How earth leakage and trip switches work
This is hopefully the first in a series of posts where I better understand something by explaining it.
A trip switch is a safety mechanism for preventing electrical shocks, it’s something I’ve used many times with out really considering how it works.
After a chat with my dad and grappling with a few high school science concepts I’d long forgotten, I think I have a pretty good grasp, here goes:
Electrical current runs in a circuit from live (brown wire) to some appliance like a light bulb which applies resistance to the current before travelling back through neutral (blue wire). This is because potential energy like electricity moves from high potential (positive) to a low potential (neutral).
The ground on which we stand has a neutral charge so given the opportunity any positively charged electrons will move from the live wire to the earth. Much like lightning.
For this reason it’s dangerous to touch a live wire while simultaneously touching the earth. As the electricity will pass through you instead of it’s intended circuit.
As a preventative measure many electrical systems like you’re house are fitted with a earth leakage switch.
Most houses will have a copper rod stuck in the ground. This rod is connected to a sensor which reads the current travelling through it, if the current exceeds 30 Ω’s it considers this an anomaly, flipping the trip switch and shutting off the power.
So if you put you finger to the live socket of a plug point, the electricity will pass through you, from live to ground, positive to neutral, but instead of dissipating into the earth and continually flowing through you. It will pass through the ground and find the copper rod, which offers little resistance to the current, remember electricity will always follows the path of least resistance. It takes the easy path and travels up the rod to a sensor which reads the current, if it is above the 30 Ω threshold, the sensor flips a switch breaking the circuit and potentially saving your life.
Usually electricians will also try earth most of the conductive materials in your house. For instance metal pipes are conductive so if exposed to some part of the circuit would carry current, if the pipe is also connected to ground it’ll carry the stray current to ground and trigger the sensor again. If there was no trip switch this leakage would likely go unnoticed.
A couple follow up questions you may have:
If the current passes through your body regardless of whether there is a trip switch why does it matter that you switch off the power.
It’s true in both cases you are shocked, but a short shock is a lot safer than a prolonged one. Your heart’s muscles pump in reaction to electrical impulses in the body, when current passes through the body it interrupts electrical pulses meaning the muscles are no longer instructed to pump and your heart stops. So short shocks are definitely better.
Why do you need to run an earth cable (the green-yellow wire) all the way to a plug point if your whole house earthed?
Right, you don’t really need to. But it does shorten the circuit if you touch the live wire and the earth at the same time. The electricity will just run through you hand and up the earth cable to the trip switch sensor.
Why do the lights work but the plugs don’t?
Normally the plugs are connected to earth and the trip switch but the reset of the electrical lines are not. That why a tripped circuit will only affect the plugs and not the lights and other circuits.
Why does it trip when I’m out of the house?
Anything that conducts electricity and runs uninterrupted from the plug to the ground could complete the circuit and cause a trip. A water leak is a common cause.