2300V and above motor grounding requirements 2

[JBUDA54]

On a particular project that I am on we have a number of medium voltage motors that I will supply an additional #4/0AWG bare copper ground that will bolt on the the motor frame and connect to nearest grounded building steel. Does anyone know if there is an IEEE standard that recommends motors greater than 460V to have these grounding requirements? I was told by a co-worker that it is standard practice to provide an additional ground in addition to the ground in the feeder cable to motors that are 2300volts and above. Any suggestions, references, personal experiences with this is greatly appreciated.

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Jacobs Engineering Group Inc.
1041 East Butler Road
Post Office Box 5456
Greenville South Carolina 29606-5456
864·676·6458 Phone 864·676·4789 Fax
E-mail: jason.buda@jacobs.com

Jason W. Buda
Electrical and Control Systems Department
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[JBUDA54]

Just an additional comment: I have not looked extensively in the National Electric Code on this, but I have found a remark (Blue Text) in the NEC Handbook stating "ANY motor in a wet location and subject to contact by personnel constitutes a serious hazard and, unless it is isolated, elevated, or guarded from reach, is required to be grounded." (This was underneath the Article 430-142 Stationary motors, and is specifically addressing the motor frame grounding). I still would like to know if there are any specific requirements for motor frame grounding for 2300V and above motors? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jacobs Engineering Group Inc.
1041 East Butler Road
Post Office Box 5456
Greenville South Carolina 29606-5456
864·676·6458 Phone 864·676·4789 Fax
E-mail: jason.buda@jacobs.com

Jason W. Buda
Electrical and Control Systems Department
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 

[electricpete]

I believe the requirement for equipment (safety) ground should apply even for motors not in a wet location.

I haven't ever heard any discussion of whether or not motor frames were required to be grounded... only what size ground conductor to be used.

I'll look for some references.

 

[busbar]

This practice may not be covered in IEEE standards but there is probably good reason why it’s written into medium-voltage specs. NEC article 250 generally mandates equipment grounding conductors be routed with phase conductors. Reactance of equipment-ground cabling is fixed and potentially excessive for low-voltage systems; undesirably increasing clearing time for protective devices. The higher driving voltage of MV circuits makes the reactance of a larger ‘loop’ of less effect on ground-fault clearing times. This type of 'jumper' is not intended to be used in place of equipment grounds routed with circuit conductors.

4/0-AWG exposed bare copper (often somewhat more corrosion-resistant 7-strand) and heavy IEEE-837 connectors are boilerplate in quite a few engineering designs. It allows for ready visual inspection of equipment condition, and limits touch-step potential during ground-fault incidents. It also generally represents a smaller portion of circuit per-unit cost where more costly shielded cables fill a given raceway, compared to, say, 600V THWN. {It’s cheap insurance, so to speak.}

As evidenced by thousands of miles of 5-35kV overhead distribution, higher-reactance ground returns are suitable for effective protective-device operation.

 

[jbartos]

Suggestion: #4/0AWG has allowable short circuit currents:
~10kA for 100cycles=1.6667seconds
~20kA for 30cycles=0.5seconds
~100kA for 1cyle=0.0167seconds
which appears to be practical and more than enough for many grounding purposes.

 

[edengineer]

Why do we gound medium voltage cable?

 

[ScottyUK]

Hi edengineer,

Are you an archaeologist? You've dug up a thread over four yers oild to ask a question that isn't really even related to the orignal content. Well done for using the search to find it, but don't be afraid to start a new thread to ask your question.


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