The earth conductor carries no current in normal operation. Its job is to provide a low-resistance path so that if a live conductor touches an exposed metal part, enough current flows to operate the protective device quickly — turning a lethal fault into a tripped breaker. It is a safety mechanism that works by making the fault worse, deliberately, so that something notices.
That is a genuinely counter-intuitive design, and it explains why an earth fault that does not trip anything is more dangerous than one that does.
Why is a residual current device different?
Because it does not measure fault current at all — it measures imbalance. An RCD or GFCI compares the current going out on the live with the current returning on the neutral, and trips if they differ by more than a small threshold, typically 30 mA.
That difference means current is going somewhere else, and the most consequential somewhere else is a person. A 30 mA device trips far below the current needed to be lethal and far below what a breaker would notice.
| Device | Detects | Protects against |
|---|---|---|
| Fuse or breaker | Overcurrent | Overload and short circuit — fire |
| RCD / GFCI | Live–neutral imbalance | Electric shock |
| Combined RCBO | Both | Both, per circuit |
The two protect against different things, which is why having one is not a substitute for the other. A breaker will not notice a person; an RCD will not notice an overloaded cable heating up inside a wall.
Why does a breaker trip on a working appliance?
Usually inrush rather than fault. A motor drawing 6 A running can pull six or seven times that for a second or two at startup, and a device with a large reservoir capacitor looks briefly like a short circuit — the capacitor article covers why.
That is what breaker curves exist for. A type B trips quickly on a modest multiple of its rating; a type C tolerates a larger, briefer surge; a type D more still. Fitting a type B on a circuit feeding motors produces nuisance trips that are not faults.
A breaker that trips repeatedly on the same load is either the wrong curve or a genuine problem, and the current calculator is the first place to check which — the 125 per cent rule exists precisely because a continuous load at the rating leaves nothing in reserve.
What is the difference between a fuse and a breaker?
Speed and reusability, mostly. A fuse melts and has to be replaced; a breaker operates a mechanism and can be reset. For very high fault currents a fuse is actually faster, which is why they persist in supply cutouts and in some industrial applications.
The important shared property is that both are protecting the cable rather than the appliance. A 32 A breaker on a circuit wired for 20 A is a fire risk regardless of what is plugged into it, which is why the cable sizing article treats capacity and protection as one decision.
Why do wire colours differ?
History, and the changes are recent enough to meet both conventions in the same building. European harmonisation moved the UK from red and black to brown and blue in 2004, and three-phase from red-yellow-blue to brown-black-grey.
North America uses a different scheme again — black or red for live, white for neutral, green or bare for earth — which is why an imported appliance or a piece of equipment wired to another standard is a genuine hazard rather than an inconvenience.
The rule that survives every scheme: identify by testing rather than by colour in any installation whose age you do not know. Colours are documentation, and documentation can be wrong.
What does bonding do?
Ties metalwork together so that everything a person can touch simultaneously is at the same potential. It is a different idea from earthing: earthing provides a fault path, bonding removes the voltage difference that would drive current through someone bridging two metal parts.
That is why pipework, structural metal and incoming services are bonded in a way that looks redundant. The protection is not that current cannot flow, but that there is no voltage across the person for it to flow through.
Questions people ask
Why does an RCD trip with nothing obviously wrong? Accumulated small leakage across several appliances can reach the threshold together. Splitting circuits across separate RCDs is the standard fix, and it also stops one fault killing the whole installation.
Are two-pin appliances unsafe? Not necessarily — double-insulated equipment provides two independent layers of insulation instead of an earth, which is a recognised and equally valid approach.
Can I test an earth with a multimeter? You can check continuity, which is not the same as measuring loop impedance. The figure that matters for whether protection will operate needs an instrument designed for it.
Is fixed wiring work I can do myself? It is regulated work in most jurisdictions, with the specifics varying substantially. Calculating the design is one thing and installing it is another.
What is an arc fault detection device? A newer class that looks for the electrical signature of arcing rather than for current or imbalance — the failure mode that starts fires without ever drawing enough current to trip anything else.
The earth exists to make the protection operate, and the RCD exists because the protection cannot see a person. The current and power calculators cover the load side, the cable size and voltage drop calculators the conductor, and the 230 V power calculator the circuit ceilings the protection is sized against.