Ground size

[tulum]

Hello,

I am from the resistance grounded world, and this is the first time I have to install a solidly grounded system. It is a 4160V-600V 650kVA transformer.

I look in table 17 of the CEC and it states that over 475 the ground conductor should be 000 at amperages over 475, is this suffecient.

Regards,
TULUM

 

[waross]

Table 16 is the size of the conductor connecting the neutral to the grounding grid or electrodes.
Table 17 is the size of the conductor used for equipment grounding.
In the event of a short to ground in a piece of grounded equipment, the fault current must flow in this conductor and return to the source via the neutral.
The size of this conductor is given in table 16 and is dependant on the size of breaker protecting the circuit.
The connection from neutral to the ground electrodes maintains the grounded surfaces of the equipment at ground potential. System fault currents in a system in good order do not flow in this conductor, except for a possible back feed of a small percentage of the fault current from the primary multiple earthing system. High currents in this conductor are rare and are typically much less than the currents that may flow in an equipment grounding conductor.
For this reason this conductor is not required to be larger than 00 AWG. (AMMENDED IN 2006)
Canadian Electric Code, 2006 Edition.
Table 17, Amended in the 2006 edition.
Ampacity over 800 Amps, minimum size of copper conductor: 00 AWG.
Rule 10-802 requires this conductor to be of copper.
Rule 10-804 describes the items and materials that are acceptable for bonding or grounding equipment. Copper is not required for these conductors.
respectfully

 

[tulum]

Thanks Waross,

The situation is that the transformer is a 4160V-600V 500/650kVA solidly grounded transformer that has a 800A Thermal Mag MCCB with a 600A trip.

If I look at table 16 (and use the frame size instead of trip - for good luck), I would require a 1/0 from the xo point to the ground bus.

Seem reasonable?

With that said grounding always throws me off. I would assume that under a faulted condition the fault current would flow back through this conductor to the xo and back to the breaker? To me this makes sense...this is why I chose table 17?

Regards,
TULUM

 

[waross]

As luck would have it, table 16 seems to give the same value as table 17 (2006 edition) IN THIS INSTANCE. For currents above 800 amps and below 100 amps the tables do not agree.
Table 17 MUST be copper. (Connection to the grid or electrodes).
X0 to the ground bus will be table 16.
The ground buss to the ground electrodes will be table 17.
hope this helps
respectfully

 

[RalphChristie]

Waross:

System fault currents in a system in good order do not flow in this conductor, except for a possible back feed of a small percentage of the fault current from the primary multiple earthing system. High currents in this conductor are rare and are typically much less than the currents that may flow in an equipment grounding conductor.

Not sure if I interpret it correctly, but are you refering to the conductor between the neutral point on the transformer and the ground?

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376

 

[waross]

Hi Ralph;
It's good to see you active on the forum again.
The conductor from the ground bus to the transformer neutral (X0) Does carry the fault current.
In an installation in an industrial plant, the equipment grounding conductors should carry the majority of a fault current. The most common fault to ground in many industrial plants is a fault to grounded equipment. The current path of least impedance is the path through the equipment grounding conductor.
The ground grid and the conductors to it will carry very little current.
This statement applies to secondary installations in a building or industrial plant.
This does not apply to primary distribution systems or transmission systems.
In North America, there may be some exceptions where a fault current returns via grounded structural steel. The code in Canada allows such alternate grounding methods for grounding equipment. The alternate grounding path must have adequate ampacity. It should have a connection to the ground bus, and from there back to X0. The current should not flow in the ground grid or the connection to it.
This is reflected in the code requirements for grounding conductor sizing.
The connection from the ground bus to X0 is sized according to the size of the breaker feeding the circuit or system.
A grounding conductor for a system or equipment protected by a 6000 amp breaker must be 800 kcmil. Ampacity from column 4 of table 2, 515 amps. (X0 to ground bus or ground buss to equipment)
The connection from the ground bus to the ground grid does not have to be over 00 AWG for any size system. Ampacity from column 4 of table 2, 185 amps.
In wye wye transformer installations whith the neutrals or X0s interconnected, there is a low impedance connection from a secondary ground grid, through the ground bus, through the inteconnected neutrals and back to the primary neutral. This is in effect another ground point on the primary multiple earthing system. In the event of a serious earth fault on the primary system, The fault current will return to the neutral via the multiple earthing points. Some of this current may flow in the conductor from a secondary ground grid through the conductor in question to the secondary neutral point and back to the primary neutral.
If you can suggest exceptions to this statement, please do. I am aware that our grounding methods are not universal, internationally.
Note, the OP asked a specific question concerning the Canadian code.
I suspect that this answer may not be valid for grounding systems in your area. If so, I will be glad to hear from you. The more I can learn about different grounding systems around the world, the better.
Respectfully

 

[RalphChristie]

Warross:

Sorry for the small misunderstanding. I agree with your comments. Your grounding methods are very similar to ours, there might be just little differences between the grounding conductor sizes specified.





Have I ever told you the struggle to keep up with your way of doing things? Although I think you are feeling the same way at that side!

Like:
wire sizes, mm2 vs awg
metric vs imperial
grounding vs earthing

and so much more.....

Hehehehehehehe

Regards
Ralph


[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376