250.30(a)(4)

[pfalcon]

Confusion abounds, and will continue...

:grin:

It is but ONE grounding electrode for the site. To be compliant with current code, there has to be at least one other.

Right now I'm trying to deal with the internal grounding of the machine tools rather than the facility. Outside the walls I can't describe well. Inside, the only grounding electrode present, or even capable of being present, is the building steel itself. You can't drive a rod or any other conductor deep enough to reach what you need. The grid was established with heavy equipment during construction and extensive bonding of building components to each other.

This is seldom true. Typically, if there is a grounding conductor which accompanies the service conductors, it is an Equipment Bonding Jumper (EBJ).

Trying to find the correct terminology here. I can't see the building steel being anything other than the "Grounding Electrode" for the machines. There are only three buss-bars present for attachment. They are 480VAC three-phase. No other conductors are present. A fourth conductor is brought from the building steel (Grounding Electrode?) to the main bonding point on the machine tool. What do you call that conductor?

 

[pfalcon]

Gonna take this in pieces as I decipher it. Thanks.

Let?s start with the grounded circuit conductor:
The grounded conductor serves as the return path to the source to complete a circuit, also as a neutral it provides a 0 volt reference point when it is connected as the center point of a center tapped transformer, this is when it is a true neutral.
The NEC requires it to be run with the ungrounded supply conductors of a circuit.

But there are times when it can also serve as an EGC, and/or as a GEC, but this is only allowed ahead of the main service disconnect with the exception of existing ranges and dryers where it can serve as the EGC.

It is intended to have normal mode current on it, that is to say it should have current on it if there is a load on that circuit.

Power available for the machine tools primarily comes from 3-bar ungrounded Delta or 3-bar high-resistance Wye. There is no grounded conductor present as described above.

The Delta supplies have ground-fault monitoring between the transformer taps and the building grid. The Wye areas have a high-resistance connection between the transformer neutral and the building grid - the neutral is not available to the power buss in the plant.

From your description, there is no grounded circuit conductor present.

 

[pfalcon]

Grounding Electrode Conductor:
is a connection thru one or more electrodes to earth to stabilize the voltage of a system to earth, to protect against voltages of a higher source, and to provide a low impedance path in the event of a lightning strike on the system, is not intended to carry current in a normal mode as it is for a grounded circuit conductor, or is it intended to carry fault current.

Now let me expand on this, there is nothing that says it wont carry normal mode current, or fault current, but that is the effect of a conductor serving a dual role such as ahead of the service disconnect. Or as a bond connection to a water pipe that is also the GEC connection, and a fault happens to the water pipe. As we can see there will be many times a conductor will be installed for a given purpose but will criss cross with serving the purpose of other conductors, but for each purpose it must meet or exceed the NEC requirements for each purpose it will serve.

The connection from the building steel to the machine bonding point serves this function.

 

[pfalcon]

Equipment Grounding Conductor:
The purpose is to simply carry a return fault current back to the source to facilitate the opening of the short circuit protection device or circuit.
Again it is not intended to have normal mode current on it. Like the grounded circuit conductor it too is also required by the NEC to be run with the circuit conductors it protects, from the MBJ to the load it serves.

The conductor from the building steel to the machine bonding point serves this function.

 

[pfalcon]

Bonding conductor:

Is a conductor that does nothing more then bring two or more conductive surfaces to the same potential, so a difference of potential can not exist between them, it could be a bond between two electrodes as an extension of the GEC, or a bond between two sections of equipments as an extension of the EGC, or it could be neither just an equal potential bond, as used in article 680.
This does not mean it will not carry fault current, nor does it mean it won?t be part of a GEC path way, as in some cases it will serve both.

These are present throughout the machine tool radiating from the machine bonding point.

 

[Smart $]

Right now I'm trying to deal with the internal grounding of the machine tools rather than the facility. Outside the walls I can't describe well. Inside, the only grounding electrode present, or even capable of being present, is the building steel itself. You can't drive a rod or any other conductor deep enough to reach what you need. The grid was established with heavy equipment during construction and extensive bonding of building components to each other.

Inside the facility, for separately derived systems, you only need one electrode. Either metal water pipe or structural steel.

Trying to find the correct terminology here. I can't see the building steel being anything other than the "Grounding Electrode" for the machines. There are only three buss-bars present for attachment. They are 480VAC three-phase. No other conductors are present. A fourth conductor is brought from the building steel (Grounding Electrode?) to the main bonding point on the machine tool. What do you call that conductor?

The problem is that the structural steel is a grounding electrode. You cannot use it as a connection for an Equipment Grounding Conductor (EGC). If the busbars are in duct (altogether called bus duct), and there are only the three power busbars, the duct serves as the EGC of the supply. You have to connect to the duct for EGC purposes. I realize the bus duct may be welded or otherwise have a solid electrical connection to structural steel, but I don't make the rules.

If you are using the structural steel to ground a separately derived system (e.g. a control transformer), that would be via Grounding Electrode Conductor (GEC). There has been many a debate about using one conductor to serve both purposes. Having taken part in a few of those, I'd say the majority believe you cannot, while I and a few others are among the minority that believe one conductor can serve both purposes... as long as it meets all the requirements of both.

 

[don_resqcapt19]

The conductor from the building steel to the machine bonding point serves this function.

The code does not permit the connection to the building steel to perform the function of the equipment grounding conductor. The EGC will end up being connected to the building steel, at least indirectly, if the steel is part of the grounding electrode system. The EGC must be run with the supply circuit conductors. This EGC may be in the form of a wire conductor or it may be in the form of a metallic raceway.

 

[hurk27]

In an older "ungrounded system" like above we must have an EGC run with the circuit conductors, this is to prevent fault current from a second fault from flowing through other pathways to return to source which could cause a shock or fire hazard, the idea is to have a code compliant EGC that is design to carry the second fault current, where other pathways such as the building steel pipes and other cables that might share a common return path back to source might not be able to safely.

 

[pfalcon]

The code does not permit the connection to the building steel to perform the function of the equipment grounding conductor. The EGC will end up being connected to the building steel, at least indirectly, if the steel is part of the grounding electrode system. The EGC must be run with the supply circuit conductors. This EGC may be in the form of a wire conductor or it may be in the form of a metallic raceway.

Don, your answers get confusing because this is a code compliant system. Telling me that it's illegal or non-compliant gets me no where. I'm trying to assign the correct terms to the correct parts of the system. It's not the system that's faulty, it's my terminology and description.

 

[pfalcon]

Inside the facility, for separately derived systems, you only need one electrode. Either metal water pipe or structural steel.


The problem is that the structural steel is a grounding electrode. You cannot use it as a connection for an Equipment Grounding Conductor (EGC). If the busbars are in duct (altogether called bus duct), and there are only the three power busbars, the duct serves as the EGC of the supply. You have to connect to the duct for EGC purposes. I realize the bus duct may be welded or otherwise have a solid electrical connection to structural steel, but I don't make the rules.

If you are using the structural steel to ground a separately derived system (e.g. a control transformer), that would be via Grounding Electrode Conductor (GEC). There has been many a debate about using one conductor to serve both purposes. Having taken part in a few of those, I'd say the majority believe you cannot, while I and a few others are among the minority that believe one conductor can serve both purposes... as long as it meets all the requirements of both.

Somewhere I mentioned earlier but it's gotten lost. The 3-bars are encased in buss duct that is exothermically welded to the building steel. The 4th tap is physically part of the buss duct.

 

[pfalcon]

Originally Posted by pfalcon
(Modified again)
Three buss bars and a buss frame are all there is to attach. It took a lot of research but to my best estimates the plant grounding system is an TN-C system while the machine tools are a TN-S system.

The progression at my site was
1) Ungrounded Delta (IT) 3-bar - Buss duct serves in place of the 4th bar
2) Solidly Grounded Wye (TT) 4-bar
3) High-Resistance Wye (IT) 3-bar - Buss duct serves in place of the 4th bar

Machine tools are all TN-S with the 2nd generation areas using the 4th bar. 1st and 3rd generation use the buss frame exothermically welded to the building steel.

Grounding Electrode :: Building Steel
EGC :: Buss Duct
? :: Single conductor drop from the Buss Duct to the machine bonding point
GEC :: ?

 

[Smart $]

Grounding Electrode :: Building Steel
EGC :: Buss Duct
? :: Single conductor drop from the Buss Duct to the machine bonding point
GEC :: ?

First ? — Machine EGC
Second ? — None*

*While highly debated, the Machine EGC may also serve as a Machine C-Xfmr GEC if it meets ALL the requirements for both EGC and GEC.

An attempt to preclude this co-application has been accepted in principle for the 2011 NEC.

 

[don_resqcapt19]

Don, your answers get confusing because this is a code compliant system. Telling me that it's illegal or non-compliant gets me no where. I'm trying to assign the correct terms to the correct parts of the system. It's not the system that's faulty, it's my terminology and description.

You are telling me that you don't have an EGC run with the power circuit to the equipment and you are telling me the system is code compliant. It is not possible for both statements to be correct. The power supply circuit to the equipment must have an EGC installed along with the ungrounded conductors. This is required by the NEC for both grounded and ungrounded systems.

 

[don_resqcapt19]

Grounding Electrode :: Building Steel
EGC :: Buss Duct
? :: Single conductor drop from the Buss Duct to the machine bonding point
GEC :: ?

Now that is different...the bus duct may be suitable for use as the EGC for the circuit.

I am not sure what you mean by the single conductor drop from the bus duct to the machine. If this drop is intended to be the EGC for the machine, it must be run in the same cable or raceway as the power supply conductors for the machine.

The grounding electrode conductor is run to the service equipment and connected to the EGCs at that point.

If the machine has transformers that are SDS, then a grounding electrode conductor would be required to the SDS. This conductor could be run from the building steel if the building steel qualifies as a grounding electrode.

 

[pfalcon]

Don,
Yes, this is what I mean about the system being right and my terminology being wrong. I've been trying to narrow the descriptions to get the pieces identified correctly. Yes the machine EGC/GEC combined conductor is dropped with the 480VAC conductors through a common conduit. So:

The building steel serves as the Grounding Electrode.
The buss duct frame serves as the Equipment Grounding Conductor (EGC) combined with the Grounding Electrode Conductor (GEC) wrapped around the 3-bar supply.
The buss head taps the 3-bar supply and provides the tie point to the buss frame for the 4-wire drop to the machine tool through a common conduit.
The 3 supply conductors go to the machine disconnect. The EGC/GEC conductor goes to the machine bonding point.

The plant system is therefore a TN-C grounding system.
Thanks Smart $

 

[shepelec]

You do not need a GEC if your control transformer is 1000va or less.
You would only need EGC if you comply with 250.30(3) ex 3.

 

[pfalcon]

The plant system is therefore a TN-C grounding system.
Thanks Smart $

With a better understanding of the terminology of the plant; and with a further study of the grounding systems:

The ungrounded Delta and high impedance Wye sections of the plant are IT grounding systems.

The solidly grounded Wye section is a TT grounding system.

 

[Smart $]

With a better understanding of the terminology of the plant; and with a further study of the grounding systems:

The ungrounded Delta and high impedance Wye sections of the plant are IT grounding systems.

The solidly grounded Wye section is a TT grounding system.

How can you have a 3-busbar HIG Wye if the busduct is exothermically bonded to a grounding electrode???

 

[Smart $]

Are you saying the only connection of the system to [earth] ground is via resistor at source N, and no other connections to N, including the busduct frame?

Even so, would not that also be considered an IT grounded system?

 

[pfalcon]

http://www.schneider-electric.com/s...ility-safety/low-voltage-minus-1kv/ect172.xml

See page 8.

The High Impedance Wye neutral is connected to the grounding system through a high impedance resistance that effectively isolates it from ground. It therefore acts like an ungrounded system. There is no other connection to the Wye neutral. Yes it is considered an IT grounding system.

The ungrounded Delta is also an IT grounding system.

The solidly grounded Wye is the only TT grounding system of the three at our site.