What Is High R Value Insulation?

Insulation resistance meters, also known as megohmmeters, shake meters, and megger meters. Insulation resistance meters are mainly composed of three parts. The first is a DC high-voltage generator to generate DC high voltage. display.

Used to measure the maximum resistance value,

(1) DC high voltage generator

To measure the insulation resistance, a high voltage must be applied to the measuring end. This high voltage value is specified in the national standard of insulation resistance meters as 50V, 100V, 250V, 500V, 1000V, 2500V, 5000V ...

There are generally three methods of generating DC high voltage. The first hand-cranked generator. About 80% of the megohmmeters produced in China adopt this method (source of shaking table name). The second type is to boost the DC voltage through the mains transformer and rectify it. The method adopted by general utility-type megohmmeters. The third method is to use a transistor oscillator or a dedicated pulse-width modulation circuit to generate a DC high voltage. The method used by general battery-type and commercial-type insulation resistance meters.

(2) Measurement circuit

In the shaking table (Megohmmeter) mentioned above, the measurement circuit and display part are combined into one. It is completed by a flow ratio meter head. This meter head consists of two coils with an included angle of 60 ° (about). One coil is connected at both ends of the voltage and the other coil is connected in series to the measurement circuit. middle. The deflection angle of the pointer of the meter head is determined by the current ratio in the two coils. Different deflection angles represent different resistance values. The smaller the measured resistance value is, the larger the coil current in the measurement loop is, and the larger the deflection angle of the pointer . Another method is to use linear

The megohmmeter generates high voltage when it is working, and the measurement object is electrical equipment, so it must be used correctly, otherwise it will cause personal or equipment accidents. Before using, you must first make the following preparations:

(1) Before the measurement, the power of the equipment under test must be cut off and the ground must be short-circuited to discharge. The equipment must not be subjected to live measurement to ensure the safety of people and equipment.

(2) For equipment that may induce high voltage electricity, this possibility must be eliminated before measurement. (3) The surface of the test object should be cleaned to reduce the contact resistance and ensure the accuracy of the measurement results.

(4) Before the measurement, check whether the megohmmeter is in a normal working state, and mainly check the two points of "0" and "". That is, shake the handle to make the motor reach the rated speed. The megohmmeter should be in the "0" position when short-circuited and in the "" position when open.

(5) The megohmmeter should be placed in a stable and stable place when used, and away from large external current conductors and external magnetic fields. After doing the above preparations, you can measure. When measuring, pay attention to the correct wiring of the megohmmeter, otherwise it will cause unnecessary errors or even errors.

There are three terminals of megohmmeters: one is L, which is the line end, one is E, which is the ground end, and the other is G, which is the shielding end (also called the protection ring). Generally, the insulation resistance to be tested is connected Between the "L" and "E" terminals, but when the surface of the insulator to be tested has serious leakage, the shield ring of the measured object or the part that does not need to be measured must be connected to the "G" terminal. In this way, the leakage current flows directly back to the negative terminal of the generator via the shield terminal "G" to form a loop, instead of flowing through the measuring mechanism (moving coil) of the megohmmeter. In this way, the effect of surface leakage current is fundamentally eliminated.In particular, it should be noted that when measuring the insulation resistance between the cable core and the appearance, the shield end button "G" must be connected well because when the air humidity is high or the cable is insulated When the surface is not clean, the leakage current on the surface will be very large.To prevent the impact of the measured object on its internal insulation measurement due to leakage, generally a metal shield ring is added to the outer surface of the cable, and the "G "End connected.

When using a megohmmeter to measure the insulation resistance of electrical equipment, it must be noted that the "L" and "E" terminals cannot be reversed. The correct connection method is: the "L" wire end button is connected to the conductor of the device under test, and "E" The grounding button grounds the equipment case, and the "G" shield terminates the insulation part of the device under test. If "L" and "E" are reversed, the leakage current flowing through the insulator and the surface is collected to the ground through the shell, and the ground flows into the measurement coil through "L", so that "G" loses its shielding effect and is given to the measurement tape. Comes with great error. In addition, because the degree of insulation between the internal lead of the "E" terminal and the casing is lower than that of the "L" terminal and the casing, when the megohmmeter is placed on the ground and the correct wiring method is used, the "E" terminal is opposite to the instrument casing. The insulation resistance between the ground and the shell is equivalent to a short circuit, which does not cause errors. When "L" and "E" are connected in reverse, the insulation resistance of "E" to ground is connected in parallel with the measured insulation resistance, which makes the measurement result biased. Small, bring large errors to the measurement.

It can be seen that if you want to accurately measure the insulation resistance of electrical equipment, you must use the megohmmeter correctly, otherwise you will lose the accuracy and reliability of the measurement.

Method and requirements of megohmmeter:

1. Before measuring, the megohmmeter should be kept in a horizontal position, hold the meter body with your left hand, and shake the megohmmeter handle with your right hand at a speed of about 120r / min. The pointer should point to infinity (), otherwise the megohmmeter is faulty.

2. Before the measurement, the power of the tested electrical equipment and circuit should be cut off, and the related components should be temporarily grounded to ensure the safety of the person and the megohmmeter and the accuracy of the measurement results.

3. The wiring must be correct when measuring. The megohmmeter has 3 terminals (L, E, G). When measuring the resistance of the circuit to ground, the L terminal is connected to the bare conductor of the circuit, and the E terminal is connected to the ground wire or metal shell. When measuring the insulation resistance of the circuit, the first and tail ends of the circuit are respectively connected to L and E; the insulation of the measurement cable In the case of resistance, to prevent the leakage current on the cable surface from affecting the measurement accuracy, the shield layer of the cable should be connected to the G terminal.

4. The measuring flexible wires leading out of the megohmmeter terminals should have good insulation, and an appropriate distance should be maintained between the two wires and between the wires and the ground to avoid affecting the measurement accuracy.

5. When shaking the megohmmeter, do not touch the terminals of the megohmmeter and the circuit under test to avoid electric shock. 6. After shaking the megohmmeter, do not short the terminals to avoid damage.

Insulation resistance meter selection requirements and inspection before use

(1) The voltage level of the instrument should be selected according to the rated voltage of the electrical component under test when it is working. When measuring the insulation resistance of the thermal element embedded in the winding and other heating elements, a 250V insulation resistance meter should generally be used.

(2) Before use, check the meter and its lead-out wires for normality. Short the two lead wires, shake the meter or turn on the power switch of the meter to enter the measurement state, the pointer of the meter is deflected to 0 or the digital indication value is 0, then the two lead wires are disconnected for measurement. The description is normal. As shown in Figure 2.

Insulation resistance meter wiring and measurement

(1) When measuring general electrical appliances such as motors, the L terminal of the instrument is connected to the component under test (such as a winding), and the E terminal is connected to the casing; in addition to the above requirements, the G terminal of the instrument should be Sheath connection of the cable under test. When using the hand-cranked megohmmeter, the speed of hand-cranking should be about 120r / min, and read after shaking until the indicated value is stable.

(2) After the measurement, use a conductor to discharge between the device under test (such as the winding) and the case, and then remove the lead wires. Direct disconnection may cause electric shock from stored charge ^[1] .