It has been awhile do I remember correctly. On the secondary of a transformer I ground xo (the neutral) to earth ground? Then I bond the neutral to ground again in the first panel or disconect as it has become a seperatly derived system and they need to be bonded together. It seems redunant to me?
Transformer grounding
- Thread starter arnettda
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eric9822
Senior Member
- Location
- Camarillo, CA
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- Electrical and Instrumentation Tech
Close. You bond the neutral to ground at the transformer OR at the first panel, never both.
250-30
250-30
NEC requires both an earth ground and an equipment ground. They have different purposes and circuit paths.
The equipment ground is a low impedance path back to the source to facilitate source breaker tripping in a fault. The earth ground is a low impedance path "to the earth", mostly for atmospheric effects and cleanliness of the ground for signaling and sensitive electronic equipment.
Don't have a code book handy but from memory look at 250-4 for the requirement for an earth ground and 250-30 (A) for the requirements for connecting the neutral to the earth. 250-30 requires the secondary neutral, X0, bonding jumper to the common grounding busbar and requires a "grounding electrode conductor", 250-30 (A) 3, to effectively grounded electrode, building structural steel or cold water pipe in contact with the earth.
The equipment ground, EGC, is not the grounding electrode conductor, GEC. I say this because this is one of the most common omissions I see.
Do's and Don'ts. The separately derived system's neutral connection to ground is a single point of connection to ground and insulated isolated everywhere else. Grounding the neutral to ground at more than one point places the neutral conductor in parallel with the ground plane and the parallel paths becomes a shunt conductor for neutral current, neutral current on the ground paths, very bad.
For NEC purposes, the earth itself is not a parallel path for neutral current. That's why service transformers are grounded at the pole and again at the building (one point of connection only). The overhead coming in is a grounded neutral and has to hit porcelain insulators and insulated conductors at the building.
250-30
NEC requires both an earth ground and an equipment ground. They have different purposes and circuit paths.
The equipment ground is a low impedance path back to the source to facilitate source breaker tripping in a fault. The earth ground is a low impedance path "to the earth", mostly for atmospheric effects and cleanliness of the ground for signaling and sensitive electronic equipment.
Don't have a code book handy but from memory look at 250-4 for the requirement for an earth ground and 250-30 (A) for the requirements for connecting the neutral to the earth. 250-30 requires the secondary neutral, X0, bonding jumper to the common grounding busbar and requires a "grounding electrode conductor", 250-30 (A) 3, to effectively grounded electrode, building structural steel or cold water pipe in contact with the earth.
The equipment ground, EGC, is not the grounding electrode conductor, GEC. I say this because this is one of the most common omissions I see.
Do's and Don'ts. The separately derived system's neutral connection to ground is a single point of connection to ground and insulated isolated everywhere else. Grounding the neutral to ground at more than one point places the neutral conductor in parallel with the ground plane and the parallel paths becomes a shunt conductor for neutral current, neutral current on the ground paths, very bad.
For NEC purposes, the earth itself is not a parallel path for neutral current. That's why service transformers are grounded at the pole and again at the building (one point of connection only). The overhead coming in is a grounded neutral and has to hit porcelain insulators and insulated conductors at the building.
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
I have never heard of "atmospheric effects"
Do you mean lightning?
Indoor transformers do not have much issues with lightning. Stabilization of voltage during normal use is one I would think of as being important.
Regarding direct strikes, that is correct... but lightning-induced voltage spikes (transient voltage) can be passed on through distribution, service, and feeder conductors. Of course lightning events have a greater impact in an inverse relation to distance and resistance
250-30
250-30
At work contractor's installed qty. 3 PDU's at 250 kva each with no grounding electrode conductors per 250-30, which would require 2/0 copper to the building structural steel.
Sure, there are haphazard connections to the building steel from the primary side equipment ground, but that's not a GEC that's in the code, it's an error of omission. I wrote it up but the powers that be refuse to install the 2/0 copper GEC. Looking through the forums here I see it's also controversial on the boards, that omission of the GEC required by 250-30 (A) 3 is acceptable to some.
So that's what I mean, the grounding electrode conductor is required "shall connect, shall comply" in the manner specified. A wire sized by table connected to endpoints as specified.
I was told this is a gray area and pushing it makes me not a team playa. Employment issue.
The ionosphere is charged to hundreds of kV by radiation from space and the sun, mostly protons.
In dry air outside if you string a single conductor between trees on porcelain anchors, the wire will become charged to several kV from capacitive coupling to the ionosphere. Since dry air is a insulator, it will not pass enough power for a load. But if the wire charges then discharges through sensitive electronics, it may be enough to damage a tuner. Antennas are required to have earth grounding also.
If the same outside wire had a battery and lamp circuit but no earth ground, the entire circuit could operate at a static voltage well above ground. Moisture in the air would be enough to relieve this static charge.
A long time ago on the job, a nearby lightning strike hit the ground but there were several miles of overhead well pump control wires that I was called to work on to find out why they would not keep working. The overhead pump control wires picked up an induced voltage from the strike that came right out of the box I was working on. I was bent down over my tools at the time and the blast passed right over my back, turned everything a really pretty blue, and hit an antique 50 kW Terry steam turbo generator whose base I was using for a tool space. I was on the steam side when it hit. The plant manager had a chicken feed sized bag of bridge rectifiers that kept blowing out for that circuit.
There are other exercises. During fault clearing the equipment ground will have an elevated voltage due to the IR drop of current flow on the ground path. This is mitigated by complying and installing the GEC.
I write this because I am professionally curious about the "grey area". What are the reasons or code exceptions that allow not installing the GEC. I see this is controversial on these boards and at work.
250-30
At work contractor's installed qty. 3 PDU's at 250 kva each with no grounding electrode conductors per 250-30, which would require 2/0 copper to the building structural steel.
Sure, there are haphazard connections to the building steel from the primary side equipment ground, but that's not a GEC that's in the code, it's an error of omission. I wrote it up but the powers that be refuse to install the 2/0 copper GEC. Looking through the forums here I see it's also controversial on the boards, that omission of the GEC required by 250-30 (A) 3 is acceptable to some.
So that's what I mean, the grounding electrode conductor is required "shall connect, shall comply" in the manner specified. A wire sized by table connected to endpoints as specified.
I was told this is a gray area and pushing it makes me not a team playa. Employment issue.
The ionosphere is charged to hundreds of kV by radiation from space and the sun, mostly protons.
In dry air outside if you string a single conductor between trees on porcelain anchors, the wire will become charged to several kV from capacitive coupling to the ionosphere. Since dry air is a insulator, it will not pass enough power for a load. But if the wire charges then discharges through sensitive electronics, it may be enough to damage a tuner. Antennas are required to have earth grounding also.
If the same outside wire had a battery and lamp circuit but no earth ground, the entire circuit could operate at a static voltage well above ground. Moisture in the air would be enough to relieve this static charge.
A long time ago on the job, a nearby lightning strike hit the ground but there were several miles of overhead well pump control wires that I was called to work on to find out why they would not keep working. The overhead pump control wires picked up an induced voltage from the strike that came right out of the box I was working on. I was bent down over my tools at the time and the blast passed right over my back, turned everything a really pretty blue, and hit an antique 50 kW Terry steam turbo generator whose base I was using for a tool space. I was on the steam side when it hit. The plant manager had a chicken feed sized bag of bridge rectifiers that kept blowing out for that circuit.
There are other exercises. During fault clearing the equipment ground will have an elevated voltage due to the IR drop of current flow on the ground path. This is mitigated by complying and installing the GEC.
I write this because I am professionally curious about the "grey area". What are the reasons or code exceptions that allow not installing the GEC. I see this is controversial on these boards and at work.
Would like to add
Would like to add
Would like to add ...
I changed the bridge rectifier at the plant end quickly but the pumps would not come on. The only control they could keep on the circuit at that end was a 30 amp single pole switch with a log book.
The plant manager drove me 5 miles down the hill to the pump house. I was impressed that they had 10 or 12 boxes of 30 amp 250 volt fuses ready to go in, sitting there on the table.
That impression lasted less the two seconds until I looked at the control board. It was 1" thick black slate with surface mounted small copper busbars and basketball sized carbonization imprints centered right where the control fuses plugged in.
I hesitated and then removed the old fuse, a Gould Shawmut 250kA AIC rated fiberglass body fuse. All the fiberglass body was gone and only the metal endcaps remained (along with the slate and copper hardware).
Would like to add
Would like to add ...
I changed the bridge rectifier at the plant end quickly but the pumps would not come on. The only control they could keep on the circuit at that end was a 30 amp single pole switch with a log book.
The plant manager drove me 5 miles down the hill to the pump house. I was impressed that they had 10 or 12 boxes of 30 amp 250 volt fuses ready to go in, sitting there on the table.
That impression lasted less the two seconds until I looked at the control board. It was 1" thick black slate with surface mounted small copper busbars and basketball sized carbonization imprints centered right where the control fuses plugged in.
I hesitated and then removed the old fuse, a Gould Shawmut 250kA AIC rated fiberglass body fuse. All the fiberglass body was gone and only the metal endcaps remained (along with the slate and copper hardware).
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
I am not sure what this has to do with...whatever.
For grounded systems, follow 250.50
Then go back to Part III of Article 250 to help determine what is, and what is not to be grounded.
Metallic parts will be grounded. Whether to have a grounded conductor for the system or not is a design issue.
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