25 OHMS RESISTANCE

[bobby ocampo]

What is the basis of the Code for the 25 ohms requirement?

 

[petersonra]

What is the basis of the Code for the 25 ohms requirement?

No one seems to know with any degree of certainty.

I have seen some speculation that it may be based on telegraph specs that only had one wire and used the earth as the second conductor so a decent connection to the earth was required.

 

[kwired]

No one seems to know with any degree of certainty.

I have seen some speculation that it may be based on telegraph specs that only had one wire and used the earth as the second conductor so a decent connection to the earth was required.

something relating to telegraph specs as the origin has been posted here before.

 

[dereckbc]

What is the basis of the Code for the 25 ohms requirement?

Old habits die hard and slow.

I have searched for the answer for 30 years and all roads have led to either telegraph where only one circuit conductor and earth used as the return conductor, or the very first electric power lines only used a single conductor to the transformer and used earth as the return conductor.

Those are the only two credible and technical reasons I have been able to come up with in 30 years. If it was telegraph that spec had to come from the civil war era. I have searched both Edison, Westinghouse, and Tesla archives and no mention of 25 ohm ground.

Now the real mystery for me from working in the telephone and power generation industry for 30 years is where the heck did 5 ohm ground requirement come from and why in the telecom industry. About 16 years ago I found the answer from an ole timer at a Telecom Power Conference. He was a retired Ma Bell Power Protection Engineer like myself and enlightened. Back in the very early days about all telephone circuits were Party Lines. Remember those? Anyway in those days Ring Voltage was applied between to the either Tip or Ring conductor and Earth. Thus the telephone office had to have a low resistance to earth. This allowed the telco to run 9 circuits on a single line and the ability to ring a one specific phone shared with 8 others.

 

[bobby ocampo]

What is the basis of the Code for the 25 ohms requirement?

Any information if the Code or the IEEE have done research as to the validity of this 25 ohm requirement? Is 25 ohm safe? Is it recommended for residential only?

If the ground resistance should be as low as practically as possible, why 25 ohms? Have somebody recommended to make it lower or higher in the Code revision?

 

[zbang]

rathole alert

Back in the very early days about all telephone circuits were Party Lines. Remember those? Anyway in those days Ring Voltage was applied between to the either Tip or Ring conductor and Earth. Thus the telephone office had to have a low resistance to earth. This allowed the telco to run 9 circuits on a single line and the ability to ring a one specific phone shared with 8 others.

The joys of selective ringing-
you could connect the ring-supply between tip (or ring) and ground
there were frequency-selective ringers (resonated at different frequencies)
there were "electron tubes" (gas diode) and silicon diodes that would let you do half-cycle selection (plus or minus side)

combine all of that and different ring cadences and the CO side could get rather complex The station side is described in BSP 500-114-100 "Ringer Limitations". OTOH a quick skim of that doesn't mention ground resistance; will have to check the AT&T Green book when I have a chance.

 

[GoldDigger]

And for those who wonder how they managed to put ring voltage just from tip to ground or just from ring to ground when there was a DC circuit in the phone and the central office between tip and ring, the answer is that the phones all had to be "on hook" for any ringing to work properly. And when the ringing machine was connected to a subscriber line, the normal DC loop for audio and supervision was effectively disconnected at the central office.

The campus radio station at Stanford, where I was a volunteer techie, had a complete 100+ line Strowger Switch (step-by-step) PBX which was assembled from surplus equipment.
It was enhanced to support flash transfer, interface to the college PBX lines and Telco lines with Direct Inward and Outward Dialing, music on hold, and access to the AP news feed audio from internal phones among other things. Originating Sleeve Control for some phones.
Nice bank of NiCad batteries too that also powered the station during grid failures.

 

[bobby ocampo]

What version of the Code did this 25 ohms first appeared? :?

 

[dereckbc]

Any information if the Code or the IEEE have done research as to the validity of this 25 ohm requirement? Is 25 ohm safe? Is it recommended for residential only?

If the ground resistance should be as low as practically as possible, why 25 ohms? Have somebody recommended to make it lower or higher in the Code revision?

No real reason to make it higher or lower as it does not have much of a function. If anything an upper limit like 500 Ohm's which is about what you get out of an average ground rod. Anything 500 or less with the current requirements is adequate.

If you get it to low say less than 1 then you actually make safety and operational problems as significant current will be flowing through earth and if you were to loose your service grounded conductor may go undetected like in the ole days when when water pipes were metallic and water departments did not install isolation bushings in water meters.

However there is no need to change the 25 ohm requirement because it is meaningless paper tiger. Bond the water pipe, rebar if available, drive two grounds rods and call it done. It could be 5000 or 5 ohms and it complies.

 

[bobby ocampo]

No real reason to make it higher or lower as it does not have much of a function. If anything an upper limit like 500 Ohm's which is about what you get out of an average ground rod. Anything 500 or less with the current requirements is adequate.

If you get it to low say less than 1 then you actually make safety and operational problems as significant current will be flowing through earth and if you were to loose your service grounded conductor may go undetected like in the ole days when when water pipes were metallic and water departments did not install isolation bushings in water meters.

However there is no need to change the 25 ohm requirement because it is meaningless paper tiger. Bond the water pipe, rebar if available, drive two grounds rods and call it done. It could be 5000 or 5 ohms and it complies.

If it is meaningless then why require 25 ohm? Why not 500 ohms as you said?

What do you mean sir of loosing the service grounded conductor? Is this the neutral?

 

[don_resqcapt19]

If it is meaningless then why require 25 ohm? Why not 500 ohms as you said?

What do you mean sir of loosing the service grounded conductor? Is this the neutral?

The code doesn't really require a ground resistance of 25 ohms or less. It just says that if a singe driven rod or pipe or a buried plate type electrode has a resistance greater than 25 ohms that you have to provide a second electrode.

 

[Pharon]

I thought it came from trying to get a value lower than that of the human body, which comes in around 300-1,000 ohms. In that respect, shooting for 25 leaves a bit of wiggle room.

 

[dereckbc]

If it is meaningless then why require 25 ohm? Why not 500 ohms as you said?

As I said old habits die hard and hard to break. It is old language based on antiquated technology. As Don stated there is no requirement to achieve 25 ohm. All you have to do is drive two ground rods, collect a check, and call it a day.

It would be really difficult for most EC's to prove 25 ohms or less on a single rod as most do not have the equipment, nor the time and training to perform valid test and in most cases impossible to get valid test. Much easier, less time, and less expense to just drive two rods and call it done. To perform a Dead Fall Potential Test can take all day and most of the readings would be invalid and meaningless in an urban environment. It is just simple economics and good biz to just drive two rods to comply.

 

[dereckbc]

What do you mean sir of loosing the service grounded conductor? Is this the neutral?

Correct. But I left out a word in my haste: Grounded Circuit Conductor

 

[kwired]

I thought it came from trying to get a value lower than that of the human body, which comes in around 300-1,000 ohms. In that respect, shooting for 25 leaves a bit of wiggle room.

Grounding electrodes are not about clearing faults, if they were they would be required to be very low resistance - much less than 25.

Though 25 is less then the resistance of the human body, it still allows for enough voltage drop between the GEC and true earth to still be a shock hazard should you contact both while energized.

 

[valassi]

25 ohms requirement

25 ohms requirement

I remember asking this question to Mr Collagero (NEC Handbook Editor) between a seminar, he said, "I have been reaching files to know about it" but he seminar was going to start and said, let's wait for the next break, well, the answer is simple, there was a survey in U.S. to know the average of the resistance with the water pipes available in the country, 25 ohms was resistance measurement that was feasible for most of the services in the country, and it has been a lot of speculation about this. it is simple, you need a solid connection to ground, to the soil, that is why is said "no 25 ohms, place other rod and go for dinner"
Well, this apply to most of cases.

 

[don_resqcapt19]

I thought it came from trying to get a value lower than that of the human body, which comes in around 300-1,000 ohms. In that respect, shooting for 25 leaves a bit of wiggle room.

Why would that make any difference?

 

[kingpb]

We have a grounding expert in our organization, I have asked him the question. Unfortunately he is out of office until 28th so will have to wait for a reply.

 

[dereckbc]

answer is simple, there was a survey in U.S. to know the average of the resistance with the water pipes available in the country, 25 ohms was resistance measurement that was feasible for most of the services in the country

I am certainly not saying that is true or false because first time I ever heard that, but is pointless today as water systems are isolated from the electric systems. To many plumbers and water department personnel were shocked or electrocuted when working on water meters or main supply to the house. Water departments installed Isolation Bushings on water meters many moons ago.

Stop and think about this for a moment. What would the fault current be on a 125 volt receptacle with a 25 ohm fault to ground? What would be the voltage at the point of contact?

Did you come up with 5 amps and 125 volts? So how would that prevent you from being shocked or electrocuted on say a 15 amp circuit if not on a GFCI circuit? Keep in mind OCPD are mot intended to prevent a electrical shock as they are not capable of doing that. OCPD is there to protect the wiring and help prevent a electrical fires.

 

[bobby ocampo]

As I said old habits die hard and hard to break. It is old language based on antiquated technology. As Don stated there is no requirement to achieve 25 ohm. All you have to do is drive two ground rods, collect a check, and call it a day.

It would be really difficult for most EC's to prove 25 ohms or less on a single rod as most do not have the equipment, nor the time and training to perform valid test and in most cases impossible to get valid test. Much easier, less time, and less expense to just drive two rods and call it done. To perform a Dead Fall Potential Test can take all day and most of the readings would be invalid and meaningless in an urban environment. It is just simple economics and good biz to just drive two rods to comply.

While it is true that it is easier to drive two rods, what is the reason behind this requirement of the code of 25 ohms? Why then would the code allow 2 driven rods that will higher than 25 ohms and not allow one driven rod which may be a little bit higher than 25 ohms?

What is the intent of the code for this 25 ohms? Any scientific basis?