Fault protection (Whats that about)

[ceraunic_level]

Quite a few questions here, any help would be great

Have I got this right
 
Short circuit fault between phase and neutral causes a massive in rush of current which activates the protective device. This is a fault between R1 and Rn and has nothing to do with R2  i.e. does not involve the CPC of the circuit and the earthing conductor or the earth fault loop path. So the fault is dealt with at the CU. PSSC.
 
Now a phase earth fault will send the fault current from the origin of the fault down the CPC, through the earthing conductor, back to the transformers earth then back down the phase conductor from the transformer through the supply fuse to the protective device, which breaks the circuit. PEFC
 
Now if this earth fault is on a TT system, there is no physical connection to the transformer, no conductor.  The electricity finds its way back to the transformer via the actual ground we stand on.
 
That is what I am finding hard to understand. I would have thought the electricity would just be absorbed I guess, be nothing left by the time it got to the transformer, or just shoot off in all directions.

What are the shock implications for people standing near the Earthing rod, or is it straight away at earth potential, which if it is how has it got the energy to get back
 
The transformer I  guess could be quite some distance away (what sort of distance could be involved?)
 
So there must be more resistance to electricity in that little bit of basic insulation (>200Mohms) around the conductors than in say 200 meters of earth (fault to transformer) where 200ohms is considered unstable. ??
Hence RCD protection
 
Neutral to earth fault not sure what happens there.

Thanks

 

[tim_s_uk]

This is my understanding (I'm happy to be corrected):

Short circuit fault between phase and neutral causes a massive in rush of current which activates the protective device.

Correct. This large current should operate the over-current device e.g. Fuse. It would not trip an RCD.

A phase earth fault will send the fault current from the origin of the fault down the CPC, through the earthing conductor...

If the earth is connected to the neutral, either at the user's end (PME) or at the substation (TN-S) then the current should be large enough to blow a fuse. If an RCD is fitted then it will normally trip first, because the current supplied by the phase conductor is not returning down the neutral conductor.

If this earth fault is on a TT system, there is no physical connection to the transformer, no conductor. The electricity finds its way back to the transformer via the actual ground we stand on.

If an earth rod is used then the resistance of the ground will be significant and only a small current will pass into the CPC and down the earth rod. In this case an earth leakage circuit breaker (ELCB) should detect the current in the earth wiring and break the circuit. An RCD would trip in this case too.

That is what I am finding hard to understand. I would have thought the electricity would just be absorbed I guess, be nothing left by the time it got to the transformer, or just shoot off in all directions.

The electricity supplied by the phase conductor is only interested in getting back to the transformer it was supplied from. This is the only way it can complete a circuit. At the substation neutral and earth are the same thing, so if the phase in your house can't get back via the neutral conductor it will travel through the earth (at a reduced current). It will hopefully use the CPC to get to earth, but it won't mind using you or me instead.

What are the shock implications for people standing near the Earthing rod

If you're touching the top of the earth rod then any current reaching the rod will choose to go down the rod or through you to earth. As always, it will choose both, with the current being proportional to the resistance. Since the resistance you present is much more than the resistance of the earth rod, the current through you will be much less. So much less you wouldn't notice it.
 

So there must be more resistance to electricity in that little bit of basic insulation

Correct - The earth is a much better conductor than PVC insulation.

Neutral to earth fault not sure what happens there.

If neutral and earth are connected then the phase will return down both. The current will be normal, so it won't blow a fuse, but an RCD should trip because not all the current supplied by the phase is returning down the neutral.

 

[JohnD]

standing near an earth rod that's taking current is fairly safe if you have only two feet, close together, and you are wearing shoes and dry socks.

However the voltage falls off quite quckly as it dissipates into the ground, so there is a voltage difference between "close" and "further away".

This can be fatal if you have four legs, with a 2m spacing, and are wearing no shoes, or, worse, steel ones nailed to your feet.

Or, of course, if you touch a metal object such as a pipe or a fence wire.

 

[Spark123]

If an earth rod is used then the resistance of the ground will be significant and only a small current will pass into the CPC and down the earth rod. In this case an earth leakage circuit breaker (ELCB) should detect the current in the earth wiring and break the circuit. An RCD would trip in this case too.

(Voltage operated) ELCBs are no longer used as parallel paths in the installation can render them useless, Current operated ELCBs are what RCDs were called before they were called RCDs.

 

[ban-all-sheds]

That is what I am finding hard to understand. I would have thought the electricity would just be absorbed I guess, be nothing left by the time it got to the transformer, or just shoot off in all directions.

This might interest you...

 

[roberta1000]

What are the shock implications for people standing near the Earthing rod, or is it straight away at earth potential

Usually you'd be OK because the earth is a good enough conductor that the voltage across a short span (like your stride) is small even at quite high current densities.

Of course this might be of interest http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2008/02/26/ndogs126.xml

So maybe not a good idea to pee on the earth rod eh?

 

[djrichie_t]

ahh but if you pee on an earth rod whilst youre installing it, you will get excellent readings which will stay permanent. Chemical reaction with urine/soil makes for very low resistance indeed!

Its an old electricity board trick i learnt when i did my electrical fitting apprenticeship. Especially earthing the neutral in a substation in dry ground, there would be a line of us stood where each rod was going to go, taking a leak on the spot lol!

As for electricity going through earth, just think yourselves lucky that in the UK we dont have whats known as SWER systems - 'single wire earth return' pretty dodgy if you ask me, take a look at the wiki...

http://en.wikipedia.org/wiki/SWER

 

[ban-all-sheds]

Its an old electricity board trick i learnt when i did my electrical fitting apprenticeship. Especially earthing the neutral in a substation in dry ground, there would be a line of us stood where each rod was going to go, taking a leak on the spot lol!

No female apprentices in those days, I guess....

As for electricity going through earth, just think yourselves lucky that in the UK we dont have whats known as SWER systems - 'single wire earth return' pretty dodgy if you ask me, take a look at the wiki...

They work fine for HVDC interconnects...

 

[JohnD]

Capital costs are roughly 50% of an equivalent two-wire single-phase line. They can be 70% less than 3-wire three-phase systems. Maintenance costs are roughly 50% of an equivalent line.

I think this has to be nonsense. He is only including the cost of the wire, not the labour cost, the transport and equipment, or the additional trasformers and earth spikes. I doubt maintenance costs are different at all.

 

[ban-all-sheds]

It seems reasonable that not erecting, or laying one of the cables is going to save on labour and transport.

And why do you need extra transformers?

 

[flyingsparks]

Apparantly the resistance of earth after 15m doesnt get any higher...not exactly sure why this is, perhaps someone can clarify and explain this, if i'm correct......

 

[ceraunic_level]

thanks for all the replies

 

[bernardgreen]

Apparantly the resistance of earth after 15m doesnt get any higher...not exactly sure why this is, perhaps someone can clarify and explain this, if i'm correct......

The flow of electrons that is an electric current can be compared to the flow of water molecules in flowing water. Thin pipes = high resistance so less flow etc etc

The analogy of the earth as a large tank of electrons to a tank of water explains the way current flows in the earth. The water molecules in the tank move to keep a level surface after some have been added or removed The electrons in the earth move in the same way to keep an equal pressure ( voltage level ) across the earth. The ripples on the water caused by drips into the tank are very similar to the voltage waves caused by pulses of current into an earth rod.

 

[roberta1000]

Its an old electricity board trick i learnt when i did my electrical fitting apprenticeship. Especially earthing the neutral in a substation in dry ground, there would be a line of us stood where each rod was going to go, taking a leak on the spot lol!

So how much beer does it take to wet the ground to a useful level assuming say a 1m earthing rod?

 

[tim_s_uk]

I'll remember to keep my feet together when standing near an earth rod during a thunderstorm!

Apparantly the resistance of earth after 15m doesnt get any higher...

The following website explains the principle: http://www.duncaninstr.com/Gr_article.htm

"Calculations for the case of an hemispherical electrode show that 90% of the total resistance is concentrated in the nearest region, up to a distance equivalent to 10 times the electrode radius."