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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.
It's 2:47pm on a hot Tuesday, and the building management system (BMS) in the office tower shows an alarm: CHWP-2 TRIP. Down in the chiller plant room, the TRIP lamp on its panel is lit. The quickest reaction is to press reset and hope the pump runs. But a trip means a protective device saw something wrong.
Back in Part 1, a trip at the MSB got a row of thumbs up in the WhatsApp group. This time we'll do better, with a worked case in electrical fault reporting and maintenance. As Part 1 promised, we'll trace the tripped pump through the drawings and breakers, and write a fault report the chargeman can use. So when a pump trips, what should we record, who should we tell, and what should we leave alone?
Reporting a fault well is something every facility team member can do. Finding and fixing the cause is work for an authorised competent person registered with Suruhanjaya Tenaga (ST), and in the tower that's the site's chargeman. This case shows where one job ends and the other begins.
How the case works
We get the same information a facility team would have. That's the BMS alarms, what we can see at the panel, the single line diagram (SLD) and the maintenance log. Then we work through six tasks. Try each one first with a notepad, because we won't need a calculator. Then open the worked answer underneath to check your thinking.
We'll only need ideas from earlier parts:
- Reading the SLD to find where a piece of equipment gets its supply, from How to Read a Single Line Diagram (Part 3).
- Breakers and discrimination, where the device nearest a fault should be the one that opens, from Circuit Breaker Types (Part 4).
- What's inside a starter panel, the contactor, overload relay and control circuit, from Inside an Electrical Control Panel (Part 9).
- Reading a control circuit, including the CHWP-2 tracing exercise, from How to Read Control Wiring Diagrams (Part 10).
The case
The date is Tuesday 15 September 2026. 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.
So the operator walks to the chiller plant room. The CHWP-2 starter panel door is closed. There's no smell of burning and no unusual noise from the panel, and the pump motor is silent.
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
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 doesn't it tell you?
Show the worked answer
1. 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.
2. 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's 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.
3. An overload trip means the motor drew more current than the relay's setting, for long enough to trip it. So it points to a sustained overcurrent. A short circuit would normally have opened the short-circuit protection instead. But the trip doesn't tell us 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
- an overload setting that doesn't match the motor
Finding which one is the competent person's job. A trip is a clue, not a diagnosis.
Task 2: where it is on the SLD
Here's the part of the tower's SLD that feeds the chilled-water pumps. The full drawing is in How to Read a Single Line Diagram.
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's the effect on the building, and what should happen about it?
Show the worked answer
1. The path is 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.
2. CHWP-1 and CDWP-1 are fed from the same sub-switchboard, SSB-CH, and they're 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. That matches discrimination from Part 4. The device nearest the problem, here the overload relay, operated, and the devices further up stayed closed.
3. 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 doesn't involve the tripped pump. In the tower, the operating procedure lets the duty operator start the standby pair from the BMS after informing the chief engineer. The operator does that at 2:58pm, and records it.
Task 3: who does what
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.
- From where you stand, note whether there's a smell, noise or heat near the pump and panel.
- Put the CHWP-2 selector in OFF so the BMS can't keep trying to start it.
Show the worked answer
The operator may do this: 2 (photographs), 5 (standby pair under the operating procedure) and 6 (smell, noise or heat from where you stand). Number 7 (selector to OFF) is also fine, but 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's 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, and 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) don't involve opening anything or changing any protection.
Task 4: the fault record
Now write the fault record for this trip, using everything above. Keep it short and factual, and try it before you read on.
A good fault record answers what happened, when, where, what you saw, what was done and what's known from before. Here's one version:
Item | Record |
|---|---|
Date and time | Tue 15 Sep 2026. First alarm 14:47:12 (BMS clock) |
Reported by | Duty operator, from BMS alarms |
Equipment | CHWP-2, primary chilled-water pump, chiller plant room |
SLD reference | TR1 → MSB section A → SSB-CH → outgoing breaker → CHWP-2 starter panel |
Indications | 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. So it can 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: the chargeman's checks
The chargeman arrives at 3:20pm. We won't do these checks, but we 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?
Show the worked answer
1. 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 that the setting matches the motor.
- Inspecting the panel with the circuit isolated, locked off and proved dead, looking at the connections, the contactor contacts and any 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.
2. 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, the protection settings and the dated test reports. As we saw in Part 6, Regulation 110 of the Electricity Regulations 1994 sets the minimum intervals. It requires competent-person checks and tests of non-domestic installations at least every five years. It also 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 and starter panels
- alarms on the automatic transfer switch (ATS) and the uninterruptible power supply (UPS), as in When the Mains Fails (Part 8)
- the status windows on surge protective devices, where fitted, as in Lightning and Surge Protection (Part 7)
- 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 (Inside the Consumer Substation, Part 2)
The substation is a restricted area, so those readings are taken by or with the authorised person. And thermographic surveys of live panels, like all testing, are done by trained, authorised staff.
Task 6: 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.
Show the worked answer
Here's one version.
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, and standby CHWP-3 and CH-3 have run 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, and where it is on the SLD. It says what state things are in now, what has and hasn't been done, and what you're asking for. It gives facts, not guesses about the cause.
What happened next
So how does our made-up case end? With the panel isolated, locked off and proved dead, the chargeman found the cause. 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 hadn't yet shown visible damage, so nobody found the cause.
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.
So the repeat trip was the clue. Because the operator recorded it and didn't 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, and that's what an infrared (thermographic) survey looks for. Inside the Consumer Substation introduced these surveys. That's why the chiller plant panels went on the next survey, because 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:
- Task 1: How to Read Control Wiring Diagrams and Inside an Electrical Control Panel
- Task 2: How to Read a Single Line Diagram and Circuit Breaker Types
- Task 3: Earthing and RCDs (Electricity Fundamentals) for why the boundary matters
- Task 5: Neutral and Earth: Why They Stay Separate (Part 6) for the checks a competent person makes on earthing and protection
For the wider rules on safe practice in non-domestic installations, the reference is ST's Non-Domestic Electrical Installation Safety Code.
And if all six tasks made sense, you've finished the course. Think back to that WhatsApp message in Part 1: "Supply tripped at the MSB." Now we can say which board and which breaker that means, what it feeds, and what's still live. We can find the switchboards, breakers, drawings and control panels in our own building, and read what they're telling us. And when something trips, we can hand the competent person the record they need, which is a lot more useful than a thumbs up.
Check your understanding
1. An MCCB feeding a pump panel trips instead of the overload relay. Is that the same kind of problem as in this case?
Show the answer
Probably not. An overload relay trips on a sustained current somewhat above the motor's rating. If the breaker opened and the overload relay didn't, a short circuit or another large fault current is more likely. Breakers also have overload elements of their own, though. That's a different and often more serious problem. So record which device operated, because it tells the competent person where to start.
2. A pump trips once, is reset, and runs normally for three weeks. Should the first trip still go in the fault log?
Show the answer
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 and the log entry from 25 August made the chargeman look for a cause instead of resetting again.
Recap
- A trip is a clue, not a diagnosis.
- Find which device operated from the lamps and the BMS, and use the control circuit to see why the equipment won't restart.
- Use the SLD to find the supply path and to see what's 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.
This is Part 11 of 11 in Cobler's Building Electrical Fundamentals course. Previous: How to Read Control Wiring Diagrams.
Cobler builds CobiNeural, a platform that shows a facility team its building's energy, water and indoor air data as live numbers across the whole site. To see how your building performs, talk to us.
FAQ
Frequently asked questions
Record the date and time, from the BMS alarm log if you have one, and who noticed and how. Record the equipment name and location, and its SLD reference, which is the supply path from the transformer to the equipment. Note what the lamps, selector and isolator show, the BMS alarms, and which other equipment is still running. Add any smell, noise or heat you noticed from outside, what was done and by whom, and any earlier trips of the same equipment. Keep it factual and do not guess the cause.
Only if the site's written procedure allows it, and never as a way of testing whether the fault has gone. A trip means a protective device detected a problem, and the cause should be found first, especially after a repeat trip. Opening panels, changing protection settings and investigating the fault are work for an authorised, ST-registered competent person such as the site's chargeman.
The motor drew more current than the overload relay's setting for long enough to trip it. That points to a sustained overcurrent rather than a short circuit. Possible causes include a mechanical problem with the pump or motor, or a loose connection or lost phase. Low or unbalanced voltage, or a setting that does not match the motor, can also cause it. The trip tells you where to look, not what the cause is.
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.
Say what failed, when, and where it is on the SLD. Give the state of the equipment now: lamps, selector, isolator and any standby plant running. Say what has and has not been done, list any earlier trips, and state what you are asking for, such as an investigation before restart. Attach photos and the BMS alarm log, and state facts rather than guesses about the cause.
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