Bonding Jumper Required?

[jmartin6261]

Is a bonding jumper required between the XO terminal and ground on a wye secondary transformer if there is no neutral conductor installed in the secondary feeder?

Installation consists of a 460 delta ~ 460Y/265 volt, 3 phase isolation transformer with electrostatic shield feeding a variable speed drive on an extruder. There is no neutral conductor installed to the extruder as it does not require one.

If it is not required, wouldn't it be advisable to install for clearing of ground faults?

 

[paul]

You need the jumper and you have to run the neutral to the first disconnecting means.

See 250.20(D), 250.30 and 250.104.

 

[derf48]

With the info supplied I am going to guess that it is not a utility transformer, but a separately derived system covered by 250.30. This system must be grounded and bonded, the choice is yours as to where. If you bond at the transformer then there is no requirement to extend the xo to the first disconnect. You must then extend a EGC from the transformer to the entire circuit as it is your effective ground fault path. If you bond at the first disconnect then the xo must be extended to that disconnect as it is your fault current path between the transformer and disconnect.

Fred Bender

 

[raider1]

You need the jumper and you have to run the neutral to the first disconnecting means.

See 250.20(D), 250.30 and 250.104.

Which condition of 250.20(B) requires that a 460Y/265 volt 3 phase system that doesn't require a neutral, be grounded?

I agree that I would recomend that the transformer be grounded, but I don't see where it is required.

Also there is no requirement that a grounded conductor be brought past the system bonding jumper. If the system bonding jumper is installed at the transformer then you don't need to bring the grounded conductor to the first disconnecting means.

Chris

 

[jmartin6261]

Paul/Derf48,
Thank you both for you replies.

The NEC is somewhat confusing to me in this area as 250.20(B)(2) and (3) specify that the system is required to be grounded if the system neutral is used as a circuit conductor. So it would seem that since the systems I described do not have any phase-to-neutral loads that the bonding jumper isn't required. However, if this were the case, why doesn't the NEC state this?

The installation I described is in a factory that has 50 extruders, each with a dry type isolation transformer feeding the VFD. 18 of the transformers are delta~wye, while the remaining are delta~delta. Not sure why they are different.

None of the delta-wye transformers have a bonding jumper, nor do most of the transformers have a proper connection to the building grounding electrode system. I am going to recommend to the owner that these installations be brought up to current NEC requirements.

 

[paul]

I retract that the neut. needs to be taken to the nearest disconnect, but I still feel that it needs to be bonded.

 

[raider1]

None of the delta-wye transformers have a bonding jumper, nor do most of the transformers have a proper connection to the building grounding electrode system. I am going to recommend to the owner that these installations be brought up to current NEC requirements.

Ungrounded systems are still required to be connected to a grounding electrode system (Earth). (See 250.4(B)(1))

You are also required to bond all non-current carrying conductive materials, equipment ect... of the ungrounded system together. (See 250.4(B)(2), (3) and (4))

Again I don't see anything in 250.20(B) that requires that the transformers you described be grounded. This would be a design issue.

Unless there is a reason for the system to be ungrounded I would suggest that you ground the transformers to provide an effective ground fault current path.

Hope this helps,

Chris

 

[raider1]

but I still feel that it needs to be bonded.

I agree,

I would suggest that the transformer be grounded in accordance with 250.30 unless there is a design issue that makes an ungrounded system desireable.

I just don't see in section 250.20(D) and 250.20(B) where the system described is required to be grounded.

Chris

 

[iwire]

Chris, I agree it could be ungrounded but will point out 250.21 requires ground detectors.

It sounds like the installation would fall under 250.21(2).

 

[raider1]

Chris, I agree it could be ungrounded but will point out 250.21 requires ground detectors.

It sounds like the installation would fall under 250.21(2).

You are correct, you would need to install ground detectors.

I was just trying to point out that it wasn't required to be grounded, and didn't think to add that if you choose to not ground the system you would need to install ground detectors, my bad.

Chris

 

[paul]

Chris, I agree it could be ungrounded but will point out 250.21 requires ground detectors.

It sounds like the installation would fall under 250.21(2).

I'll buy that.

 

[tom baker]

This is not one of the systems that is required to be grounded.
It can be optionally grounded, or use a ground fault detection.
Keep in mind if ungrounded, you will have 460 volts to ground and this will create a higher voltage stress, and in a ground fault, capacitance ringing can result in up to 2,200 volts fault.
Experience shows grounding, via the system bonding jumper is desirable, but if shutdowns can't be tolerated, then a impedance grounded system is best.

 

[jmartin6261]

Thank you all for your replies. I've been wrestling with this issue for several weeks now.

Iwire - all of these transformers were installed in the mid-to-late '90's before ground detectors were required.

I'm going to recommend to the owner that the wye transformers have a bonding jumper installed as well as all of the transformers be connected to the building grounding electrode system.