The structure and application of clamp ammeter

When we electricians need to measure current without disconnecting the circuit, we need to use a clamp meter (sometimes referred to as a clamp meter or clamp meter). A clamp meter is a meter used to measure the current in an operating electrical circuit and is a very common measuring tool for electricians. Clamp meters are divided into two categories: clamp meters for AC and clamp meters for DC. Some can also measure AC voltage.

structure

Clamp meters are usually used as AC ammeters. There is a clamp head on the meter head. When measuring current, the clamp meter does not need to be connected to the circuit to be measured. You only need to pass the power supply wire (only one) through the jaws to directly measure the current.

The clamp meter is mainly composed of clamp head, clamp head trigger, hold button, function knob, LCD display, test lead jack, and red and black test leads. The clamp head is used to clamp the wire to measure current, and the red and black test leads are mainly used to connect the clamp meter to measure resistance and voltage.

The clamp head of the clamp meter is mainly used to clamp the measured wire when measuring AC current, and use the current transformer principle to sense the wire current.

The clamp head trigger of the clamp meter is mainly used to open and close the clamp head. The clamp head opens when pressed and closes when released.

The hold button of the clamp meter is mainly used to hold the measured data when observing the electronic circuit so that the recorded data can be read at the right time.

The function knob of the clamp meter is mainly used for multiple purposes, and the corresponding range is set for different observations.

The LCD display of the clamp meter is mainly used to display information such as measured data, data units, and selected range.

The test lead jack is mainly used to connect the lead plug of the test lead and insulation test accessories. The red test lead is connected to the VΩ jack, and the black test lead is connected to the ground terminal.

The clamp type AC ammeter is essentially composed of a current transformer and a rectifier. The current-carrying wire being measured corresponds to the primary winding of the current transformer, and the secondary winding of the current transformer is on the iron core. The secondary winding is connected to the rectifier. According to the certain change ratio relationship between the primary and secondary windings of the current transformer, the rectifier can display the current value of the measured circuit.

The clamp-on AC/DC meter is an electromagnetic system. The current-carrying wire to be measured is placed in the jaws as an excitation coil. The magnetic flux forms a loop in the iron core. The electromagnetic measuring device is located at the base of the notch of the iron core and deflects under the influence of the magnetic field to obtain the reading. Because its deflection is not affected by the measured current, it can measure AC and DC currents.

Working principle of clamp meter:

The working principle of the clamp ammeter is based on the working principle of the current transformer. When the clamp ammeter wrench is tightened, the iron core of the current transformer can be opened, and the wire of the measured current enters the jaws as the primary winding of the current transformer. When the wrench is loosened and the iron core is closed, an induced current is generated on its secondary winding according to the principle of the transformer, and the ammeter pointer deflects, thereby indicating the value of the measured current.

It is worth noting that: because its principle is to use the principle of mutual inductance, whether the core is tightly closed and whether there is a lot of residual magnetism has a great influence on the measurement effect. When measuring a small current, the measurement error will increase. At this time, the measured wire can be wound around the core a few more times to change the current ratio of the mutual inductance to increase the current range.

How to use

The clamp ammeter is easy to use, as shown in the figure on the right. When measuring current, you only need to clamp the wire to be measured into the clamp core of the clamp ammeter, and then read the reading on the digital display or indicator panel. It is very simple to use, just clamp the measuring wire. However, the widespread use of digital clamp ammeters now adds many multimeter functions to the clamp meter, such as voltage, temperature, resistance, etc. (sometimes this type of multi-function clamp meter is called a clamp multimeter, as shown in the figure on the right, there are two test pen jacks on the surface), and different functions can be selected through the knob, and the method of use is almost the same as that of an ordinary digital multimeter. For the meaning of some special function buttons, please refer to the corresponding instructions. In addition, the following questions should be paid attention to when using the clamp ammeter:

Choose the appropriate range. Do not use a small range to measure a large current. If the current to be measured is small, you can wind the current-carrying wire a few more times and put it into the jaws for measurement. However, the reading should be divided by the number of windings to get the actual current value. After the measurement, put the adjustment switch in the maximum range position (or closed position) for safe use next time. Do not switch the range during the measurement process. Pay attention to the voltage on the circuit to be lower than the rated value of the clamp meter. Do not use the clamp ammeter to measure the current of a high-voltage circuit, otherwise, it is easy to cause an accident or cause a risk of electric shock.

Using clamp meters in residential applications:

For residential electricians, clamp meters are an essential tool for measuring the load on each branch circuit at the switchboard. While checking the current is often sufficient, sometimes this check does not provide a complete picture as the load is switched on and off, through several cycles, etc. The voltage in the electrical system should be stable, but the current varies greatly.

To see the peak or worst-case load on a circuit, use a clamp meter with a min/max function designed to measure high currents that exist for longer than 10 ms, or about 8 cycles. These currents cause intermittent overload conditions that can cause nuisance tripping of circuit breakers.

Make the measurement on the load side of the circuit breaker or fuse. The circuit breaker will open the circuit in the event of an accidental short. This is especially important for any type of direct touch voltage measurement. Even though the clamp meter's jaws are insulated and thus provide a level of protection not available with direct touch voltage measurements, caution is still necessary.

A common problem in electrical work in residential installations is matching electrical outlets to circuit breakers. A clamp meter is very useful in identifying which circuit a particular outlet is located on. First, get a baseline reading of the circuit's current at the distribution panel. Then, put the clamp meter in min/max mode. Go to the outlet in question and plug in a load (a hair dryer is ideal) and power it up for a minute or two. Check to see if the clamp meter's maximum current reading changes. A hair dryer will typically draw 10-13 amps, so there should be a detectable difference. If the readings are the same, the incorrect circuit breaker is being used.

Using Clamp Meters in Commercial Applications:

Clamp meters are used to measure circuit loads on feeders and branch circuits at the distribution panel. Measurements on branch circuits should always be made on the load side of the circuit breaker or fuse.
1. The feeder cables should be checked for balance and loading: the currents on all three phases should be roughly the same to minimize the current returning to the neutral.
2. The neutral should also be checked for overload. With harmonic loads, the neutral may carry more current than the feeder even if the feeder phases are balanced.
3. Each branch circuit should also be checked for possible overloads.
4. Finally, the ground loop should be checked. There should be very little current to ground.

Clamp meter measurement:

Test leakage current:

To see if there is leakage current on a branch circuit, place the front and neutral wires in the jaws of the clamp meter. Any current measured is leakage current, that is, current returning to the ground loop. The supply current (black wire) and the return current (white wire) generate opposing magnetic fields. The currents should be equal (and opposite), and the opposing magnetic fields should cancel each other. If there is no cancellation, it means that some current (called leakage current) is returning from another path, and the only other path is the ground loop. If you see a net current between the supply current and the return current, you need to consider the nature of the load and circuit. A miswired circuit can allow up to half the total load current to flow through the ground system. If the current measured is very high, there is most likely a wiring problem. Leakage current can also be caused by load leakage or poor insulation.

Worn windings in the motor or moisture in the clamping arrangement are common culprits. If you suspect a leak, a de-energized test with a megohmmeter will help evaluate the integrity of the circuit insulation and help determine if and where the problem exists.

Measuring each load:

To measure individual loads, you can use a pigtail at the outlet. It is simply an extended cable with the outer insulation stripped away to expose the black, white, and green wires. This is much easier than pulling the outlet out to touch the wires. Plug the load onto the cable and the cable into the outlet. To measure the load current, clamp the black wire. Make a ground current check either directly on the green wire or on the black wire in combination with the white wire.