A surge suppressor was installed in the service meter box. A water line ground and 8'ground rod were connected to neutral in the service meter box. A clamp-on ground tester (AEMC) was used to measure the ground rod resistance (37 ohms) and the resistance of the water line ground(.3 ohms).With these measurements ,is the ground rod necessary given its resistance is over 100 times that of the water line ground? What does NEC 250 say about this situation?
clamp-on ground resistance measurements
- Thread starter theodore
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: clamp-on ground resistance measurements
The NEC says you must install a second ground rod.
Don
The NEC says you must install a second ground rod.
Don
charlie
Senior Member
- Location
- Indianapolis
Re: clamp-on ground resistance measurements
Putting an extra grounding electrode doesn't make any sense until you consider that the water line could have a piece of plastic inserted in it either inside or outside. This would cause the customer to lose his grounding electrode if he didn't have a backup.
Re: clamp-on ground resistance measurements
I do a lot of modifications and repairs in older homes, some as old as 100 years. In the past 5 years I bet that over 20 homes that I have worked in didn't have a earth ground. This was beacuse the original installation was grounded to the incoming metal water line and at some point the underground water line was changed to plastic.
I do a lot of modifications and repairs in older homes, some as old as 100 years. In the past 5 years I bet that over 20 homes that I have worked in didn't have a earth ground. This was beacuse the original installation was grounded to the incoming metal water line and at some point the underground water line was changed to plastic.
Re: clamp-on ground resistance measurements
The 8 foot steel rod was removed and a 16 foot copper rod was installed resulting in a "clamp on resistance measurement" of 17.3 ohms.We believe this satisfied NEC code requirements and safety issues connected with the future potential degradation of the water pipe ground.
The 8 foot steel rod was removed and a 16 foot copper rod was installed resulting in a "clamp on resistance measurement" of 17.3 ohms.We believe this satisfied NEC code requirements and safety issues connected with the future potential degradation of the water pipe ground.
karl riley
Senior Member
Re: clamp-on ground resistance measurements
Theodore, you have solved your problem and met Code. Some research I am doing causes me to ask this question: what "safety issues" are related to the rod?
This is like a survey question about the functions of ground rods.
Thanks, Karl
Theodore, you have solved your problem and met Code. Some research I am doing causes me to ask this question: what "safety issues" are related to the rod?
This is like a survey question about the functions of ground rods.
Thanks, Karl
G
Guest
Guest
Re: clamp-on ground resistance measurements
Sounds like S/he's worried about electrolysis on the water pipe. Over time the pipe would wear away and burst. Far fetched but that's my read.
../Wayne
Sounds like S/he's worried about electrolysis on the water pipe. Over time the pipe would wear away and burst. Far fetched but that's my read.
../Wayne
Re: clamp-on ground resistance measurements
The safety issues are clearly related to changes in the conductivity of the water pipe paths to ground.The conductivity of these paths may be disrupted by replacing copper pipe with synthetic pipe,corrosion of water pipe grounding connectors, breaking of ground wires to connectors.Surge supression devices will not operate effectively unless a satifactory path to ground is available.
The safety issues are clearly related to changes in the conductivity of the water pipe paths to ground.The conductivity of these paths may be disrupted by replacing copper pipe with synthetic pipe,corrosion of water pipe grounding connectors, breaking of ground wires to connectors.Surge supression devices will not operate effectively unless a satifactory path to ground is available.
karl riley
Senior Member
Re: clamp-on ground resistance measurements
Theodore, thank you for replying. I am interested in the one safety issue you mention, that surge suppression devices will not operate without a "satisfactory" connection to earth.
(By the way, on this forum some members like to be argumentative. I am simply seeking clarification for our general increase of knowledge about grounding. So please don't think I am pulling you into a dispute).
My first question is, how do the surge suppression specs define "satisfactory"?
(The NEC 25 ohms seems to be some historical left-over. No one has yet explained it).
And maybe Dereck can comment on what the grounding connection has to do with surge suppression operation.
Karl
Theodore, thank you for replying. I am interested in the one safety issue you mention, that surge suppression devices will not operate without a "satisfactory" connection to earth.
(By the way, on this forum some members like to be argumentative. I am simply seeking clarification for our general increase of knowledge about grounding. So please don't think I am pulling you into a dispute).
My first question is, how do the surge suppression specs define "satisfactory"?
(The NEC 25 ohms seems to be some historical left-over. No one has yet explained it).
And maybe Dereck can comment on what the grounding connection has to do with surge suppression operation.
Karl
G
Guest
Guest
Re: clamp-on ground resistance measurements
While you're waiting for your replies from others I will add that it's my understanding that some surges have L-G MOV's-- and those MOV's would be moot if there wasn't an L-G path.
Many electronic devices function better with a ground-- they rely on it as reference point.
../Wayne
[ September 20, 2003, 10:45 AM: Message edited by: awwt ]
While you're waiting for your replies from others I will add that it's my understanding that some surges have L-G MOV's-- and those MOV's would be moot if there wasn't an L-G path.
Many electronic devices function better with a ground-- they rely on it as reference point.
../Wayne
[ September 20, 2003, 10:45 AM: Message edited by: awwt ]
Re: clamp-on ground resistance measurements
Theodore, I am interested in knowing why you think a low impedance ground is nessecary for surge protection.
Consider this; according to ANSI/IEEE 95% of all lightning surge events occur ahead of the service transformer. This means that the transient event appears on the transformer secondary side in the differential mode (L-L & L-N) rather than the common mode (L-G & N-G). At the service disconnect means you should have a N-G bond.
Now for transients that are generated on the secondary side of the transformer, the EGC impedance, at the point of use, is to high to offer any assistance. True ANSI/IEEE recommends a common mode SPD (surge protection device) at the "point of use device", but it is not to discharge to ground, but rather than clamp the voltage at the point of use. What this means is at the point of use the ground reference may rise to 10,000 volts, but the voltage between N-G and L-G will be clamped within safe operating limits.
Dereck
Theodore, I am interested in knowing why you think a low impedance ground is nessecary for surge protection.
Consider this; according to ANSI/IEEE 95% of all lightning surge events occur ahead of the service transformer. This means that the transient event appears on the transformer secondary side in the differential mode (L-L & L-N) rather than the common mode (L-G & N-G). At the service disconnect means you should have a N-G bond.
Now for transients that are generated on the secondary side of the transformer, the EGC impedance, at the point of use, is to high to offer any assistance. True ANSI/IEEE recommends a common mode SPD (surge protection device) at the "point of use device", but it is not to discharge to ground, but rather than clamp the voltage at the point of use. What this means is at the point of use the ground reference may rise to 10,000 volts, but the voltage between N-G and L-G will be clamped within safe operating limits.
Dereck
- Location
- Massachusetts
Re: clamp-on ground resistance measurements
It has to do with the EGC and the bonding of the neutral to the EGC.
Bob
Agreed, but that really has nothing to do with a ground rod or the GEC.
It has to do with the EGC and the bonding of the neutral to the EGC.
Bob
Re: clamp-on ground resistance measurements
Karl,Thank you for your interest in my comments.
Surge protection is of increasing importance in residential homes due to the proliferation of sensitive digital and analog electronics used in: TV's,fax,telephones,VCR's,printers,refrigerators,microwaves,washing machines,water sofeners,personal computers,and LAN's.Fatalities have actually occured in situations where a telephone was being used during a lightning storm.It is very common to loose a VCR after an electrical storm. Prior to installing my surge suppresor I lost 3 VCR's and 2 telephones and 2 TV's over a 10 year period all of them after an electrical storms.The magnitude of a surge voltage is a function of the inductive parameters of the distributed circuits,the spectral content of the surge and the amplitude of the strike.These 3 components combine to determine the voltage the suppresor device must deal with.The suppresor device is essentially a threshold trggered switch directing the energy in the surge to ground where ground is path of low resistance with the capacity to provide a sink for dissipation of the energy.If the ground is not low resistance the clamp or switch will not effectively short circuit the voltage to to a low enough level to protect the electronics in the home.The reason the electronics fail is due to voltages which have not been clamped to ground or a zero potential.When I used the word safety I was thinking about the safety of equipment and people.
Karl,Thank you for your interest in my comments.
Surge protection is of increasing importance in residential homes due to the proliferation of sensitive digital and analog electronics used in: TV's,fax,telephones,VCR's,printers,refrigerators,microwaves,washing machines,water sofeners,personal computers,and LAN's.Fatalities have actually occured in situations where a telephone was being used during a lightning storm.It is very common to loose a VCR after an electrical storm. Prior to installing my surge suppresor I lost 3 VCR's and 2 telephones and 2 TV's over a 10 year period all of them after an electrical storms.The magnitude of a surge voltage is a function of the inductive parameters of the distributed circuits,the spectral content of the surge and the amplitude of the strike.These 3 components combine to determine the voltage the suppresor device must deal with.The suppresor device is essentially a threshold trggered switch directing the energy in the surge to ground where ground is path of low resistance with the capacity to provide a sink for dissipation of the energy.If the ground is not low resistance the clamp or switch will not effectively short circuit the voltage to to a low enough level to protect the electronics in the home.The reason the electronics fail is due to voltages which have not been clamped to ground or a zero potential.When I used the word safety I was thinking about the safety of equipment and people.
Re: clamp-on ground resistance measurements
tonyi,yes you are correct! The meter installation is indeed an arrestor connected on the line side and the suppressors are installed inside the home at each sensitive load portal. Each of the portals must be kept at the same reference ground otherwise harmful potential differences may occur.The power system ground and the portal ground must be tied together to a common local reference point; i.e. the ground rod.
tonyi,yes you are correct! The meter installation is indeed an arrestor connected on the line side and the suppressors are installed inside the home at each sensitive load portal. Each of the portals must be kept at the same reference ground otherwise harmful potential differences may occur.The power system ground and the portal ground must be tied together to a common local reference point; i.e. the ground rod.
pierre
Senior Member
- Location
- Westchester County, New York
Re: clamp-on ground resistance measurements
Surge Arresters can be installed on the load side of a service, 280.22.
Surge Arresters can be installed on the supply or load side, TVSS can be installed on the load side only.
That is my understanding of one of the basic differences between the two.
Pierre
Surge Arresters can be installed on the load side of a service, 280.22.
Surge Arresters can be installed on the supply or load side, TVSS can be installed on the load side only.
That is my understanding of one of the basic differences between the two.
Pierre
- Status