Reference & definitions
How the gear works and the principles behind it — devices and circuits explained in plain English, with diagrams. Cross-linked to the faults they relate to.
56 entries
Transformer
Transfers electrical energy between circuits by magnetic coupling, stepping voltage up or down.
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Relay
A small electrically-operated switch — like a miniature contactor — used to switch or route control signals.
VSD (Variable Speed Drive)
Controls the speed of an AC motor by converting the supply to a variable frequency and voltage.
Soft starter
Reduces motor starting current by ramping the voltage up, then often hands over to a bypass contactor.
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Overload relay
Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.
RCD (safety switch)
Detects earth leakage and disconnects fast to protect people from electric shock.
RCBO
Combines an RCD and a circuit breaker in one device — earth-leakage plus overload/short-circuit protection.
PLC (Programmable Logic Controller)
An industrial computer that reads inputs, runs a program, and drives outputs to control machinery.
HMI (operator panel)
A touchscreen/operator interface that lets people monitor and control a machine or process.
SSR (Solid-State Relay)
Switches a load electronically with no moving parts — fast, silent, ideal for frequent switching like heaters.
Timer relay
A relay that switches its contacts after a set delay, enabling sequenced and timed control.
Isolator / disconnector
A switch whose job is safe isolation — visibly and securely disconnecting a circuit for work.
Proximity sensor
Detects the presence of a target without contact — inductive, capacitive, or photoelectric.
Limit switch
A mechanical switch operated by a moving part reaching a position — often for end-of-travel and safety.
Encoder
Gives precise position or speed feedback by producing pulses (or a coded value) as a shaft turns.
Thermostat
Switches a heating or cooling load on and off to hold a temperature at a setpoint.
Thermocouple
A temperature sensor that produces a tiny voltage proportional to the temperature at its junction.
Heating element
A resistive conductor that turns electrical energy into heat — the business end of most electric heating.
Phase failure / monitoring relay
Watches a three-phase supply and disconnects the load if a phase is lost, unbalanced, or in the wrong sequence.
Surge protective device (SPD)
Diverts transient over-voltages (from lightning or switching) to earth to protect equipment.
Two-way switching
Controlling one light from two switches — flick either one to change the state.
Start/stop circuit (seal-in)
A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.
Forward / reverse circuit
Two contactors run a motor in either direction; reverse swaps two phases, and an interlock prevents both closing at once.
Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
Single-phasing
What happens when a three-phase load loses one phase — and why it's so damaging to motors.
Three-phase power
Three AC supplies offset in time, giving smooth power and a rotating field for motors.
AC vs DC
Alternating current reverses direction many times a second; direct current flows one way. Why it matters on site.
Series vs parallel circuits
In series, current is shared and voltage divides; in parallel, voltage is shared and current divides.
Ohm's law & power
The relationship between voltage, current and resistance — and how it gives you power.
Voltage drop
Volts lost along a cable's resistance under load — why the far end of a long run can misbehave.
Earthing & bonding
Connecting exposed metal to earth so a fault blows protection fast and metalwork can't become live.
Neutral vs earth
The neutral carries return current; earth is for safety. Confusing them causes real, dangerous faults.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.
Interlocks
Logic that prevents an unsafe or impossible combination of states — like two contactors closing together.
How an RCD protects you
An RCD compares current out and back; any imbalance means leakage to earth, so it disconnects fast.
Inrush current
The brief, high current many loads draw at switch-on — and why it trips protection if not allowed for.
No-volt release
After a power cut, equipment stays off until deliberately restarted — preventing dangerous auto-restart.
Diversity & load balancing
Not everything runs at once, and loads should be spread across phases — the basis of sizing and balance.
Distribution & sub-mains
How power is split from the main board into final circuits and sub-boards, with protection at each level.
How HVAC control works (electrical view)
A thermostat/controller calls for heating or cooling; the electrical side energises the unit — refrigerant is separate.
How emergency lighting works
Self-contained fittings charge a battery in normal use and light automatically when the supply fails.
Power factor
How much of the current actually does useful work — and why a poor figure costs you capacity and money.
Real, apparent & reactive power (kW · kVA · kVAr)
Three different 'powers' on an AC system — what each one is, and why they don't simply add up.
Harmonics & non-linear loads
Why electronic loads distort the current waveform — and the overheating and nuisance tripping that follows.
Motor slip & torque-speed
Why an induction motor must run slightly slower than its field, and how its torque changes from start to full speed.
Back-EMF & inductive kick
Why a spinning motor generates its own voltage, and why switching a coil produces a damaging spike.
Discrimination (selectivity)
Arranging protection so only the device nearest a fault trips — not the whole board above it.
Earth-fault loop impedance
The path a fault current takes back to the source, and why its impedance decides whether protection trips in time.
Phase sequence & rotation
What 'phase rotation' means on a three-phase supply, and why getting it wrong reverses motors.
Transients, surges & overvoltage
Brief voltage spikes far above normal — where they come from and how equipment is protected from them.
Cable current capacity & derating
Why the same cable can safely carry less current in some installations than others — it all comes down to heat.
Maximum demand
The realistic peak load a supply has to carry — the figure that sizes the mains, the main switch and the protection.
Safety systems (E-stops, guards & safety relays)
The protective layer that stops a machine safely — built to fail safe and never to be defeated, not ordinary control wiring.