Insulation is the protection of equipment from the passage of electrical current through it. When electrical installations operate, their insulation and structure are exposed to the environment, aging and wear due to heating. All this negatively affects the performance of the equipment, so it is important to measure the insulation resistance of the cable from time to time. We have provided the measurement methodology below.
Content:
- What devices are used?
- Test procedure
- How often are measurements taken?
- Who conducts the audit and why is it needed?
What devices are used?
Before starting work, you need to measure the temperature of the ambient air. What is it for? If the cable line has water particles during negative temperatures, then they turn into pieces of ice under the influence of frost, and ice is a dielectric that has no conductivity. Therefore, when the resistance is measured at a negative temperature, these pieces of ice will not be detected.
Then, in order to measure the insulating layer of the wiring (its resistance), it is necessary to have special devices and means for diagnostics. You can measure the resistance with a special device called a megohmmeter (pictured below).
With a megohmmeter, you can measure the resistance for a voltage of 2500 V (insulation of low-voltage and high-voltage lines). The measurement takes place for a voltage of 500-2500 V of control power lines (control circuits, power circuits, short-circuits etc.).
Such devices must undergo state verification every year, as a result of which a stamp is put, which indicates the serial number and the date when it is necessary to pass the next verification. Each cable has its own standards, GOST and PUE, according to which checks and tests of wires are carried out.
Test procedure
Before measuring the insulation resistance of wires and cables, do the following:
- Check the condition of the device. To do this, check the direction of the arrow with open (arrow points to infinity) and closed (points to zero) wires.
- Check for power failure. The wire must not be live.
- Ground the cable to be tested.
The measurement differs depending on the classification of the lines of force, but these differences are not significant. For example, the control cable has its own distinctive feature: in order to measure resistance, the wire does not need to be disconnected from the circuit.
The insulation of the devices is checked using special devices, which must not be touched during the tests. Readings should be taken only when the pointer of the device takes a stable position. The measurement is carried out within one minute. With electronic devices, things are faster and the result is displayed immediately on the screen. All data should be recorded in a notepad.
After all the data have been received, it is necessary to draw up an act and a test report. First of all, the obtained values should be compared with existing norms and requirements. Then make a conclusion: is the cable suitable for further operation. And only then draw up a protocol for measuring the cable insulation resistance. A sample of the protocol is shown in the photo below:
More details about how to use a megohmmeter, you can find out from our article!
How often are measurements taken?
In small organizations, resistance is measured at intervals of once every three years (according to GOST and PTEEP). The insulation of the wiring is recorded in the protocol, which, in addition to measurements, indicates and checking the serviceability of the RCD.
Insulation resistance measurement at hazardous facilities should be carried out every year. These are rooms where there is high humidity or high temperature. In industrial plants, such a measurement will prevent or avoid equipment shutdown. After the equipment has been inspected, a special report is drawn up, which fully indicates the condition of the electrical installations.
Measurement should be carried out according to the specified time. After all, thanks to this, it is possible to avoid in advance various emergency situations that can have serious consequences. Also, untimely verification carries fines that are imposed by the relevant authorities.
Below is a diagram of the frequency of checks, depending on the classification and category of the room:
Who conducts the audit and why is it needed?
In order to measure resistance, you must have a special permit and access. Based on this, the cable can only be tested by special companies and organizations that have qualified employees. They must receive appropriate training and receive the required electrical safety grade.
Measurement is necessary in order to identify damage in the equipment in advance. After all, insulation plays a significant role in the safety of working with electrical equipment. If the cable or wire is damaged, then the electrical installation becomes dangerous during operation. After all, a wire or cable can catch fire and cause a fire. If you check the cable in advance for the serviceability of the insulating layer, this will prevent such troubles as:
- premature failure of equipment;
- short circuit wiring;
- electric shock to the employee;
- emergency situations of various nature.
That is why it is very important to measure the insulation resistance of the cable. Finally, we recommend watching a useful video on the topic:
Now you know how to measure the insulation resistance of wires and cables. We hope the provided instruction was useful and interesting for you!
You probably don't know:
- The method of testing cable lines with increased voltage
- Phase-zero loop resistance measurement
- How to use a multimeter correctly
