Electrical test technology exchange meeting and product exhibition

2021-09-15

Electrical test technology exchange meeting and product exhibition

After removing the cable connecting the sewage pump under the thermal relay, the author used a multimeter and a 500V megohmmeter to test the load cable and the sewage pump, but no abnormality was found! So I had to test the electric control circuit. After measuring the resistance of the multimeter, I found that the electric control circuit was also normal. However, I decided to test the power transmission of the electric control circuit first, but an unexpected scene happened - after the start button was pressed when the air switch was powered on with no load, it still tripped! This made me think that there was a problem with the air switch, and I immediately replaced it with a new air switch of the same model. Unexpectedly, under the no-load test of closing, the air switch still tripped! Faced with such a situation, I had to take the ultimate skill: throwing electrical devices and cables one by one from the bottom to the top. During the power-on test after disconnecting the thermal relay from the line, the air switch did not trip! Observe again that the six main terminals of the thermal relay are normal. Use the multimeter resistance gear to measure the interphase resistance of the thermal relay in two, and it still shows infinity! After looking around the scene, I thought for a second and then copied the 500V megohmmeter around me and carried out the phase-to-phase insulation measurement of the thermal relay. After only two turns of the megohmmeter, a surprising scene happened - the insulation resistance measured by the multimeter was safe, but it went to zero under the pressure of DC500V, and there was a intermittent and subtle "hissing" discharge sound inside the thermal relay!

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What are the correct steps for measuring insulation resistance with HD2705 megohmmeter

2021-09-15

What are the correct steps for measuring insulation resistance with HD2705 megohmmeter

（1） Before the test, remove the resistance of the tested power cable and all external wires, and discharge them to the ground. The discharge time shall not be less than 1min, and the power cable with large capacitance shall not be less than 2min to ensure safety and accurate test results. （2） Wipe away the dirt on the power cable terminal sleeve or core wire and its insulation surface with a dry and clean soft cloth to reduce surface leakage. （3） Place the tramegger in a stable and horizontal position to avoid shaking the tramegger with uneven force during operation, resulting in inaccurate readings. （4） Under no-load condition, turn the crank of the megger to the rated speed (120r/min) and adjust the pointer to "∞". （5） For multi-core power cables, the insulation resistance of each phase of core wire shall be tested separately. At this time, connect the outgoing line of the measured core wire to the terminal (L) of the megger, and connect the other core wires to the ground (lead package) and then connect them to the ground terminal (E) of the megger. In order to avoid the influence of leakage current on the insulation surface of power cable, the shake down screen should also be used to knock the wall (G) to completely knock the surface insulation out of the indication of the megger. For the power cable that has not yet been laid, a protective ring can be added on the insulation of the two sides of the tested core wire, and the two protective rings can be connected to the shielding terminal (G) of the megger. For the laid power cable, a protective ring can be added on the sleeves or insulation at both ends of the tested core wire with metal flexible wire, and the protective ring at both ends is connected with the shielding terminal of shake down, while another power cable core is used as the circuit of the shielding wire.

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Short circuit fault that must be solved with the aid of a megger

2021-09-15

Short circuit fault that must be solved with the aid of a megger

I believe that the majority of electricians have experienced short circuit fault in their daily work. I'm afraid the scene when the fault occurs can only be described as thrilling! As for the identification of short circuit fault, the electrical equipment to be treated can generally be known by sight and nose. Only in the process of handling the short circuit fault of electronic circuit must the multimeter and other instruments be used for maintenance. But recently, the author encountered a short circuit fault that can only be found with the help of a megohmmeter. Its maintenance process is really unforgettable. A few days ago, I received a call from a customer asking me to take a tool to the coal washing plant of a unit: an air switch of a sewage pump tripped and could not operate. After arriving at the site, I checked the on-site distribution cabinet and learned that the sewage pump control circuit is the most basic AC contactor start/stop self-locking circuit. However, at the time of initial assembly, a thermal relay was installed after the contactor to prevent the pump from blocking the rotation. In order to prevent failures caused by human factors (similar situations caused by human misoperation have been encountered in the practice), I have operated and checked in person. The result is consistent with the customer's report: close the air switch, press the start button, and the contactor will close after about 1 second delay, and the air switch will trip. In view of this situation, according to inertia thinking, I think the probability of short circuit fault of sewage pump is 80%, and the probability of short circuit fault of load cable is 20%.

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How to use the universal meter

2021-09-15

How to use the universal meter

Measuring voltage: select a good range when measuring voltage (or current). If you use a small range to measure large voltage, there will be a danger of burning the meter; If a large range is used to measure small voltage, the pointer deflection is too small to read. The selection of measuring range shall make the pointer deflect to about 2/3 of the full scale. If the magnitude of the measured voltage is not known in advance, the highest range gear shall be selected first and then gradually reduced to the appropriate range. A. Measurement of AC voltage: place one change-over switch of the multimeter at the AC and DC voltage range, and the other change-over switch at the appropriate range of AC voltage. The two probes of the multimeter can be connected in parallel with the circuit or load under test. B Measurement of DC voltage: place one change-over switch of the multimeter at the AC and DC voltage ranges, and the other change-over switch at the appropriate range of DC voltage, and connect the "+" probe (red probe) to the high potential, and the "-" probe (black probe) to the low potential, that is, let the current flow in from the "+" probe and out from the "-" probe. If the probe is connected reversely, the pointer of the meter head will deflect in the opposite direction, which is easy to bend the pointer. (6) Current measurement: when measuring the DC current, place one change-over switch of the multimeter at the DC current gear, and the other change-over switch at the appropriate range of 50uA to 500mA. The current range selection and reading method are the same as the voltage. When measuring, the circuit must be disconnected first, and then the multimeter is connected in series to the circuit under test in the direction of current from "+" to "-", that is, the current flows in from the red probe and out from the black probe. If the multimeter is connected in parallel with the load by mistake, the internal resistance of the meter head is very small, which will cause short circuit and burn the instrument. The reading method is as follows: actual value=indicated value × Range/full deviation (7) Measuring resistance: When measuring resistance with a multimeter, the following methods shall be followed: A Select the appropriate magnification gear. The scale line of the ohm gear of the multimeter is uneven, so the selection of the magnification gear should make the pointer stay at the thinner part of the scale line, and the closer the pointer is to the middle of the scale, the more accurate the reading is. Generally, the pointer should be between 1/3~2/3 of the scale. B Ohm zero. Before measuring the resistance, the two probes should be short-circuited, and the "ohm (electrical) zero adjustment knob" should be adjusted at the same time, so that the pointer just points to the zero position on the right of the ohm scale line. If the pointer cannot be adjusted to the zero position, the battery voltage is insufficient or there is a problem inside the instrument. And every time the magnification gear is changed, the ohm shall be adjusted to zero again to ensure accurate measurement. C Reading: the reading of the meter head multiplied by the multiplier is the resistance value of the measured resistance.

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How to use the universal meter

2021-09-15

How to use the universal meter

For the measurement of current and voltage, many people know that a universal meter should be used. Generally, a universal meter can measure DC current, DC voltage, AC voltage, resistance and audio level. Some can also measure AC current, capacitance, inductance and some parameters of semiconductors. It is a very practical tool (1) Be familiar with the meaning of each symbol on the dial and the main functions of each knob and selector switch. (2) Perform mechanical zero adjustment. (3) Select the gear and range of the transfer switch according to the type and size of the measured, and find out the corresponding scale line. (4) Select the position of the probe socket.

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