earthing calculation

mivey

Senior Member
If the ground resistance is beyond a minimum value, the EM relays, unlike the static relays, do not clear the earth fault.
But EM relays do provide sensitive earth fault protection. Electronic relays do not protect for all faults either, but they do provide better and more protection options; that is why the EM relays are being replaced with electronic relays.
 

kwired

Electron manager
Location
NE Nebraska
NEC is enforced directly by building codes and indirectly by OSHA.
POCO would still have to satisfy OSHA with their own safety program even if they are are not separately required to follow NESC.
OSHA is concerned about safety of POCO employees while on the job, not the way the installation of their distribution system may impact the customers served. I can't say that I know much about NESC, but I kind of assume it is a lot like the NEC and primarily is concerned with safety in the use of the system and has little to do with safety of the installer, but maybe has some safety in mind of a person performing maintenance, like having disconnecting means for certain equipment. Like the NEC it may tell us we need a disconnect, but doesn't tell us anything about verifying the status or securing of that disconnect before beginning any work. That is where OSHA steps in.
 

Sahib

Senior Member
Location
India
But EM relays do provide sensitive earth fault protection.
Not residually connected EM relay but one with a CT on the neutral but that too requires a minimum ground resistance unlike a static relay which requires none.
 

JoeStillman

Senior Member
Location
West Chester, PA
Is it truly adopted as the law like the NEC often is? Or does the industry just follow the NESC in general?
As far as I know, compliance with the NESC is only voluntary. But it is also the only standard I know of that has some scientific basis for determining the adequacy of a grounding grid. My main point was that the 4-ohm requirement seemed quite arbitrary - and could be proved so if the available ground-fault current was known.
 

JoeStillman

Senior Member
Location
West Chester, PA
...this would require that this low impedance path to be at least .005 ohms or less which would give you a 60 volt touch/step potential between the grid and earth until the GF relay cleared the circuit, adding another load of 4 ohms to this 60 volts would not drop it any further at least noticeable if you consider .005 volts off the 60 volts much with 12ka behind the 60 volts.
IEEE 80 gives some criteria for deciding what your tolerable step and touch potentials are; it doesn't come right out and say because there are so many variables. However, in their example calcs, the tolerable potentials are in the range of kilo-volts.

Again as I said before earth would not play much of a roll in protection against touch/step potential
Quite true. The earth is not a good enough conductor to equalize step and touch potential, which is why we need metallic grids.
 

kwired

Electron manager
Location
NE Nebraska
As far as I know, compliance with the NESC is only voluntary. But it is also the only standard I know of that has some scientific basis for determining the adequacy of a grounding grid. My main point was that the 4-ohm requirement seemed quite arbitrary - and could be proved so if the available ground-fault current was known.
Which makes it a little like OSHA and 70E. Many think OSHA mandates using 70E, but in reality they just require an electrical work safety program. 70E just happens to be the most popular publication that covers things pretty well and is more likely to be accepted as a valid standard in a court of law than "Joe's" written safety policy. Even if Joe goes to the same extent and details as 70E does when writing his policy, it would cost much less to just adopt 70E than it would to pay Joe for his work, plus 70E will be updated periodically where it will cost more again if you want Joe to keep things updated.

So POCO's generally do not have any code they are required to follow, but most choose to follow NESC because it is a well recognized standard and by doing so they reduce their liability.
 

topgone

Senior Member
Which makes it a little like OSHA and 70E. Many think OSHA mandates using 70E, but in reality they just require an electrical work safety program. 70E just happens to be the most popular publication that covers things pretty well and is more likely to be accepted as a valid standard in a court of law than "Joe's" written safety policy. Even if Joe goes to the same extent and details as 70E does when writing his policy, it would cost much less to just adopt 70E than it would to pay Joe for his work, plus 70E will be updated periodically where it will cost more again if you want Joe to keep things updated.

So POCO's generally do not have any code they are required to follow, but most choose to follow NESC because it is a well recognized standard and by doing so they reduce their liability.
IMHO, every POCO just needs to follow what OSHA says. They need to prevent accidents from happening and they will be okay. Whatever happens, it will be the General Duty Clause that will be applied when things get bad in any workplace.
 

kwired

Electron manager
Location
NE Nebraska
IMHO, every POCO just needs to follow what OSHA says. They need to prevent accidents from happening and they will be okay. Whatever happens, it will be the General Duty Clause that will be applied when things get bad in any workplace.
But what does OSHA have to do with installation codes of utilities or any electrical installation for that matter? OSHA is all about occupational safety, and could or at least should care less about installation type codes, and only focus on safety of the workers, which could have some impact on certain aspects of the installation but not all aspects of it. Why should OSHA care much about conductor ampacity, overcurrent protection, and issues like that, those are equipment issues not employee safety issues. Things they are interested is proper personal protection when working live, or deenergizing instead of working live, and making sure when something is deenergized that it can not accidentally become energized until it is intended in the interest of protecting workers, or proper procedures to prevent falling, or entrapment of workers.
 

topgone

Senior Member
But what does OSHA have to do with installation codes of utilities or any electrical installation for that matter? OSHA is all about occupational safety, and could or at least should care less about installation type codes, and only focus on safety of the workers, which could have some impact on certain aspects of the installation but not all aspects of it. Why should OSHA care much about conductor ampacity, overcurrent protection, and issues like that, those are equipment issues not employee safety issues. Things they are interested is proper personal protection when working live, or deenergizing instead of working live, and making sure when something is deenergized that it can not accidentally become energized until it is intended in the interest of protecting workers, or proper procedures to prevent falling, or entrapment of workers.
Let's just KISS it. All engineering calculations result into safer and proper operation of electrical equipment. You don't size it smaller than what standards say it should, else you'll burn the thing, you set the protections just enough to allow safe operations but not too high for holy smokes to drift in the air, etc.

One thing leads to another, to another, blah, blah, and that's the electrical life we have chosen to be in! It doesn't need rocket science to help people rationalize those!
 

kwired

Electron manager
Location
NE Nebraska
Let's just KISS it. All engineering calculations result into safer and proper operation of electrical equipment. You don't size it smaller than what standards say it should, else you'll burn the thing, you set the protections just enough to allow safe operations but not too high for holy smokes to drift in the air, etc.

One thing leads to another, to another, blah, blah, and that's the electrical life we have chosen to be in! It doesn't need rocket science to help people rationalize those!
For the most part I agree with that, my point was OSHA doesn't set the rules for public safety, their mission is employee safety, which has more to do with work practices than how a distribution system is designed and performs. If a lineman is changing a fuse OSHA doesn't look at whether or not he is replacing it with the correct fuse, NESC may determine what the correct fuse is. OSHA is more concerned with how he will be protected from electric shock, arc flash, or even fall potential while performing this task.
 

mivey

Senior Member
Not residually connected EM relay but one with a CT on the neutral but that too requires a minimum ground resistance unlike a static relay which requires none.
Please give your definition of static relay. I have said that sensitive ground fault detection is also dome with EM relays. You seem to disagree for some reason and I'm not following what you are trying to say.
 

JoeStillman

Senior Member
Location
West Chester, PA
Which makes it a little like OSHA and 70E. Many think OSHA mandates using 70E, but in reality they just require an electrical work safety program. 70E just happens to be the most popular publication that covers things pretty well and is more likely to be accepted as a valid standard in a court of law than "Joe's" written safety policy. Even if Joe goes to the same extent and details as 70E does when writing his policy, it would cost much less to just adopt 70E than it would to pay Joe for his work, plus 70E will be updated periodically where it will cost more again if you want Joe to keep things updated.

So POCO's generally do not have any code they are required to follow, but most choose to follow NESC because it is a well recognized standard and by doing so they reduce their liability.
Also, Joe can't afford that much liability insurance - if he could buy it at all.
 

Sahib

Senior Member
Location
India
Please give your definition of static relay. I have said that sensitive ground fault detection is also dome with EM relays. You seem to disagree for some reason and I'm not following what you are trying to say.
Static relay is digital relay in this case. Even when sensitive ground fault detection is also dome with EM relays, the EM earth fault relay may not operate for such ground fault as caused by a snapped conductor on high resistivity ground. More information is given in pages 470-471 of
http://books.google.co.in/books?id=...tor&source=bl&ots=CiRgCoiUFf&sig=daT8Xb3pqKPJ
 

mivey

Senior Member
Even when sensitive ground fault detection is also done with EM relays, the EM earth fault relay may not operate for such ground fault as caused by a snapped conductor on high resistivity ground.
As I thought, you have nothing backing up your statement that EM relays are not being used for sensitive earth fault detection.

The technology focus today is digital because they are better and provide more options. Before that era, EM relays provided the sensitive earth fault detection and they are still in operation in many places.
 

Sahib

Senior Member
Location
India
As I thought, you have nothing backing up your statement that EM relays are not being used for sensitive earth fault detection.

The technology focus today is digital because they are better and provide more options. Before that era, EM relays provided the sensitive earth fault detection and they are still in operation in many places.
What is your comment about the link in post#75 in respect of feeble earth fault clearance by EM and digital earth fault relays?
 

mivey

Senior Member
What is your comment about the link in post#75 in respect of feeble earth fault clearance by EM and digital earth fault relays?
I made my comments in post #76 but will clarify.

Your reference does not say EM relays are not used for sensitive ground fault detection so your claim is still not supported.

Your reference supports my statements that modern digital relays provide better protection and are replacing the old EM relays.
 

Sahib

Senior Member
Location
India
Your reference does not say EM relays are not used for sensitive ground fault detection so your claim is still not supported.
EM relays are used for sensitive ground fault detection when the ground resistance is low.
Your reference supports my statements that modern digital relays provide better protection and are replacing the old EM relays.
Modern digital relays provide better protection and are replacing the old EM relays in places where the ground resistance is high.
 
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