Switchgear and Switchboard Inspection and

Testing Guide

Switchgear maintenance is essential for continued reliable operation. Photo: Twins Chip Electrical Industry

Substations and switchgear in an electrical system perform the functions of

voltage transformation, system protection, power factor correction metering, and
circuit switching.

Electrical power aparatus, 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.

Safety Considerations
Warning: Only qualified electrical personnel familiar with the equipment, its
operation and the associated hazards should be permitted to work on
switchboards and switchgear. Always be certain that the primary and secondary
circuits are de-energized before attempting any testing or maintenance.

Switchgear and Switchboard Inspection and Testing Guide Contents

Visual/Mechanical Inspections
• General Visual and Mechanical Inspections

• Moisture and Corona Inspections

• Wiring and Bolted Connection Checks

• General Wiring Checks

• Moving Parts and Interlocks

• Insulators and Barrier Checks

Electrical Tests
• Bolted Connection Electrical Tests

• Insulation Electrical Tests

• Dielectric Withstand Tests

• Control Wiring Electrical Tests

• Instrument Transformers

• Circuit Breakers and Switches

• Control Power Transfer Scheme

• Metering Electrical Tests

• Current Injection Tests

• System Function Test

• Cubicle Heaters

• Surge Arresters

• Dual-Source Phasing Check

General Visual and Mechanical Inspection of

Switchgear
Switchgear should be inspected for proper anchorage, alignment, grounding and required clearances. Photo: General
Electric.

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 electircal industry accepted methods of cleaning.
3.) For initial acceptance, verify that fuse and/or circuit breaker sizes, types,
and protective device settings match the project drawings and coordination
study. Circuit breaker's 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.
Many electric inspection protocols call for using ultrasound to test enclosed electric gear before opening to prevent
arc flash incidents. Video: UE Systems Europe.

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.
 Key Interlock Scheme Example. Photo: Kirk Key Interlocks

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 manufacturer standard accepted lubrications and
techniques.

Inspect the lubrication state of the circuit breaker operating mechanism latch faces and rollers. Photo Credit: ABB
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:
Wickens 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.
Example of switchgear shutter operation. Video by Twins Chip Electrical Industry.

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 microhms, B-phase bus measures 90 microhms, C-

phase bus measures 135 microhms. Investigate values that deviate by more than
50% of 90 microhms (90 * 1.5 = 135 microhms).

Insulation Electrical Tests for Switchgear and

Switchboards
1.) Insulation–resistance tests should be performed with a megohmmeter 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 depends 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
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.
 Photo: AC Hipots are recommended for dielectric withstand testing circuit breakers. Photo: HV, Inc.

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.

Important: Dielectric withstand voltage tests should not proceed until

insulation-resistance levels are raised above the recommended minimum
values. Dielectric Withstand is an optional test when performing routine
maintenance per ANSI/NETA-MTS 2019 Section 7.1.B.3.

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.

Important: Units with solid-state components could be damaged if not properly

isolated (via removal of plugs and/or fuses) before applying test voltage. Be sure
to follow all manufacturers' recommendations when performing dielectric tests on
solid state components.

Solid-state components could be damaged if not properly isolated before applying test voltage. Photo: Square D.

2.) Minimum insulation-resistance values of control wiring should be comparable

to previously obtained results but not less than two megohms. This test is
optional for both maintenance and initial acceptance. Refer to NETA-ATS/MTS
Section 7.1.B.4 for more information.

Instrument Transformer Tests for Switchgear and

Switchboards
 Current transformers are just some of the many instrument transformers found in switchgear and switchboards.
Photo: ABB.

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

• Insulation Resistance Test

• 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 / Magnetron Atmospheric Condition (MAC)

• 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.

Recommended Reading:

• 10 Critical Tests for New Power Circuit Breakers

• Primary vs. Secondary Injection Testing for Circuit Breakers

• 5 Often Overlooked Tests for Power Circuit Breakers

• 3 Basic Electrical Tests for Medium-Voltage Circuit Breakers

• 4 Methods of Medium-Voltage Breaker Design

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 a fall-of-potential or lternative ground resistance test in accordance

with 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 IEEE Standard 142 for more information on this topic.

Metering Electrical Tests for Switchgear and

Switchboards

Metering devices are verified using secondary voltage and current levels. Photo: EATON

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 watthour 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.

Tip: Infrared cameras are the easiest way to verify heater functionality without
making contact with energized electrical equipment.

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.

Recommended: Surge Arrester Field Maintenance and Testing

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.

Remember to always follow safe work practices when performing

energized work!
References
• ANSI/NETA Standard for Acceptance Testing Specifications 2013
Edition

• ANSI/NETA Standard for Maintenance Testing Specifications 2015

Edition

• NFPA-70B Recommended Practice for Electrical Equipment

Maintenance