Electrical knowledge necessary for solar panel installers - measuring instruments

After the photovoltaic system is installed, it cannot be turned on and connected to the grid immediately. It is necessary to test whether the system is safe and qualified before it is connected to the grid. If there are short-circuit and grounding problems during the installation of the system, all of them must be found out and eliminated one by one. This requires electrical measuring instruments.

Instruments that measure various amounts of electricity are collectively referred to as electrical measuring instruments. There are many kinds of electrician measuring instruments, the most common is the one that measures the basic electricity. There are many kinds of electrical instruments according to the measurement method, instrument structure, and instrument use. In general, electrical instruments are used to measure physical quantities such as current, voltage, electrical power, electrical power, power factor, frequency of electricity, resistance, and insulation conditions in circuits. As a result, there are instruments named after various physical quantities to be measured, such as ammeters, voltmeters, etc.

1. Multimeter

A multimeter is a portable instrument. Because of its ability to measure AC, DC voltage or current parameters, as well as resistance in circuits, it is called a multimeter.

digital multimeter

1. Precautions for using the multimeter

1) Before use, you should be familiar with the functions of the multimeter, and select the correct gear, range and test lead jack according to the object to be measured. Beginners should pay special attention not to insert the test leads backwards.

2) When the size of the measured data is unknown, the range switch should be set to the maximum value, and then switched from the large range to the small range.

3) When measuring the resistance, touch the two test pens to make the pointer point at the zero position. If the display is not zero, you should use the "Zero" button to reset the pointer to zero, so as to ensure accurate measurement results. If it cannot be adjusted to zero or the digital display emits a low voltage alarm, it should be checked in time.

4) When measuring the resistance of a circuit, the power supply of the circuit under test must be cut off, and no live measurement is allowed.

5) When using the multimeter to measure, pay attention to the safety of the person and the instrument and equipment. Do not touch the metal part of the test pen with your hands during the test, and it is not allowed to switch the gear switch with electricity to ensure accurate measurement and avoid accidents such as electric shock and burning of the instrument. .

2. Measurement of voltage, current and resistance

1) Measurement of DC voltage, such as battery, Walkman power supply, etc. First, insert the black test lead into the "COM" hole, and the red test lead into "V Ω". Select the knob to a range larger than the estimated value (note: the values ​​on the dial are the maximum range, "V-" means DC voltage range, "V~" means AC voltage range), then connect the test leads to the power supply or both ends of the battery ; keep the contact stable. The value can be read directly from the display screen. If it is displayed as "1", it means that the range is too small, so it is necessary to increase the range before measuring. If "-" appears on the left side of the value, it means that the polarity of the test lead is opposite to that of the actual power supply. At this time, the red test lead is connected to the negative pole.

2) Measurement of AC voltage. The test pen jack is the same as the DC voltage measurement, but the knob should be turned to the required range at the AC gear "V~". There is no positive or negative AC voltage, and the measurement method is the same as the previous one. Whether measuring AC or DC voltage, pay attention to personal safety and do not touch the metal part of the test pen with your hands.

3) Measurement of current. Insert the black test lead into the "COM" hole first. If measuring the current greater than 10A, insert the red test lead into the "10A" jack and turn the knob to DC "A"; if measuring the current less than 200mA, insert the red test lead into the "mA" jack, turn the knob on to a suitable range within DC mA. Once adjusted, you can measure. String the multimeter into the circuit, keep it stable, and you can read it. If the display is "1", then the range should be increased; if "-" appears on the left side of the value, it means that the current flows into the multimeter from the black test lead.

To measure currents greater than 10A, a clamp-on ammeter is required.

4) Resistance measurement: Insert the test leads into the "COM" and "VΩ" holes, turn the knob to the desired range in "Ω", and connect the test leads to the metal parts at both ends of the resistor, which can be touched by hand during measurement. Resistance, but do not touch both ends of the resistance with your hands at the same time, this will affect the measurement accuracy, the human body is a conductor with a large but limited resistance. When reading, keep the test leads in good contact with the resistance, and pay attention to the unit: the unit digit is "Ω", the thousand digit is "KΩ", and the mega digit is "MΩ".

2. Clamp ammeter

The clamp meter can be changed to different ranges through the shift of the switch. However, it is not allowed to operate with power on when shifting gears. The accuracy of the clamp meter is generally not high, usually 2.5 to 5 grades. For the convenience of use, there are also switches of different ranges in the meter for measuring different levels of current and measuring voltage.

The clamp meter was originally used to measure AC current, but now the multimeter has all the functions it has, which can measure AC and DC voltage, current, capacitance, diode, triode, resistance, temperature, frequency and so on.

Clamp-on ammeters can measure photovoltaic DC current and inverter output AC current. Special attention should be paid to the fact that some clamp-on ammeters do not have DC function, and the jaws must be closed tightly, and the range cannot be changed while electrified.

3. Shake table (megger)

When exposed to heat and moisture, the insulating material ages, resulting in a decrease in insulation resistance, resulting in electrical leakage or short-circuit accidents in electrical equipment. In order to avoid accidents, it is required to frequently measure the insulation resistance of various electrical equipment. Determine whether the insulation degree meets the equipment needs. There are two ways to measure common resistance: low voltage measurement and high voltage measurement. However, due to the generally high value of insulation resistance (usually megohm level), the measured value under low voltage cannot reflect the real insulation resistance value working under high voltage conditions. Megohmmeter, also called insulation resistance meter, is the most commonly used instrument for measuring insulation resistance. It has a high-voltage power supply itself when measuring insulation resistance, which is the difference between it and a resistance measuring instrument. Megohmmeters are convenient and reliable for measuring insulation resistance.

When the megger is working, it generates high voltage itself, and the measurement object is electrical equipment, so it must be used correctly, otherwise it will cause personal or equipment accidents. Before use, first make the following preparations:

1) Before the measurement, the power supply of the device under test must be cut off, and the short-circuit discharge to the ground must be performed.

2) For equipment that may induce high-voltage electricity, this possibility must be eliminated before measurement can be performed.

3) The surface of the object to be measured should be clean to reduce the contact resistance and ensure the correctness of the measurement results.

4) Before measurement, check whether the megger is in normal working condition, mainly check its "0" and "∞" points. That is, shake the handle to make the motor reach the rated speed. The megohmmeter should point to the "0" position when it is short-circuited, and it should point to the "∞" position when it is open.

5) When measuring the insulation resistance of electrical equipment with a megohmmeter, be sure to pay attention that the "L" and "E" terminals cannot be reversely connected. The correct connection method is: "L" wire E" ground terminal button grounded equipment shell, "G" shield terminal connected to the insulating part of the equipment under test. If the "L" and "E" are reversed, the leakage current flowing through the insulator and the surface will be collected to the ground through the shell, and flow into the measuring coil from the ground through "L", so that the "G" loses its shielding effect and supplies the measuring tape to the ground. come a big error.