TN earthing resistance

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Sahib said:
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.
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Yes, you do need to know the rating of the protective device. But you can't calculate earth loop impedance like that.
If, for example, it was 2.19 ohms, a 100A protective device would take hours to operate. If at all.

In the UK and, I suppose the EU since we are supposed to be harmonised, the maximum operating tome for a 100A protection device on a 230/220V system is 0.4 seconds. To get the protection device to operate in that time requires some multiples of its rated operational current. About 15 times for a BS88 fuse and around 10 times for a C-curve BS EN 60898 breaker or 61009 RCBO. So, we're looking at a fraction of an ohm.
The actual requirements in the regs 0.05 ohms for a 100A breaker.

I don't know where the OP is from but 220V and the term "earthing" sounds like EU land or one of the ex-British colonies where British standards might apply.

I'm done.
 
electrics said:
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...
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minimum earth resistance of the utility here is 5 ohms, as an acknowledgement for electronics equipment that require a lower earth resistance than the old 25 ohm requirement.
 
Besoeker said:
Yes, you do need to know the rating of the protective device. But you can't calculate earth loop impedance like that.
If, for example, it was 2.19 ohms, a 100A protective device would take hours to operate. If at all.

In the UK and, I suppose the EU since we are supposed to be harmonised, the maximum operating tome for a 100A protection device on a 230/220V system is 0.4 seconds. To get the protection device to operate in that time requires some multiples of its rated operational current. About 15 times for a BS88 fuse and around 10 times for a C-curve BS EN 60898 breaker or 61009 RCBO. So, we're looking at a fraction of an ohm.
The actual requirements in the regs 0.05 ohms for a 100A breaker.

I don't know where the OP is from but 220V and the term "earthing" sounds like EU land or one of the ex-British colonies where British standards might apply.

I'm done.
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overcurrent protection here is supposed to be 8 cycles at 60Hz.
 
TN Earthing Resistance

TN Earthing Resistance

The recommended Value in Ohms to Ground can be found in the IEEE Green Book. The Volume is IEE 147 and it goes into detail the hows and whys of grounding and it makes recommendations as to grounding resistance for various types of buildings.
 
robbietan said:
minimum earth resistance of the utility here is 5 ohms, as an acknowledgement for electronics equipment that require a lower earth resistance than the old 25 ohm requirement.
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Why would an electronic device care what the resistance to earth is (can you give some examples of the types of equipment and why they care)? For dissipating static build up, a number way higher than that would be sufficient.
 
suemarkp said:
Why would an electronic device care what the resistance to earth is (can you give some examples of the types of equipment and why they care)? For dissipating static build up, a number way higher than that would be sufficient.
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Most electronic devices that I can think of would be more affected by differences in ground voltages than absolute levels. In those cases the ground network's ability to provide a low equipotential grid resistance is more important than it resistance to earth.
But equipment that interacts with devices which are actually referenced to a separate earth ground (directly or via wiring or plumbing, for example) could have problems not with a high ground resistance by itself but rather with a high voltage offset from earth ground caused by small stray (non-fault) currents.
 
electrics said:
but I can not understand why? Is it just economical reasons? For saving money or what?
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The NEC caters to politics, which ultimatly caters to money. If RCD was required on all branch circuits many, many would be against it. Recently in the past 4 years the NEC has begun to require AFCI (Arc Fault Circuit Interrupters) on most residential branch circuits, and as a result many builders are speaking out against it because of the price. 1 is state not even allowing that code section to remain while many areas do not even want to enforce the AFCIs all together. Simply put American builders do not want to spend the money.

It is important to note that TT systems are forbidden by the NEC no matter what. IT systems are not allowed in residential either. Our systems are TN-CS. A few ITs are used in industrial but that is it.

In my opinion your system is way better than ours hands down, but there is not much the American public can do about it.
 
iceworm said:
Robbie -
A little more please. OCP of what? At what current level?

Utility? Industrial? Residential? All?

ice
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The class A GFCI per UL shall operate for a ground leakage current of 6 mA or more and its operating time in seconds shall not exceed (20/I) raised to the power of 1.43, where I is in mA greater than or equal to 6 mA.

So the operating time stated in post #21 is for over current protection and not for ground fault protection.

I clarified myself. :happyyes:
 
Neutral amperage

Neutral amperage

Im studying electrical theory after 10 years of practicing electrical work. Im curious as to why the neutral at my service entrance has a reading of 6 amps. How can one test the "actual" voltage between the neutral and the "ground"? Why would the ground(aka earth) have current, and or amps? Is this reading just the unused electricity flowing back from the unbalance of phases?
 
flappert said:
Im studying electrical theory after 10 years of practicing electrical work. Im curious as to why the neutral at my service entrance has a reading of 6 amps. How can one test the "actual" voltage between the neutral and the "ground"? Why would the ground(aka earth) have current, and or amps? Is this reading just the unused electricity flowing back from the unbalance of phases?
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A couple of points. Neutral isn't ground. It may be connected to ground. Grounded rather than ground.
If your service is live and neutral, the current in the live and neutral will be the same.
If it's 3-wire, say 120-0-120, the neutral will carry the difference in current between the two hots.
 
flappert said:
Im studying electrical theory after 10 years of practicing electrical work. Im curious as to why the neutral at my service entrance has a reading of 6 amps.
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There will current on the neutral unless the current on the ungrounded conductors is exactly equal.
How can one test the "actual" voltage between the neutral and the "ground"?
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You would simply place your meter between the neutal and a grounding conductor. When you measure a voltage between the neutral and a grounding conductor, you are actually measuring the voltage drop on the neutral between the location of the main bonding jumper and the point where you are reading the voltage.

If you are reading voltage between the grounded conductor and the earth itself, you will be reading the voltage drop on the utility primary grounded conductor and/or the secondary grounded conductor from the utility transformer
Why would the ground(aka earth) have current, and or amps? Is this reading just the unused electricity flowing back from the unbalance of phases?
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In NEC compliant systems the earth is a parallel current path for the neutral conductor current between the utility transformer and the point where the grounding electrode conductor is connected to the grounded conductor at the building. There are also other parallel paths that may exist, for example a common metal underground water piping system or the shield of the TV cable coax. Both are required by the NEC to be bonded to the buildings electrical grounding system. With the metal underground water pipe system, it would not be unusual to find 20% or more of the grounded conductor current flowing on the water pipe.
 
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