Grounded or Floating Secondary

[EE_FSU]

I can't decide whether or not the secondary neutral (on a delta-to-wye isolation xfrmer) should be bonded/grounded to the panel chassis or left ungrounded. It appears as though NEC 250-20(D) (separately derived systems), which applies to 250-20(B) (AC system of 50-1k volts), provides insight. I don't understand what is meant in 250-20(B)(2) by "circuit conductor" though. Also, note that earth ground is not an option - has to be bonded at the panel (if it is to be grounded).

 

[don_resqcapt19]

What is the secondary voltage of the transformer?

 

[Smart $]

I can't decide whether or not the secondary neutral (on a delta-to-wye isolation xfrmer) should be bonded/grounded to the panel chassis or left ungrounded. It appears as though NEC 250-20(D) (separately derived systems), which applies to 250-20(B) (AC system of 50-1k volts), provides insight. I don't understand what is meant in 250-20(B)(2) by "circuit conductor" though. Also, note that earth ground is not an option - has to be bonded at the panel (if it is to be grounded).

Essentially it means that if a conductor is connected and run from X0 to the first disconnecting means, the system is required to be grounded... though some will say that an actual load must be connected to the neutral before it is truly a circuit conductor.

No earth ground is not an option. Not sure of section offhand, but the GEC is required to be run to the nearest 1) water-pipe electrode or 2) building steel electrode (or their GEC if irreversibly connected) if they exist... otherwise, the GES of the SDS supply.

 

[Smart $]

What is the secondary voltage of the transformer?

Doesn't matter for a wye secondary where X0 is connected to a circuit conductor.

 

[EE_FSU]

Essentially it means that if a conductor is connected and run from X0 to the first disconnecting means, the system is required to be grounded... though some will say that an actual load must be connected to the neutral before it is truly a circuit conductor.

If I understand this correctly: The neutral is considered a circuit conductor when it is intended to carry current. This is why it would connect directly to the first disconnecting mean (such as the main circuit breaker). Also, as some would say, it needs to be loaded as to draw a current.
It seems as though 250-20(B)(2) does not apply to the situation at hand.

No earth ground is not an option. Not sure of section offhand, but the GEC is required to be run to the nearest 1) water-pipe electrode or 2) building steel electrode (or their GEC if irreversibly connected) if they exist... otherwise, the GES of the SDS supply.

It seems like bonding the neutral at the service panel and having the GEC run to ground would be the best/safest option as this would create an effective ground fault current path. Under what circumstances would one leave the secondary neutral ungrounded?

 

[Smart $]

If I understand this correctly: The neutral is considered a circuit conductor when it is intended to carry current. This is why it would connect directly to the first disconnecting mean (such as the main circuit breaker). Also, as some would say, it needs to be loaded as to draw a current.
It seems as though 250-20(B)(2) does not apply to the situation at hand.

If not, then we are back to what Don brought up [250.20(B)(1)]. If you ground the neutral and it results in a line-to-ground voltage not more than 150V, then you are required to ground the system.

It seems like bonding the neutral at the service panel and having the GEC run to ground would be the best/safest option as this would create an effective ground fault current path. Under what circumstances would one leave the secondary neutral ungrounded?

In a production or other environment where discontinued operation caused by one trip of an overcurrent device can shut down an entire process or ruin product until fixed.

When using an ungrounded system, grounding continuity of non-current-carrying conductive parts is still required... there is simply no direct connection, no direct path... until the first line-to-ground fault occurs. The system is required to have a ground detector installed which will indicate this line-to-ground fault. This type of protection will permit continued operation during this single line to ground fault condition, and permit a coordinated shutdown to fix it.

 

[GoldDigger]

If a second failure occurs before the first one is fixed, there will be an immediate unplanned shutdown instead.

 

[EE_FSU]

What is the secondary voltage of the transformer?

200/115VAC

 

[Smart $]

200/115VAC

For that voltage, you are required to ground X0 at the transformer or the neutral at the first disconnecting means, in the same enclosure where the system bonding jumper is located.

 

[EE_FSU]

For that voltage, you are required to ground X0 at the transformer or the neutral at the first disconnecting means, in the same enclosure where the system bonding jumper is located.

Does this requirement fall under NEC 250-20(B)(1) or another section?

 

[Smart $]

Does this requirement fall under NEC 250-20(B)(1) or another section?

250.20(B)(1)
250.30(A)