Grounding Main-Tie-Main w/ Two Separate Gears

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Sean.Day72

Member
Location
Florida
I read around on here but couldn't find something specific for this situation.

I have two 2500KVA transformers. One transformer is normal utility power, the other transformer is Fed From a large generator facility for emergency power. I also have two pieces of Switch Gear. One switch gear has a Main 4000A breaker and is fed from the Normal Utility transformer. The other switch gear has a 3000A main breaker is fed from the emergency transformer. The Gears are located in separate rooms. The Normal Gear has a 3000A breaker which feeds a 3000A tie breaker on the emergency gear w/ 8 Sets using 4#400, #400G cables. The Tie breaker is normally closed to feed the loads on the emergency gear from normal utility power. If power goes out the tie breaker opens, main breaker on emergency gear closes and emergency loads are fed from generator. Note the loads on emergency gear are things like elevators, security systems, etc so they operate during normal and emergency situations. Also there will be neutral loads (4 wire system).The ground buses in each gear are connected with a cables.

Question the system Ground-Neutral bonding jumper should be at the tie breaker in the emergency gear correct? Any system bonding should be "ahead" of the ground fault sensors if I'm understanding correctly. I'm intending to use 4-pole breakers with an internal CT on the neutral. There should also be a CTs on the Ground-Neutral bonding jumper. These connect to each breakers trip unit to compare ground to neutral currents correct? In this case that would be 4 breakers. Any special considerations when setting up the trip units as to not mess up breaker coordination?

I using "single point grounding scheme" since the sources are not parallel. I can draw up a basic one line if my explanation is not clear.
 

david

Senior Member
Location
Pennsylvania
I read around on here but couldn't find something specific for this situation.

I have two 2500KVA transformers.

One transformer is normal utility power, the other transformer is Fed From a large generator facility for emergency power.

I also have two pieces of Switch Gear. One switch gear has a Main 4000A breaker and is fed from the Normal Utility transformer. The other switch gear has a 3000A main breaker is fed from the emergency transformer.

The Gears are located in separate rooms. The Normal Gear has a 3000A breaker which feeds a 3000A tie breaker on the emergency gear w/ 8 Sets using 4#400, #400G cables. The Tie breaker is normally closed to feed the loads on the emergency gear from normal utility power.

If power goes out the tie breaker opens, main breaker on emergency gear closes and emergency loads are fed from generator.

Note the loads on emergency gear are things like elevators, security systems, etc so they operate during normal and emergency situations. Also there will be neutral loads (4 wire system).The ground buses in each gear are connected with a cables.

Question the system Ground-Neutral bonding jumper should be at the tie breaker in the emergency gear correct?

Any system bonding should be "ahead" of the ground fault sensors if I'm understanding correctly.

I'm intending to use 4-pole breakers with an internal CT on the neutral. There should also be a CTs on the Ground-Neutral bonding jumper. These connect to each breakers trip unit to compare ground to neutral currents correct?

In this case that would be 4 breakers. Any special considerations when setting up the trip units as to not mess up breaker coordination?

I using "single point grounding scheme" since the sources are not parallel. I can draw up a basic one line if my explanation is not clear.

get back to you
 

david

Senior Member
Location
Pennsylvania
Question the system Ground-Neutral bonding jumper should be at the tie breaker in the emergency gear correct?

In your set up the 3000 amp gear is a sub- panel feed off of your 4000 amp service. You do not re- bond the neutral load side of the 4000 amp service gear
 

david

Senior Member
Location
Pennsylvania
One transformer is normal utility power, the other transformer is Fed From a large generator facility for emergency power.

the other transformer is Fed From a large generator facility for emergency power.



Is the normal service 120/ 208 3 PH.? Is the generator 480 Volt 3 PH with three conductor no grounded conductor supplying an exterior transformer?

Where is this other transformer located at?


Is the normal service 480/277 3 PH.? Is the generator 120 / 208 3 PH?
 
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david

Senior Member
Location
Pennsylvania
I'm intending to use 4-pole breakers with an internal CT on the neutral.

Sounds like you have a ground fault detection system on a 4 pole transfer switch

The transfer gear does not involve the generator directly; this is just an initial guess
.
I would think the generator has a grounding electrode conductor and grounding electrode at the generator.

You would bond the secondary neutral at the transformer your supply side system bonding jumper would bond to the secondary neutral in the transformer enclosure.

You would bond your secondary neutral to your building steel if any, you would bond your secondary neutral to your buildings metal water system if any.

you would bond your secondary neutral to your equipment grounding buss in your 3000 amp gear supply side bonding jumper

If your 4000 amp service is set up using 250.64 (F) (3) you could just run one secondary side bonding jumper from your transformer secondary neutral to that busbar location. Bonding jumper to the transformer enclosure. One supply side bonding jumper to your equipment ground buss in the 3000 amp gear

250.64 Grounding Electrode Conductor Installation.(F) Installation to Electrode(s).
(3) Bonding jumper(s) from grounding electrode(s) shall be permitted to be connected to an aluminum or copper busbar not less than 6 mm × 50 mm (1/4 in. × 2 in.). The busbar shall be securely fastened and shall be installed in an accessible location. Connections shall be made by a listed connector or by the exothermic welding process. The grounding electrode conductor shall be permitted to be run to the busbar. Where aluminum busbars are used, the installation shall comply with 250.64(A).
 

Sean.Day72

Member
Location
Florida
The Normal and Emergency transformers are adjacent to each other in the yard . There is a 13.2KV Normal utility Loop and a 13.2kv emergency loop around the campus that feed all the normal and emergency transformers on the campus. Both transformers are 3ph 480/208v. The actual generator building is far away (it’s a huge central energy plant with very large generators). My concern is when the “tie” breaker is open during a power outage the ground fault current has to return to the emergency transformer . During normal operation the main breaker on the emergency gear is open and fault current returns to the utility transformer. Given the only one main breaker (normal or emergency) can be closed at a time the fault current only has one transformer to go back to . I want to make sure my ground-neutral bond is done in the correct location to operate GFI breakers properly


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Sean.Day72

Member
Location
Florida
I attached a basic one line. I want to make sure my main bonding jumper is in the correct location so the GFI works properly. Can I make this connection ahead of the main breakers in the switchgear? Or do I have to do this in the transformer?
 

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david

Senior Member
Location
Pennsylvania
I attached a basic one line. I want to make sure my main bonding jumper is in the correct location so the GFI works properly. Can I make this connection ahead of the main breakers in the switchgear? Or do I have to do this in the transformer?

The bonding can be done in the 3000 amp switch gear

250.24 Grounding Service-Supplied Alternating-Current Systems.
(2) Outdoor Transformer. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building.

250.30 Grounding Separately Derived Alternating-Current Systems. (1) System Bonding Jumper

Exception No. 2: A system bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.
 
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david

Senior Member
Location
Pennsylvania
I attached a basic one line. I want to make sure my main bonding jumper is in the correct location so the GFI works properly. Can I make this connection ahead of the main breakers in the switchgear? Or do I have to do this in the transformer?

700.5 Capacity.(D) Use. Transfer equipment shall supply only emergency loads.

NEC Handbook NFPA 70-2008
"Although the alternate power source is permitted to supply emergency loads as well as other loads, the transfer switch used for the emergency system is strictly limited to emergency loads, that is, loads classified as emergency in accordance with 700.1. Other loads, such as legally required standby loads or optional standby loads (covered by Articles 701 and 702), are not permitted to be supplied from the emergency system transfer switch. Where a single generator is used to supply both emergency and nonemergency loads, multiple transfer switches are required."

If a ground fault activates your normal 4000 amp GF protection device and you provide a GF protection device on your 3000 amp distribution” N/E” panel your Gf Protection device may take that distribution out.

700.7 Signals.
(D) Ground Fault. To indicate a ground fault in solidly grounded wye emergency systems of more than 150 volts to ground and circuit-protective devices rated 1000 amperes or more. The sensor for the ground-fault signal devices shall be located at, or ahead of, the main system disconnecting means for the emergency source, and the maximum setting of the signal devices shall be for a ground-fault current of 1200 amperes. Instructions on the course of action to be taken in event of indicated ground fault shall be located at or near the sensor location.


700.26 Ground-Fault Protection of Equipment.
The alternate source for emergency systems shall not be required to have ground-fault protection of equipment with automatic disconnecting means. Ground-fault indication of the emergency source shall be provided per 700.7(D).


Edit: I cant read your one line diagram
 
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Sean.Day72

Member
Location
Florida
I attached a jpeg of the one line if that helps. So the Main breaker on the emergency gear will have a ground fault detector but will not open the breaker. It will only sounds an alarm in a ground fault to keep the emergency system operational. Has anyone designed a system like this that does not use a transfer switch? Open the tie breaker, before closing the Emergency Breaker.
 

david

Senior Member
Location
Pennsylvania
I attached a jpeg of the one line if that helps. So the Main breaker on the emergency gear will have a ground fault detector but will not open the breaker. It will only sounds an alarm in a ground fault to keep the emergency system operational. Has anyone designed a system like this that does not use a transfer switch? Open the tie breaker, before closing the Emergency Breaker.

You have to transfer to emergency loads with in 10 Sec. of loss of normal power

III. Sources of Power
700.12 General Requirements.
Current supply shall be such that, in the event of failure of the normal supply to, or within, the building or group of buildings concerned, emergency lighting, emergency power, or both shall be available within the time required for the application but not to exceed 10 seconds. The supply system for emergency purposes, in addition to the normal services to the building and meeting the general requirements of this section, shall be one or more of the types of systems described in 700.12(A) through (E). Unit equipment in accordance with 700.12(F) shall satisfy the applicable requirements of this article.

700.6 Transfer Equipment.
(A) General. Transfer equipment, including automatic transfer switches, shall be automatic, identified for emergency use, and approved by the authority having jurisdiction. Transfer equipment shall be designed and installed to prevent the inadvertent interconnection of normal and emergency sources of supply in any operation of the transfer equipment. Transfer equipment and electric power production systems installed to permit operation in parallel with the normal source shall meet the requirements of Article 705.
 
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