Series vs parallel circuits
In series, current is shared and voltage divides; in parallel, voltage is shared and current divides.
Series
In a series circuit, components are connected end-to-end so there's only one path for current. The same current flows through every component, and the supply voltage divides across them in proportion to their resistance.
Break a series circuit anywhere and everything stops — which is why a single open (a blown lamp in an old series string, a broken connection) kills the whole path.
Parallel
In a parallel circuit, components are connected across the same two points, so each has its own path. Every branch sees the full supply voltage, and the total current is the sum of the branch currents.
Most wiring (lights, sockets) is parallel: each load gets full voltage and one failing doesn't stop the others.
Why it matters
Recognising whether something is in series or parallel tells you how a fault will behave — whether one open stops everything, and how voltage and current share out. Control circuits mix both: contacts in series form an 'AND', branches in parallel form an 'OR'.
Related faults
No supply at a socket-outlet or point
A socket-outlet or point is dead — nothing plugged in works — while other points may be fine. A bread-and-butter 'trace it back' fault.
All lights in one area out (others fine)
Every light in one part of the house is dead while power points and other lighting still work — points at that lighting circuit's protective device or a shared fault.
Related definitions
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.
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.
Start/stop circuit (seal-in)
A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.