Structure Bonding

[kevin458]

In NEC we bond the exposed non-current carrying metal parts (like steel structure) to the ground electrode. What if a structure has both 480V equipment and 4160V equipment; should we ground the metal of this structure to the ground electrode of 480V system or to the ground electrode of 4160V system? Thanks.

 

[GoldDigger]

In NEC we bond the exposed non-current carrying metal parts (like steel structure) to the ground electrode. What if a structure has both 480V equipment and 4160V equipment; should we ground the metal of this structure to the ground electrode of 480V system or to the ground electrode of 4160V system? Thanks.

Unless you are talking about a local separately derived system, under the NEC there is only one building GES which is bonded to both service neutrals.

 

[kevin458]

Yes I am talking about a local separately derived system. There is one 4160V separately derived system which feeds a couple of large loads, and then this 4160V is changed to 480V system which feeds most of the plant motors and other loads.

 

[GoldDigger]

Yes I am talking about a local separately derived system. There is one 4160V separately derived system which feeds a couple of large loads, and then this 4160V is changed to 480V system which feeds most of the plant motors and other loads.

So there is no utility service to building? Are you familiar with the definition of Separately Derived System (SDS) in the NEC?

 

[kevin458]

I just checked the definition and you are right, it is not a separately derived system because it does not have its own generator. So should we connect the ground grid/ring of 4160V system to the ground grid/ring of 480V system using a loop conductor sized per Table 250.66, and then connect the exposed metal parts (steel structure) to this loop conductor?

 

[petersonra]

I just checked the definition and you are right, it is not a separately derived system because it does not have its own generator. So should we connect the ground grid/ring of 4160V system to the ground grid/ring of 480V system using a loop conductor sized per Table 250.66, and then connect the exposed metal parts (steel structure) to this loop conductor?

Suppose a plant had a 4160V service that fed a 4160-480 V transformer. Would the 480V be an SDS or a service?

In any case, there is only one GES to a structure and whatever grounding electrodes are present at the structure have to be bonded together. There cannot be separate 4160V and 480V GES.

 

[don_resqcapt19]

I just checked the definition and you are right, it is not a separately derived system because it does not have its own generator. ...

I think you need to re-visit the definition...most transformers are separately derived systems.

 

[kevin458]

Yes it makes sense that for structure there will be only one ground. If there are two motors in the same structure (one 4000V motor and the other 460V motor) then we can ground the frame of 4000V motor to the ground grid of 4160V substation, and the frame of 460V motor to the ground grid of 480V substation.


And yes the transformer is a separately derived system because it has its own grounded and ungrounded conductors. I think the concept of service is more confusing than separately derived. Service for industrial plants means the primary of the first transformer; but the service for the purpose of calculating the voltage drops would be the secondary of transformer because primary is not at the same voltage level as the loads.

 

[petersonra]

Yes it makes sense that for structure there will be only one ground. If there are two motors in the same structure (one 4000V motor and the other 460V motor) then we can ground the frame of 4000V motor to the ground grid of 4160V substation, and the frame of 460V motor to the ground grid of 480V substation.

What do you mean by "ground grid"? The motor frame in both cases has to be bonded to the equipment grounding conductor that is run with the branch circuit conductors. It can optionally be bonded (or grounded) to something else, but that is never required by the code.

And yes the transformer is a separately derived system because it has its own grounded and ungrounded conductors. I think the concept of service is more confusing than separately derived. Service for industrial plants means the primary of the first transformer; but the service for the purpose of calculating the voltage drops would be the secondary of transformer because primary is not at the same voltage level as the loads.

I think you should look at the definition of service in the NEC. It has nothing whatsoever to do with voltage level, or what side of a transformer the voltage is on.

 

[GoldDigger]

The ground grid mentioned is, I think, the grid at the substation or line transformer which is not at the customer site but is in series with the earth return path if the wire neutral is compromised.

 

[Smart $]

Yes it makes sense that for structure there will be only one ground. If there are two motors in the same structure (one 4000V motor and the other 460V motor) then we can ground the frame of 4000V motor to the ground grid of 4160V substation, and the frame of 460V motor to the ground grid of 480V substation.


And yes the transformer is a separately derived system because it has its own grounded and ungrounded conductors. I think the concept of service is more confusing than separately derived. Service for industrial plants means the primary of the first transformer; but the service for the purpose of calculating the voltage drops would be the secondary of transformer because primary is not at the same voltage level as the loads.

Your starting post refers to one structure having both 4160V and 480V equipment. Now your most recent post mentions substations. Please describe what you mean by a substation. Are you dealing with a power company substation or a consumer installation? About the only time I've heard the term substation used in a discussion of consumer installation is heavy industrial (steel mill, oil refinery, and such) and large commercial (campuses, or campus-type building arrangements).

 

[kevin458]

By ground grid I meant to say grounding electrode (as attached.) There is one ground electrode on the HV side of the transformer and another on the LV side. So if a motor is fed by the HV side of the transformer, then should we bring the equipment grounding conductor of this motor back to electrode 1 or electrode 2, or does it not matter?

 

[GoldDigger]

Electrode 1 and electrode 2 must be bonded together, so the question of which to connect where is somewhat academic.
The only electrodes not unconditiomally required to be bonded together into one GES are lightning protection electrodes and auxiliary electrodes. Yours are neither.

 

[Smart $]

By ground grid I meant to say grounding electrode (as attached.) There is one ground electrode on the HV side of the transformer and another on the LV side. So if a motor is fed by the HV side of the transformer, then should we bring the equipment grounding conductor of this motor back to electrode 1 or electrode 2, or does it not matter?

The diagram you posted only shows the secondary side of a single [service] transformer and distribution thereof.

In order to provide appropriate advice, we need to know more detail about your installation. First, is your installation under the CEC (as noted in the diagram) or the NEC? Most advice you get here will be NEC specific.

Next, we need to know the exact arrangement of the service, or services. For example, separate 4160V and 480V services, or a single 4160V service with 480V [consumer] separately derived system (SDS).

 

[Smart $]

Electrode 1 and electrode 2 must be bonded together, so the question of which to connect where is somewhat academic.

Basically true. However there are some situations where two or more GEC's are bonded to the grounded service conductors at different points at or prior to the MBJ.

In any case however, an EGC (which is always a load-side grounding conductor) must always bonded to the MBJ/SBJ on the side opposite the grounded conductor (assuming it is a grounded system, of course*).

*In the case of an ungrounded system, everything is the same except there is no MBJ/SBJ.

The only electrodes not unconditiomally required to be bonded together into one GES are lightning protection electrodes and auxiliary electrodes. Yours are neither.

Auxiliary electrodes are bonded to the GES through its "GEC" and the circuit EGC. Additionally, Code has no provision(s) that I'm aware of to not bond lightning protection electrodes from the GES that qualify as Code "defined" grounding electrodes at the structure.

 

[kevin458]

I think it makes sense now because as you guys have mentioned, the EGC should always be bonded on the load side.

 

[kevin458]

Sorry I did not explain it correctly before. In the attached sketch: in NEC, should the EGC of motor M1 be connected to electrode E1, or can it also be connected to electrode E2?

 

[Smart $]

Sorry I did not explain it correctly before. In the attached sketch: in NEC, should the EGC of motor M1 be connected to electrode E1, or can it also be connected to electrode E2?

At the very least, your drawing needs to indicate service point and service disconnect to be give an accurate account.

Assuming the service point is after the 4160 secondary terminals (i.e. there is a service drop or lateral involved), E1 would not be part of the NEC-required GES. An EGC is never connected ahead of the MBJ, which is required to be in the service disconnecting means enclosure.

Assuming the 4160-480V transformer is an SDS, it must be bonded to the same GES as the primary [250.30]

 

[Smart $]

At the very least, your drawing needs to indicate service point and service disconnect to be give an accurate account....

Also draw a dashed rectangle(s), representing structure bounds or building envelope.

 

[kevin458]

See the revised sketch. Unfortunately I don't know which is the service point. This is an industrial facility so I don't think we have any service, service drop, MBJ, etc. We only have a main bonding bar. Is the attached sketch Ok?