Understanding Earthing: Risks and Safety in Un-Earthed Energy Networks

[Bringitondown]

Hi All,

First post of many on the forum.
On the Subject of earthing I had always been led to believe that electricity would always go to ground (if of course ground was the lower potential)
I was talking to someone the other day who was discussing un-earthed transmission systems and live working on such systems. They seemed to infer that there was no danger of becoming part of the current path when acting as a route to Earth from a live conductor to ground of an un-earthed system.
Is this possible?
Any insight would be greatly appreciated

 

[stedwards]

For current to flow there needs to be a loop. If you tough either side of an isolated secondary transformer and Earth ground, there's no return path.

However there could be a ground fault in which case you get zapped. There could be capacitive coupling between circuit and Earth ground, forming a weak return path.

 

[jim hardy]

we answer this question a lot.

here's a quote from one of them
https://www.physicsforums.com/threa...ound-rather-than-negative-pole.798525/#post-1

Many people think "ground" has some magical affinity for electric current.
I think that comes from the "Water Analogy" .
Water runs out the end of our garden hose and is pulled to ground by gravity. And when we lift water above ground we increase its gravitational potential energy.

Electric charge is (so far as i know) unaffected by gravity .

"Ground" (i much prefer to call it Earth) is, for circuits, nothing but another wire that happens to go almost everywhere.
If current can get back to its source by going through ground, Kirchoff's Laws say it probably will.
But it has no reason to prefer that path homeward over a copper wire.

Your flashlight is oblivious to whether it's resting on the ground or hung from a skyhook.
There's a lot to be learned from the humble flashlight, and we've had long threads about it before.

and another from https://www.physicsforums.com/threads/how-do-you-ground-a-current-in-a-circuit.757305/#post-1

"Ground" is a sloppy term , misused almost universally.

What it usually means is "Circuit Common" or "Power Supply Return". That may or may not be connected to the earth.

If you believe Kirchoff's current law, you know that current must get back to where it started. If it leaves the positive end of a flashlight battery it must get back to the negative end of that same battery.

It is traditional to tie the negative side of a power supply or battery to a point that collects all the currents returning from various loads and call that point "Circuit Common" or "power supply return". A sloppy draftsman or engineer will call it "Ground", even though it has no connection to the earth...

Car companies still call "Chassis" "Ground" though it's insulated from ground by the tires.
Missile and aviation stuff that I've seen calls their chassis more correctly, "Vehicle Skin". Obviously an airplane in flight isn't grounded.
The British use the term "Earth" instead of "Ground" which i like.

So please form the habit early in your career of calling circuit common by that name, it'll save you confusion later on.

I use the term "Earth Ground" for a wire sunk into the earth, and i correct people (to their occasional annoyance) who speak of circuit common as "ground". I'm old enough to get away with that but you may not be so proceed judiciously.

The telephone company does tie their circuit common to earth. So does power company.
But your TV set's circuit common is very well insulated from Earth ground. Else it'd have a three prong power plug.

If you encounter an old Chrysler or Volkswagen you will find the battery's positive terminal tied to chassis instead of the negative and the car described as "Positive Ground". That's blown up a lot of aftermarket radio installations .

I know above sounds overly-fastidious but "ground" an important concept. And I'm a potentate-level nerd.

check these threads
https://www.physicsforums.com/threads/why-do-we-ground-an-electronic-circuit.810961/#post-1
https://www.physicsforums.com/threads/why-ground-split-phase-residential.767111/#post-1
https://www.physicsforums.com/threads/not-fully-understanding-grounds.622107/#post-1

and come back with questions on anything that's unclear ?.

 

[jim hardy]

They seemed to infer that there was no danger of becoming part of the current path when acting as a route to Earth from a live conductor to ground of an un-earthed system.
Is this possible?
Any insight would be greatly appreciated

With how powerful a microscope do you want to examine that question ?

If there's absolutely no circuit from that power system to Earth besides you, then no current can flow through you.
He'd be right.

All circuits have area, so does earth, so there is some capacitance between that power system and earth.
That capacitance can complete a circuit and allow current to flow through you.
He's not mentioned that.
So long as that capacitance limits current to less than your can feel (a few microamps) it won't shock you.
Your doorbell is a small enough power system system it won't shock you. Not enough capacitance.
But a mid-size industrial system might push a goodly fraction of an amp through its capacitance and that's plenty to be lethal.

That's one of the reasons we Earth power systems and equip them with "ground detectors".
Check out "IEEE GREEN BOOK" for a really thorough lesson.

 

[Bringitondown]

Sorry for the late reply, but thanks for all the replies much appreciated, although maybe created more questions, thanks all

 

[William White]

Hi All,

First post of many on the forum.
On the Subject of earthing I had always been led to believe that electricity would always go to ground (if of course ground was the lower potential)
I was talking to someone the other day who was discussing un-earthed transmission systems and live working on such systems. They seemed to infer that there was no danger of becoming part of the current path when acting as a route to Earth from a live conductor to ground of an un-earthed system.
Is this possible?
Any insight would be greatly appreciated

Its quite safe to work on live conductors as long as you are at the same potential as the conductor and inuslated from any other potential.

This is how linesman are able to work on EHV lines.The term Earth is defined in British law:
http://www.legislation.gov.uk/uksi/1989/635/regulation/8/made
"A conductor shall be regarded as earthed when it is connected to the general mass of earth by conductors of sufficient strength and current-carrying capability to discharge electrical energy to earth. "If the system is completely isolated from earth, then current will not flow through you to earth, because that is an open circuit. Its a method used to achieve safety in explosive atmospheres where stray Earth currents are very dangerous.

However, the reason we Earth power distribution networks, and are obliged to do so by law; is because an undetected Earth fault can arise. If you then become part of the circuit to earth, (ie another fault) current will flow.