Electrical Switchgear

Switchboards and switchgear in an electrical system perform the functions of voltage transformation, system protection, power factor correction metering, and circuit switching. Electrical power apparatus, such as transformers, regulators, air switches, circuit breakers, capacitors, and lightning arresters comprise the components necessary to perform these functions.

This guide provides a general overview for the inspection, testing and maintenance techniques used on switchgear and switchboard assemblies, and their associated components.

General Visual and Mechanical Inspection of Switchgear

1.) Inspect the physical, electrical, and mechanical condition of switchgear or switchboard, including its anchorage, alignment, grounding and required clearances. When performing acceptance testing, verify that the equipment nameplate data matches project drawings and specifications. This is important because switchboards are designed and rated for specific applications and should not be used otherwise, unless explicitly approved by the manufacturer.

2.) The unit should be clean and all shipping braces, loose parts, and documentation shipped inside the cubicles removed. Keep all documentation in a safe location for maintenance personnel in the future while loose parts and switchgear tools should be safely stored outside of the enclosure for easy access. When performing maintenance programs, clean the assembly using industry accepted methods of cleaning.

3.) For initial acceptance, verify that fuse and/or circuit breaker sizes, types, and protective settings match the project drawings and coordination study. Circuit breakers equipped with microprocessor-communication packages should be programmed with the proper digital address. All instrument transformer current and voltage ratios should also correspond to project drawings.

Moisture and Corona Inspections for Switchgear and Switchboards

If corona occurs in switchgear assemblies, it is usually localized in thin air gaps that exist between a high-voltage bus bar and its adjacent insulation or between two adjacent insulating members. Corona might also form around bolt heads or other sharp projections that are not properly insulated or shielded. Corona in low-voltage switchgear is practically nonexistent.

1.) Inspect for evidence of moisture or corona when performing maintenance inspections. On outdoor assemblies, roof or wall seams should be checked for evidence of leakage, and any leaking seams should be sealed with weatherproof caulk.

Prolonged leakage can be identified by rust or water marks on surfaces adjacent to and below leaky seams. The assembly base should be checked for openings that could permit water to drain into the interior, and any such openings should be caulked or grouted. Larger openings should be sealed to prevent rodent intrusion.

2.) All interior and exterior lighting should be checked for proper operation. It is essential for personnel safety that the area be well lit at all times in case of emergency response and other security reasons.

Wiring and Bolted Connection Checks for Switchgear and Switchboards

1.) Bolted electrical connections should be inspected for high resistance, either by use of a low-resistance ohmmeter (DLRO), calibrated torque-wrench, or infrared scan. Loose bolted electrical connections can lead to higher energy consumption and eventual equipment failure if not properly addressed.

When using a low-resistance ohmmeter, investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.

Bolt-torque levels should be in accordance with manufacturer’s published data. Use NETA Table 100.12 in the absence of manufacturer’s data.

General Wiring Checks for Switchgear and Switchboards

Loose control wires can lead to catastrophic failure if they are part of a critical protective circuit, such as a protective relay for a circuit breaker. Other critical functions like electrical charging and re-closing of circuit breakers can be inhibited if poor connections overheat and lose integrity.

1.) Check that all wiring connections are tight and that wiring is secure to prevent damage during routine operation of moving parts, especially when removing draw-out circuit breakers or opening and closing cubicle doors. Gently tug on control wires to ensure a tight connection or use a screwdriver to gently verify torque on the connection. Infrared scans are also very effective for finding loose wires in control circuits.

Moving Parts and Interlock Checks for Switchgear and Switchboards

1.) Confirm the correct operation and sequencing of electrical and mechanical interlock systems. Attempt closure on locked-open devices and attempt to open locked-closed devices.

2.) Test key interlock systems by making key exchanges with all devices included in the interlock scheme as applicable. All of these systems are essential for safety of both the operator and the equipment.

Lubrication of Switchgear and Switchboards

1.) Check for appropriate lubrication on moving current-carrying parts and moving/sliding surfaces to keep everything operating smoothly. This includes hinges, locks, and latches when performing maintenance tests. Lubricate as necessary using industry accepted lubrications and techniques.

Insulators and Barrier Checks for Switchgear and Switchboards

Tracking is an electrical discharge phenomenon caused by electrical stress on insulation. This stress can occur phase-to-phase or phase-to-ground. Although tracking can occur internally in certain insulating materials, these materials as a rule are not used in medium- or high-voltage switchgear insulation. Tracking, when it occurs in switchgear assemblies, normally is found on insulation surfaces.

Accumulated dirt, oil or grease might require liquid solvents or other alternative methods to be removed. Photo Credit: Wickes Dry Ice Blasting

1.) Electrical insulators should be inspected for evidence of physical damage or contaminated surfaces. Damage caused by electrical distress is normally evident on the surface of insulating members in the form of corona erosion or markings or tracking paths.

2.) Inspect barrier and shutter assemblies for proper installation and operation. All active components should be exercised; mechanical indicating devices should be inspected for correct operation.

3.) Ensure that vents are clear and filters are in place. Screens covering ventilation openings should be in place to prevent entry of rodents or small animals.

Bolted Connection Electrical Tests for Switchgear and Switchboards

1.) Perform resistance measurements through bolted electrical connections with a low-resistance ohmmeter. Measure line/load bus resistance end-to-end and to each distribution section.

2.) Verify dual-source switchgear bussing is correct at the tie breaker. Compare resistance values to values of similar connections and investigate values that deviate by more than 50 percent of the lowest value.

Example: A-phase bus measures 109 micro Ohms, B-phase bus measures 90 micro ohms, C-phase bus measures 135 micro Ohms.

Investigate values that deviate by more than 50% of 90 micro ohms (90 * 1.5 = 135 micro ohms).

Insulation Electrical Tests for Switchgear and Switchboards

1.) Insulation–resistance tests should be performed with a Meg ohm meter for one minute on each bus section, phase-to-phase and phase-to-ground. The test voltage to be used is dependent on the rating of the equipment and should be applied in accordance with manufacturer’s published data. ANSI/NETA Table 100.1 can be used as a guideline if manufacturer’s data cannot be found.

2.) Insulation-resistance values of bus insulation depend on voltage class and should be in accordance with manufacturer’s published data or ANSI/NETA Table 100.1. Values of insulation resistance less than Table 100.1 or manufacturer’s recommendations should be investigated.

Dielectric Withstand Test for Switchgear and Switchboards

Photo: AC Hipots are recommended for dielectric withstands testing circuit breakers. Photo: HV, Inc.

1.) Perform a dielectric withstand voltage test on each bus section, each phase-to-ground with phases not under test grounded, using a test voltage in accordance with manufacturer’s published data. If no manufacturer recommendation for this test exists, reference ANSI/NETA Table 100.2.

2.) Apply test voltage for one minute. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application, the test specimen is considered to have passed the test.

Control Wiring Electrical Tests for Switchgear and Switchboards

1.) Perform insulation-resistance tests on control wiring with respect to ground. Apply 500 volts dc for 300-volt rated cable and 1000 volts dc for 600-volt rated cable for one minute each.

2.) Minimum insulation-resistance values of control wiring should be comparable to previously obtained results but not less than two Meg ohms. This test is optional for both maintenance and initial acceptance.

Instrument Transformer Tests for Switchgear and Switchboards

Current transformers are just some of the many instrument transformers found in switchgear and switchboards.

The procedure for inspecting and testing instrument transformers is beyond the scope of this guide as each type has its own procedure. Instrument transformers generally include current transformers, voltage transformers, and control power transformers. Conduct electrical tests on instrument transformers in accordance with ANSI/NETA Section 7.10. Where applicable, testing of instrument transformers generally include:

  • Visual/Mechanical Inspection
  • Dielectric Withstand
  • Turns Ratio Tests
  • Excitation Tests
  • Burden Test
  • Power/Dissipation Factor
  • Secondary Wiring Integrity

Results of electrical tests on instrument transformers should be in accordance with ANSI/NETA Section 7.10.

Circuit Breaker and Switch Tests for Switchgear and Switchboards

It’s essential that circuit breakers be tested and maintained to ensure proper operation during electrical faults. Photo: Vacuum Interrupter Testing

The procedure for the inspection/testing of circuit breakers and switches is beyond the scope of this guide as each type and voltage class has its own procedure. Conduct electrical tests on circuit breakers in accordance with ANSI/NETA Section 7. Where applicable, testing of circuit breakers generally include:

  • Visual/Mechanical Inspection
  • Insulation Resistance
  • Dielectric Withstand
  • Contact/Pole Resistance
  • Electrical Operations
  • Vacuum Integrity
  • Power/Dissipation Factor
  • Protective Devices and Instrument Transformers

Results of electrical tests on circuit breakers and switches should be in accordance with ANSI/NETA Section 7.

Control Power Transfer Scheme Test for Switchgear and Switchboards

1.) Switchgear and switchboard assemblies equipped with multiple control power sources should be checked for proper function of the control transfer scheme by connecting a rated secondary voltage to each source. Transfer relays should perform as designed when the primary source is lost.

Ground Resistance Electrical Tests for Switchgear and Switchboards

1.) Perform resistance measurements through bolted ground connections with a low-resistance ohmmeter, if applicable. Compare bolted connection resistance values to values of similar connections and investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.

2.) Determine the resistance between the main grounding system and all major electrical equipment frames, system neutral, and derived neutral points by means of point-to-point testing using a low-resistance ohmmeter. Values which exceed 0.5 ohm should be investigated.

3.) Perform fall-of-potential or alternative ground resistance test in accordance with ANSI/IEEE 81 on the main grounding electrode or system. The resistance between the main grounding electrode and ground should be no greater than 5 ohms for large commercial or industrial systems and 1 ohm or less for generating or transmission station grounds, unless otherwise specified by the owner. Reference ANSI/IEEE Standard 142 for more information on this topic.

Metering Electrical Tests for Switchgear and Switchboards

Metering device inspections and tests are beyond the scope of this guide. Generally, metering devices are verified using secondary voltage and current levels supplied by a relay test set or other secondary source.

  1. Determine accuracy of all meters and calibrate watt hour meters in accordance with ANSI/NETA Section 7.11.

Current Injection Tests for Switchgear and Switchboards

Current-injection tests will prove current wiring is in accordance with design specifications. This is an optional test according to ANSI/NETA.

1.) Perform current-injection tests on the entire current circuit in each section of switchgear by secondary injection with magnitudes that produce a minimum current of 1.0 ampere flows in the secondary circuit. Verify correct magnitude of current at each device in the circuit.

System Function Test for Switchgear and Switchboards

The procedure for System Functional Testing exceeds far beyond the scope of this document. System function tests should be performed in accordance with ANSI/NETA-ATS Section 8 during initial switchgear/switchboard acceptance. Results of system function tests should be in accordance with ANSI/NETA-ATS Section 8.

Cubicle Heater Tests for Switchgear and Switchboards

Moisture accumulation is prevented by heat and air circulation. It’s important, therefore, to make sure the heating and ventilating systems are functioning properly to reduce internal condensation.

1.) The operation of switchgear/switchboard heaters should be verified along with their controller. Heaters should be operational.

Surge Arrester Tests for Switchgear and Switchboards

Inspection and testing procedures for surge arresters exceeds the scope of this document. Surge Arresters should be performed in accordance with ANSI/NETA-ATS Section 7.19. Testing these devices typically consist of applying a high voltage across the arrester to ground and observing the leakage current.

Dual-Source Phasing Check for Switchgear and Switchboards

1.) During initial acceptance, perform phasing checks on double-ended or dual-source switchgear to insure correct bus phasing from each source. Phasing checks should prove the switchgear or switchboard phasing is correct and in accordance with the system design.

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