Part 2 – De-Energized Maintenance
In part one of this series we discussed electrical maintenance options that can be performed with the system energized. For review these were Infrared Survey, Millivolt Drop, Ultrasonic Monitoring, Partial Discharge Testing and Protective Relay testing. In Part 2 of this series we will discuss electrical maintenance activities that can be performed while the system is deenergized. This is a broad topic so for the sake of brevity I will focus on low voltage switchboards.
Let’s start with some definitions. The U.S. National Electrical Code (NEC) defines a switchboard as “a large single panel, frame, or assembly of panels on which are mounted, on the face, back, or both, switches, over-current and other protective devices, buses, and usually instruments”. A switchboard rated to less than 1000 volts is a low voltage switchboard. NFPA 70E defines and Electrically Safe Work Condition to be” a state in which an electrical conductor or circuit part has been disconnected from energized parts, locked/tagged in accordance with established standards, tested to ensure the absence of voltage, and grounded if determined necessary.” There is a widely accepted practice for checking for a zero-energy state known as live-dead-live. This practice involves the use of a meter or a voltage detection device to test for voltage. The user will test a known energized circuit to assure the meter or voltage detection device is reading accurately, then test the area to be worked on and then test the known energized circuit again. This assures the meter was working properly while testing the area to be worked on preventing an accident from occurring from a faulty meter or voltage detection device.
Before we get into the various test methods and the associated benefits, let’s discuss the importance of regular preventive maintenance of your electrical system. Quite often the electrical system gets ignored because there are few or no moving parts and if the power is on, nobody really thinks about it. The truth is, left unchecked, every electrical system will have a failure, at some time and that failure will always cause more damage and interruption than if it were prevented. Dust and dirt on buss work alone will eventually cause a failure. Thermal stresses from heating and cooling cause bolted connections to loosen and create higher than acceptable resistance. Operating mechanisms in breakers and switches become inoperable over time. For more information regarding recommended maintenance intervals and recommended testing refer to the standards set forth by the International Electrical Testing Association (NETA) www.netaworld.org.
Insulation Resistance (Meggar)
The Insulation Resistance test, commonly known as the Meggar test, measures the integrity of the switchboard insulation using a megohmmeter. The bussing, phase to ground and phase to phase is subjected to voltages as high as 10,000 volts to stress the insulation. is This test cannot be performed with the switchboard energized because the testing requires direct contact with the bus. When performed properly, the Insulation Resistance Test can identify areas in the switchboard where the insulation is breaking down which will eventually cause a flashover. The most common source of failure is dirt, contamination or carbon tracking from an energized component to a grounded component.
Contact Resistance (Ductor)
The contact resistance test or (Ductor) test is performed with a Digital Low Resistance Ohmmeter. The purpose of this test is to measure the resistance between bolted or mechanical connections such as buss joints. Generally speaking, the lower the resistance the better. The NETA standard allows for the use of a DLRO to measure resistance or a calibrated torque wrench to verify tightness of connections. I prefer measuring contact resistance over checking tightness with a torque wrench because it produces less risk. No matter how many times I read the torque specs for a 3/8” stainless steel bolt, my human nature takes over and I find myself over torqueing a bolt because 259-inch pounds just doesn’t seem tight enough. In addition, if the contact area of the buss was not properly prepared before installation, no matter how tight the bolts, you will not get a good connection.
Ground Fault Test
The National Electrical Code (NEC) requires Ground Fault Protection for equipment in Articles 215.10, 230.95, 240.13 and 517.17. NEC Article 230.95 requires that all service disconnects over 1,000-amps must have Ground Fault Protection in addition to regular overcurrent protection systems. The Ground Fault protection system is designed to detect low magnitude faults that could cause harm to personnel. The system can be built into a circuit breaker or it can be a system separate from the breakers in the switchboard. For this discussion we will focus on separately derived ground fault systems. The Ground Fault protection system is made of three main parts, A current transformer, a relay and a monitoring panel. The relay is set to operate and open a protective device when it senses a ground current, from the CT at predetermined magnitude and duration. The test panel usually has indicator lights that display the status of the system and allows operators to perform a no-trip test of the system. To verify the relay and the balance of the system are operating as designed a test technician injects current of a given magnitude through the current transformer and records the magnitude of current when the protective device opens. The delay or duration is also recorded. Ground fault relay settings should be developed by a qualified Engineer to prevent unnecessary nuisance tripping while providing adequate protection for personnel as required by the NEC.
Over-Potential Test (Hipot)
The Over-Potential test is similar to the Insulation Resistance test but is performed at a much higher voltage. The maximum Insulation Resistance test voltage for a low voltage switchboard (600V class) is 1000V while the overpotential can be as high as 2,200V, per the latest International Electrical Testing Association (NETA) Acceptance Testing Standard. The Over-Potential test measures leakage current between a current carrying conductor and any other conductive material that is at a different potential than the section under test. Usually each phase of a system is tested phase to phase and phase to ground. This is a pass-fail test, if the leakage current is above the acceptable limit the equipment is failed and repairs/corrections need to be made.