Electrical Fault Reporting and Maintenance: A Worked Case | Cobler
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A Trip Is a Clue: Fault Reporting and Electrical Maintenance
The final part of Building Electrical Fundamentals is a worked case. A chilled-water pump in our example office tower trips on overload. Read the lamps and the BMS alarms, find the circuit on the SLD, write the fault record and the escalation brief, and see what the competent person checks.
Tan Kok XinBuilding Electrical Fundamentals
Part 11 of 11 in Cobler's Building Electrical Fundamentals course. New here?See the course page.
Part 10 ended with chilled-water pump CHWP-2 stopped and its TRIP lamp lit. The lamp tells you which device operated, but not why. When a pump trips, what should you record, who should you tell, and what should you leave alone?
The quickest reaction is to press reset and hope the pump runs. But a trip means a protective device saw something wrong. This final part is a worked case in electrical fault reporting and maintenance, built on an overload trip of the office tower's CHWP-2. You get the same information a facility team would have: alarms from the building management system (BMS), what can be seen at the panel, the single line diagram (SLD) and the maintenance log. You then work through six tasks, each with a worked answer.
Reporting a fault well is something every facility team member can do. Finding and fixing the cause is work for an authorised, Suruhanjaya Tenaga (ST) registered competent person. In the tower, that is the site's chargeman. This case shows where one job ends and the other begins.
How to use this exercise
Read the case, then try each task before reading its Worked answer. You need a notepad, not a calculator.
Breakers and discrimination: the protective device nearest a fault should be the one that opens. From Circuit Breaker Types (Part 4).
What is inside a starter panel: contactor, overload relay, control circuit. From (Part 9).
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It is Tuesday 15 September 2026, a hot afternoon. The tower's chiller plant is running two chillers, CH-1 and CH-2, each with its own primary chilled-water pump, CHWP-1 and CHWP-2. The third chiller and pump, CH-3 and CHWP-3, are on standby. Each chiller has a flow switch that stops it if its pump stops.
At 2:47pm, the duty operator in the BMS room sees these alarms:
Time
BMS alarm or reading
14:47:12
CHWP-2 TRIP
14:47:12
CHWP-2 run command ON, run status OFF
14:47:40
CH-2 stopped: low chilled-water flow
14:55:00
Chilled water supply temperature above its 6 °C design value and rising
CHWP-1 and the condenser water pump CDWP-1 still show as running.
The operator walks to the chiller plant room. The CHWP-2 starter panel door is closed. There is no smell of burning and no unusual noise from the panel. The pump motor is silent.
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What the operator can see at the CHWP-2 starter panel without opening it.
The maintenance log has one earlier entry for this pump: "25 Aug 2026, 3:10pm. CHWP-2 tripped on overload. Reset by chargeman. No fault found. Returned to service."
Task 1: Which device operated, and what does that tell you?
Use the panel door, the alarms and the CHWP-2 control circuit from Part 10.
Which protective device has operated?
Why is the pump still stopped even though the BMS is still giving a run command?
What does the trip tell you about the cause, and what does it not tell you?
Worked answer
The overload relay FC1 has tripped. The TRIP lamp PF2 is lit through the overload relay's normally open contact FC1 97-98, which closes only when the relay trips. The BMS TRIP alarm agrees.
When FC1 tripped, its normally closed contact FC1 95-96 opened. That contact sits between the live rail and wire 2, before the circuit splits into the Hand and Auto branches. So there is no path to the contactor coil QA1, whatever the BMS relay KF1 does. The contactor opened, the pump stopped, and the RUN lamp went out. The BMS sees its command ON but the status OFF, which is why it also raised the mismatch alarm.
An overload trip means the motor drew more current than the relay's setting for long enough to trip it. It points to a sustained overcurrent, not a short circuit: a short circuit would normally have opened the short-circuit protection instead. It does not tell you why the current was high. Possible reasons include a problem with the pump or its motor, such as a worn bearing; an electrical problem, such as a loose connection or a lost phase; low or unbalanced supply voltage; or an overload setting that does not match the motor. Finding which one is the competent person's job. A trip is a clue, not a diagnosis.
Task 2: Where is the pump on the SLD, and what else is affected?
Only the CHWP-2 branch has stopped; the transformer, the MSB and SSB-CH are still supplying the other pumps.
Write down the supply path to CHWP-2, from the transformer to the pump.
CHWP-1 and CDWP-1 are still running. What does that tell you about where the problem is?
What is the effect on the building, and what should happen about it?
Worked answer
TR1 → MSB section A → feeder breaker → SSB-CH → outgoing breaker → CHWP-2 starter panel → CHWP-2 motor. This line is the SLD reference for the fault record.
CHWP-1 and CDWP-1 are fed from the same sub-switchboard, SSB-CH, and they are still running. So TR1, the MSB and SSB-CH are still supplying power. The problem is on the CHWP-2 branch: in its starter panel, its cable or its motor and pump. This matches the idea of discrimination from Part 4: the device nearest the problem, here the overload relay, operated, and the devices further up stayed closed.
With CH-2 stopped, the plant has lost one of its two running chillers on a hot afternoon. The chilled water is warming up, and the building will soon feel it. Starting the standby pair, CHWP-3 and CH-3, is an operating task, not electrical work, and it does not involve the tripped pump. In the tower, the operating procedure allows the duty operator to start the standby pair from the BMS after informing the chief engineer. The operator does that at 2:58pm, and records it.
Task 3: What may the operator do, and what is for the competent person?
A colleague suggests several things. Sort them into "the operator may do this" and "leave this to the competent person".
Press the overload reset button and try Start, to see if it was a one-off.
Photograph the panel door, the motor nameplate and the BMS alarm screen.
Open the panel door to look for burn marks.
Turn the overload relay's setting up a little so it stops tripping.
Start the standby pump and chiller from the BMS under the operating procedure.
Note whether there is a smell, noise or heat near the pump and panel, from where you stand.
Put the CHWP-2 selector in OFF so the BMS cannot keep trying to start it.
Worked answer
The operator may do this: 2 (photographs), 5 (standby pair under the operating procedure), 6 (note smell, noise or heat from where you stand) and 7 (selector to OFF), but 7 only if the site procedure allows it.
Leave this to the competent person: 1 (reset and restart), 3 (open the panel) and 4 (change the overload setting).
Here is why:
Reset and restart (1). This is the second overload trip in three weeks. Restarting without finding the cause could damage the motor further or cause a more serious fault. Whether an operator may ever press a reset is set by the site's written procedure. Here the decision belongs to the chargeman.
Opening the panel (3). The panel contains 400 V and 230 V parts, and the isolator is still ON. Opening it is work for the competent person, who isolates, locks off and proves the circuits dead first.
Changing the setting (4). The overload setting is chosen to match the motor. Turning it up hides the problem and removes the motor's protection. Only the competent person, working to the design, may change a protection setting, and the change must be recorded.
Selector to OFF (7). This is a sensible step, but it changes the state of the equipment. Do it only if the site procedure allows the operator to, and record that you did. Otherwise, ask the chargeman.
Observing (2, 6) and operating the standby plant (5) do not involve opening anything or changing any protection.
Task 4: Write the fault record
Using everything above, write the fault record for this trip. Keep it short and factual.
Worked answer
A good fault record answers: what happened, when, where, what you saw, what was done and what is known from before. Here is one version:
TRIP lamp lit, RUN lamp off, selector AUTO, isolator ON. BMS: CHWP-2 TRIP; command ON, status OFF; CH-2 stopped on low flow at 14:47:40
Other equipment
CHWP-1 and CDWP-1 running normally. SSB-CH still supplying
Conditions
Hot weekday afternoon, two chillers on duty
Observations
No smell, noise or visible damage from outside the panel. Motor silent. Photos taken
Actions
Panel not opened, not reset. Standby CHWP-3 and CH-3 started from the BMS at 14:58 under the operating procedure, chief engineer informed
History
Previous overload trip 25 Aug 2026, reset, no fault found
Three details make this record useful to the competent person:
The time comes from the BMS alarm log. It can then be matched against other data, such as the chiller plant sub-meter trend.
The SLD reference lets anyone find the circuit on the drawing and in the building.
The history shows this is a repeat trip, which changes how the fault is treated.
Task 5: What will the competent person check, and what should the records show?
The chargeman arrives at 3:20pm. You will not do these checks, but you should know what to expect and what records to have ready.
What kinds of checks is the chargeman likely to make?
Which records should the facility team be able to hand over?
Worked answer
The chargeman decides the checks, and the site's procedures and their certificate set what they may do. Typical checks for a repeat overload trip include:
Comparing the overload setting with the motor nameplate full-load current, to check the setting matches the motor.
Inspecting the panel with the circuit isolated, locked off and proved dead: connections, contactor contacts, signs of overheating.
Insulation tests on the motor and its cable, with the circuit isolated.
Checking the current in each of the three phases during a controlled test run. Any measurement with the supply on is done under a controlled procedure by authorised personnel.
Asking the mechanical team to check the pump and motor bearings, because a mechanical problem can make the motor draw more current.
Useful records include:
the current SLD and the panel's control drawings (Part 10)
the protection settings, including the overload setting for CHWP-2
dated test and inspection reports for the installation
the fault log, with this record and the one from 25 August
the planned maintenance history for the pump and panel, including any thermographic (infrared camera) surveys of the switchboards and panels
Records and intervals. Keep the current single-line diagram, protection settings and dated test reports. Regulation 110 of the Electricity Regulations 1994 requires competent-person checks and tests of non-domestic installations at least every five years. It requires checks, tests and calibration of protective relays and devices at least every two years, or as directed by ST. Routine checks and any shorter maintenance intervals still apply, and a defect should never wait for the next scheduled test.
Routine rounds help find problems before they cause a trip. From outside the equipment, a facility team can record:
panel lamps and any alarms on switchboards, starter panels, the automatic transfer switch (ATS) and the uninterruptible power supply (UPS), see When the Mains Fails (Part 8)
smells, noises, heat or discoloured covers around switchboards and panels
readings in the substation, such as the transformer temperature gauge and any fault passage indicators, taken by or with the authorised person, because the substation is a restricted area, see Inside the Consumer Substation (Part 2)
Thermographic surveys of live panels, and all testing, are done by trained, authorised staff.
Task 6: Write the escalation brief
The chief engineer asks for a short message to the chargeman and to the maintenance contractor. Write it in five lines or fewer.
Worked answer
CHWP-2 (primary chilled-water pump 2) tripped on overload at 14:47 today, Tue 15 Sep. TRIP lamp lit, RUN off, selector AUTO, isolator ON; supply path TR1 → MSB section A → SSB-CH → CHWP-2 panel. CH-2 stopped on low flow; standby CHWP-3 and CH-3 running since 14:58. Panel not opened or reset. This is the second overload trip since 25 Aug, so please investigate the cause before any restart. Photos and BMS alarm log attached.
A good brief says what failed, when, where on the SLD, what state things are in now, what has and has not been done, and what you are asking for. It gives facts, not guesses about the cause.
What happened next
This is how our fictional case ends. With the panel isolated, locked off and proved dead, the chargeman found that one of the three motor cable connections in the starter panel was loose and discoloured by heat. A loose connection adds resistance in one phase and unbalances the motor currents, so the other phases carry more current than normal and the overload relay trips. At the first trip, the connection had not yet shown visible damage, so the cause was not found.
The chargeman remade the connection and checked the overload setting against the nameplate. The mechanical team found the pump bearings in good condition. CHWP-2 returned to duty after a controlled test run. The fault record was closed with the finding, and the chiller plant panels were added to the next thermographic survey.
The repeat trip was the clue. Because the operator recorded it and did not reset the relay, the cause was found before it did more damage.
Worth knowing: A loose connection often shows up as a hot spot before it trips anything, which is what an infrared (thermographic) survey looks for (Inside the Consumer Substation introduced these surveys). That is why the chiller plant panels were added to the next survey: it can catch the next loose connection before a pump stops.
Where to go next
If one task gave you trouble, go back to the part it draws on:
If all six tasks made sense, you have finished the course. You can find the switchboards, breakers, drawings and control panels in your own building, read what they are telling you, and give a competent person the information they need. For the wider rules on safe practice in non-domestic installations, the reference is ST's Non-Domestic Electrical Installation Safety Code.
Check your understanding
An MCCB feeding a pump panel trips instead of the overload relay. Is that the same kind of problem as in this case? Probably not. An overload relay trips on a sustained current somewhat above the motor's rating. If the breaker opened and the overload relay did not, a short circuit or another large fault current is more likely, although breakers also have overload elements of their own. That is a different and often more serious problem. Record which device operated, because it tells the competent person where to start.
A pump trips once, is reset, and runs normally for three weeks. Should the first trip still go in the fault log? Yes. A single trip may have been a one-off, but if it happens again, the earlier record shows a pattern. In this case, the second trip, together with the log entry from 25 August, is what made the chargeman look for a cause instead of resetting again.
Recap: A trip is a clue, not a diagnosis. Identify which device operated from the lamps and the BMS, and use the control circuit to understand why the equipment will not restart. Use the SLD to find the supply path and to see what is still live. Record the time, the indications, the device, the SLD reference, what was done and the history. Observe from outside and operate standby plant under the procedure; leave resets, opening panels and protection settings to the competent person. Keep the SLD, protection settings, test reports and fault log up to date: Regulation 110 sets the minimum test intervals, and a defect should never wait for the next scheduled test.
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Tan Kok XinBuilding Electrical Fundamentals
