According to the National Electrical Code (NEC), completed wiring installations should not have any short circuits, ground faults, or unauthorized connections to ground. These types of issues can be dangerous and can cause damage to the electrical system.
It is important to ensure that all wiring is properly installed and free from these issues to ensure the safety and reliability of the electrical system. The NEC provides guidance on the proper installation and grounding of electrical wiring to help prevent these types of problems from occurring.
Wiring Integrity (NEC 110.7)
Insulation failure is a common issue in electrical systems, both high voltage and low voltage. The main causes of insulation failure are heat, moisture, dirt, physical damage, chemical attack, sunlight, and excessive voltage stresses. These issues can occur during or after installation, and can lead to problems in the electrical system. To prevent insulation failure, it is important to properly install and maintain the insulation on electrical conductors and equipment, and to protect them from damage and exposure to adverse conditions.
To prevent damage to insulated conductors, it is important to select and coordinate overcurrent protective devices using tables of insulation thermal withstand ability. These tables, called “Allowable Short-Circuit Currents for Insulated Copper (or Aluminum) Conductors,” can be found in the publication ICEA P-32-1982 from the Insulated Cable Engineers Association. The tables provide information on the maximum allowable short-circuit currents for different types of insulated copper or aluminum conductors. This information can help ensure that the damage point of the insulated conductor is not reached, which is important for the safety and reliability of the electrical system. The National Electrical Code (NEC) provides additional guidance on the selection and coordination of overcurrent protective devices and other circuit componentsmeg in Article 110.10.
Insulation tests are conducted on electrical installations to assess the quality or condition of the insulation on conductors and equipment. One type of insulation test is an insulation resistance test, which applies a voltage across the insulation and measures the resistance using a device called a megohmmeter. The voltage used in the test typically ranges from 100 to 5000 volts, although for systems of 600 volts or less, a voltage of 500 to 1000 volts is usually sufficient. The megohmmeter indicates the insulation resistance directly on a scale calibrated in megohms. The quality of the insulation is evaluated based on the level of insulation resistance measured during the test. This information can be used to determine the condition of the insulation and identify any issues that may need to be addressed.
The insulation resistance of many types of insulation can vary with temperature, so it is important to correct the field data obtained during insulation tests to the standard temperature for the equipment being tested. This helps ensure that the results of the test accurately reflect the condition of the insulation. The insulation resistance measured during an insulation test is also inversely proportional to the volume of insulation being tested. This means that, all other conditions being equal, a larger volume of insulation will have a lower insulation resistance than a smaller volume of insulation. For example, a cable that is 1000 feet long would be expected to have a lower insulation resistance than a cable that is 100 feet long, if all other factors are the same.
The National Fire Protection Association’s publication 70B, “Recommended Practice for Electrical Equipment Maintenance,” provides guidance on test methods and establishing a preventive maintenance program for electrical equipment. This publication contains useful information on the proper use of insulation testers and the importance of having a thorough understanding of these devices in order to obtain meaningful test results. Instrument manufacturers can also provide additional information on specific test methods and the use of insulation testers. An insulation tester, such as a megohmmeter, is a device used to measure the insulation resistance of electrical conductors and equipment. It is important to have a thorough understanding of how to use these testers and to follow proper test procedures in order to obtain accurate and reliable results.
