Grounding

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donnohpp

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Hello All
We have a 13.8V Delta Primary to 480V Y Secondary, 3 Phase Transformer with Equipment Grounding conductors feeding a 480 V power panel. Is it nessesary to ground the XO on the 480V Secondary when the Neutral in not required.
 
Is it nessesary to ground the XO on the 480V Secondary when the Neutral in not required.
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Yes, without a system bonding jumper you would not be able to trip an overcurrent device on the secondary of the transformer in a ground fault situation.

Take a look at 250.30(A)(1).

Chris
 
donnohpp said:
Hello All
We have a 13.8V Delta Primary to 480V Y Secondary, 3 Phase Transformer with Equipment Grounding conductors feeding a 480 V power panel. Is it nessesary to ground the XO on the 480V Secondary when the Neutral in not required.
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No, it is not necessary, as long as there is no connection to the X0 at all. You will need to provide ground fault detection. Your installation must be industrial, and the system must be maintained by qualified individuals.

Why do you not want to bond the X0?
 
raider1 said:
Yes, without a system bonding jumper you would not be able to trip an overcurrent device on the secondary of the transformer in a ground fault situation.

Take a look at 250.30(A)(1).

Chris
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You won't have a devastating arcing ground fault (or need to worry about the OCP operating)as the first faulted conductor is now the grounded conductor.

Depending on the situation and engineers design would determine, grounded, ungrounded or high resistance grounded.
 
brian john said:
You won't have a devastating arcing ground fault (or need to worry about the OCP operating)as the first faulted conductor is now the grounded conductor.

Depending on the situation and engineers design would determine, grounded, ungrounded or high resistance grounded.
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I should have been more specific, under most circumstances a grounded system is preferable so a system bonding jumper is required for a grounded system.

There are times when an ungrounded system is permitted and desired.

Chris
 
Grounding

The reason the XO was not grounded is the Power Distribution Dept made this decision. The 480 V Panel will now power 480V lighting and also a 480/208/120V transformer.
 
Pierre C Belarge said:
My kneejerk reaction is the same. I do not see many industrial locations, and the ones I do see usually perform grounding.
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That is pretty much what I see as well. There are a couple of industrial installations close by my area that do have ungrounded systems, one is a foundry and the other a solid rocket motor plant.

Chris
 
donnohpp said:
Hello All
We have a 13.8V Delta Primary to 480V Y Secondary, 3 Phase Transformer with Equipment Grounding conductors feeding a 480 V power panel. Is it nessesary to ground the XO on the 480V Secondary when the Neutral in not required.
Click to expand...

Typically yes the NEC requires this, with some an allowance for a 'ungrounded' system where desired.
The rules for ungrounded systems are in 250.32(B). On an ungrounded system, the load side bonding is done exactly the same as a grounded system, but no system bonding jumper is installed. You still have to install a GE and GEC.

This is critical that you understand this: With no system bonding jumper (SBJ), a line to case fault will not open the overcurrent device. The SBJ provides the return path back to the transformer windings.

Often systems are operated ungrounded for continuity of power.
A good compromise between ungrounded and grounded,
is the high impedance grounded system
 
raider1 said:
I should have been more specific, under most circumstances a grounded system is preferable so a system bonding jumper is required for a grounded system.

There are times when an ungrounded system is permitted and desired.

Chris
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Chris: I knew you knew, I was expanding for the OP.

95% of all ungrounded systems I see are ungrounded because the installer did not know what he was doing or they are a wye to delta step up operated ungrounded.

Primarily what I have seen are resistance grounded systems.
 
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brian john said:
95% of all ungrounded systems I see are ungrounded because the installer did not know what he was doing or they are a wye to delta step up operated ungrounded.
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Agreed, that is why my initial responce was kind of a knee jerk reaction in the direction of needing a system bonding jumper.

Chris
 
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