Lessons · Engineering · Earthing and the fault path
A fault needs somewhere to go, fast
Earthing gives a fault current a low-resistance path back to the source so that protection sees a large current and disconnects quickly. Without that path, exposed metal can sit at a dangerous voltage while the breaker, seeing only a small current, never operates.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is the difference between a fault that trips a breaker in milliseconds and one that waits silently on a metal case for somebody to touch it. The danger is not the fault; it is the fault that does not disconnect.
How to think about it
Follow the fault current the way you would follow a load path: from the live conductor, through the fault, back to the source. If any part of that route is missing or high resistance, the protection will not operate in time.
Worked example
A fault is a connection that should not be there.Live to metal, usually.
A low-resistance earth path gives a large fault current.Large current is what operates protection.
Protection then disconnects in a fraction of a second.This is the whole mechanism.
A broken or high-resistance earth leaves metal live.With nothing tripping.
Your turn
What does a low-resistance earth path cause that makes protection operate?
It makes the fault large enough to operate protection
Solve one, graded on the server
The trap
Assuming an earth connection is either present or absent. A corroded or undersized one passes a test lamp and still will not carry enough fault current to trip anything.