Basic Electrics

01The course and what you need

The route through the course, in order:

1. Attend 14 days of online theory training (webinars).
2. Complete the homework on your portal.
3. Book your exams at your chosen centre, then take them.
4. Once all exams are passed, book practical training (weekend or midweek).
5. Attend 10 days of practical training.
6. Book and attend 4 days of final practical assessment.

Qualifications you receive: Building Regulations (L3) · BS 7671 (L3) · In-service Inspection and Testing of Electrical Equipment (L3) · Fundamental Inspection and Testing (L3) · Initial Verification (L3) · Condition Reporting, periodic inspection and testing (L3).

What they let you do: join a local electrical firm (usually as a trainee at first, because you lack experience), work for yourself, or join an agency and work as a subcontractor. You can work domestically, commercially and industrially.

Timetable: webinars run 9–12 and 1–4. Practical training is 10 days, either two weeks Monday to Friday or five weekends. You can't book practical training until all your exams are passed.

02Voltage

The session opened with a question: what three things make up an electrical circuit? The answer is voltage, current and resistance.

Voltage is the electrical pressure that pushes electricity around a circuit. It is measured in volts, V.

Symbols

V in general formulas. U is how voltage is written in electrical books like BS 7671.

U₀

The UK nominal voltage, 230 V (line to earth).

U

The line-to-line voltage, 400 V (three-phase).

EMF

Electromotive force, the voltage a source produces (battery, generator, transformer winding). Still measured in volts, and written E in formulas. Don't think of it as a force in newtons. It's the push that drives electricity round the circuit.

Potential difference

The difference in voltage between 2 points. For example, 230 V on the line and 0 V on the neutral gives a PD of 230 V.

03Current

Current is the flow of electrons through a conductor. It is measured in amperes (amps), A. The symbol in formulas is I, from the old word "intensity". You'll see I everywhere, for example Iₙ for a device's rating, Ib for design current, and IΔn on RCDs and RCBOs.

Conventional current

Drawn flowing positive → negative. This is the direction everyone assumes on drawings.

Electron flow

What the electrons actually do: negative → positive.

DC

Electrons flow in one direction only. Batteries are always DC.

AC

Electrons flow backwards and forwards. UK mains is AC.

04Resistance and impedance

Resistance

Opposition to current flow. Measured in ohms, Ω. Symbol R.

Impedance

Opposition to current in an AC circuit only (never DC). Symbol Z, also in ohms. It accounts for loads, such as the inductor in a fluorescent fitting, that shift current and voltage out of step. You'll meet Ze and Zs in fault loop impedance testing, and R when testing continuity.

Think of rocks in a stream: the water can't flow straight and fast, so it slows down. Every conductor has resistance, and so do cables, loads and your body. Increase resistance and current goes down, as long as the voltage stays the same.

• Low resistance is good in neutrals and earth conductors (CPCs). If there's a fault, enough current has to flow to trip the protective device quickly.
• Your body has resistance too, and it's much lower when wet, for example straight out of the shower. That's why wet skin is so dangerous.
• A protective device (breaker or fuse) is there to protect the cable from carrying too much current. More current means more heat.

05The basic supply circuit

This diagram came up throughout both sessions. It shows where your 230 V actually comes from.

• The star point is where the neutral and earth join at the transformer. From there it goes down into the ground. The electricity doesn't disappear: it's a continuous loop.
• The coil in this drawing is the transformer's secondary winding. The secondary side is always star.
• A circle with an X is the symbol for a lamp. Lamps are resistive loads.
• The symbol with three horizontal lines is earth (ground).
• It's called line (it used to be called phase), not "live". Both the line and the neutral are live conductors, and you can get a shock from either.

06Ohm's law

V = I × R     volts = amps × ohms
I = V ÷ R     amps  = volts ÷ ohms
R = V ÷ I     ohms  = volts ÷ amps

Supply 230 V, lamp resistance 6 Ω
I = V ÷ R = 230 ÷ 6 = 38.33 A

Supply 230 V, current measured 12 A
R = V ÷ I = 230 ÷ 12 = 19.17 Ω

A 6 A lighting circuit doesn't mean the lights draw 6 A. They might only draw 2.5 A. The 6 A breaker is sized to protect the cable.

07The power triangle

Power is the rate at which energy is used. It is measured in watts, W, and 1 kW = 1000 W.

P = I × V     watts = amps × volts
I = P ÷ V     amps  = watts ÷ volts
V = P ÷ I     volts = watts ÷ amps

I = P ÷ V = 20 ÷ 230 = 0.087 A
  → round to 2 decimal places: 0.09 A  (≈ 87 mA)

P = I × V = 16.92 × 230 = 3891.6 W
                         = 3.892 kW

• Power is used by the loads, not by the cables. To get total power, add up every load. Three 20 W downlights = 60 W.
• Work to 2 decimal places. If you round, round properly (0.087 → 0.09). Otherwise use the full number.

08Series circuits

In a series circuit the loads are connected one after another in a single path. There's only one route for the current. Fairy lights and old Christmas tree lights are the classic example: when one bulb goes, they all go out.

Electricians don't wire circuits in series. It isn't efficient, one fault kills everything, and if the loads are different they don't all get the same voltage, so they aren't all equally bright.

Given: 230 V supply · cables (line + neutral) = 30 Ω
       lamps = 5 Ω, 6 Ω, 7 Ω

1) Total resistance (add everything, cables included)
   Rt = 30 + 5 + 6 + 7 = 48 Ω

2) Total current (Ohm's law)
   I = V ÷ Rt = 230 ÷ 48 = 4.79 A

3) Voltage dropped across each part (V = I × R)
   Lamp 1:  4.79 × 5  =  23.95 V
   Lamp 2:  4.79 × 6  =  28.74 V
   Lamp 3:  4.79 × 7  =  33.53 V   → lamps total 86.22 V
   Cables:  4.79 × 30 = 143.70 V

   Check: 86.22 + 143.70 = 229.92 ≈ 230 V ✓  (Kirchhoff's voltage law)

09Series recap: three 12 Ω lamps

The afternoon opened with a practice question using this method: 1. find total resistance → 2. find total current → 3. calculate the voltage dropped across each lamp.

Rt = 12 + 12 + 12 = 36 Ω
I  = V ÷ Rt = 230 ÷ 36 = 6.38 A
V per lamp = I × R = 6.38 × 12 = 76.66 V
Check: 76.66 × 3 ≈ 230 V ✓

When all the loads are the same, you can shortcut: 230 ÷ 3 = 76.67 V each. When they're different, you can't, and you have to do V = I × R for each one.

The tutor put a cross on one lamp. Because there's only one path, a break anywhere, even after the last lamp, stops current to every lamp. That's the big weakness of series wiring, and it's why the circuits in a building are wired in parallel.

10Parallel circuits

Each load gets its own connection directly across line and neutral, so each one sees the full 230 V. This is how electricians wire circuits in buildings.

Open circuit

There's a break in the path, so there's no potential difference and no current flows.

Closed circuit

The path is complete, so current can flow.

In parallel, if the cable to one lamp breaks, the other lamps still have their own path to line and neutral, so they stay on. A blown bulb in the bedroom doesn't put out every light in the house.

1) Total resistance
   1/Rt = 1/30 + 1/15 + 1/9
        = 0.0333 + 0.0667 + 0.1111 = 0.2111
   Rt   = 1 ÷ 0.2111 = 4.74 Ω     (lower than 9 Ω ✓)

2) Total current (Ohm's law)
   It = V ÷ Rt = 230 ÷ 4.74 = 48.52 A

3) Branch currents (Kirchhoff's current law check)
   I₁ = 230 ÷ 30 =  7.67 A
   I₂ = 230 ÷ 15 = 15.33 A
   I₃ = 230 ÷ 9  = 25.56 A
   Total ≈ 48.5 A ✓

20 Ω, 10 Ω, 121 Ω:
   1/Rt = 0.05 + 0.1 + 0.0083 = 0.1583 → Rt = 6.32 Ω

7 Ω, 11 Ω, 36 Ω:
   1/Rt = 0.1429 + 0.0909 + 0.0278 = 0.2615 → Rt = 3.82 Ω

25 Ω, 42 Ω, 21 Ω  (try it yourself first)
   1/Rt = 0.04 + 0.0238 + 0.0476 = 0.1114 → Rt = 8.97 Ω

A voltmeter is always connected across a load, in parallel with it, because voltage is constant in parallel.

11Power ratings and 4 A per kW

Working out current from an appliance's power rating is how you size protective devices and cables in domestic installations.

I = P ÷ V = 10,000 ÷ 230 = 43.48 A

Later in the course this feeds into choosing a cable's CSA (cross-sectional area). That depends on the current, but also on the cable's length and how it's installed.

12Number systems and prefixes

This is engineering notation: electricians work in powers of 3, so each step is ×1000. Moving to a bigger prefix makes the number smaller, and moving to a smaller prefix makes it bigger.

• Positive power (10⁶): the zeros go before the decimal point, so it's a big number. Negative power (10⁻⁶): the zeros go after the decimal point, so it's a tiny number.
• Capital M = mega (million) and small m = milli (thousandth). Mixing them up is a big mistake.
• You'll see MΩ when insulation resistance testing, and Ω when continuity testing.

8.4 kW   in watts      → 8400 W
100 mA   in amps       → 0.1 A
0.03 A   in milliamps  → 30 mA
0.5 MΩ   in kilo-ohms  → 500 kΩ
80 mA    in amps       → 0.08 A  (= 80 × 10⁻³)

13Conductors and insulators

• A voltage is put across a circuit or load. Current flows through it.
• A conductor is any material that allows current to flow through it. Good conductors (copper, aluminium, most metals) have low resistance.
• An insulator is any material that doesn't allow current to flow. Good insulators (plastics, rubber, wood, glass) have very high resistance.

A cable uses both: copper conductors to carry current, and PVC insulation and sheath to keep it where it should be.

14DC and AC

Rectifier

Converts AC → DC. Your phone charger has a small bridge rectifier inside the plug to charge the DC battery from AC mains.

Inverter

Converts DC → AC. Solar panels produce DC, and an inverter turns it into AC for the home.

If DC isn't fully smoothed you get a rectified waveform: humps that stay on one side of zero, because the current never reverses.

15Magnetism and induction

• Like poles repel, unlike poles attract. With two N poles together the flux lines push outwards away from each other. With N and S together the flux lines run straight between them, and they attract.
• The lines of magnetic flux run from the North pole round the outside into the South pole.
• Flux density is the amount of flux in a given area. Lines close together mean a strong field, and lines far apart mean a weak one.
• When a conductor carries a current, a magnetic field forms around it. The field is circular and runs the whole length of the conductor. It goes clockwise or anticlockwise depending on the direction of the current. Motors and transformers both rely on this.
• Flux lines are invisible. Sprinkle iron filings over a magnet (Faraday's school-physics experiment) and you can see their shape.

Electromagnetism is how UK electricity is made: in a power station, a coil rotates inside magnetic fields, generating at about 25,000 V.

Magnetic induction: how a generator makes AC

Magnetic induction is one of the main sources of EMF. A loop of wire is mechanically rotated between the N and S poles of a magnet. As it cuts the lines of flux, a voltage is induced, which is the electrical output.

• One full turn gives one cycle. At 50 Hz the loop turns 50 times a second, giving 50 cycles and 100 peaks (one each way per cycle).
• That's why lights sometimes flicker when you look at them through a phone camera. The camera is catching the 50 Hz cycle.
• The peaks at 90° and 270° are current going one way, then back the other way. It isn't "positive then negative electricity".

16Transformers and impedance

A transformer converts an alternating voltage at one level to an alternating voltage at a higher or lower level (step up or step down). It uses mutual induction. A continually changing flux in the primary winding induces a voltage in the secondary winding. That's why a transformer only works on AC: DC doesn't produce a changing flux.

V₁ / V₂ = N₁ / N₂
110 / V₂ = 50 / 25   →   V₂ = 55 V

• Fewer turns on the secondary means step down, and more turns means step up. You can tell which one a transformer is by comparing the windings.
• On drawings, the primary is usually on the left and the secondary on the right.
• The two coils never touch. The voltage is induced across through the magnetic field in the core.
• The core is laminated (thin lacquered layers) to reduce losses.

Types you'll see: step-up, step-down and isolating transformers. An isolating transformer separates you from earth, so there's no path through your body to earth. A bathroom shaver socket has one inside. You'll also see pole-mounted three-phase transformers, current transformers (CTs), potential transformers, and the big fenced-off "Danger, high voltage" substations near housing estates.

17Generation, transmission and distribution

1. Generate: fossil fuels or nuclear energy make steam, which drives a turbine and generator at about 25 kV. Solar and hydro are other sources.
2. Step up to 400 kV (275 kV is also used) to transmit across the country, say from Cornwall to Birmingham. A higher voltage means lower current for the same power, so thinner cables and smaller losses.
3. Regional transformers step down to 132 kV, then local ones to 33 kV or 11 kV. This is distribution. A big factory might take 11 kV directly.
4. Step down again for streets: 230 V for domestic, and 400 V for commercial and industrial properties (shops, schools, factories).

Most houses have a 60, 80 or 100 A main fuse. Transformers only work on AC.

18Three-phase supply, star and delta

A generator has three sets of windings, each creating an EMF set 120° apart. So a three-phase supply is effectively three supplies all rising and falling at different times.

• The star system produces 230 V and 400 V. 230 V × √3 (1.732) ≈ 400 V.
• Single-phase houses on a street are each connected to one line plus neutral, spread across L1, L2 and L3 to keep the load balanced. Any line to neutral = 230 V.
• Three-phase commercial and industrial supplies use all three lines. Any line to line = 400 V.
• The phases are 120° apart because 360° ÷ 3 = 120°.
• Why 400 V and not 460 V (230 + 230)? Two phases never reach their peaks at the same moment. They cross before either one peaks, so the difference between them tops out at 400 V, not double. (415 V is the old figure and no longer used.)
• Delta (the primary side) is balanced, with the same current on every line, so it has no neutral and only gives 400 V.
• Star (the secondary side) has a neutral. Houses put different loads on each phase, so the phases become unbalanced, and the neutral carries that out-of-balance current back.
• Two-phase supplies (two lines plus neutral) exist but are rare, only for very large properties.

19Kirchhoff's laws

20Formula sheet and self-test

Unofficial student notes written from the Module 1 Day 1 webinar recordings, shared free for fellow learners. Not produced or endorsed by the training provider. Always check against your course books, BS 7671 and your tutor. Spotted a mistake? Let us know so it can be fixed for everyone.

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