Grounding Skid / Structural Steel for large electric motor

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DanHart

Member
Location
Colorado
During the construction of a new heating plant it was brought up that the skid and structural steel were not earth grounded. These are 300+ hp pumps and the argument was that the case and skid should be earth grounded due to inductive charge build up. As far as I can tell, and others have verified, the grounding / bonding for the distribution is spot on to code. But checking the old heating plant, all large equipment frames, skids and structural steel are indeed grounded in addition to normal grounding/bonding.

My questions are:

1) Does the casing/frame need to be earth grounded at the motor skid?
2) If the answer to the first question is no, does the existing grounding in the old plant pose any possible problems?
3) Is this something that changed a while back? (the old plant was built in 1900 is but has been updated in 1970's and in 1992)

I know that this is a broad question with a bunch of variables, but I am hoping that there is someone out there that has seen this specific problem and could walk me through it. Thanks in advance.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Look at 250.112 (K)

Note that it says "shall be connected to an equipment grounding conductor". It does not specify just how this connection is to be made.
 

nhee2

Senior Member
Location
NH
I think it is common for large skidded equipment to be tied to the facility ground electrode system or ground ring at industrial facilities, although I believe if an EGC is installed and properly sized per 250.122 then this GEC connection is redundant and not required by NEC.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I think it is common for large skidded equipment to be tied to the facility ground electrode system or ground ring at industrial facilities, although I believe if an EGC is installed and properly sized per 250.122 then this GEC connection is redundant and not required by NEC.

A GEC is not required IMO.

An EGC is required, and a GEC cannot be a substitute for an EGC (although I gather that the latest code suggests they can at least sometimes be the same conductor).
 

DanHart

Member
Location
Colorado
The equipment ground is pulled through the conduit in addition to the 3 phase supply, and is connected. In the old plant, there is a ground rod driven into each pad that is also connected to the skid.
 

nhee2

Senior Member
Location
NH
This seems to fall into the same category as separate ground rods at light poles - something that is frequently spec'd and installed, but not required by code, and is not a substitute for an EGC.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The equipment ground is pulled through the conduit in addition to the 3 phase supply, and is connected. In the old plant, there is a ground rod driven into each pad that is also connected to the skid.

I am not convinced that the wire connected to this rod is a GEC.

take a look at 250.54.
 

Tony S

Senior Member
For the sake of preventing argument and to comply with the plants ?normal? practice would it be a great hardship to drive a couple of ground rods in?
They wouldn?t be detrimental to the existing GEC, in fact they would be adding to its effectiveness.

Something years in heavy engineering has taught me, sometimes it?s easier to comply with ?custom and practice?. I may not agree with it but for the sake of an easy life?.
At one time I would have argued, I?ve mellowed.
 

Smart $

Esteemed Member
Location
Ohio
I am not convinced that the wire connected to this rod is a GEC.

take a look at 250.54.
My thought is, if one end of a conductor connects to a grounding electrode, it IS a grounding electrode conductor. What else could it be? The only question left in my mind is, exactly how does it get sized...???
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
My thought is, if one end of a conductor connects to a grounding electrode, it IS a grounding electrode conductor. What else could it be? The only question left in my mind is, exactly how does it get sized...???

Grounding Electrode. A conducting object through which
a direct connection to earth is established.

Grounding Electrode Conductor. A conductor used to
connect the system grounded conductor or the equipment to
a grounding electrode or to a point on the grounding electrode
system.

I don't see how this conductor can be a GEC as it does not connect to the system grounded conductor or to the GES, but it does allow for a GEC being connected to the equipment from a GE. Kind of vague as it is not clear if an aux GE is the same thing as a regular GE.

250.54 Auxiliary Grounding Electrodes. One or more
grounding electrodes shall be permitted to be connected to
the equipment grounding conductors specified in 250.118
and shall not be required to comply with the electrode
bonding requirements of 250.50 or 250.53(C) or the resistance
requirements of 250.53(A)(2) Exception, but the
earth shall not be used as an effective ground-fault current
path as specified in 250.4(A)(5) and 250.4(8)(4).

250.53
(C) Bonding Jumper. The bonding jumper(s) used to connect
the grounding electrodes together to form the grounding
electrode system shall be installed in accordance with
250.64(A), (B), and (E), shall be sized in accordance with
250.66, and shall be connected in the manner specified in
250.70.

There does not appear to be any sizing criteria at all in the code for auxiliary grounding electrodes since they are specifically exempted from the sizing requirements found in 250.66.
 

Smart $

Esteemed Member
Location
Ohio
I don't see how this conductor can be a GEC as it does not connect to the system grounded conductor or to the GES, but it does allow for a GEC being connected to the equipment from a GE. Kind of vague as it is not clear if an aux GE is the same thing as a regular GE.
And there you have it.

There are only two types of conductors that can connect to a grounding electrode, and per 250.53, a bonding jumper cannot connect to an auxiliary electrode. ;)

There does not appear to be any sizing criteria at all in the code for auxiliary grounding electrodes since they are specifically exempted from the sizing requirements found in 250.66.
Exactly.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Think carefully about this.

250.54 Auxiliary Grounding Electrodes. One or more
grounding electrodes shall be permitted to be connected to
the equipment grounding conductors

Grounding Electrode Conductor. A conductor used to
connect the system grounded conductor or the equipment to
a grounding electrode or to a point on the grounding electrode
system.

Nowhere in the definition of GEC does it say the GEC is connected to the EGC, while the conductor attached to an auxiliary GE is required to be connected to the EGC.
 

DanHart

Member
Location
Colorado
Thanks for all of the responses. I was just a little concerned that it would cause parallel ground paths on the equipment causing problems with ground fault detection.
 

Smart $

Esteemed Member
Location
Ohio
Think carefully about this.





Nowhere in the definition of GEC does it say the GEC is connected to the EGC, while the conductor attached to an auxiliary GE is required to be connected to the EGC.
I believe that was put in there intentionally to indicate there must be an EGC and this auxiliary GE cannot be used as a substitute. Hardly any AGE that I've seen was ever connected to the EGC itself. Is not a connection to the EGC also a connection to the equipment? And in a lesser sense, also a connection to the grounded conductor in a grounded system?

You'll find quite a few Article 250 grounding connections are not meant ultra literally. For example, in most cases one does not literally connect a GEC to the grounded conductor, but rather a terminal of the same busbar, or two lug terminals bonded to the can, etc.
 
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