What are current clamps and what measurements can be made with them? How to use them to the maximum effect? Which current clamp is best suited for specific conditions? This review aims to provide answers to all these questions.
With the introduction of technological advances in electrical equipment and circuits, electricians and technicians face new challenges. Progress requires not only great capabilities from modern measuring instruments, but also great skills on the part of the people who use them. Electricians with a good knowledge of the basics of test equipment are better equipped to measure and troubleshoot. Clamps are one of the most important and common tools to be found in their arsenal today.
This device is a meter that combines a clamp-on voltmeter and ammeter. Like a multimeter, having passed through the analog period, it entered the world of digital measurements. Created primarily as a versatile tool for electricians, modern models have become more accurate and have acquired many additional features,some of which are very special. Today, current clamps duplicate many of the basic functions of a DMM, but differ from it by having a built-in current transformer.
Working principle
The ability to measure large AC currents with current clamps is based on the simple action of a transformer. When the clamps are closed around the conductor, the current is in the device like the iron core of a power transformer, and flows through the secondary winding connected through the input shunt. A much smaller current is supplied to the input of the device due to the ratio of the number of turns of the secondary winding to the number of turns of the primary. Usually the primary winding is represented by one conductor, around which the tongs are clamped. If the secondary winding has 1000 turns, then the secondary current is 1/1000 of the primary, or, in this case, the conductor. Thus, 1 A is transformed into 0.001 A or 1 mA at the input of the device. This method makes it easy to measure large currents by increasing the number of secondary turns.
Choice
Purchasing current clamps requires not only familiarization with their specifications, but also an assessment of their functionality and quality provided by the design of the device and its production technology.
Reliability of the tester, especially in difficult conditions, is more important today than ever. Engineers, when developing measuring instruments, must test them not only for electrical, but also for mechanical strength. For example, Fluke current clamps before being sent to storesundergo a rigorous testing and evaluation program.
User safety should be the primary consideration when choosing this instrument or any other electrical measuring equipment. In addition, digital clamp meters must not only be produced according to the latest standards, but each instrument must be tested and certified by testing laboratories such as UL, CSA, VDE, etc. Only in this way can you be sure that the tool meets all new safety requirements and standards.
Resolution and measuring range
The resolution of an instrument indicates how accurate its measurements are. It determines what is the minimum signal change that can be registered. For example, if the resolution of the current clamp is 0.1 A in the range of 600 A, then a current of about 100 A is measured with an accuracy of 0.1 A.
Who needs a ruler marked in centimeters if you need to determine the size of an object a few millimeters in size? Likewise, you should choose an instrument that can display the required resolution.
Error
This is the maximum allowable error that can occur under certain operating conditions. In other words, it is a measure of how closely the measured value matches the actual value.
Instrument error is usually expressed as a percentage of the reading. For example, if it is 1%, then for 100 amps the actual current value is between 99up to 101 A.
In addition to the error in the specifications, it can be indicated how much the indication changes in the rightmost digit of the measured value. For example, if the accuracy is specified as ± (2% + 2), then for 100.0 A, the actual current is in the range 97.8 - 102.2 A.
Crest factor
With the rise of electronic power supplies, currents drawn from modern distribution systems are no longer pure 50Hz sine waves. They have become quite distorted due to the harmonics these power supplies generate. However, the electrical components of the network, such as fuses, busbars, conductors, and circuit breaker thermal elements, are designed for rms current, since their main limitation is related to heat dissipation. If you need to check the electrical circuit for overload, then you need to measure the rms current and compare the resulting value with the nominal value. Therefore, modern test equipment must be able to accurately measure the true magnitude of a signal, regardless of the degree of signal distortion.
Crest factor is the ratio of peak current or voltage to their RMS value. For a pure sine wave, it is 1.414. However, a signal with a very sharp pulse will cause the crest factor to be high. Depending on the pulse width and frequency, ratios of 10:1 and higher can be observed. In real power distribution systems, crest factors of more than 3 are rarely encountered. Thus, the coefficientamplitude is a sign of signal distortion.
These measurements can only be made by instruments capable of measuring true RMS. It shows how distorted the signal can be and register it according to the error of the instrument. Most current clamps are capable of measuring crest factors of 2 or 3. This is sufficient for most applications.
Alternating current
One of the main purposes of current clamps is the measurement of alternating current. Usually such measurements are carried out on the branches of the electrical distribution system. Determining the strength of the current flowing through various circuits is a routine task for an electrician.
To measure you need:
- Select AC mode.
- Open the jaws and close them around one conductor.
- Read the readings on the display.
By measuring the current along a section of the circuit, you can easily determine how much power each load draws.
When a circuit breaker or transformer overheats, it is best to measure the load current. However, you must ensure that true RMS values are recorded to accurately measure the signal that heats up these components. A conventional instrument will not give a true reading if the current and voltage are not sinusoidal due to non-linear loads.
Voltage
Another common function of the instrument is to measure voltage. Modern current clamps are able to determine the constant and variablevoltage. The latter is usually created by a generator and then distributed over the network. The job of an electrician is to be able to take measurements throughout the electrical system in order to find troubleshooting. Another use of the device is to check the battery charge. In this case, it is necessary to measure direct current or direct voltage with a current clamp.
Troubleshooting a circuit usually starts with checking the network parameters. If there is no voltage, if it is too high or too low, this problem must be resolved before continuing the search.
The ability of a current clamp to measure AC voltage is affected by the frequency of the signal. Most testers of this type can accurately determine this parameter at frequencies of 50-500 Hz, but the DMM has a bandwidth of 100 kHz or more. That is why measuring the same voltage with testers of different types gives different results. The DMM allows high frequency voltage to be applied to the circuit while the current clamp filters out the portion contained in the signal above their bandwidth.
When troubleshooting VFDs, the instrument's input bandwidth can be essential to obtaining a meaningful reading. Due to the high harmonic content of the signal coming out of the frequency converter, the DMM, depending on its input bandwidth, will measure most of the voltage. Recording VFD parameters is not a common task. Motor connected to frequencyconverter responds only to the average value of the signal, and to register this power, the input bandwidth of the tester must be narrower than that of the multimeter. The Fluke 337 Clamp is specifically designed for testing and troubleshooting this type of problem.
Measure voltage as follows:
- Select the appropriate current clamp mode: DC Volts DC (V) or AC Volts AC (V ~).
- Connect the black wire of the test probe to the COM input jack and the red wire to the V jack.
- Touch the probe tips to the circuit on opposite sides of the load or power source (parallel to the circuit).
- Read readings, paying attention to the unit of measure.
- Press the HOLD button to fix the result. After that, you can disconnect the probes from the circuit and take readings at a safe distance.
Measuring the voltage at the input of the circuit breaker before and after connecting the load allows you to determine its drop. If significant, it indicates how well the load is functioning.
Current Clamps: Instructions for Measuring Resistance
Resistance is measured in ohms. Its value can vary from a few milliohms for contacts to billions of ohms for insulators. Most current clamps measure resistance with a resolution of 0.1 ohms. When its value exceeds the upper limit or the circuit is open, the display shows OL.
This parameter should be measured whenpower off, otherwise the instrument or circuit will be damaged. Some devices provide resistance measurement protection in case of contact with voltages. Depending on the model, the level of protection may vary greatly.
The most common requirement is to determine the electrical resistance of a contactor coil.
The measurement order is as follows:
- Turn off circuit power.
- Select resistance measurement mode.
- Connect the black wire of the probe to the COM jack and the red one to the Ω jack.
- Touch the probe tips on both sides of the element or section of the circuit for which you want to determine the resistance.
- Read the instrument readings.
Chain integrity
This is a quick resistance test that can detect an open circuit.
Audible current clamp makes many of these tests quick and easy. The device signals when it detects a closed circuit, so you do not need to look at the display when checking. The level of resistance required to trigger the device may vary. Typical is a value not exceeding 20-40 ohms.
Special functions
A rather popular functionality of current clamps, according to user reviews, is the determination of the frequency of alternating current. To do this, you need to close the "jaws" around the conductor and turn on the frequency measurement mode. The signal frequency will appear on the display. This function is very useful for determiningsource of harmonic problems in the electrical network.
Another feature of some models (eg current clamp Mastech MS2115B) is the recording of minimum and maximum values. When this feature is enabled, each reading is compared to previously stored readings. If the new value is higher than the maximum, then it replaces it. The same comparison is made for the minimum reading. As long as the MIN MAX function is active, all measurements are processed in this way. After some time, you can call each of these values on the display and determine the highest and lowest readings for a certain period of time.
For electricians who work with motors, the ability to record the current drawn by a motor during startup can tell a lot about its condition and load. The Fluke 335, 336 and 337 Clamps can measure it "in motion". To do this, you need to close them around one of the input wires of the motor, activate the in-rush mode and turn on the engine. The instrument display will show the maximum current drawn by the motor during the first 100ms of its start cycle.
Uni-T UT210E current clamps allow you to determine the presence of an alternating voltage or an electromagnetic field in a non-contact way. To do this, bring the device closer to the tested object at a distance of 8–15 mm. The device distinguishes 4 voltage levels, gives a corresponding sound signal and indicates the intensity of the field with a light indicator.
DT-3347 current clamp supports temperature measurement function.
Safety
Safe measurement starts with choosing the right instrument for the environment in which it will be used. Once the correct tool has been found, it should be used according to the recommended procedure.
The International Electrotechnical Commission has set new standards for safety when working on electrical systems. It must be ensured that the instrument being used conforms to the IEC category and voltage rating approved for the environment in which the measurement is to be made. For example, if measurements are being made on a 480-volt electrical panel, then a category III 600-volt clamp meter should be used. This means that the meter's input circuitry is designed to withstand the transient voltages typically found in this environment without harm to the user. Choosing a tool in this class that is also UL, CSA, VDE or TUV certified means that it has not only been designed to IEC standards, but has been independently tested and found to comply with these standards.
Safety regulations
- Must use current clamps that meet accepted safety standards for the environment in which they will be used.
- Check the probe wires for physical damage before taking a measurement.
- Make sure the wire is intact using current clamps.
- Do not use probes with bare connections and no finger protection.
- Must applydevices with recessed input sockets only.
- Current clamps must be in working condition.
- Always disconnect the hot (red) test lead first.
- You can't work alone.
- Must use a meter with overload protection in resistance measurement mode.
Special Features
The following special features can facilitate the use of current clamps:
- On-screen icons let you know at a glance what is being measured (volts, ohms, etc.).
- The data hold function will freeze the reading on the display.
- One switch makes it easy to select measurement functions.
- Overload protection prevents damage to the instrument and circuit, and protects the user.
- Automatic range detection ensures correct range selection at all times. Manual setting allows you to fix the range for repeated measurements.
- Low battery indicator ensures timely replacement of batteries.
