Before performing this test, the position of the main switch, fuses, switches, etc. The line terminal of the megger is connected to phase terminal of the installation and the earth terminal of megger is connected to neutral wire.
The insulation resistance so measured should not be less than 0. In a low voltage installation, this test is performed to verify that all single pole switches have been connected to phase wire throughout the installation. It is very necessary to place all switches on phase so that when a switch is made OFF, the connected appliance is quite dead.
If the switch is connected to the neutral wire then the connected appliance will get phase even if the switch is in OFF position and remain alive. There is absolutely no difference in the functioning of the switch in either case, but from the safety point of view to avoid shock, etc.
The simple method of conducting the polarity test is by using a test lamp. Before performing this test the position of the main switch, fuses, switches, etc. One end of the test lamp is connected to earth wire and the other end to the incoming terminal of the switch.
If the lamp lights, it indicates that the switch is connected to phase wire, otherwise to neutral wire. To perform this test with the help of megger, the main switch is opened, the main fuses are withdrawn, all the switches are made ON and all the lamps are put in position.
In this test, megger should indicate a resistance value between 0. If it will be in bad condition then it will offer high resistance to the current. I have explained this test earlier in an article. You can refer to that article for earth resistance test. It is a attractive article because it gives true and insightful information regarding new electrical installation for house.
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I would surely recommend others to go through this link. This resource Insightelectrical. Manufacturer f ield t ests : More commonly specified than factory witness tests are requirements for the electrical equipment manufacturer to require factory-trained technicians to perform additional testing and adjustments in the field during or after installation. The most common tests are functional tests and demonstrations for the owner and staff.
Additional work may include relay- and protective-device settings. Contractor f ield t ests : Field testing by the installing electrical contractor is a common specification requirement.
Some of the more common contractor field tests include medium-voltage cable testing, load balancing, phase rotation, and infrared IR scanning of terminations and connections. The point of this testing is to check the integrity of cables usually medium-voltage immediately after pulling but before energizing, or as a condition assessment after years of operation.
Years ago, direct current dc high-voltage also called hi-pot was the common test for medium-voltage cables. The simplest way to explain hi-pot testing is the application of a very high voltage to cables to see if the insulation breaks down or remains intact.
It is basically a pass-or-break test. Concerns over damage to cables from hi-pot testing—especially on existing cables—has led to the preferred use of partial-discharge PD testing. There are several PD testing methods depending on whether the cable is live. Commonly in pre-energized new installs, PD involves the use of very low-frequency voltage, which is raised to a minimal level at which partial discharges would appear in insulation weakness, and sensors measure where this takes place.
PD testing also has the benefit of being used on new, old, and live cables. Load balancing is another electrical test that is specified by consulting engineers. During design, electrical engineers will design loads across a 3-phase system, typically on panelboard schedules, to be relatively balanced among each phase.
Overheating, power-quality issues, and energy loss can occur with unbalanced loads. During construction, however, loads often change from what is in original design documents, and it is good practice to specify final load balance testing.
IR scanning has become a very common requirement over the years. It is used in both new installation start-ups and ongoing maintenance. IR thermography images show excess heat is present in electrical systems due to unbalanced loads, loose or defective electrical connections, harmonics, overloads, and more. IR testing is an important part of pre-occupancy testing, when loads are being energized but the facility is not fully operational and can be worked on.
Soils testing: Often overlooked by electrical engineers is the option to specify soils testing for conductivity and thermal resistivity. An effective grounding system is one of the more critical factors for a trouble-free and safe electrical system. Soils vary greatly by region. Soil conditions also vary over seasonal time frames, rain, drought, and the application of landscaping materials and chemicals.
NEC Article discusses the need to achieve a maximum earth ground of 25 ohms. However, the origin of this number is unknown. Power cables generate heat, and heat affects resistivity and current-carrying capacity. The NEC has numerous tables adjusting capacity depending on the method of running those conductors in conduit, including underground. The best time to require electrical conductivity and thermal characteristic tests is during geotechnical testing. Electrical engineers should coordinate with their civil or structural engineering team members to include these electrical tests.
For some types of critical facilities, such as health care, data centers, laboratories, and government facilities, it is important to specify additional electrical testing by independent third-party contractors. It is beneficial for the engineer to specify that this third-party testing firm is an accredited member with documented and verified experience and abilities to objectively validate the electrical system. The consulting engineer may wish to confine the testing to certain parts of the system, such as emergency or standby systems, or may wish to limit the testing to certain voltage levels or amperage ratings.
A discussion with the building owner can also help in defining the limits of NETA testing. Electrical commissioning may be optional or required depending on the code or certifications required by the project.
The commissioning agent CxA or commissioning provider CxP is a critical part of the design and construction team and should be engaged early in the design and construction process. Some of the codes or standards that provide requirements or recommendations for commissioning include:. For electrical systems, this includes lighting, controls, daylight-harvesting controls, and any sustainable green power systems, like solar photovoltaic systems. Fundamental commissioning activities performed by the CxA include:.
LEED-rated facilities also offer additional credits for Enhanced Commissioning, which can provide additional points. It should be noted that enhanced commissioning is done by a third party independent of the design and construction teams. The third-party professional will:. Hospitals are key mission critical facilities where commissioning is required.
ASHE publishes the Health Facility Commissioning Program , an extensive document that includes recommendations for the CxA to be involved in all phases of the facility design including schematic, design development, and construction documents.
Data centers are another type of mission critical facility with some of the more rigorous requirements for commissioning electrical power systems. While some data centers will also pursue LEED certification, the commissioning process is much more extensive and includes many of the testing requirements identified here.
The data center commissioning process is commonly expressed in levels. Level 1 : Factory w itness t esting: The consulting engineer should outline the testing protocol in bid specifications. Level 2 : Site -a cceptance i nspection : When the equipment arrives onsite, it is inspected to confirm it meets specified requirements. Although Level 1 can minimize issues found in Level 2, changes often occur after factory testing and shipping have caused damage or because shipped items have missing parts.
Level 3 : Pre- f unctional t esting PFT : This level involves the inspection of the initial install equipment to verify that all equipment is installed properly and that installation meets specified standards and requirements. Equipment also is started for the first time to check basic functionality.
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