Aircon DB box: finding the switch that feeds your unit
Every electrical fault conversation opens with someone saying to check the DB box, and nobody explains what the panel is. Reading it takes no tools and no risk. The labelling is the part that misleads.
By Team Snowflake | Updated 5 Aug 2026
What the DB box is doing in the flat
A DB box is a splitting point, not a control panel. One supply arrives at the flat. The board divides it into the separate circuits that serve the rooms, and hands each circuit its own protective device on the way out. Nothing else happens there.
Three names describe the same object and Singapore uses all three. DB box is the everyday term, short for distribution board. Consumer unit is the phrasing that turns up on paperwork and in product literature. Electricians move between them mid-sentence without meaning anything by it.
The board holds no intelligence about the aircon. There is no setting on it, no timer, and no memory of what failed. Every device on the rail either passes current or does not. A homeowner sent to look at it expecting an explanation will not find one there.
What is on offer is better than an explanation for the first phone call. The board is the only place in the flat where something has already measured the fault and acted on it. One device answered. Which one answered narrows the problem before anybody has driven over.
What sits inside a Singapore consumer unit
Three kinds of device occupy the rail, in a layout that barely varies between flats. A main switch commands the whole board. One or more earth-leakage devices sit over groups of circuits. A row of individual breakers takes one circuit each.
The main switch is the largest body on the rail and usually sits at one end. Everything below it goes dead when it opens. Its amp figure describes the flat's approved loading, not any single circuit. Singapore boards carry either 30 A or 40 A, and which one a flat holds depends on how it was built.
The device sitting wider than the rest, with a test button on its face, is the earth-leakage protection. It covers a group of circuits rather than one, which is why a single leaking appliance can take several rooms dark together. An earth leakage trip is a different event from an overload, and the two lead to different repairs.
The narrow row along the rest of the rail is the individual breakers, one circuit each. Every face carries an amp figure chosen for the cable behind it, not for the appliance at the far end. Reading an aircon breaker rating as a statement about the aircon is the most common misunderstanding at this board.
The earth arrangement is the one part that does not show on the front at all. It has its own bar behind the cover and its own conductor reaching every circuit. Aircon earthing deserves understanding on its own terms, and nothing about it can be confirmed by looking at a closed board.
| What you are looking at | How much of the flat it holds | What its trip has already narrowed |
|---|---|---|
| What you are looking atMain switch, the largest body, one end of the rail | How much of the flat it holdsEverything | What its trip has already narrowedNothing about a single circuit, only that the whole board lost supply |
| What you are looking atEarth-leakage device, wider face, test button on it | How much of the flat it holdsA group of circuits sitting under it | What its trip has already narrowedCurrent left a circuit instead of returning, and the source need not be the aircon |
| What you are looking atA breaker in the narrow row | How much of the flat it holdsOne circuit | What its trip has already narrowedThat circuit alone, either drawing past its rating or shorted |
| What you are looking atAn unlabelled position between labelled ones | How much of the flat it holdsUnknown until a schedule is produced | What its trip has already narrowedNothing at all, which is the situation this page exists for |
Why the labelling is wrong so often
The labels are the least reliable thing on the board and the first thing anybody reads. They were written by hand, by whoever wired the flat, on the day the board went in. Nothing in the system keeps them true afterwards.
Renovation moves circuits and rarely moves the writing. A wall comes down, a kitchen gets rearranged, a socket circuit is extended into a room it never used to reach. The electrician relabels the positions that changed if there is time and a marker within reach. Frequently there is neither.
An aircon fitted after the flat was wired is the single largest source of a wrong label. The system needs a supply, it takes one from a spare position or from a circuit already in use, and the writing still describes what that position did beforehand. A breaker marked for the kitchen can be the one feeding the condenser.
Shorthand does the rest of the damage. AC, aircond, ledge, room 2 and a blank all turn up on the same rail in the same flat, written by different hands at different stages of its life. Faded ink and a newer sticker over the top are ordinary. A blank position is more honest than a wrong one, and considerably rarer.
Treat a label as evidence rather than proof. It costs nothing to read and it is right more often than not. It is also why technicians arrive at flats where the aircon runs from a position marked for a water heater, and nobody in the household ever had reason to doubt it.
Working out which one is the aircon
Identification is done by reading, and the cover stays on throughout. Everything useful is printed or written on the front face of the board. None of it calls for touching a switch.
Read every label, not only the one that looks promising. The full set describes what the person writing it believed the flat contained. A rail with seven positions named and one left blank makes the blank worth asking about, and that inference is only available to someone who read the whole rail.
Note the amp figure and the shape of the device next. An aircon circuit usually sits among the higher-rated positions on a domestic board, because it was sized around a compressor starting rather than a lamp switching on. That is a narrowing, not an identification. A water heater and a cooker hob live in the same range.
Then set what the flat contains against what the rail offers. Hob, oven, water heater and aircon each want a circuit to themselves. If the rail holds more positions than the household can name heavy appliances for, one of the spares is probably the aircon, and the installation schedule will say which.
When it stays unclear, the answer comes from whoever installed the system rather than from experiment. The aircon installer, the renovation contractor, or the electrician who wired the flat holds that record. In a resale flat none of them may still be reachable, and identification then becomes work for a licensed worker holding an instrument.
Switching things off to see what stops is not a substitute for that record. It is a test run on a live installation by somebody with no way to interpret an ambiguous result. On a board that has already tripped once, it also disturbs the very thing the next person needs to find intact.
- Every label on the rail, not only the one nearest the position in question
- The amp rating stamped on each device face
- Whether a device is a narrow single breaker or the wider earth-leakage body with a test button
- Which device is down, and which way the rest are pointing
- Whether the writing looks original to the board or was added later in a different hand
What to write down before calling anyone
One observation separates a whole-board fault from a circuit fault, and it disappears the moment somebody resets the board. Which device went, and how much of the flat went with it.
The split is between the earth-leakage device and a single breaker, and it changes who the suspect is. If the wide device dropped, current was escaping somewhere underneath it. That somewhere need not be the aircon, since the device covers several circuits at once. If one narrow breaker dropped while every other room kept its power, the fault belongs to that circuit and the list of candidates is short.
What else lost power is the same question asked from the room, and a household can answer it without going near the board. Lights out everywhere, or only the aircon stopped while the television stayed on. That answer is available from memory, and it survives even after somebody has reset everything.
Add the surrounding conditions while they are fresh. What was running at the moment it went. Whether the outdoor unit was starting at that instant. Whether it had been raining. Those details decide which test gets done first, and they are the first thing a household forgets.
| What went off at the board | What lost power in the flat | What that already rules out |
|---|---|---|
| What went off at the boardOne narrow breaker | What lost power in the flatOnly the aircon | What that already rules outThe board and the incoming supply, leaving one circuit and one machine |
| What went off at the boardOne narrow breaker | What lost power in the flatThe aircon and other rooms | What that already rules outA circuit of its own, since that breaker is feeding more than the aircon |
| What went off at the boardThe wide device with the test button | What lost power in the flatSeveral circuits together | What that already rules outA plain overload, and it puts the whole group under suspicion rather than the aircon |
| What went off at the boardThe main switch | What lost power in the flatEverything | What that already rules outAny single circuit, and it points instead at the board or the supply into it |
| What went off at the boardNothing moved on the board | What lost power in the flatOnly the aircon stopped | What that already rules outThe board entirely, leaving the isolator, the machine, or its own controls |
Reading the board is fine, opening it is not
The line sits at the cover, and it is not a question of confidence. Everything described so far happens with the board shut. Everything past the cover is licensed work, and the reason is physical rather than procedural.
The terminals behind the front stay live when every device on the rail is off. Supply reaches the main switch from outside the flat, and throwing that switch does not stop it arriving. A board that looks entirely dead from the front is not dead inside.
Singapore reserves work on a fixed electrical installation to a Licensed Electrical Worker, and the board sits squarely inside that definition. Fitting an aircon and altering a circuit are separate trades with separate paperwork behind them. The aircon contractor credentials question is worth settling before booking, since a firm that cannot say which of the two it holds has answered it already.
What a homeowner is being asked for is not a diagnosis. Read the front, note what went, note what else stopped, and leave it there. That short account beats any attempt to narrow it further, because the board has already taken the only measurement available without instruments.
Getting the board labelled once, properly
The moment to fix the labelling is when a licensed worker has traced the circuits for some other reason. Tracing is the expensive half of that job and it has just been paid for. Writing the result down adds almost nothing on top.
Ask for a written schedule, position by position, rather than fresh marker on the rail alone. Writing on a board gets covered by the next sticker and rubbed away by the next pair of hands. A sheet kept with the flat's documents outlasts both, and it outlasts the next owner too.
Have the aircon entry name the machine it feeds. A multi-split flat can have one circuit serving the condenser and no separate provision for the indoor units, which reads as a missing circuit to anyone checking later. What a dedicated aircon circuit was meant to cover belongs in that record beside the position number.
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