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 . We have an electrical system phase neutral 220 volt (a dedicated transformer secondary is connected to the general earthing rods and electrodes ) and I am asking what must be the value when we check the resistance value in the final stage in which we bond general earthing with iron frame of the building...
TN earthing resistance
- Thread starter electrics
- Start date
- Status
JoeStillman
Senior Member
- Location
- West Chester, PA
Here in NEC land, the minimum resistance is 25 ohms. See article 250.53(A)(2)
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
not really true. that is the minimum resistance if you want to use a single rod style electrode. in general in NEC applications the ground resistance just is not very important.
Last edited:
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
RCD are not found in NEC systems.
TN is a designation not used in the NEC as well. That should have been a tip off to me initially that your question is likely outside of the scope of this forum which is the NEC.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
short circuit and overload protection is supplied by fuses or circuit breakers.
ground fault protection is a different issue. on grounded electrical systems, the fuses and circuit breakers supply it inherently. there are additional ground fault circuit interrupters required in certain cases that provide protection at much lower levels of fault current.
some circuits have no ground fault protection at all.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
it is just a different system.
- Location
- Illinois
- Occupation
- retired electrician
The majority of our (NEC) systems are grounded systems. The fault clearing path for a "ground fault" does not depend on the connection to earth. It is via the Equipment Grounding Conductor, back to the main bonding jumper and then to the "grounded" conductor. The connection to earth does not play much of a part in clearing ground faults.
- Location
- Illinois
- Occupation
- retired electrician
We use GFCIs for people protection with a trip of ~5 mA.
The only way a GFCI/RCD provides fire protection is from ground faults. It appears to me that most of the fires that are of electrical origin involve a poor connection on a current carrying conductor, not a high impedance fault to the grounding conductor.
Indeed, "the earth is not a bonding jumper."
heavyaslead
Member
Ground resisitance
Ground resisitance
Grounding resistance is recommended no greater than 25 Ohms, however not a requirement (as in it cannot be met with additional rods).
This would be a requirement by the client or some engineered specification.
Many industrial facilities for example like to have no more than 5 Ohms.
Sensitive (or costly electronics) may desire 1/2 Ohm, it just depends.
Keep in mind grounding resistance is a guide for best equipment reference earth ground operation but more importantly is equipotential grounding.
Ground resisitance
Grounding resistance is recommended no greater than 25 Ohms, however not a requirement (as in it cannot be met with additional rods).
This would be a requirement by the client or some engineered specification.
Many industrial facilities for example like to have no more than 5 Ohms.
Sensitive (or costly electronics) may desire 1/2 Ohm, it just depends.
Keep in mind grounding resistance is a guide for best equipment reference earth ground operation but more importantly is equipotential grounding.
growler
Senior Member
- Location
- Atlanta,GA
I have tried to work on a military project where we were expected to follow both BS7671 and the NEC at the same time and this is just not possible.
I have nothing against the British system but if you are going to use BS7671 then you have to stick with that system and forget about the NEC.
Example: the very minute you use British color codes you have already violated the NEC. Neutral as blue or in the old days black?
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
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.
(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).
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
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.
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
Last edited:
handy10
Senior Member
- Location
- western suburbs, chicago
Don't we mean the maximum resistance?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If you must, at a minimum, provide a 25 ohm resistance, that makes it a maximum resistance.
The minimum performance required of a ground rod is to have a 25 ohm resistance.
Wonderful language.
It is the earth loop resistance (impedance) starting from the supply transformer conductor rather than the earth resistance that should be maintained at a specific value with respect to a feeder protective device rating in a TT power system in which neutral is not used as protective conductor in contrast to TN power system (adopted by NEC) in which neutral is used as protective conductor. In advanced TT system, GFCI/RCD is incorporated in the circuits so that the necessity of maintaining low earth loop resistance does not exist.
You need to know the protective device rating and the earth loop resistance. Say the rating of your protective device is 100 A. The earth loop resistance should be less than 220/100=2.2 ohms. If the actual earth resistance (measured by an earth loop tester ) is more than 2.2 ohms, action should be taken to reduce it below 2.2 ohms. Otherwise the protective device (fuse or CB) would not operate to clear a ground fault with danger to life/property.
Note: I assume yours is a TT power system and not a TN system.
Last edited:
- Status