Measuring current in electrical networks. What is current

Current strength is measured in amperes and characterizes the load on electrical networks. The need to measure current arises to check whether the load on the cable is acceptable. Cables of various sections are used for installation of electrical wiring. Permissible currents for cables with polyvinyl chloride insulation laid over the air are:

When the load is exceeded cable line acceptable, the cable will heat up and its insulation will deteriorate. Ultimately this will lead to short circuit, and the cable will have to be replaced with a new one.

Therefore, after replacing the cables, measure the current flowing through it when connecting all electrical appliances. If the electrical wiring is old, then when connecting an additional load to it, you also need to check whether the currents in it correspond to permissible values.

At the maximum load of the electrical wiring, you can check whether the current through circuit breakers their nominal data. If exceeded rated current The automatic operation of the machine due to overload is inevitable.

Measuring current is required to determine the operating modes of electrical appliances. Measurement of load currents of electric motors is carried out not only to monitor their serviceability (currents in all phases must be the same), but also to determine the presence of overload due to increased torque on the shaft. For a heater, measuring the current will show whether all its heating elements are working properly. Only by measuring the load current can you find out whether the heated floor is working.

Electric current power

Power is the work done by electric current per unit time. It is measured in Watts (W, W). It is theoretically possible to measure power directly, but for this purpose special devices are used - wattmeters, which measure the current through the load and the voltage across it. They give readings in Watts, but connecting them is too difficult. Therefore, they are used for measurements at predetermined nodes of the electrical network, connecting to them once and for all.

For domestic use, the power is calculated after measuring the current consumed by the load and the voltage across it, which for simplicity can be taken equal to 220 V.

This method does not always give accurate results. If there is inductive reactance in the load active power influences the power factor. Some electrical appliances consume non-sinusoidal current (LEDs and energy saving lamps, computer and television equipment), which is not all measuring instruments designed to measure AC voltage, are measured correctly.

Instruments for measuring current strength

You can measure current using the following instruments:

— ammeters. Like wattmeters, they are used for stationary measurements.

— multimeter– multifunctional device with digital liquid crystal display ();

— tester– a device that measures several quantities, but, unlike a multimeter, has a dial indicator;

— current clamp- a device that allows you to measure current without interruption electrical circuit.

Current measurement methods

Unlike measuring voltage, current is measured not when the device is connected in parallel to the load, but when connected in series. This means that the measuring device must be connected to the break of any of the power wires of a single-phase consumer. With three-phase power, the same must be done for each phase. In this case, the current in neutral wire is not measured, since with a symmetrical load it is zero. Sometimes it is necessary to measure the current in neutral conductor, but for a group of consumers it is impossible to turn off the zero for measurements.

All these reasons lead to the fact that testers and conventional multimeters are rarely used to measure current. They can only be used for a single consumer or for DC measurements.

In all other cases, current clamps or multimeters containing them are used. To take measurements, simply press the button to release the clamps, place the conductor with the current being measured inside the measuring circuit, and release the button. The magnetic circuit of the clamps will close and the display (there are clamps with a scale and arrow) will show the measured value.

When using current clamps, you need to carefully ensure that only the conductor in which the current is measured gets inside the magnetic circuit. When two or more conductors get inside, the clamps will measure the sum of the currents in them, and also the vector one. This means that by placing a two-core cable with a load inside the magnetic circuit of the clamp, we will measure a current equal to zero. Clamps, like an RCD, will add up the current flowing through the phase conductor towards the load and the same current with the opposite sign returning back.

Clamps are intended for measuring alternating current only. At direct current, an attempt to use them will lead to the magnetic circuit closing with an irresistible force. It will not be possible to open it with your hands until the current is turned off.

To measure current, a measuring device called is used. Current strength has to be measured much less often than voltage or resistance, but, nevertheless, if you need to determine the power consumption of an electrical appliance, then without knowing the amount of current it consumes, the power cannot be determined.

Current, like voltage, can be constant or variable, and different measuring instruments are required to measure their values. Current is designated by the letter I, and to the number, to make it clear that this is the current value, a letter is added A. For example, I=5 A means that the current in the measured circuit is 5 Amps.

On measuring instruments to measure alternating current, the letter A is preceded by the sign " ~ ", and intended for measuring DC is put " – ". For example, -A means that the device is designed to measure direct current.

You can read about what current is and the laws of its flow in a popular form in the website article “The Law of Current Strength”. Before taking measurements, I strongly recommend that you read this short article. The photo shows an ammeter designed to measure direct current up to 3 Amperes.

Circuit for measuring the amount of current flowing with an Ammeter

According to the law, current flows through wires at any point closed circuit the same size. Therefore, in order to measure the current value, you need to connect the device by breaking the circuit in any convenient place. It should be noted that when measuring the current value, it does not matter what voltage is applied to the electrical circuit. The current source can also be a 1.5 V battery, car battery 12 V or household power supply 220 V or 380 V.

The measurement diagram also shows how the ammeter is indicated on electrical diagrams. This uppercase letter And surrounded by a circle.

When starting to measure the current in a circuit, it is necessary, as with any other measurements, to prepare the device, that is, set the switches to the current measurement position, taking into account its type, constant or alternating. If the expected current value is not known, the switch is set to the maximum current measurement position.

How in practice
measure the current consumed by an electrical appliance

For the convenience and safety of measuring current consumption by electrical appliances, it is necessary to make a special extension cord with two sockets. By appearance homemade extension cord no different from an ordinary extension cord.

But if you remove the covers from the sockets, it is not difficult to notice that their terminals are connected not in parallel, as in all extension cords, but in series.

As you can see in the photo, the mains voltage is supplied to the lower terminals of the sockets, and the upper terminals are connected to each other by a jumper made of wire with yellow insulation.

Everything is ready for measurement. Insert the plug of the electrical appliance into any of the sockets, and the ammeter probes into the other socket. Before measurements, it is necessary to set the device switches in accordance with the type of current (AC or DC) and to the maximum measurement limit.

As can be seen from the ammeter readings, the current consumption of the device was 0.25 A. If the device scale does not allow direct reading, as in my case, then it is necessary to calculate the results, which is very inconvenient. Since the ammeter's measurement limit is 0.5 A, to find out the division value, you need to divide 0.5 A by the number of divisions on the scale. For this ammeter it turns out 0.5/100=0.005 A. The needle has deviated by 50 divisions. So now you need 0.005×50=0.25 A.

As you can see, taking current readings from pointer instruments is inconvenient and you can easily make a mistake. It is much more convenient to use digital instruments, such as the M890G multimeter.

The photo shows a universal multimeter turned on in the AC current measurement mode to a limit of 10 A. The measured current consumed by the electrical device was 5.1 A at a supply voltage of 220 V. Therefore, the device consumes power of 1122 W.

The multimeter has two sectors for measuring current, indicated by letters A- for DC and Ah~ to measure a variable. Therefore, before starting measurements, you need to determine the type of current, estimate its magnitude and set the switch pointer to the appropriate position.

Multimeter socket with inscription COM is common to all types of measurements. Sockets marked mA And 10A are intended only for connecting a probe when measuring current. For a measured current of less than 200 mA, the probe plug is inserted into a mA socket, and for a current of up to 10 A, into a 10 A socket.

Attention, if you measure a current that repeatedly exceeds 200 mA when the probe plug is in the mA socket, the multimeter can be damaged.

If the value of the measured current is not known, then measurements should be started by setting the measurement limit to 10 A. If the current is less than 200 mA, then switch the device to the appropriate position. Switching multimeter measurement modes can only be done by de-energizing the circuit being measured..

How to calculate the power consumption of an electrical appliance
by current consumption

Knowing the current value, you can determine the power consumption of any consumer electrical energy, be it a light bulb in a car or an air conditioner in an apartment. It is enough to use a simple law of physics, which was established simultaneously by two physicists, independently of each other. In 1841 James Joule, and in 1842 Emil Lenz. This law was named after them - Joule–Lenz law.

For many centuries, humanity has tried to understand what current is. Today, scientists have given a specific definition of this physical phenomenon. So what exactly is current, and how is it measured?

We also know from the school physics course that electric current is considered to be nothing more than the movement of particles having a charge in a certain direction, in a certain conductor. For such a movement to occur, there must be electric field. At the same time, charged particles can appear everywhere, due to the close contact that occurs between different substances.

Charges are able to move freely between a variety of particles in conductors or not be able to move in insulators. Metals, salt and acid solutions act as conductors. An example of an insulator is many gases. Ebonite, quartz, amber, and some artificial materials also do not allow electrical charges to pass through: polyvinyl chloride, polyethylene, etc.

To measure current, various parameters are used, which include: voltage, force, resistance, power, frequency, etc. Let's look at the main ones.

Current strength

That's what it's called physical quantity, measurable. It is equal to the ratio between the amount of available electric charge, which is capable of passing through a conductor, or rather its cross section, in a specified period of time relative to the value of this time interval. To measure this quantity, physicists use a unit called Ampere (A).

Current power

Power is usually called the work that current particles do relative to the resistance of electricity. As a result of this work, thermal energy is released. We can also say that current power is the amount of thermal energy that is released over a set period of time. In physics, it is customary to measure power in units called Watts (W).

Voltage

This concept defines the ratio of the work of the current, which is carried out relative to the charge in a particular section of the electrical circuit. Since the unit of measurement of charge is considered to be the Coulomb (C), and the unit of work is the Joule (J), the unit of 1J/1C, which is equal to 1 Volt (V), is used to measure voltage.

Electrical resistance

While conducting various experiments with electric current, Georg Ohm noted that depending on the electrical circuits used, the instruments show different current strengths. Thus, proof emerged that each conductor has its own resistance. To calculate it, the length of the conductor must be divided by its cross-sectional area. The unit of resistance measurement is considered to be Ohm.

Already from the name it becomes clear that direct current is the one that, regardless of external factors, does not change direction and frequency. Since direct current has no frequency, it is generally accepted to be zero.

Respectively, AC– a current capable of changing its magnitude and direction within a specified unit of time. Its frequency is the number of cycles of change over a certain period of time. This value is measured in Hertz (Hz).

. Current or amperage determined by the number of electrons passing through a point or circuit element within one second. For example, about 2,000,000,000,000,000,000 (two trillion) electrons pass through the filament of a burning incandescent lamp of a flashlight every second. However, in practice it is not the number of electrons that is measured, but their movement, expressed in amperes(A).

Ampere is a unit electric current, which was named after the French physicist and mathematician A. Ampere, who studied the interaction of conductors with current. It has been experimentally established that with a current of 1A, about 6,250,000,000,000,000,000 electrons pass through a point or circuit element.

In addition to the ampere, smaller units of current are also used: milliampere(mA) equal to 0.001 A, and microampere(μA) equal to 0.000001 A or 0.001 mA. Hence: 1 A = 1000 mA = 1,000,000 µA.

1. A device for measuring current strength.

Like voltage, current can be constant And variable. Instruments used to measure current are called ammeters, milliammeters And microammeters. Just like voltmeters, ammeters are arrow And digital.

On electrical diagrams, devices are indicated by a circle and a letter inside: A(ammeter), mA(milliammeter) and µA(microammeter). Next to symbol ammeter is indicated by its letter designation “ PA" and the serial number in the diagram. For example. If there are two ammeters in the circuit, then next to the first one they write “ PA1", and about the second " PA2».

To measure current, the ammeter is turned on directly into the circuit in series with the load, that is, a break in the load power supply circuit. Thus, for the duration of the measurement, the ammeter becomes like another element of the electrical circuit through which current flows, but at the same time the ammeter does not make any changes to the circuit. The figure below shows a diagram of connecting a milliammeter to the power circuit of an incandescent lamp.

You must also remember that ammeters are available in different ranges (scales), and if you use a device with a smaller range in relation to the measured value when measuring, the device can be damaged. For example. The measurement range of a milliammeter is 0...300 mA, which means that the current strength is measured only within these limits, since if the current is measured above 300 mA, the device will fail.

2. Measuring current with a multimeter.

Measuring current with a multimeter is practically no different from measuring with an ordinary ammeter or milliammeter. The only difference is that a conventional device has only one measurement range, designed for a certain maximum current value, while a multimeter has several ranges, and before measuring you have to determine which range to use at the moment.

Ordinary multimeters, not professional ones, are designed to measure direct current and have four subranges, which is quite sufficient at the household level. Each subrange has its own maximum measurement limit, which is indicated by a digital value: 2m, 20m, 200m, 10A. For example. At the limit " 20m» DC current can be measured in the range 0...20 mA.

For example, let's measure the current consumed by a conventional LED. To do this, we will assemble a circuit consisting of a voltage source (pen-AA battery) GB1 and LED VD1, and connect a multimeter to the open circuit PA1. But before including the multimeter in the circuit, we will prepare it for measurements.

We insert the measuring leads into the sockets of the multimeter, as shown in the figure:

red the dipstick is called positive, and it is inserted into the socket, opposite which there are icons of the measured parameters: “ VΩmA»;
black the dipstick is minus or general and it is inserted into the socket, opposite which it is written “ COM" All measurements are made relative to this probe.

In the DC measurement sector, select the limit " 2m", the measurement range of which is 0...2 mA. We connect the multimeter probes according to the diagram and then apply power. The LED lit up and its current consumption was 1.74 mA. That, in principle, is the whole measurement process.

However, this measurement option is suitable when the current consumption is known. In practice, a situation often arises when it is necessary to measure the current in some section of the circuit, the value of which is unknown or known approximately. In this case, the measurement starts from the highest limit.

Let's assume that the current consumption of the LED is unknown. Then we move the switch to the limit " 200m", which corresponds to the range 0...200 mA, and after that we connect the multimeter probes to the circuit.

Then we apply voltage and look at the multimeter readings. In this case, the current readings were “ 01,8 ", which means 1.8 mA. However, a zero ahead indicates that you can go down to the limit " 20m».

Turn off the power. We move the switch to the limit " 20m" Turn on the power and take the measurement again. The reading was 1.89 mA.

There is often a situation when, when measuring current or voltage, the indicator appears unit. A unit indicates that a low measurement limit is selected and it is less than the value of the measured parameter. In this case, you need to go to a higher limit.

There may also be a moment when the measured current is above 200 mA and it is necessary to go to the measurement limit " 10A" However, there is a nuance that needs to be remembered. In addition to the fact that the switch is moved to the limit " 10A", it is also necessary to move the positive (red) probe to the leftmost socket, opposite which there is a alphanumeric value "10A", indicating that this socket is intended for measuring high currents.

And one more piece of advice. Make it a rule: when you finish all measurements at the limit « 10A» Immediately move the positive (red) probe to its normal place. This will save you nerves, probes and a multimeter.

Well, that’s basically all I wanted to say about measuring current with a multimeter. The main thing to understand is that when the voltmeter is connected parallel to the load or voltage source, whereas when measuring current, an ammeter connected directly to the circuit and current flows through it, which powers the circuit elements.

Well, to reinforce what you’ve read, I suggest watching a video that uses circuit examples to talk about measuring voltage and current with a multimeter.