Electrical preventive maintenance: the program along the power path, from the supply to the motors
The electrical program as a path from the incoming supply to the motors, with the dominant failures and the tasks for each class, the four kinds of task that do most of the work, the hidden protection functions that need a test rather than an inspection, motors and drives where the volume is, and how the statutory floor relates to the program.
Key takeaways
- Electrical equipment fails quietly and then all at once. Most of its failures are connections, insulation and mechanisms that have not moved, and all three can be found before they fail.
- The program follows the power path: supply and protection, transformers, switchboards, motor control and drives, motors, with cables and earthing between everything and the standby supplies beside it.
- Four kinds of task do most of the work: thermography under load, insulation tests when stopped, operate-and-test for anything that sits idle, and oil analysis for anything with oil in it.
- Protection is a hidden function. A relay that has not been injection tested, a breaker that has not tripped and a battery that has not been load tested are each a protection that may not be there.
- Statutory and insurer requirements set a floor, not the program. The program comes from the failure modes, the same as for any other equipment.
Electrical equipment gives less warning than mechanical equipment and more than most people think. A loose connection runs hot for months before it arcs. Insulation ages for years before it breaks down. A breaker that has not operated in a decade is stiff long before the fault that asks it to trip. None of that is visible on a walk through the switchroom, and all of it is visible to the right test.
This guide sets out the electrical preventive maintenance program as a path, from the incoming supply to the motors, with the dominant failures and the tasks for each class of equipment on it. It covers the four kinds of task that do most of the work, the hidden functions that need a test rather than an inspection, how the program sits with the statutory and insurer requirements, and where programs fail. It extends the preventive maintenance guide, whose checklist workbook now carries these tasks in its library.
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Preventive maintenance checklist template
The site's preventive maintenance checklist workbook, whose task library now carries the electrical rows from this guide: transformers, switchboards, protection, drives, motors, UPS and generators, each with a failure mode and a pass criterion.
The program follows the power path
An electrical program is easiest to build along the path the power takes. The incoming supply and its protection, the transformers, the switchboards, the motor control centres and drives, the motors and the loads. Cables and earthing run between everything, and the emergency and standby supplies sit beside the path, idle until the day they are needed. Each class has a short list of dominant failures, and each failure has a task that finds it.
The table gives the classes, the failures and the tasks, with the reference each draws on. Intervals are deliberately not in the table, because they come from the environment, the duty and the consequence, by the same reasoning as for any other equipment. The rows in the checklist workbook carry typical starting intervals, marked as such.
1Incoming supply and protection
Fails by
Protection relay settings drifted or never set, batteries in the tripping supply dead, earthing degraded
Tasks
Secondary injection test of protection relays, tripping battery capacity test, earth grid resistance test, thermographic survey under load
2Transformers
Fails by
Insulation ageing, moisture and acid in the oil, loose connections, tap changer wear, cooling blocked
Tasks
Oil sample for dissolved gas, moisture and acidity, insulation resistance and polarisation index, thermography of connections and radiators, tap changer inspection, cooling fan and pump test
3Switchboards and switchgear
Fails by
Loose or corroded connections, contaminated insulation, breaker mechanisms stiff from not operating, protection untested
Tasks
Thermographic survey under load, de-energised clean and torque check, breaker operate and timing test, protection relay secondary injection, partial discharge survey on medium voltage, arc flash labels current
4Motor control centres and drives
Fails by
Cooling filters blocked, capacitors aged, fans failed, loose power connections, contactor contacts eroded
Tasks
Filter and fan check, thermography, capacitor condition by age and ripple, contactor inspection, parameter backup, drive event log review
5Motors
Fails by
Bearing failure first, then winding insulation, then rotor bar and connection faults
Tasks
Vibration readings, greasing to the chart, insulation resistance and polarisation index, motor current signature where fitted, thermography of the terminal box, winding temperature trend
→ Power flows right. Thermography under load reads every joint on the path in one survey
Cables and earthingBetween everything
Insulation resistance on circuits that can be isolated, thermography of terminations, visual inspection of trays and supports, earth continuity and bond resistance tests
Emergency and standby suppliesWhat runs when the supply does not
Battery capacity test and replacement by age, generator start and load test on load bank, transfer switch operation under test, fuel quality
| Class | Dominant failures | Tasks | Reference |
|---|---|---|---|
| Incoming supply and protection | Protection relay settings drifted or never set, batteries in the tripping supply dead, earthing degraded | Secondary injection test of protection relays, tripping battery capacity test, earth grid resistance test, thermographic survey under load | Network operator connection rules, AS 2067, the site's protection study |
| Transformers | Insulation ageing, moisture and acid in the oil, loose connections, tap changer wear, cooling blocked | Oil sample for dissolved gas, moisture and acidity, insulation resistance and polarisation index, thermography of connections and radiators, tap changer inspection, cooling fan and pump test | AS 2374 and IEC 60076 series, the transformer maker's schedule |
| Switchboards and switchgear | Loose or corroded connections, contaminated insulation, breaker mechanisms stiff from not operating, protection untested | Thermographic survey under load, de-energised clean and torque check, breaker operate and timing test, protection relay secondary injection, partial discharge survey on medium voltage, arc flash labels current | AS 2467, AS/NZS 3000, NFPA 70B |
| Motor control centres and drives | Cooling filters blocked, capacitors aged, fans failed, loose power connections, contactor contacts eroded | Filter and fan check, thermography, capacitor condition by age and ripple, contactor inspection, parameter backup, drive event log review | The drive maker's schedule, AS/NZS 3000 |
| Motors | Bearing failure first, then winding insulation, then rotor bar and connection faults | Vibration readings, greasing to the chart, insulation resistance and polarisation index, motor current signature where fitted, thermography of the terminal box, winding temperature trend | IEEE 3007.2, the motor maker's schedule, AS/NZS 1359 for new motors |
| Cables and earthing | Mechanical damage, moisture in terminations, thermal ageing at overloaded joints, earth continuity lost at bonds | Insulation resistance on circuits that can be isolated, thermography of terminations, visual inspection of trays and supports, earth continuity and bond resistance tests | AS/NZS 3000, AS/NZS 3017 for verification |
| Emergency and standby supplies | UPS batteries aged, generator fails to start or to take load, transfer switch untested | Battery capacity test and replacement by age, generator start and load test on load bank, transfer switch operation under test, fuel quality | AS/NZS 3010 for generating sets, AS 2676 for batteries, NFPA 70B |
Four kinds of task do most of the work
Thermography under load is the condition task that reads the whole path. Every connection, every breaker contact, every overloaded conductor and every blocked cooling path shows as heat against its neighbour, and the survey needs no outage. Done yearly, and more often on critical boards, it finds most of the connection failures that would otherwise arc. It is only useful under load, which is why it is scheduled for a production day rather than a shutdown.
Insulation tests when stopped read the slow failures. Insulation resistance and polarisation index on motors, transformers and cables, trended over years rather than compared with a limit once, show the ageing long before breakdown. Oil analysis on transformers does the same for the one piece of insulation that can be sampled while running: dissolved gases say what is happening inside, and moisture and acidity say how the oil is ageing.
Operate-and-test is the failure-finding task for anything that sits idle. A breaker is operated and timed. A protection relay is injection tested against the settings in the protection study. A transfer switch is transferred. A generator is started and loaded on a load bank. A battery is discharged to prove it carries the load. None of these can be inspected into confidence; each has to be made to work.
Fixed interval servicing is the smallest share. Breaker lubrication and contact inspection, drive filter and fan changes, capacitor replacement by age, greasing to the chart. The intervals come from the maker and the duty, and they are corrected on findings like any other.
A switchroom that looks fine has told you nothing. The faults are at the temperatures and resistances you did not measure.
Protection is a hidden function
Most electrical equipment fails evidently: the motor stops, the lights go out. Protection fails hidden. A relay with the wrong settings, a breaker whose mechanism is stiff, a tripping battery that is flat, an earth grid that has corroded, all look exactly the same as working ones until the fault that asks them to act. The consequence of that fault with the protection absent is the fire or the arc flash the protection existed to prevent.
Hidden functions are managed by failure-finding tasks at an interval set from the unavailability the site can tolerate, the same reasoning as in the preventive maintenance guide and the FMECA that identifies them. In practice that means every protection relay injection tested and its settings reconciled to the protection study, every breaker operated and timed, every tripping supply load tested, and the earthing measured, on a schedule that is kept even when nothing has ever been found. Nothing found is the point.
Motors and drives: where the volume is
A plant has a few transformers and hundreds of motors, and most of the electrical work orders are on motors and the drives that feed them. Motor failures run in a known order: bearings first, by a long way, then winding insulation, then rotor and connection faults. The program follows that order. Vibration readings and correct greasing on the bearings, because over-greasing fails as many bearings as neglect. Insulation resistance trended yearly on the windings. Current signature analysis where it is fitted for the rotor. Thermography on the terminal box for the connections.
Drives fail by heat and by age. Blocked filters and failed fans cook the power electronics; capacitors age whether or not the drive runs. A six-monthly inspection of the cooling path, a capacitor replacement by age, and a backup of the parameters after every change, which is the one task that turns a drive failure from a week of reconfiguration into an afternoon, cover most of it. The drive's own event log, read at the same visit, is the best condition data on the motor it feeds and is almost never looked at.
The floor and the program
Statutory, regulatory and insurer requirements set a floor under an electrical program, and the floor is real. Workplace electrical safety regulations require equipment to be kept in a safe condition. In-service testing of plug-in equipment follows AS/NZS 3760. High voltage installations carry their own obligations, the network operator sets conditions at the point of connection, and an insurer will have its own schedule for switchgear and transformers. A program that does not meet the floor is a liability.
A program that only meets the floor is not a program. The floor is written for the regulator's purposes and the insurer's risk, not for the plant's failure modes, and it says nothing about the drive that keeps tripping or the motor that fails every two years. The tasks come from the failure modes, class by class, the way the table above sets them out, and the floor is checked against the result rather than used as the result. NFPA 70B, now a standard rather than a recommended practice, is the most complete published reference for building it that way.
Where programs fail
- Thermography done during the shutdown, with no load on the board, so every joint reads cold.
- Insulation readings compared with a limit once and never trended, so the decline is seen only when it crosses the line.
- Protection relays never injection tested, and settings in the relay that nobody has reconciled to the study.
- Breakers that have not operated since commissioning, assumed to trip.
- Standby generators started weekly and never loaded, so the one thing not tested is taking the load.
- UPS batteries replaced on failure, which is the one time they cannot be replaced.
- Drives with no parameter backup, so a failure is a week of reconfiguration from memory.
- Motors greased by the fitter's judgement rather than the chart, with the quantity unknown.
- A program built from the insurer's list and nothing else.
Common questions
What is electrical preventive maintenance?
The scheduled inspection, testing and servicing of a plant's electrical equipment, from the incoming supply to the motors, to find degrading connections, insulation and mechanisms before they fail. It uses the same four kinds of task as any preventive program: fixed interval servicing, inspection, condition monitoring such as thermography and oil analysis, and failure-finding tests on protection and standby equipment. NFPA 70B is the most complete published guide to it; in Australia the installation standards and AS 2467 for switchgear set the floor.
How often should a switchboard be maintained?
A thermographic survey under load every year, or every six months on boards carrying critical loads, because loose and corroded connections are the dominant failure and they show as heat under load. A de-energised clean, inspection and torque check, with the breakers operated and timed and the protection relays injection tested, every three years or so, aligned with a shutdown. The interval is tightened where the environment is dirty, hot or vibrating, and AS 2467 gives the basis.
What does thermography find on electrical equipment?
Resistance where it should not be: a loose or corroded connection, an overloaded conductor, a failing breaker contact, an unbalanced load, a blocked cooling path on a transformer or a drive. All of them show as a temperature rise against an adjacent phase or a reference point, under load. It is the most valuable single survey in an electrical program because it reads every joint on the power path without an outage, and it is the one most often skipped because nothing is visibly wrong.
How often should motors be tested?
Vibration readings on the bearings every four weeks or so for critical motors, because bearings are the first thing to fail. Greasing to the maker's chart, with the quantity and grease specified, because over-greasing fails more bearings than under-greasing. Insulation resistance and polarisation index yearly when the motor can be stopped, trended rather than compared with a limit alone. Motor current signature analysis where it is fitted picks up rotor bar and eccentricity faults while running.
What is secondary injection testing?
A test of a protection relay by injecting currents and voltages into its inputs from a test set, to confirm it operates at the set values within tolerance and in the set time, without disturbing the primary circuit. It proves the relay and its settings, and is the failure-finding task for a protection function that otherwise sits idle for years. The settings tested are checked against the protection study, which is how settings that drifted or were never applied are found.
Is electrical maintenance a statutory requirement?
Parts of it are, and the parts vary by jurisdiction and site. Australian workplace electrical safety regulations require electrical equipment to be maintained in a safe condition, in-service testing of some equipment follows AS/NZS 3760, and high voltage installations carry specific obligations. Insurers add their own. Those requirements are a floor. A program built only to meet them maintains the regulator's list, not the plant's failure modes.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
- NFPA 70B, Standard for electrical equipment maintenance (NFPA)
- AS 2467-2008 Maintenance of electrical switchgear (Standards Australia)
- IEEE 3007.2 Recommended practice for the maintenance of industrial and commercial power systems (IEEE)
- ISO 14224:2016 Collection and exchange of reliability and maintenance data for equipment (ISO)
- Best practices, metrics and guidelines for maintenance and reliability (SMRP)
Related case studies and tools
- Reliability engineering and asset strategy program (case study)
Related reading
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