1/ Any ground drying out or freezing will at most only affect the first metre, it's extremely rare for a 3m/10' or deeper rod to be affected to the extent of no longer being able to provide a sufficient low resistance earth path, to protect the installation it's connected to under a fault conditions.
Depends on the climate and soil. In some areas passing 100ma is no problem all year round, others like say the Country of Norway getting even 5ma might be a challenge.
2/ A driven earth rod will actually improve over the 1st year of installation as the soil around it consolidates around the rod. Maybe after 30 years you may find a bit of deterioration, but only if the soil make-up has a corrosive element. All earth conductor connections to rods should be protected if they are of the bolted/mechanical type connection, usually with two or three layers of amalgamating tape. What's the chances of a bolted or exothermic weld connection becoming disconnected, ...near Zero!!
If undisturbed and driven deep enough it can improve as soil settles.
However, in the US I see a lot of disconnected ground rods, it happens. Yes they should be protected, but when comparing a main bonding jumper to a ground rod, the bonding jumper is 1000s of times more fail safe. I have seen even undisturbed ground rods corrode.
3/ Now that is something i wouldn't even think of considering to do. I don't know what the DNO's are like in the States, but in the UK (or anywhere else i can think of) they wouldn't take interfering with a TX's neutral-earth point too kindly, to say the least....
I never said breaking the neutral to ground bond at a transformer. In the US we leave one, two, or three phases out of the weather head plus a neutral and pocos hook it up to their wires. In underground services its the same, they either hook their wires (1,2 or 3 hots plus neutral) at the meter or in an underground splice box.
You mentioned that DNOs would get upset if we did something to the grounding system within the home or building (at least that is what I was trying to say) but in the US that is not a POCOs responsibility.
1/ External and internal parts of an electrical installation warrants the same degree of testing, in fact the external side of things will often dictate the safety of the whole installation. What do you mean by pointless??
In a TT system the external ground rods play a role to fault clearing yes. The lower the ground rod resistance in a TT system the better, here low Z is of value. But in TN-S and TN-C-S system external grounding makes no major difference. Assuming the N or PE never breaks, whats the advantage? And if the N did break in TN-C-S I can see the ground rod drying up if the condition persisted and it probably would because a low Z would mask a broken N until something made it noticeable.
A decent 0.5 - 1 ohm TT system is Far from pointless it is in many cases better than is seen on a TN-S system and probably better than some of your TNC-S supplies if they don't have sufficient N-E links along the supply line....
What do you mean seen better on TT than TN-S probably better than TNC-S? Confused. Why would more or fewer N-E links along the supply line matter?
If you are talking about making the earth another neutral conductor to relieve voltage drop across the neutral conductor a low Z ground rod wont matter to much. All the potential rise relative to remote earth would still persist to some degree or another. A metal water line might help though...
What will prevent a neutral from shocking people in a building is bonding to all the metal parts if a TN-C-S system. And to be honest if you are getting a lot of voltage drop on a neutral it is either undersized or broken. And that brings up another issue. In TN-C-S earthing a broken neutral might get covered up by many ground rods in parellel that have a low Z. It just food for though.
2/How do you think the DNO maintain a low Ze level on a TNC-S supply if they don't N-E link with earth rod(s) at intervals along the supply lines??
Simple. The condutor itself. That is a job of the conductor. Relying on earth for low Z is asanine. Its a waste of money to pound so many ground rods that the earth now has an impedance similar or lower to the neutral. In some soil conditions it can not be done. If the neutral is undersized it is a lot cheaper to upsize it than to drive long or many ground rods at every pole. How is an aluminum neutral more in cost (that could even be pareleleld to the old one) cheaper than driving copper ground rods? Further, if the neutral broke on a low Z earth rod, who would notice?
Also, useing the earth as a conductor has draw backs. Not only are you creating magnetic fields and loseing the ability to detect downed conductors through zero sequnce (RCD) protection but also creating stray voltage problems. If you dont beleive me Google Stray voltage or stray current. In the US where TN-C is the norm for utilities this is beoming a major issue in some palces.
Personally, one ground rod 4 times per mile is enough. Maybe a few extras for lightining protection but thats it.
3/ I'm not saying and never said you NEED to employ a TT system be it rods ,Ufer, tape etc when a TNC-S or TN-S supply is available...
Ok, makes sense.
4/ What do you mean by ground rods offer ''Some'' help on a Lightning Protection system?? It's the ''ONLY'' protection that an LP system is based and relies on. So where do you think all that incredible amount of energy is going to go without those ground electrodes then?? More to the point what would you think would happen with that incredible amount of energy without those rods being in place??
It is the basis for lighting protection, but do 2 ground rods help over many? Yes, lighting dispsipates into the earth, but what point do we spend money on ground rods and testing for something that may never happen. And if lighting did strike a home directly, it doesnt matter how many gorund rods or low Zs. A direct hit will destropy everything. I have seen direct lighting hits on homes with UFERs and city water pipes which as the best ground eletodes on earth, litterally. Yet the damage is obscene either way. Also, keep in mind what Mike Holt said. A well casing has a lower Z, but because lighting is a short pulse, it will go to something that is closer by.
Ok, I will agree one this, if you have 2 ground rods at 1ohm, they will help with lightning protection.
1/ Main earthing and main bonding are two completely different animals each having it's own task. They are also of equal importance.
In terms of human safty bonding is what protects, grounding does nothing about that.
2/ I'm afraid you are Wrong a good earth electrode connected to the MET will indeed make a difference, while not entirely removing the danger it will reduce considerably the voltage potential...
If the Z is low enough between the transormer and service to the point it has a few ohms or lower it certianly does help. In fact the higher the Z the more voltage rise. However, if the Z is high itself to the transfmomer the earth rod will not do much even close to the structure.
Sorry, but you're wrong again soil resistance has everything to do with a TT system!! Do you not think that any extraneous earthing brought into a property via metallic water, gas, or oil services pipework will not also help a TT system just as much or as little as it will a TN earthing system??
I am not disagreeing that it is everthing in a TT system. Pipes certainly help a TT system. In fact metal water or gas pipes can act as a 2nd neutral during an open service neutral in a TN-C-S system provided they are continous to other structures. In the US I have seen an open serice neutral not get noticed baecause the city metal water pipe acts as a perfect conductor.
The fact is, Bonded services (equipotential zone) should not be considered as part of the fault path for tripping breakers
This part I absolutly disagree with. How does an equal potenital zone not help clearing a breaker? Its pure copper that is less than 0.1 ohm? And again, a bond is less than 0.1 ohm, so it will trip a breaker no questions asked. An equal potential zone is based on excaptioonally low Z bonding (copper wire) and is always counted as a current carrying during a fault.
1/ So what else do you rely on if they are providing a TN earthing point??
For fault clearing you rely on the neutral they give you, or in the case of TN-S the ground POCO gives you. If the neutral can handle 100amps at 230 volts without issue it can definelty clear a fault. When RCDs are not present this is all that you rely on. Yes metal water pipes can compliment it, but your neutral is what clears the fault.
2/ Again you do NOT rely on the main bonding conductors (extraneous earthing) to provide a fault path to trip breakers, that is not what they are intended for....
Than what is the intention. I dont know. Again, what is better than a main bonding jumper with an impedance of 0.0001 ohms and a neutral or ground wire that goes back to the XO of 0.01 ohms???? How is even a best case ground rod senerio of 1 ohm better than that.
Keep in mind, ground (soil) is nothing more than a resitor. It has no special properties or electron ambitions.
3/ Exactly that, you are relying on a network operators earthing point when provided.
We are not relying on their "earthing" point. We realy on their neutral conductor. A neutral condutor is not an earthing point, it is a low impedane path that allows for fault current to go back to its source.
The problem you have in the US is that you don't test anything.
I agree, this is an issue.
How many electricians in the States own a Earth loop Impedance tester??
Few, if any.
Virtually none, so you have no idea what the Ze or the Zs of an installation is. The NEC doesn't call for any testing, apart from ground rods , so none get's done.
Wrong. The NEC does not require testing ground rods. All it says that if you can prove on ground gorund rod is less than 25 ohms you dont need another. If you cant test a ground rod or dont want to, drive 2 rods and you are done.
Truth is we dont have to worry about Zs. Most of our serivces are oversized, the neutral conductors are often as big as the phases, poco transfomers are close by, and above all we dont ever rely on the earth for anything. As a result, I can short circuit my home panel and easilly get 8,000amps phase to ground which is bonded to the low impedance neutral. Even at the branch level we use full sized earth wires, and even when we dont like with circuits over 40amps our EGCs are still plenty low Z. Voltage drop is our biggest factor since we use a lot of 120, so when that forces use to increase the phase and nuetral size it forces the same on the earth wires forcing a lower Z which compensates for the longer run.
In truth earth wires and bonding in the NEC is over engenered by requirments, so a fualt always draws well over 10 times the breaker handle rating anywhere in the system tripping the circuit in 1/60th of a second. And yes, most of the time we meet IEC disconnect requrments. And even when we do not, out systems are so well bonded that the voltage rise between metal points is still very low.
If we did TT earthing we might then have to worry about Zs but dont.
Every electrician in the UK and Europe owns either a MFT (multifunction tester) or a set of separate testers to test Insulation Resistance, ELI, RCD's of most types, Voltage, etc....
This would help, in other cases like ensuring the installtion is free of defect like an open earth wire.
1/ On small installations such as domestics we use DP isolators rather than a main breaker. On a TT system it is prudent to use an upfront 100mA time delayed RCD to compensate for a 30mA failing to operate.
Good info! Thanks!
But again, this costs more.
1/ I was talking about as a primary main supply system
Ok.
I don't think you've ever worked on IT centres from what you have stated above.
RCD's will have no effect whatsoever on the quality of any grounding system
So your saying that when a neutral touches a ground (earth wire) after an RCD, it doesnt trip? How is a grounding system with neutral current on it quality when you can measure voltage gradients and magnetic fileds. Heck take an amp meter and you will read current on metal parts.
Isolated grounding in the USA only came form that misunderstood concept. In the USA RCDs are nonexistent so many buildings have standing neutral to ground faults.
Can you explain that comment please??
Nothing misunderstood about an isolated/separated/dedicated earthing system...
When RCDs are not used, neutral to ground faults go unoticed as well as high Z hot to ground faults. This puts current on the grounding system. Such current flows on all metal objects such as audio equipemnt shields inducing counter currents in the vary conductors being sheilded. Current also results in voltage drop, causeing voltage differnces between grounding points. Even 1 volt can cause problems. These voltages interfere with signal transfer in senesitive equipment. Thus electronic equipment malfunctions.
Isolated grounding came about as a code compliant method which allowed to help break the differnece in potential by going all the way back to the serice entrance yet still having an effectinve low Z ground fault currnt path. It helps somewhat, but not much. Every time you take an isolated ground machine and have it contact something elese in the facility you loose that iso ground. And it still doenst correct the issue at hand.
Dirty grounding is nothing more than a ground that has a voltage difference between other metal objects which is caused by current being placed on the grounding system in some way. Current seaks all paths, so when its travelling on the grounding system any extra path like shileded cable or metal racks will take some of that current. The magnetic feilds that AC current produce travelling on the grounding system also wrecks havoc. Old CRT computer monitors will jitter from it for example, and audio equipemt will produce a 50/60hz hum.
The misunderstianding came when it was beleived this was being caused by other issues like not having an ISO ground. Most American electricans beleive ISO grouninding is a scam.
Are you saying that GFCI's (RCD's) are non existent in the US now??
They are none existent on branch circuits. They exist near sinks and outdoor sockets for personal proectection, but other than that 99% of circuits do not have any RCD protection.
