Electronic – How does equipment ground protects us from electrical shock

electricalgroundsafety

What I have always learned is current leave the source and return back to the source, forming a closed circuit, here are the two thought experiments I had and got confused:

Scenario 1:

Here I have a black (hot), white (neutral) and green (ground) wires coming from the wall and into the box where the ground wire is connected to the chassis of the metal test box.

In the case if the black wire touches the metal casing and a person is touching the case, then the ground wire would lower the entire resistance since the person and the ground forms a parallel resistor of the circuit and drop all the voltage upstream so the person would be safe

enter image description here

Scenario 2:

But what if the ground wire isn't connected to the chassis? and somehow the hot wire made a connected to the chassis?

enter image description here

My inclination would be the current travels through the person, but here is the dilemma in my head, how does the current return back to the source from the person standing on the concrete ground? are we really saying the current travels from the concrete, into the building and somehow back to upstream?

If the person is receiving an electric shock from scenario 2, then how does the current get back to the source?

If there is no close circuit for the fault current to return to source, shouldn't the person be safe? since it is an open circuit?

Best Answer

Not quite.

In the case if the black wire touches the metal casing and a person is touching the case, then the ground wire would lower the return circuit resistance, causing a very large current to flow. This will blow the fuse in the plug (UK BS1363 plugs) or the much larger fuse or trip the breaker in the house circuit, keeping the person safe even before they touch the enclosure.

If the ground wire isn't fault current rated or the breaker is too high current for the circuit, a fire ensues instead.


In the second circuit, only a small current is required to kill the person. If the concrete floor provides a very high resistance, the person feels an unpleasant tingling but probably survives.

If they touch some grounded metal with the other hand, the current increases. Now the RCD (UK) or GFCI (US) can see current flow on Live which isn't balanced by current returning on Neutral, so it trips at 20 mA or so : unpleasant but not usually fatal.

(This may also happen in the first scenario : there's a race between the protection mechanisms. Older houses may not have RCD protection so fuses/overcurrent breakers are still a good idea)


In either scenario the person is protected IF the protection mechanisms are properly installed, tested and working.