Capstone: Read a Building's Electrical Story

Part 15 of 15 in Cobler's Electricity Fundamentals course. New here? Start with the course map.

When someone switches on the kettle in the office tower's level 12 pantry, power stations on the grid have to produce a little more electricity almost at once. Between the power station and the kettle sit four or five transformers, several voltage levels and a chain of protective devices, and you have now met every one of them. Part 14 ended by asking you to put it all together: can you trace the electricity from the power station to the pantry kettle, and read the numbers on the way? This final part does that as a building electricity worked example in five tasks.

How to use these tasks

Each task starts with a puzzle and some information from the office tower. Try the questions with a calculator and a notepad first. The Worked answer comes straight after each task, so you can check your working. All the numbers are fictional but consistent with each other and with the rest of the course.

You only need tools the course has already given you:

• Watts = volts × amps. From Volts and Amps (Part 2).
• Energy (kWh) = power (kW) × hours. From Power vs Energy (Part 5).
• Power factor = kW ÷ kVA, so kVA = kW ÷ power factor. From kW, kVA and kVAR (Part 11).

Task 1: How does electricity get from the power station to the pantry kettle?

The kettle in the level 12 pantry is rated 3 kW at 230 V. It plugs into a socket on the pantry's final circuit, which is protected by a 20 A RCBO with a 30 mA residual current function. Here are the stages the electricity passes through, in the wrong order:

• A. The level 12 east distribution board (DB-12E)
• B. A generator at a power station, making three-phase AC at 50 Hz
• C. The tower's own transformer, 1,500 kVA, 11 kV to 400 V
• D. The pantry socket and the 20 A, 30 mA RCBO on its circuit
• E. A step-up transformer and the 275 kV National Grid lines
• F. TNB substations that step the voltage down, ending at the tower's 11 kV intake and TNB meter
• G. The tower's main switchboard and the riser switchboard that feeds levels 11 to 20

1. Put the stages in order, from the power station to the kettle.
2. Why is the voltage raised to 275 kV, only to be lowered again later?
3. Inside the tower, the supply from the main switchboard is three-phase 400 V. Why does the kettle get 230 V?
4. How much current does the kettle draw?
5. The kettle's lead is damaged, and a small current leaks through a person's hand to earth. Which part of the RCBO responds, and why would an ordinary breaker not?

Worked answer

1. B, E, F, C, G, A, D. The generator makes three-phase AC (Part 7). A step-up transformer raises it to 275 kV for the National Grid, substations step it down to 11 kV at the tower's intake (Part 12), and the tower's own transformer brings it to 400 V (Part 8). The main switchboard and riser feed the floor's distribution board, and a final circuit takes it to the pantry socket (Part 3).
2. At a higher voltage, the same power needs less current, and less current means much less energy lost as heat in the long lines (Part 6). A 275 kV line cannot supply a building, so the voltage comes back down in steps near where it is used. Transformers can do this only because the supply is AC.
3. Three-phase 400 V is the voltage between any two of the three live conductors. A single-phase circuit such as the pantry socket uses one live conductor and the neutral, and the voltage between them is 230 V (Part 9).
4. Watts = volts × amps, so amps = watts ÷ volts: 3,000 ÷ 230 ≈ 13 A. That is inside the 20 A rating of the circuit's breaker, but other appliances on the same circuit add their own current.
5. The residual current (RCD) function. It compares the current going out on the live conductor with the current coming back on the neutral. Current leaking through a person to earth creates an imbalance, and a correctly selected, working 30 mA device trips when that imbalance meets its operating conditions. The breaker part reacts only when the total current is too high, and a leak through a person can be far too small for that (Part 4). An RCD adds protection, but it cannot prevent every shock, so a damaged lead must be taken out of use.

Task 2: How much energy does one office floor use in a day?

A facility engineer has measured some of the loads on level 12, east half. The figures are average electrical input over a normal weekday, taken from the floor's sub-meters and the building management system (BMS).

Assume 22 working days in a month, and ignore weekends and night-time standby loads.

1. How many kWh do these three loads use on one weekday?
2. How many kWh do they use in a month?
3. Why is the socket figure a measured average rather than the total of every appliance's rating?

Worked answer

Multiply each load's power by its hours:

• Lighting: 12 kW × 10 h = 120 kWh
• Sockets: 15 kW × 10 h = 150 kWh
• AHU-12A fan: 6 kW × 11 h = 66 kWh

1. One weekday: 120 + 150 + 66 = 336 kWh.
2. One month: 336 kWh × 22 days = 7,392 kWh.
3. If you added up the ratings of every computer, screen and printer on the floor, you would get a much bigger number that never occurs in practice, because they are not all drawing full power at the same time. The meter records what is actually drawn, and that is what adds up to kWh.

Task 3: Why doesn't the pump's meter agree with its nameplate?

The tower's chiller plant has condenser water pumps that run at a fixed speed. One pump motor has this nameplate:

• Rated output: 30 kW
• 400 V, three-phase, 50 Hz
• Rated current: 54 A
• Power factor: 0.86
• Efficiency: 93.6%

A meter on the pump's feeder cable shows the motor drawing 27.0 kW of electrical input while it runs.

1. What does the 30 kW on the nameplate describe?
2. How much electrical input would the motor draw at full load?
3. Is the motor running at, above or below full load? Roughly how much power reaches the pump shaft?
4. To estimate this pump's monthly kWh, should you use 30 kW or 27.0 kW?

Worked answer

1. The 30 kW is the rated output: the mechanical power the motor can deliver at its shaft, continuously, at full load. It is not the electrical power the motor draws. How Electric Motors Work (Part 10) explains how a motor turns electrical input into shaft output.
2. The motor loses some energy as heat, so the input must be larger than the output. At full load: input = output ÷ efficiency = 30 ÷ 0.936 = about 32.1 kW.
3. The meter reads 27.0 kW, which is less than the 32.1 kW full-load input. So the motor is running below full load, at about 27.0 ÷ 32.1 ≈ 84% of it. If we assume the efficiency stays close to the rated 93.6% at this loading, the shaft receives about 27.0 × 0.936 ≈ 25.3 kW. The other 1.7 kW or so becomes heat in the motor.
4. Use the measured 27.0 kW. The meter records electrical input, and electrical input is what adds up to kWh. Using 30 kW would mix up output and input, and it would still be wrong at full load, because the input would then be about 32.1 kW, not 30 kW.

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The meter measures what goes into the motor; the nameplate kW describes what the shaft can deliver at full load.

Task 4: How much of its transformers is the tower really using?

The same meter shows the pump motor's power factor as 0.85 at this load. At the tower's maximum demand of 1,600 kW, the power factor at the main switchboard is 0.92, after the capacitor banks. The tower has two transformers of 1,500 kVA each.

1. What is the pump's apparent power in kVA?
2. What is the building's total kVA at maximum demand, and what share of the two transformers' combined rating is that?
3. The capacitor banks sit at the main switchboard. Do they change the pump's own power factor of 0.85?

Worked answer

1. kVA = kW ÷ power factor: 27.0 ÷ 0.85 ≈ 31.8 kVA. The pump's cable has to carry the current for 31.8 kVA, although only 27.0 kW does useful work.
2. Building kVA: 1,600 ÷ 0.92 ≈ 1,739 kVA. The two transformers can supply 2 × 1,500 = 3,000 kVA together, so at maximum demand they are carrying about 1,739 ÷ 3,000 ≈ 58% of their combined rating. Transformers are rated in kVA because their heating follows the current, not the kW (Part 11).
3. No. The capacitor banks supply reactive power at the main switchboard, so they reduce the current in the transformers and the cables upstream of the banks. The reactive current between the switchboard and the pump still flows, so the pump's own power factor stays at 0.85.

Task 5: What may you do yourself, and what needs a competent person?

While collecting information for these tasks, you walk around level 12 and the chiller plant. Decide which of these you may do yourself, and which need the electrical competent person responsible for the installation.

1. Read the kWh and kW figures on a sub-meter display through the closed door of a distribution board (DB).
2. Read the pump motor's nameplate, which sits close to the coupling guard.
3. You notice a burning smell and a discoloured breaker inside the level 12 DB.
4. You want to clamp a portable power meter onto the pump's cable to check the 27.0 kW reading.
5. The floor's residual current device (RCD) is due for its test-button check.

Worked answer

1. You may do this. Reading meter displays, BMS screens and bills does not involve touching anything live.
2. You may read it only if you can do so without removing a guard or reaching near moving parts. Otherwise, ask for it to be read while the pump is stopped and made safe by the person responsible.
3. Do not open the board or touch the breaker. Keep people away and report it immediately to the competent person responsible. Follow the site's emergency procedure if there is smoke or fire.
4. This needs a competent person. Fitting a clamp means opening a panel and working near live conductors. The person's Suruhanjaya Tenaga (ST) registration and certificate must cover the work concerned.
5. You may press the test button if the site's procedure assigns this to you, at the interval stated by the manufacturer, after arranging for the power interruption. Operate only a safely accessible button and do not remove a board cover. If it does not trip, report it immediately and have the affected circuit made safe and checked. The test button checks the RCD's built-in test function only. Measured RCD testing and earthing tests are work for the competent person.

The rule from Earthing and RCDs applies throughout: observe, read and report, and leave anything behind a cover to the competent person. Meter readings, like the ones in these tasks, do not replace earthing and protective-device tests.

What you now understand

• Electricity reaches a kettle through a chain you can now name: generator, step-up transformer, grid, substations, the building's transformer, switchboards, a distribution board and a protected final circuit.
• Energy in kWh comes from measured power multiplied by hours, not from adding up ratings.
• A motor nameplate's kW is shaft output; the electrical input is larger, and the meter measures the input.
• kVA = kW ÷ power factor tells you how much of a transformer or cable a load really uses.
• You can read, observe and report safely, and you know which jobs belong to the competent person.

Check your understanding

1. A fan motor's nameplate says 15 kW and 93% efficiency. A meter shows it drawing 16 kW. Is the motor overloaded? Not necessarily. The 15 kW is rated shaft output. At full load, the input would be about 15 ÷ 0.93 ≈ 16.1 kW, so a 16 kW input means the motor is running close to full load, not above it. Use the measured 16 kW when estimating kWh.
2. A kettle and a phone charger are plugged into the same pantry socket circuit. Which one needs a rectifier, and why? The phone charger. A phone battery needs DC, so the charger rectifies the 230 V AC into DC (Part 13). The kettle's element heats up whichever way the current flows, so it runs directly on AC.

Where to go next

If one task gave you trouble, go back to the parts it draws on:

• Task 1: How the Malaysia Electricity Grid Reaches Your Plug, Three-Phase Power Explained and Earthing and RCDs
• Task 2: Power vs Energy: The Difference Between kW and kWh
• Task 3: How Electric Motors Work
• Task 4: kW, kVA and kVAR: The Power Triangle
• Task 5: Earthing and RCDs

If all five tasks made sense, you have finished the course. Three other courses on Cobler Learn continue from here. Understanding Your Electricity Bill (start here) turns these kW and kWh into ringgit, including maximum demand and the power factor surcharge. Building Electrical Fundamentals follows the power from the building's substation to the socket in the wall: switchboards, breakers, cables and drawings. Energy Management: The Economics of Saving Energy covers how to prove a saving and judge whether it is worth what it costs.

This is Part 15 of 15 in Cobler's Electricity Fundamentals course. Previous: How Batteries Store Energy (and Why It Is All DC).

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.