# What's the function of ground wire anyway?

I thought Ground wire in house electrical systems was used in order to discharge electrical currents to the earth. However, I've seen people saying ground wire would drive electric current to fuses, when there was a short circuit, in order to avoid damaging people, or the system. Can someone help me understanding this?

Keeping the example to mains in a house for simplicity...

UK mains consists of three wires: L(ive), N(eutral) and E(arth).

The mains electrical circuit has electrical current flowing between Live and Neutral.

At the local sub-station, Neutral is connected to Earth and Earth is connected to the ground.

In a house, Earth is connected to the metal cases of electrical equipment like cookers and to water pipes, radiators and so on.

If the mains accidentally connects to these metal cases or anything else earthed, a high current flows from Live to Earth, back to the sub-station and thence to Neutral.

In a very short time, a fuse in the mains Live should be overloaded and blow, cutting off the Live supply and rendering things safe. There is a 1, 3, 5 or 13 A fuse in every mains plug.

In the old days, the mains supply to those sockets would have a 30 A fuse in the fuse box where the house's incoming mains first goes through. That would blow if the mains plug fuse didn't. However, you needed to draw all that current to blow the fuse and if you were the thing causing the short-circuit, you might be seriously harmed or dead by the time the fuse went.

Nowadays, house 'fuse boxes' must use Residual Current Devices (RCD) instead of fuses. These are a sort of current-detecting switch and cut the mains supply to the socket when the current flowing out from that Live is sufficiently different to the current coming back on that Neutral. If Live current is being shorted down to Earth and heading off to the sub-station, it's not coming back on that Neutral.

RCDs are very fast-acting and also very sensitive, tripping on a current imbalance between Live and Neutral of milliamps. These means that far less harmful current flows off through a fault or short-circuit and for far less time, causing far less harm to people caught up in that fault.

Ground by practical terms is any 0V reference. When they connect Neutral in outside premise distribution transformer (DT) to earthing ground, it may be <100 ohms if another ground in home is via copper underground pipes with some other value <100 ohms. Normally the only current flowing thru ground is EMI filter noise up to 0.25mA or 0.5mA (stds?) per line filtered supply. The other case is if a lightning strike to ground occurs, inside home is protected. Or if line to ground fault occurs, breaker should be tripped and inside home is protected.

IF a line to GNd fault occurs, the LIne breaker would trip instantly.

But for excess ground leakage currents, a local GFI or RCD outlet or breaker in panel, (\$) is needed.

However in special cases miswired outlets or ground faults from unbalanced Hot LINe and Neutral currents, a GFI or equiv.RCD will,protect that user from humid leakage currents to ground that may occur with dust contaminating pure water and if hair dryer in bathroom does this, user is protected by GFI or equiv.RCD. It does not sense the ground current, rather the difference between Line and Return... Neutral to sense ~ 500uA or ? local std. of safe current trip level .

In electronics, we call anything with 0V and low resistance a " ground". But here we are talking about an "earthed ground" so power line transients from lightning must also consider ground wire inductance and fault to ground to see what voltage gets created to see if unsafe to equipment. Thus MOV's are used in areas with high hurricane or lightning activity to shunt induced over-voltage and protect the electronics. Since most portable devices do not have a 3 pronged plug, the protection is designed into the insulation and Breakdown Voltage bdv, rating or the device. Most are designed for 1.5kV, some are 3kV like TV's and the best are protected to 6kV where external power meter is gapped to protect at 6kV line transients and above with respect to ground and each other.