stable performance. The circuit design adopts a large-scale integrated circuit Σ/Δ analog-digital converter as the core, and features an over-load protection circuit in the whole range and a unique appearance design, making it a dedicated electro-technical instrument. It is used to measure AC/DC current, AC/DC voltage, resistance, circuit on-off, diode, capacitance, etc., and can also measure induced voltage.

The Operation Instruction includes relevant safety information and warnings. Please read relevant contents carefully and strictly observe all warnings and notices.

? Warning: Before using the clamp meter, please read relevant "Safe Operation Criterion".

II. OOBA (Out Of box Audit)

Open the packing box and take out the instrument. Please carefully check whether or not the following accessories are missing or damaged:

1. Operation Instruction: One copy
2. Probe: One pair
3. Cloth bag: One piece

Please contact your supplier in case of any lack or damage.

? Warning: In case of probe replacement, the probes replaced must meet the following or higher specification: double insulation CAT III 600V/CAT II 1000V 10A.

III. Safety Operation Criterion

Please pay attention to the "Warning Signs and Sentences". Warnings mean the situations and behaviors that endanger the user or cause losses to the instrument or the equipment to be measured.

The instrument is designed and produced in strict accordance with GB4793 Safety Requirements for Electronic Measuring Apparatus and IEC61010-1 and IEC61010-2-032 safety standards, and complies with the safety standard of double insulation, over-voltage CATII 1000V, CAT III 600V and pollution level 2. If you fail to use the clamp meter according to relevant operation instructions, the protection provided by the clamp meter is likely to be weakened or undermined.

Conforms to UL STD. 61010-1, 61010-2-030, 61010-2-032, 61010-2-033 Certified to CSA STD. C22.2 NO. 61010-1, 61010-2-030, IECSTD 61010-2-032, 61010-2-033.

CAT II: Applicable to test and measuring circuits connected directly to utilization points (socket outlets and similar points) of the low-voltage MAINS installation.

CAT III: Applicable to test and measuring circuits connected to the distribution part of the building's low-voltage MAINS installation.

Follow all safety instructions.

1. Prior to using, please check the clamp meter and probe so as to prevent any damage or abnormality. If you find the probe and insulating layer of the shell are damaged obviously, the display screen cannot work, etc., or you consider that the clamp meter cannot work well, please don't use the clamp meter any more.
2. It is strictly prohibited to use the clamp meter without the rear cover or battery cover, otherwise, shock hazard may occur.
3. When carrying out the measurement, please be sure that your fingers should not exceed the shield and not touch the bare wire and connector, unused input terminal or circuit under measurement, to prevent the electric shock.
4. Before measurement, the clamp meter must be switched to the correct tap position. It is prohibited to switch tap position during measurement, so as to prevent the clamp meter damage.
5. Do not apply DC1000V/AC750V or above voltage between the clamp meter terminal and grounding, so as to prevent electric shock and clamp meter damage.
6. When the instrument is used to measure DC voltage of higher than 42V or AC RMS voltage of higher than 30V, it is necessary to operate it carefully, for there may be danger of an electric shock.
7. Do not measure the voltage or current higher than the allowable input, and the function range switch must be placed at the maximum range position when the range of the value to be measured is not determined. Before measuring the resistance, diode or circuit on-off, it is a must to cut off all the power supplies in the circuit and discharge all capacitors, otherwise, it may lead to incorrect measurement results.
8. When the symbol of "?" is displayed on the LCD, it is necessary to replace the battery in a timely manner, so as to assure the measuring precision.
9. Please do not change the internal wiring of the clamp meter at random, to prevent instrument damage and insecurity.
10. Do not keep or use the clamp meter in an environment with high-temperature, high-humidity, inflammable, explosive or strong electromagnetic field.
11. During maintenance, please clean the shell of the clamp meter with a soft cloth soaked with neutral detergent, and do not use abradant or solvent, so as to prevent shell corrosion, instrument damage and insecurity.
12. To test the known voltage before use to confirm the product function is correct.

IV. Electrical Symbols

V. Appearance Structure

1. Clamp head: It is the sensing device for AC/DC measurement. Fix the wire into the designated position to measure the current through the wire. As the head may touch the live wire at the time of the measurement, the user's hand shall be kept below the clamp head.
2. Clamp body: It is the part held by the user's hand. The user shall hold the middle of the clamp body and not exceed the protective baffle at the jointing section between the upper part of the clamp body and the clamp head; for fear that the hand touches the dangerous area.
3. Rotary knob: It is used for the start-up and selection of basic measurement functions. The salient point on the rotary knob indicates the present gear position. When it points to "OFF", it means the clamp meter is off; when it points to other gears, the clamp meter will perform the corresponding measurement function.
4. Function key: It is used for the selection of key functions, including SELECT, REL/ZERO, HOLD/-, and FLASHLIGHT. Start up or exit the corresponding key functions by short pressing or long pressing them.
5. Display screen: It is used for the display of the measurement data and function symbols. The segment code type LCD displays measured data, unit of corresponding function gears, and symbols of key functions. The display screen also has the backlight function (?), which provides convenience for measurement in insufficient light environment.
6. Measurement at input terminal: It is at the insertion port of the probe that the voltage, resistance, capacitance, and other parameters are measured.

[Figure 1: Diagram showing the UNI-T UT256B clamp meter with numbered parts: 1. Clamp head, 2. Clamp body, 3. Rotary knob, 4. Function keys, 5. Display screen, 6. Input terminals.]

VI. Functions of Keys

• SELECT: Function selection button, which works at ACI/DCI, ACV/DCV, and RES/CNT/DIO/CAP gears.
  1. Short press SELECT button at ACI/DCI gear to switch over ACI and DCI function. When the rotary knob switches to ACI/DCI gear, it defaults to ACI function. Short press the SELECT button to switch to DCI function from ACI function; and short press the SELECT button again to switch to ACI function. The above forms a cycle.
  2. Short press SELECT button at ACV/DCV gear to switch over ACV and DCV functions. When the rotary knob switches to ACV/DCV gear, it defaults to ACV function. Short press SELECT button to switch to DCV function from ACV function and short press SELECT button again to switch to ACV function. The above forms a cycle.
  3. Short press SELECT button at RES/CNT/DIO/CAP gear to switch over RES, CNT, DIO and CAP functions. When the rotary knob switches to RES/CNT/DIO/CAP gear, it defaults to RES function. Short press SELECT button to switch to CNT function from RES function and short press SELECT button to switch to DIO function from CNT function; short press SELECT button to switch to CAP function from DIO function and short press SELECT button again to switch to RES function back from CAP function. The above forms a cycle.
• REL: Measure button of relative value, which works at other function gears except from CNT, DIO, NCV and DCI gears. Short press REL button under normal measuring mode to switch to the relative value measuring mode. Under the relative value measuring mode, it will display Dn-Df on the main display screen and display Df on the sub display screen, of which Df is the last measured value (relative value) before pressing REL button and Dn is the current measured value. Meanwhile, it will display "RELA" at the top left of the display screen, to indicate the entering of the relative value measuring mode. Under REL mode, when short press REL button again, turn the rotary knob or press SELECT button for function shift, it will exit the relative value measuring mode and return to normal operation mode. Press HOLD button under the relative value measuring mode, it will enter the data-hold mode as well, and the clamp meter will not update the measurement data; when exit the relative value measuring mode, HOLD state will be cancelled as well.
• ZERO: Base reset button; this function is used with REL and works at DCI function gear. Since the clamp head of the clamp meter generates the induced voltage due to earth magnetic field induction when it does not measure any signal, a base number exists at DCI gear generally and varies from the placement position and direction of the clamp meter. It is necessary to deduct the base number at the time of DCI measurement. Short press ZERO button under DC gear to enter the resetting mode and deduct the base number during DC measurement. Under the resetting mode, it will display Dn-Df on the main display screen, of which Df is the DCI base number before pressing ZERO button, and Dn is the current measured value. Meanwhile, it will display "ZERO" symbol at the top left of the display screen, to indicate the entering of the resetting mode at present. If press ZERO function key again under the resetting mode, it will update Df again and display updated Dn-Df. If long press ZERO function key for over one second, or turn the rotary knob or press SELECT button for function shift, it can exit the reset function. Press HOLD button under the resetting mode, it will enter the data-hold mode as well, and the clamp meter will not update the measurement data; when exit the resetting mode, HOLD state will be cancelled as well.
• HOLD: The data hold button, which works at all gears. Short press HOLD button under normal measurement mode, it will enter the data-hold mode, the clamp meter will not update the measurement data and the displayed value will be locked and remain unchanged. It will display "H" symbol at the top left of the display screen, to indicate the entering of the data-hold mode. Under the data-hold state, when short press HOLD button, or turn the rotary knob or press SELECT button for function shift, it can exit the data-hold function.
• ? (Backlight): Screen backlight control button. This function key can be used with HOLD button, and works when the clamp meter is on. Under the backlight off state, long press ? button to open the screen backlight and enter the low-light mode, under which the backlight brightness can meet the basic reading requirement, thus saving the battery capacity. Long press ? button again to enter the high-light mode, under which the backlight brightness is higher and the screen contrast is clearer, thus increasing the battery capacity consumption accordingly. Long press ? button again under the high-light mode, the backlight will be closed. The above forms a cycle.
• ? (FLASHLIGHT): LED lighting lamp on-off key, which works when the clamp meter is on. Short press the LED lighting lamp to light it up, and short press FLASHLIGHT key again to turn it off. The above forms a cycle.

VII. Technical Indexes

1. General specifications

• Liquid crystal display: Maximum display up to 5999.
• Polarity display: Automatic positive and negative polarity display.
• Over-load display: Display in "OL" or "-OL".
• Battery voltage display: When the battery voltage is low than 2.4V, it will display the low-voltage symbol (?).
• Sampling rate: Approximately 3 times/second.
• Sensor types: Hall effect sensor for DC/AC measurement.
• Measured position error: When measuring the current, ±1.0% additional error may occur because it fails to put the source to be measured into the center position of the probe.
• Impact-resistant strength: 1m height impact can be borne.
• Maximum opening dimension of clamp head: Diameter of 14.7mm.
• Predicted maximum dimension of the current wire: Diameter of 14.7mm.
• Electromagnetic field effect: When the electromagnetic field interference exists, it probably displays unstable or incorrect readings.
• Power requirements: Two AA 1.5V batteries.
• Auto OFF function: 15 min.
• Dimension: 210mm×53mm×35mm.
• Weight: Approximately 163.7g (battery included).

2. Environmental limit

• Working environment: Indoor use.
• Altitude: <2,000 m.
• Safety requirements: IEC61010-1; IEC61010-2-032; CATII 1,000V; CATIII 600V.
• Pollution degree: 2.
• Operating temperature and humidity: 0℃-30℃ (not larger than 80%RH), 30℃-40℃ (not larger than 75%RH), 40°C-50°C (not larger than 45%RH).
• Storage temperature and humidity: -20℃-+60℃ (not larger than 80%RH).

3. Electric specification

Accuracy: ± (% reading + word count), calibration period is a year.

Ambient temperature: 23℃±5℃.

Ambient humidity: ≤80%RH.

Temperature coefficient: 0.1×(accuracy)/°C.

(1). Measuring of AC current (A~)

Main display: true virtual value current.

Frequency response: 50Hz-60Hz.

(2). Measuring of DC current (A==)

DCI base number shall be cleaned up by pressing ZERO key.

(3). Measuring of AC voltage (V~)

Main display: true virtual value voltage.

Input impedance > 10MΩ.

Frequency response: 45-400 Hz (50-100Hz when ≤400mV).

(4). Measuring of DC voltage (V)

Input impedance ≥ 10MΩ.

(5). Measuring of resistance(Ω)

(6). Conductivity testing (•)))

Open-circuit voltage is about 1.2 V.

(7). Diode measurement (+)

Open-circuit voltage is about 3.3 V.

(8). Measuring of capacitance ( )

(9). Induced voltage measurement (NCV)

VIII. Measurement Operation Instructions

? Before measuring, it is necessary to ensure that the clamp meter has been switched to the correct tap position. Measurement shall be in strict accordance with the guidance herein, and it is prohibited to switch the tap position during measurement, in order to avoid injury of users or damage of clamp meter.

1. Measuring of AC Current (ACI)

1. Set measuring function range of AC current: Rotate the knob to start up and keep the pointer directing to "A~". The clamp meter will be within the measuring function range of AC current as default.
2. Clamp instrument to input current signal to be tested: Clamp single current lead to be tested and keep it at the bottom of the "U" clamp head. The clamp meter will automatically select the proper range and display the true virtual value of AC current at the central screen. Frequency response range: 50Hz-60Hz.

? Warning: The largest measuring current shall not be greater than AC 200A during the measurement of AC current.

[Figure 2: Diagram showing the UNI-T UT256B clamp meter measuring AC current with a wire clamped in the jaws.]

2. Measuring of DC Current (DCI)

1. Set measuring function range of DC current: Rotate the knob to start up and keep the pointer directing to "A=". The clamp meter will be within the measuring function range of DC current as default. Press the SELECT key to switch the clamp meter function into measuring function range of DC current.
2. Clear the base number of the measuring range of DC current: As the clamp head sensor reacts to terrestrial magnetism and surrounding magnetic field, there generally exists a base number even without measuring. Therefore, before measuring DC current, the base number shall be cleared by pressing the "ZERO" key. Measurement shall be conducted after the base number is cleared and the measured value returns to zero.
3. Clamp instrument to input current signal to be tested: Clamp single current lead to be tested and keep it at the bottom of the "U" clamp head. The clamp meter will automatically select the proper range. If the current direction is the same as that of the clamp head, it will display the positive value of AC current at the central screen; otherwise, it will display the negative current value.

? Warning: The largest measuring current shall not be greater than DC 200A during the measuring of DC current.

[Figure 3: Diagram showing the UNI-T UT256B clamp meter measuring DC current with a wire clamped in the jaws.]

3. Measuring of AC Voltage (ACV)

1. Insert meter probes: Insert the black probe into "COM" input end and insert the red probe into "V Ω" input end to prepare for the measurement.
2. Set measuring function range of AC voltage: Rotate the knob to start up and keep the pointer directing to "V~", and the clamp meter will be within the measuring function range of AC voltage as default.
3. Connect voltage to be tested: Put red and black probes at two sides of voltage to be tested, and the clamp meter will automatically select the proper range. The central screen will display the current true visual value of AC voltage. When the voltage value exceeds AC 30V, the screen will display a high-voltage alarm signal (⚡) to warn against the shock hazard. Frequency response scope: 45 Hz-400 Hz.

? Warning: AC voltage measuring range shall not be greater than AC 750 V.

[Figure 4: Diagram showing the UNI-T UT256B clamp meter connected to an AC voltage source using probes.]

4. Measuring of DC Voltage (DCV)

1. Insert meter probes: Insert the black probe into "COM" input end and insert the red probe into "V Ω" input end to prepare for the measurement.
2. Set measuring function range of DC voltage: Rotate the knob to start up and keep the pointer directing to "V=". The clamp meter will be within the measuring function range of DC voltage as default. Press the SELECT key to switch the clamp meter function into measuring function range of DC voltage.
3. Connect voltage to be tested: Put red and black probes at two sides of voltage to be tested, and the clamp meter will automatically select the proper range. The central screen will display the current true virtual value of DC voltage. If the electric potential at the red probe is higher than that at the black probe, the screen will display the positive voltage value; otherwise, the negative voltage will be displayed. When the voltage exceeds DC 42V, the screen will display the high-voltage alarming sign (⚡) to warn against the shock hazard.

? Warning: DC voltage measuring range shall not be greater than DC 1000V.

[Figure 5: Diagram showing the UNI-T UT256B clamp meter connected to a DC voltage source using probes.]

5. Measurement of Resistance (OHM)

1. Insert meter probes: Insert the black probe into "COM" input end and insert the red probe into "V Ω" input end to prepare for the measurement.
2. Set function range of resistance measurement: Rotate the knob to start up and keep the pointer directing to "Ω", and the clamp meter will be within the measuring function range of resistance measurement as default.
3. Connect resistance to be tested: Put red and black probes at two sides of resistance to be tested. The clamp meter will automatically select the proper range. The central screen will display the current resistance value.

? Warning: During the measurement of connection resistance, the power must be cut off before connecting, and the residual electric charge of all capacitors shall be fully discharged.

? Note: At the function range of 600Ω, it is suggested to firstly short-circuit the meter probe and press the REL key to clear the probe short circuit base number before measurement, to eliminate the impact of probe resistance.

[Figure 6: Diagram showing the UNI-T UT256B clamp meter connected to a resistor using probes.]

6. Conductivity Testing (CNT)

1. Insert meter probes: Insert the black probe into "COM" input end and insert the red probe into "V Ω" input end to prepare for the measurement.
2. Set function range of conductivity testing: Rotate the knob to start up and aim the pointer at ")))", the clamp meter will be within the measuring function range of conduction testing as default. Press SELECT key to switch the clamp function into conductivity testing function range.
3. Connect with the endpoints to be tested: Place the red and black probes at two sides of the endpoints to be tested. If the measured resistance is less than 10Ω, the buzzer will ring persistently; if it is between 10Ω and 1000Ω, it may or may not ring; if it is larger than 100Ω, it will not ring.

? Warning: When measuring on-off condition, the circuit power must be cut off and the residual electric charge of all electric capacitors must be fully discharged before the endpoints are connected with the product.

[Figure 7: Diagram showing the UNI-T UT256B clamp meter connected to test endpoints for conductivity testing.]

7. Diode Measurement (DIO)

1. Insert meter probes: Insert the black probe into "COM" input end and insert the red probe into "V Ω" input end to prepare for the measurement.
2. Set function range of diode measurement: Revolve the rotary knob to start up and aim the pointer at "+", and the clamp meter will be within the measuring function range of diode measurement as default. Press SELECT key to switch the clamp function into diode measurement function range.
3. Connect with the diode to be tested: Place the red and black probes at positive and negative poles of the diode to be tested. The positive conductivity voltage value will be displayed in the center of the screen. If the probes are inserted in the wrong terminals, the screen shall display the measurement sign "OL".

? Warning: When measuring the diode, the circuit power must be cut off and residual electric charge of all electric capacitors must be fully discharged before the diode is connected with the product. More accurate measurement may be available when the elements are separated from circuit.

[Figure 8: Diagram showing the UNI-T UT256B clamp meter connected to a diode using probes.]

8. Measuring of Capacitance (CAP)

1. Insert meter probes: Insert the black probe into "COM" input end and insert the red probe into "V Ω" input end to prepare for the measurement.
2. Set function range of capacitance measurement: Rotate the knob to start up and keep the pointer directing to " ", the clamp meter will be within the measuring function range of capacitance measurement as default. Press the SELECT key to switch the clamp meter function into measuring function range of capacitance measurement.
3. Connect capacitance to be tested: Put red and black probes at two sides of capacitance to be tested, and the clamp meter will automatically select the proper range. The central screen will display the current capacitance measurement value.

? Warning: During the measurement of connection capacitance, the circuit power must be cut off and residual electric charge of all electric capacitors must be fully discharged before connecting. More accurate measurement may be available when the elements are separated from circuit.

? Note: Before measuring the capacitance, REL key shall be pressed in advance to clean the open-circuit base number to eliminate the effect of probe parasitic capacitance. At the range of 60nF, it is suggested to firstly press the REL key to clear the probe short circuit base number before measurement, to eliminate the impact of probe resistance.

[Figure 9: Diagram showing the UNI-T UT256B clamp meter connected to a capacitor using probes.]

9. Measuring of Induced Voltage (NCV)

1. Set the measuring range of induced voltage: Rotate the knob to start up and keep the pointer directing to "NCV", and the clamp meter will be within the measuring function range of induced voltage as default.
2. Measuring of induced voltage: The top of the right end of the clamp head is equipped with an electromagnetic induction sensor, which is able to detect whether the alternating current magnetic field exists or not. During the measurement, the right end of the clamp head may be close to the tested conductor to conduct induction detection. The screen will display "EF" if the induced voltage is not detected. When the test distance is less than 10mm and the tested voltage is larger than 100Vrms, the clamp meter will alarm with buzz and the NCV LED alarm light will also start to flicker. According to the magnitude of induced voltage, the central screen will display four grades: "-", "- -", "- - -", "- - - -". The greater the induced voltage, the higher the grade displayed.

[Figure 10: Diagram showing the UNI-T UT256B clamp meter with NCV function active.]

IX. Maintenance and Repair

? Warning: Please pull out the test probe before uncapping the lid so as to avoid electric shock.

1. General Maintenance

1. The maintenance and other services of this clamp meter shall be accomplished by qualified professional maintenance staff or designated maintenance department.
2. The shell shall be regularly cleaned by dry cloth. Detergent which contains abradant or solvent shall not be applied.

2. The installation or replacement of batteries

The power supply of this product is two AA 1.5V batteries. Please install or replace batteries according to the orders listed below:

1. Turn off the product, and pull out the test probe which is at the input terminal.
2. Turn the product's faceplate down, unscrew the battery holder and remove the battery cover. Remove the batteries and install new batteries according to the polarity indication.
3. Please use batteries of the same model, and don't install inappropriate batteries.
4. After installing new batteries, fit on the battery cover, and lock the screws.

[Figure 11: Diagram illustrating access to the battery compartment.]

Instructions are subject to change without prior notice.

UNI-T

UNI-TREND 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