grounding transformer fed w/PVC

[sdbob]

Hi all, first time here.

I wire tilt-up buildings ranging from shells to full-blown manufacturing facilities. Often these buildings have a 480/277 service and require the use of one or more indoor, dry-type transformers, 480 delta to 120/208 wye, from 75 to 750 kva.

These transformers are typically fed from underground PVC conduits, with the secondaries leaving underground in PVC also. Usually everything over 75kva uses parallel secondaries, and over 150kva has parallel primaries as well.

For years we've grounded these the same way, pulling full sized equipment grounds in each PVC, provide ufer to transformer, and bond XO in transformer. This week I was sizing conduits for a 750kva installation and started thinking. (that's where I usually get into trouble)

Our 750 KVA will have 4 - 3" with 3 250 mcm's ea., the secondary will have 6 - 4" with 4 400 mcm's ea.

Add up the ground terminations in that transformer, there?s a bunch of ?em, especially since that?s where we?ve been bonding the neutral.

My questions are the following:

Why does the primary 3-wire circuit in PVC conduits fed from the main service (PVC the entire lenght) length need an equipment ground? The service that feeds it is grounded, and the transformer frame would be bonded via the GEC.

Why do the secondaries need equipment grounds? Exhibit 250-14 of the 2002 NEC

shows the neutral bond at the first disconnecting means, (our low voltage distribution main), with the transformer frame being bonded with the GEC from what would be our distribution.

If a GEC bonding the transformer is sufficient, and I can?t think of a reason why it wouldn?t be, can the GEC be run as a single 3/0 exposed? Remember, there?s 6 secondary conduits looping the transformer secondary with the distribution, and the two are within 4? of each other.

Lastly, where would this GEC between the transformer and main be terminated? It seems to me it should just be between the ground buss of the main and the frame of the transformer, or should it be to the grounded neutral buss in the main where the GEC from the ufer is terminated?

Thanks in advance, kinda long winded but I wanted to paint a clear picture of what I?m talking about. I?ve read other threads here on transformer bonding but nothing addressing the use of PVC.

Bob

[ June 17, 2004, 12:31 PM: Message edited by: sdbob ]

 

[don_resqcapt19]

Re: grounding transformer fed w/PVC

Why does the primary 3-wire circuit in PVC conduits fed from the main service (PVC the entire lenght) length need an equipment ground?

This is the only fault clearing path for a primary side fault.

Why do the secondaries need equipment grounds?

When XO is bonded at the first overcurrent protective device, and not at the transformer, then again the EGC between the OCPD and the transformer is the only path to clear a fault on the secondary windings of the transformer. However, where there are no conductive paths between the OCPD and the transformer, other that the circuit conductors, you are permitted to bond XO at both locations and then an EGC would not be required.
Don

[ June 17, 2004, 01:03 PM: Message edited by: don_resqcapt19 ]

 

[sdbob]

Re: grounding transformer fed w/PVC

Originally posted by don_resqcapt19:

Why does the primary 3-wire circuit in PVC conduits fed from the main service (PVC the entire lenght) length need an equipment ground?

This is the only fault clearing path for a primary side fault.

Could you please explain this to me? I'm not arguing with you but I would like to understand this statement as I've read it before. It seems to me a ground fault in this 3-wire circuit could only occur at either the main service feeding the transformer or in the transformer itself, both of which are bonded to the building's grounding electrode. A phase to phase fault would not use the ground path supplied by an equipment ground.

I hate to sound ignorant but I bet many would have a problem explaining this.

 

[sdbob]

Re: grounding transformer fed w/PVC

Let me also just add here that I understand that obviously SAFETY and redundancy are the most important things to worry about when discussing grounding methods. I just want to understand this better.

 

[don_resqcapt19]

Re: grounding transformer fed w/PVC

Bob,

It seems to me a ground fault in this 3-wire circuit could only occur at either the main service feeding the transformer or in the transformer itself, both of which are bonded to the building's grounding electrode.

The connection to a grounding electrode system will not clear a fault. The impedance of the fault return path back to the power source is too high to flow enough current for the OCPD to open the circuit. Remember that the current must return to its source, not to the earth. In most cases you can connect a hot wire from a 15 amp breaker to a driven ground rod and the breaker will not trip. The only path that can be used for fault clearing in your installation is an EGC installed with the primary and secondary feeder conductors.
Don

 

[sdbob]

Re: grounding transformer fed w/PVC

Thanks Don. I read this from the 2002 handbook:

One of the functions of an equipment grounding conductor is to provide a low-impedance ground-fault path between a ground fault and the electrical source. This path allows the overcurrent protective device to actuate, interrupting the current. To keep the impedance at a minimum, it is necessary to run the equipment grounding conductor within the same raceway or cable as the circuit conductor(s). This practice allows the magnetic field developed by the circuit conductor and the equipment grounding conductor to cancel, reducing their impedance.

Between that and your explanation I understand clearly now. Using the grounding electrode at the transfomer requires fault current generated by the primary side of the transformer to take a path separate from the conductors, resulting in high impedance.

Now why couldn't I figure that one out myself earlier?

Thanks very much for the time you spent clearing the fog.

Bob

 

[dana1028]

Re: grounding transformer fed w/PVC

I also thank you Don - your clear and concise answers make this subject much more understandable to all of us. (I copy these discussions and hand them out to my students; they too appreciate your experience and ability to clarify subjects such as this).

 

[don_resqcapt19]

Re: grounding transformer fed w/PVC

Thanks guys, for the kind words. The confusion about this subject is very wide spread. This is exactly why myself and others submitted proposals that would have changed the term "equipment grounding conductor" to "equipment bonding conductor". The use of the word grounding leads many to believe that it must be connected to earth, when in reality, the most important connection is the connection back to the source via the main bonding jumper. These proposals were rejected for the 2005 code, but a task force was appointed to study the issue for the 2008 code.
Don

 

[tom baker]

Re: grounding transformer fed w/PVC

We are fortunate to have Don as a moderator. His is responsible for a lot of good, common sense code changes. He proposal to change equipment grounding conductor to equiopment bonding conductor was 100% correct, but it was to radical for the cmp. Look how long it took to stop using the neutral to "ground" the frames of ranges and dryers...

[ June 18, 2004, 12:47 AM: Message edited by: tom baker ]