Hi, What must be the principle when calculating the minimum earthing resistance of a building electrical sysystem? I am asking this since I couldnt see any resource citing such an important issue . ...
From the west side of the Atlantic (and the northwest corner of the american continent), best I can suggest is to get IEEE 142, and IEEE 1100.
but is this based on an assumption that there is a residual current device in the main panel and it will operate in case there is a earth fault in the system. So you say there is no such a backup protection for earth faults as maybe some codes enforce it....
(paraphrased - simplified)
As noted, regulation (NEC) requires personnel ground fault protection for low level Branch circuits.
Regulation also requires equipment gfp for 1000A or above. As is known, most all faults start phase to ground. Impedance is high enough to limit current to where the CBs take a long time to trip - lots of damage. Eventually the fault goes phase to phase. Current climbs quickly - CB finally trips. So, the NEC requires monitoring ground fault current and max trip set at 1000A, 1 sec. (Could be somewhat different numbers - I didn't look it up) Keep in mind the NEC is not a design guide - it is just a set of minimum specs that may not work well, might not be reliable, and provides minimal safety. If one wishes a more reliable, safer system, there are plenty of IEEE design guides available - provided one can keep the electrical contractor from value engineering the installation to unworkable, functional death (project death that is).
excuse me, but I am asking due to my ignorance, how an electrical protection is offered by nec in short? I really wonder...
Other than small branch circuits and a few other small circuits, for a low level fault to un-bonded ground (like earth) - it doesn't
but I can not understand why? Is it just economical reasons? For saving money or what?
Yes, I think mostly money. The existing North American system, TN-C, benifits the utilities with an excellent low impedance ground grid, virtually cost free to the utilities, which they use for their relaying. But this ground grid does little to protect residential structures.
what I can not understand is why GFCI/RCD is not used in nec. ...
Hummmm .... In this matter, Americans are not as smart as Germans, Brits, and even the French? And way dumber than the Norwegians.
And Americans tend to think we are the best there is AT ANYTHING. How could anything we have possibly be second rate? And to change, would be a lot of money. So apply the principles of institutional and cultural momentum and then slam the American (and Canadian) wallets. I don't expect any change is my lifetime and likely not in my kid's lifetime.
Europe got a head start because in WWII, the infrastructure got bombed to near stoneage rubble. What was left standing was fractured, bent, condition unknown. The choices were limited.
- Condemn the still standing structures
- Completely rewire the still standing structures
- Install RCD and continue to use until replacement money was available
The Norwegians are a special case. They use IT. The whole country is built on a rock knob. A reasonable earth connection is not available. Amazingly they said, "We're not following the American or European lemmings. Let's find our own cliff to jump off." And they now have one of the best systems in the world. And they paid for it - plenty.
So, why doesn't the nec require RCD?
We did not get the snot bombed out of us.
We don't want to spend the money.
We're not as smart as the Norwegians.
We don't want to spend the money.
We Americans tend to think we are the best and brightest - how could anything we do be second rate.
We don't want to spend the money
History of world wide electrical systems - per the Worm
PS: Merlin-Gerin has several good papers, Cahier Technique. "Earthing systems worldwide and evolutions", E/CT 173, is a good one