conduit bonding
- Thread starter bwyllie
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bphgravity
Senior Member
- Location
- Florida
Re: conduit bonding
To ensure electrical continuity of the grounding path.
To provide a low imedance grounding path.
Are you refering to the requirement of 250.64(E)? Please list code references.
To ensure electrical continuity of the grounding path.
To provide a low imedance grounding path.
Are you refering to the requirement of 250.64(E)? Please list code references.
- Location
- Illinois
- Occupation
- retired electrician
Re: conduit bonding
A GEC is a single conductor that is physically isolated from the other circuit conductors. Any time the conductors of an AC circuit are physically seperated, the impedance goes up. In this case the increase in the impedance is even greater because of the conduit. The magnetic field created by a the current flow creates a "choke" when installed in a ferrous conduit. This can cause a large increase in the impedance of the GEC. When you bond at both ends, you short out the choke and the impedance is actually lower than that of the GEC itself.
Don
A GEC is a single conductor that is physically isolated from the other circuit conductors. Any time the conductors of an AC circuit are physically seperated, the impedance goes up. In this case the increase in the impedance is even greater because of the conduit. The magnetic field created by a the current flow creates a "choke" when installed in a ferrous conduit. This can cause a large increase in the impedance of the GEC. When you bond at both ends, you short out the choke and the impedance is actually lower than that of the GEC itself.
Don
- Location
- Illinois
- Occupation
- retired electrician
Re: conduit bonding
physis,
Don
physis,
That is exactly what I am saying, but only in this specific case. The problem only exists where there is a ferrous conduit and a single circuit conductor in the conduit. About the only time you can have this situation on an AC system is when the GEC is in a magnetic raceway. In normal circuits all of the circuit conductors and the EGC are in the same raceway or cable and the problem does not exist.
Don
Re: conduit bonding
Physis, when the conduit is effectively connected to the "wire conductor", the conduit given its physical area, will actually carry more current on it's surface than the "wire conductor" itself.
Roger
[ May 29, 2003, 08:58 PM: Message edited by: roger ]
Physis, when the conduit is effectively connected to the "wire conductor", the conduit given its physical area, will actually carry more current on it's surface than the "wire conductor" itself.
Roger
[ May 29, 2003, 08:58 PM: Message edited by: roger ]
- Location
- Massachusetts
Re: conduit bonding
Hi Sam,
Another way to avoid this problem is to use RNC or Aluminum RMC instead of steel raceways.
TVSS units I have installed require non-ferrous conduits for their isolated ground conductor.
The fact I can always learn something new is what keeps this trade fresh to me.
Bob
Hi Sam,
Another way to avoid this problem is to use RNC or Aluminum RMC instead of steel raceways.
TVSS units I have installed require non-ferrous conduits for their isolated ground conductor.
The fact I can always learn something new is what keeps this trade fresh to me.
Bob
- Location
- Illinois
- Occupation
- retired electrician
Re: conduit bonding
Bob,
While the use of metallic nonferrous conduits will avoid the "choke" problem, the code still requires both ends of any metallic conduit containing a GEC to be bonded to the GEC.
Don
Bob,
While the use of metallic nonferrous conduits will avoid the "choke" problem, the code still requires both ends of any metallic conduit containing a GEC to be bonded to the GEC.
Don
al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: conduit bonding
Physis,
Here's another way to understand the "choking" of a GEC in a ferrous raceway.
The current in the GEC is alternating, so the magnetic field created by the current is expanding and intensifying, then weakening and collapsing 120 times a second (each half cycle). 120 times a second there is no magnetic field.
The field moves out through the stationary metal raceway. The raceway, being also a conductor, in the presence of a moving magnetic field is a generator and has a current generated in it. This current is commonly called an eddy current.
The eddy current, flowing in the raceway metal, creates its own magnetic field, which is in the opposite direction of the magnetic field that creates the eddy current in the first place. This is the "choke".
As a side note: if the current in the GEC is DC, the magnetic field around the GEC is not changing (not moving) so it doesn't generate eddy current in the raceway, and is not choked.
The ferrous raceway concentrates the magnetic field that passes into it, as compared to aluminum or other non-magnetic conductive material. Because of the higher magnetic field density inside the ferrous metal, the eddy current created by AC is great enough to really be a problem for GECs.
When the GEC is bonded to both ends of the ferrous raceway that houses it, as the choking effect increases on the GEC, the current shifts more and more to the raceway itself.
The fact that the raceway becomes a large part of the active path in a fault condition, underscores the importance of making all the raceway joints tight, as well as bonding to both ends.
Physis,
Here's another way to understand the "choking" of a GEC in a ferrous raceway.
The current in the GEC is alternating, so the magnetic field created by the current is expanding and intensifying, then weakening and collapsing 120 times a second (each half cycle). 120 times a second there is no magnetic field.
The field moves out through the stationary metal raceway. The raceway, being also a conductor, in the presence of a moving magnetic field is a generator and has a current generated in it. This current is commonly called an eddy current.
The eddy current, flowing in the raceway metal, creates its own magnetic field, which is in the opposite direction of the magnetic field that creates the eddy current in the first place. This is the "choke".
As a side note: if the current in the GEC is DC, the magnetic field around the GEC is not changing (not moving) so it doesn't generate eddy current in the raceway, and is not choked.
The ferrous raceway concentrates the magnetic field that passes into it, as compared to aluminum or other non-magnetic conductive material. Because of the higher magnetic field density inside the ferrous metal, the eddy current created by AC is great enough to really be a problem for GECs.
When the GEC is bonded to both ends of the ferrous raceway that houses it, as the choking effect increases on the GEC, the current shifts more and more to the raceway itself.
The fact that the raceway becomes a large part of the active path in a fault condition, underscores the importance of making all the raceway joints tight, as well as bonding to both ends.
al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: conduit bonding
In simplest terms, the lower the current that can get through the choked GEC, the longer it takes for the overcurrent protection to clear the circuit.
The current can be choked so far as to leave the fault burning but unable to trip the breaker or blow the fuse.
In simplest terms, the lower the current that can get through the choked GEC, the longer it takes for the overcurrent protection to clear the circuit.
The current can be choked so far as to leave the fault burning but unable to trip the breaker or blow the fuse.
Re: conduit bonding
First, a GEC has nothing to do with clearing an internal fault, as most of you know. Its only purposes are to provide a path to ground for lightning and accidental contact with high voltage on the service, and reference the system to ground.
Since the GEC is not run with any phase conductors, it does not receive the benefit of magnetic cancellation of internal faults. But since the GEC is only to ground the system that does not matter. Where the real ?choking? or ?girdling? effect comes in to play is with lightning. DC and AC power frequencies are not affected significantly by any choking effect presented by single ground conductors ran in a ferrous metal raceway.
?Girdling or Choking? refers to the encirclement of a single grounding conductor by a ring of ferromagnetic metal. When metallic enclosures are made of magnetic material such as steel, the ?choke effect? is much greater than the metallic non-metallic materials such as aluminum. The reason for this is that the magnetic field associated with the wire is ?amplified? when it is brought within the vicinity of the magnetic material (the amount of magnetic flux available to induce current flow in the iron increases). This ?amplification? effect occurs because the ?permeability? of magnetic materials is greater than one. ?Permeability? is simply, a number that measures the ability of the material to ?amplify? a magnetic field in the presence of current. The greater the permeability of a material, the greater the amplification effect.
However, based on calculations that did not include the effects of eddy currents flowing in the girdling material. New calculations, which include this effect, indicate that the increased magnitude of the induced voltage is much less than previously thought
First, a GEC has nothing to do with clearing an internal fault, as most of you know. Its only purposes are to provide a path to ground for lightning and accidental contact with high voltage on the service, and reference the system to ground.
Since the GEC is not run with any phase conductors, it does not receive the benefit of magnetic cancellation of internal faults. But since the GEC is only to ground the system that does not matter. Where the real ?choking? or ?girdling? effect comes in to play is with lightning. DC and AC power frequencies are not affected significantly by any choking effect presented by single ground conductors ran in a ferrous metal raceway.
?Girdling or Choking? refers to the encirclement of a single grounding conductor by a ring of ferromagnetic metal. When metallic enclosures are made of magnetic material such as steel, the ?choke effect? is much greater than the metallic non-metallic materials such as aluminum. The reason for this is that the magnetic field associated with the wire is ?amplified? when it is brought within the vicinity of the magnetic material (the amount of magnetic flux available to induce current flow in the iron increases). This ?amplification? effect occurs because the ?permeability? of magnetic materials is greater than one. ?Permeability? is simply, a number that measures the ability of the material to ?amplify? a magnetic field in the presence of current. The greater the permeability of a material, the greater the amplification effect.
However, based on calculations that did not include the effects of eddy currents flowing in the girdling material. New calculations, which include this effect, indicate that the increased magnitude of the induced voltage is much less than previously thought
Re: conduit bonding
Dereck: I wish I could write like that. Very good composition.
I and some others performed tests on this phenomenon, when I was working on the Air Defense Command, radar sites, in the 1960s.
We built a brine tank load bank and pumped high current through a single conductor inside a metal conduit. The change of impedance was trivial. This choke theory is hogwash.
Dereck: I wish I could write like that. Very good composition.
I and some others performed tests on this phenomenon, when I was working on the Air Defense Command, radar sites, in the 1960s.
We built a brine tank load bank and pumped high current through a single conductor inside a metal conduit. The change of impedance was trivial. This choke theory is hogwash.
- Location
- Massachusetts
Re: conduit bonding
dereckbc, Thanks for the great info.
Don, always good to get your input.
I would like more info on this subject from anyone who can help.
When I installed the TVSSs I followed the instructions which required Aluminum RMC or RNC for the run to the XO of the transformer feeding the panel from a terminal they referred to as transient ground.
This was in addition to an equipment grounding conductor run with the ungrounded conductors.
The aluminum conduit was terminated to metal enclosures on both ends, so it was grounded but not bonded to the conductor inside.
Now it sounds like I have a violation, so would the only code compliant way to do this be RNC?
And what happens when I pass through a steel enclosure, steel pipe straps or staples do we get this choke effect?
Or is this all a bunch of theories that as Bennie suggests is a bunch of hogwash?
I am a "show me kind of guy" and the experiment Bennie describes would seem to disprove the theories.
Bennie it sounds like you had some interesting experiences, I would think "work" like that would be a lot of fun.
Just trying to learn, thanks, Bob
[ May 30, 2003, 05:23 PM: Message edited by: iwire ]
dereckbc, Thanks for the great info.
Don, always good to get your input.
I would like more info on this subject from anyone who can help.
When I installed the TVSSs I followed the instructions which required Aluminum RMC or RNC for the run to the XO of the transformer feeding the panel from a terminal they referred to as transient ground.
This was in addition to an equipment grounding conductor run with the ungrounded conductors.
The aluminum conduit was terminated to metal enclosures on both ends, so it was grounded but not bonded to the conductor inside.
Now it sounds like I have a violation, so would the only code compliant way to do this be RNC?
And what happens when I pass through a steel enclosure, steel pipe straps or staples do we get this choke effect?
Or is this all a bunch of theories that as Bennie suggests is a bunch of hogwash?
I am a "show me kind of guy" and the experiment Bennie describes would seem to disprove the theories.
Bennie it sounds like you had some interesting experiences, I would think "work" like that would be a lot of fun.
Just trying to learn, thanks, Bob
[ May 30, 2003, 05:23 PM: Message edited by: iwire ]
al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: conduit bonding
Thanks Dereck,
Can you point to a reference?
Bennie,
The salt brine tank sounds impressive. What were the electrical conditions and observations of the tests? Current levels, pipe lengths & construction, frequency, time duration, conductor size & length. . .?
Thanks Dereck,
Can you point to a reference?
This is intriguing.
Bennie,
The salt brine tank sounds impressive. What were the electrical conditions and observations of the tests? Current levels, pipe lengths & construction, frequency, time duration, conductor size & length. . .?
Re: conduit bonding
Al: I don't have any of the measurements. We used three metal drums, on a wood truck bed, filled with water. Electrodes were suspended in the barrel. Salt brine was added until the load reached the amount we needed.
We put each phase in a separate conduit and compared the current flow under the two conditions. The decrease in current was negligible.
The power was from diesel generators 480 volt, three phase wye. The brine tanks were dummy load banks.
Bonding of raceways to the equipment ground conductor, for fault purposes, makes sense.
I think someone was thinking of the same situation with the ground electrode conductor. The choke effect was a figment of someones imagination.
Al: I don't have any of the measurements. We used three metal drums, on a wood truck bed, filled with water. Electrodes were suspended in the barrel. Salt brine was added until the load reached the amount we needed.
We put each phase in a separate conduit and compared the current flow under the two conditions. The decrease in current was negligible.
The power was from diesel generators 480 volt, three phase wye. The brine tanks were dummy load banks.
Bonding of raceways to the equipment ground conductor, for fault purposes, makes sense.
I think someone was thinking of the same situation with the ground electrode conductor. The choke effect was a figment of someones imagination.
Re: conduit bonding
Thanks Bod, Al, and Bennie, wished I had a job.
Bennie, the choke or girdling is not complete hogwash, it is not as extreme as first believed, but none the less still around. The reason you did not see any effects in the salt brine is because you were running low frequency power without sudden quick changes in load level. In order for choking to be a problem takes very fast current rise times like lightning produces. The increased inductance and impedance is not noticable unitl you get into high frequencies or lightning. But I agee it has been overstated for the reasons I mentioned earlier.
Al, my source is from Bell Labs a.k.a. Bellcore TR-NWT-000295, and ANSI/IEEE Emerald Book 2004 which has not been released, still in draft form.
Bob, I can help with TVSS questions. Give me ahout. The supplemental ground you added was not needed. The fast rise times over the inductive impedance on a single ground conductor made it worthless. TVSS systems should have all phase, neutral, and ground conductors should be ran together in a straight line as possible, nearest the buss as possible, wound in a spiral to closely couple them, and no longer than 12 inches in length. Best designs have the TVSS built into the panel using a Kelvin clamp method. A Kelvin clamp means the protector modules are bolted on to the busses without leads
Thanks Bod, Al, and Bennie, wished I had a job.
Bennie, the choke or girdling is not complete hogwash, it is not as extreme as first believed, but none the less still around. The reason you did not see any effects in the salt brine is because you were running low frequency power without sudden quick changes in load level. In order for choking to be a problem takes very fast current rise times like lightning produces. The increased inductance and impedance is not noticable unitl you get into high frequencies or lightning. But I agee it has been overstated for the reasons I mentioned earlier.
Al, my source is from Bell Labs a.k.a. Bellcore TR-NWT-000295, and ANSI/IEEE Emerald Book 2004 which has not been released, still in draft form.
Bob, I can help with TVSS questions. Give me ahout. The supplemental ground you added was not needed. The fast rise times over the inductive impedance on a single ground conductor made it worthless. TVSS systems should have all phase, neutral, and ground conductors should be ran together in a straight line as possible, nearest the buss as possible, wound in a spiral to closely couple them, and no longer than 12 inches in length. Best designs have the TVSS built into the panel using a Kelvin clamp method. A Kelvin clamp means the protector modules are bolted on to the busses without leads
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