Maintenance of electrical components
In a factory, one hour of downtime costs more than a whole year of maintenance. The craft lies in detecting the problem before it stops production, and that is done with method, with instruments and with records.
01Three ways to maintain
Repairs are made when something fails. It is the only option for cheap, non-critical components, and the worst one for everything else: the failure happens at the worst moment and usually drags other damage along with it.
Work is done on a calendar or by hours of service: cleaning, re-tightening, checking, replacing wear parts. Simple to plan; sometimes parts that were still good get replaced.
Work is done according to actual condition, measured with instruments: thermography, vibration, insulation, current draw. It is the most efficient and the one that demands the best technical training.
Almost no electrical failure is sudden: there is a progressive degradation that can be measured. Contact resistance grows, insulation drops, temperature rises, current draw becomes unbalanced. All of that can be tracked over time, and a trend says far more than an isolated value. That is why the historical record is the most valuable tool in maintenance.
02Thermography as a diagnostic tool
| Difference from the matching point | Action criterion |
|---|---|
| less than 5 °C | Normal. Record it and follow it at the next inspection. |
| 5 to 15 °C | Minor anomaly: schedule the correction for the next maintenance. |
| 15 to 30 °C | Serious anomaly: correct it as soon as possible. |
| more than 30 °C | Critical: intervene immediately, there is a risk of failure and fire. |
A switchboard with no load is cold and shows nothing. The inspection is done with the installation in service and, if possible, with at least 40 % of the rated load, noting what current was flowing at that moment. Without that data, the measurement cannot be compared with the next one.
03Component by component
| Component | What is checked | Failure symptoms |
|---|---|---|
| Contactor | Condition of the main contacts, the coil and the core. Number of accumulated operations. | Humming, pitted or welded contacts, overheating, delay in closing. |
| Thermal relay | Setting to the motor's rated current, trip class, reset test. | Trips for no reason, or never trips. Setting that has drifted or was badly calibrated from the start. |
| Motor circuit breaker | Integrated thermal and magnetic protection. Setting, tightness and condition of the operating handle. | Same as the thermal relay, plus insufficient breaking capacity if it was poorly chosen. |
| Timer | Actual switching time against the set one. Condition of the output contacts. | Times that drift, sequences that get out of order, failed starts. |
| Fuses | Correct type and rating, condition of the fuse holders and contacts. | Fuse holders hot from a loose contact; fuse replaced by one of a higher rating. |
| Thermal-magnetic breakers | Terminal tightness, number of trips, signs of overheating. | Frequent tripping, stiff handle, discolored plastic around the terminals. |
| RCD | Actual tripping current and time, measured with an instrument. | Trip time above the permitted value, or it simply does not trip. |
| Terminals and wiring | Tightening torque, condition of the insulation, identification. | Hot spots, discolored or brittle insulation, illegible labels. |
| Relay class | Trip time at 7.2 · In | Application |
|---|---|---|
| 10 | less than 10 s | Normal starts: pumps, fans, conveyors. |
| 20 | 10 to 20 s | Heavy starts: compressors, mixers. |
| 30 | 20 to 30 s | Very long starts: mills, centrifuges with high inertia. |
A three-phase motor rated at 7.5 A makes the relay trip after twenty minutes of running. Measured with a clamp meter: 7.4 A – 7.6 A – 9.8 A. It is not an overload: it is imbalance.
The usual cause is a loose or pitted contact on one phase, which would also show up as a hot spot in the thermography. The relay is doing exactly what it should: the differential models detect the imbalance and protect the motor from heating that the average current does not reveal. Raising the relay setting so that it “stops tripping” destroys the motor in a few weeks.
04The maintenance plan
| Frequency | Tasks |
|---|---|
| Monthly | Visual inspection of switchboards, checking of signage and emergency lighting, testing the RCD test button. |
| Every six months | Thermography under load, cleaning of switchboards, checking ventilation and filters, checking protection settings. |
| Yearly | Re-tightening terminals with controlled torque, insulation measurement, instrument testing of RCDs, grounding measurement, inspection of contactors and contacts. |
| By hours | Replacement of contacts and coils according to the number of operations, and of the wear parts specified by the manufacturer. |
All electrical maintenance work starts the same way: disconnect, lock out with a padlock, tag, verify the absence of voltage with an instrument —testing it before and after— and, when applicable, ground and short-circuit. The lockout has a single owner: whoever placed it is the one who removes it.
When the measurement requires live equipment, such as thermography or a current measurement, work with arc-flash protective equipment, insulated tools and without opening more than necessary.
Every intervention is written down: date, equipment, task, measured values, spare parts used and who did it. That history is what makes it possible to see trends, justify a replacement before failure and detect that the same component has already failed three times —which almost always means the problem lies elsewhere—.
05In the lab
On a real switchboard at the school: identify each component, check the labeling, measure the current in each phase, check the setting of each protective device and fill in the survey sheet. Detect and list the nonconformities.
Using an adjustable current source, subject a thermal relay to 1.5, 3 and 7.2 times its setting and measure the trip time in each case. Plot it and compare it with the curve for the class declared by the manufacturer.
Take apart a contactor that is out of service: observe the main and auxiliary contacts, the arc chute, the coil and the core with its shading ring. Compare it with a new one and estimate the remaining life. Measure the contact resistance of both.
The teacher introduces faults into a training panel: loose terminal, misadjusted relay, open phase, dirty auxiliary contact. Diagnose with instruments, without opening things up blindly, and record the procedure followed and the time taken.
06Common mistakes
| Mistake | Consequence |
|---|---|
| Raising the thermal relay setting so that it does not trip | The motor's protection is eliminated and the motor ends up burning out. The relay was warning of a real problem. |
| Replacing a fuse with one of a higher rating | The protection stops protecting: the conductor can melt before the fuse does. |
| Doing thermography without load | You see nothing. Hot spots appear only with current flowing. |
| Relying only on the RCD test button | It checks the mechanism, not the sensitivity or the trip time. Those are measured with an instrument. |
| Not recording interventions | The trend is lost and the same diagnoses are repeated over and over. |
| Cleaning a switchboard with unfiltered compressed air | Moisture and compressor oil are blown onto the contacts. |
| Re-tightening without controlled torque | Loose terminals or stripped threads, depending on each person's strength. |
| Replacing the failing component without looking for the cause | If a contactor burns out three times, the problem is in the motor, in the load or in the sizing. |
07Self-assessment
What is the difference between preventive and predictive maintenance?
Preventive maintenance is done on a calendar or by hours; predictive maintenance is done according to actual condition measured with instruments, which is more efficient but demands more training.
What is compared in a thermographic inspection, and what difference is considered critical?
It is compared with the matching point —the same phase at another terminal—. A difference above 30 °C is critical and requires immediate intervention.
Why is thermography done under load?
Because with no current flowing there is no heating and the defects do not show up. The current at the time is also recorded, so that it can be compared with the next measurement.
What does the class of a thermal relay indicate?
How long it tolerates 7.2 times the rated current before tripping: less than 10 s in class 10, between 10 and 20 s in class 20, and between 20 and 30 s in class 30. It is chosen according to how demanding the start is.
A 7.5 A motor measures 7.4 – 7.6 – 9.8 A per phase. What is happening?
There is imbalance, not overload. The usual cause is a loose or pitted contact on one phase. The relay is protecting correctly: you have to look for the contact, not raise the setting.
What does the thermal relay protect, and what does the thermal-magnetic breaker protect?
The thermal relay protects the motor against overload; the thermal-magnetic breaker protects the cable and clears the short circuit.
What is checked on a contactor during maintenance?
The condition of the main and auxiliary contacts, the coil and the core, and the number of accumulated operations. Humming, pitting or welding of the contacts are signs it should be replaced.
Why is a fuse not replaced with one of a higher rating?
Because the fuse is sized to protect a particular conductor. With a larger one, the cable can melt before the protection acts.
What is the procedure before working on a switchboard?
Disconnect, lock out with a padlock, tag and verify the absence of voltage with an instrument, testing it before and after. Whoever places the lockout is the one who removes it.
What is the maintenance log for?
To see trends, justify replacements before failure and detect repetitions that indicate the cause is elsewhere.