Ground resistance for conduit/box connections
- Thread starter tg2546
- Start date
- Status
- Location
- Massachusetts
Re: Ground resistance for conduit/box connections
I do not believe that the engineer on record for a project needs an official source.
If it is put in the specifications that you bid and accepted it is as official as it needs to be.
I think .3 ohms or less is easily achieved.
I do not believe that the engineer on record for a project needs an official source.
If it is put in the specifications that you bid and accepted it is as official as it needs to be.
I think .3 ohms or less is easily achieved.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Ground resistance for conduit/box connections
Ditto, well said.
Ditto, well said.
pierre
Senior Member
- Location
- Westchester County, New York
Re: Ground resistance for conduit/box connections
If the locknuts are made up wrench tight, I could not see any measurable resistance being measured with a standard meter, between the box and conduit at that point.
If the locknuts are made up wrench tight, I could not see any measurable resistance being measured with a standard meter, between the box and conduit at that point.
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: Ground resistance for conduit/box connections
Let me make a comment on checking boxes, conduit, etc. for continuity of ohms resistance.
I worked for the DuPont Co. at the Chambers Works site, Deepwater, N.J. for 45 years.
DuPont had a procedure called "Fault & Static Inspection". This meant that we were to check every stick of conduit from the ECR to the device, being a solenoid valve or a motor, also building steel with a ohmmeter. We were to see zero ohms to ground. For example, if we came to a stick of conduit and at the coupling that may have corroded, and on one side we would see zero ohms, but on the other side, we would see a higher reading of ohms. We would install a jumper wire to correct the situation.
I might add, back a few years ago, we would relie on the conduit for our ground continuity, back to the source. Today, they still have the conduit continuity, but they install a ground wire internally in the conduit. I totally agree with installing the ground wire inside of the conduit.
Let me make a comment on checking boxes, conduit, etc. for continuity of ohms resistance.
I worked for the DuPont Co. at the Chambers Works site, Deepwater, N.J. for 45 years.
DuPont had a procedure called "Fault & Static Inspection". This meant that we were to check every stick of conduit from the ECR to the device, being a solenoid valve or a motor, also building steel with a ohmmeter. We were to see zero ohms to ground. For example, if we came to a stick of conduit and at the coupling that may have corroded, and on one side we would see zero ohms, but on the other side, we would see a higher reading of ohms. We would install a jumper wire to correct the situation.
I might add, back a few years ago, we would relie on the conduit for our ground continuity, back to the source. Today, they still have the conduit continuity, but they install a ground wire internally in the conduit. I totally agree with installing the ground wire inside of the conduit.
Re: Ground resistance for conduit/box connections
Thanks to everybody for yor opinions on whether the 0.3 ohms is a good spec or not. However, I was hoping to get your opinions on where the spec may have originated, besides an engineer of record. Does anyone remember seeing the 0.3 ohm requirement in a NEMA, UL, etc document?
Thanks to everybody for yor opinions on whether the 0.3 ohms is a good spec or not. However, I was hoping to get your opinions on where the spec may have originated, besides an engineer of record. Does anyone remember seeing the 0.3 ohm requirement in a NEMA, UL, etc document?
Re: Ground resistance for conduit/box connections
TG2546, as I stated earlier .3 ohms per joint would be high. IEEE Std 1100-2005 reccomends less than .25 ohms or less on the furthest EGC circuit. That point is measured from the N-G bond at the service entrance all the way to the most distant branch circuit. That includes all feeders, branch panels, connections, etc.
So IMHO .3 ohm's per joint is a mute point, it is not adequate. Did you make a typo? Could the spec say ".003 Ohms per joint as measured with a DLRO"?
[ August 15, 2004, 06:00 PM: Message edited by: dereckbc ]
TG2546, as I stated earlier .3 ohms per joint would be high. IEEE Std 1100-2005 reccomends less than .25 ohms or less on the furthest EGC circuit. That point is measured from the N-G bond at the service entrance all the way to the most distant branch circuit. That includes all feeders, branch panels, connections, etc.
So IMHO .3 ohm's per joint is a mute point, it is not adequate. Did you make a typo? Could the spec say ".003 Ohms per joint as measured with a DLRO"?
[ August 15, 2004, 06:00 PM: Message edited by: dereckbc ]
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: Ground resistance for conduit/box connections
It is my opinion that you will not find any spec. stating a value of ohms as you mentioned, 0.3 ohms. I feel confident that you will not find that in any official document such as, NFPA, NEC ISA, etc.
In fact, I have never heard of any company doing the "Fault & Static Inspection" that I mentioned to you. We had many hazardous areas, and we would keep records of these inspections, just in case of an explosion, fire, etc.
It is my opinion that you will not find any spec. stating a value of ohms as you mentioned, 0.3 ohms. I feel confident that you will not find that in any official document such as, NFPA, NEC ISA, etc.
In fact, I have never heard of any company doing the "Fault & Static Inspection" that I mentioned to you. We had many hazardous areas, and we would keep records of these inspections, just in case of an explosion, fire, etc.
Re: Ground resistance for conduit/box connections
Hello-
derecbc mentioned the IEEE 1100-2005 standard, which is where these numbers come from in most cases. I am not aware of any State or municipality adopting IEEE 1100 as an official standard, so your company may want to create an internal policy, otherwise the next electrician who comes along won't have a clue where you got the number from.
This data was originally developed (and then incorporated into IEEE 1100-1992 edition) by Ed Cantewll and Chris Kleronomos (sp?) during the 1970's and published as an IEEE Conference paper as a life safety issue to properly trip overcurrent devices, but has slowly been adopted as a technology performance standard as well.
Keep in mind that this is an impedance measurement and not a DC resistance measurement- that is, your standard toolbox multimeter cannot be used to take this reading. You need a ground line impedance meter to do this job.
The actual value of the ECG impedance should be looked up in the IEEE 1100 table as it is based on the OC device rating and the voltage. In most cases the real value will be less then your .3 ohms.
Best wishes,
Mark
Hello-
derecbc mentioned the IEEE 1100-2005 standard, which is where these numbers come from in most cases. I am not aware of any State or municipality adopting IEEE 1100 as an official standard, so your company may want to create an internal policy, otherwise the next electrician who comes along won't have a clue where you got the number from.
This data was originally developed (and then incorporated into IEEE 1100-1992 edition) by Ed Cantewll and Chris Kleronomos (sp?) during the 1970's and published as an IEEE Conference paper as a life safety issue to properly trip overcurrent devices, but has slowly been adopted as a technology performance standard as well.
Keep in mind that this is an impedance measurement and not a DC resistance measurement- that is, your standard toolbox multimeter cannot be used to take this reading. You need a ground line impedance meter to do this job.
The actual value of the ECG impedance should be looked up in the IEEE 1100 table as it is based on the OC device rating and the voltage. In most cases the real value will be less then your .3 ohms.
Best wishes,
Mark
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