Bonding Motors -- req'd?

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lakee911

Senior Member
Location
Columbus, OH
Do motors need to be explicitly grounded?

I have an install of a 650HP Siemens motor that has two 1/2-13 UNC ground bolts on the motor frame and I noticed that the electrical Contractor hadn't bonded the frame.

The literature on the motor simply states the following:

Motor Literature said:
Motors should be installed and grounded per local and national codes.

All that I've found in the NEC is the following:

NEC 430.12(E) said:
(E) Equipment Grounding Connections. A means for attachment of an equipment grouning conductor termination in accordance with 250.8 shall be porovided at motor terminal housings for wire-to-wire connections or fixed terminal connections. The means for such connections shall be permitted to be located either inside or outside the motor terminal housing.

Exception: Where a motor is installed as a part of factory-wired equipment that is required to be grounded and without additional connections being required at the terminal housing during equipment installation, a seperate means for motor grounding at the motor terminal housing shall not be required.

The shop drawings show four wires connected from the VFD enclosure to the motor. U-T1, V-T2, W-T3 and PE connected to T1, T2 T3 and the motor case on the motor. There is a gound symbol connected to the case along with the wire from PE.

The Contractor tells me that the plans do not show a ground cable run out to the blower motors and that there is metal to metal contact between the motor and skid.

The motor was actually field installed on the skid, and I don't think the skid was bonded. It's sitting on a concrete platform, but it's probably attached with epoxy anchors.

I'm not sure how to proceede here. Any advice?

Thanks!
 

Buck Parrish

Senior Member
Location
NC & IN
In most cases the lug on top is for bonding. In your case it sounds as if it is the only equpment ground.
Depending on the KW. You could probably use the metal conduit. Put a grounding bushing on it. Then go to the lug.
 
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kwired

Electron manager
Location
NE Nebraska
For equipment grounding you must start in art 250.

250.112(C) refers you to 430.242 for motor frames.

430.242 says:

The frames of stationary motors shall be grounded under any of the following conditions:
(1) Where supplied by metal-enclosed wiring
(2) Where in a wet location and not isolated or guarded
(3) If in a hazardous (classified) location
(4) If the motor operates with any terminal at over 150 volts to ground
Where the frame of the motor is not grounded, it shall be permanently and effectively insulated from the ground.
There is a gound symbol connected to the case along with the wire from PE.

Could you explain what you mean with this statement. PE terminal is an equipment ground terminal on many VFD's. Do you have a conductor between PE and the motor case. Do you have an equipment ground run from VFD to power source? If you have both the motor is properly grounded. Anything else depends on exactly what you have before we can tell if it is acceptable.
 

lakee911

Senior Member
Location
Columbus, OH
I've attached a couple pages from the shop drawings. Maybe this will help explain what I've described. I've highlighted relevent areas.

We have a ground run from the VFD to the motor (which goes into the peckerhead, but I've not opened it to see what is going on inside) and we have metallic conduit between the VFD and the motor. The VFD is also bonded at its source.

Thanks.
 

lakee911

Senior Member
Location
Columbus, OH
According to the motor documents, there is a "grounding lug" inside of the motor terminal box. I'm assuming that is where the ground wires from the VFD are terminated.

Thx,
Jason
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The motor supply circuit requires an EGC and this conductor must be bonded to the motor. If the supply circuit has an EGC of the wire type, it will be bonded in the motor terminal box. If the raceway is being used as the EGC, an external bonding jumper will be required around the flexible connection between the raceway and the motor.
 

Thedroid

Member
And at 650 hp you'll I imagine the ground to be a decent size, and I would bet that there's some special cable from that VFD to the motor.
 

lakee911

Senior Member
Location
Columbus, OH
The motor supply circuit requires an EGC and this conductor must be bonded to the motor. If the supply circuit has an EGC of the wire type, it will be bonded in the motor terminal box. If the raceway is being used as the EGC, an external bonding jumper will be required around the flexible connection between the raceway and the motor.

I haven't verified it myself, but I'm assuming that its bonded within the peckerhead. There is a lug inside for just such a purpose (according to the motor docs).

And at 650 hp you'll I imagine the ground to be a decent size, and I would bet that there's some special cable from that VFD to the motor.

No, we're utilizing standard wiring because the VFD is very close by. If I had to guess, we are using THWN or XHHW. It's installed in metal conduit--three parallel runs in fact.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The motor supply circuit requires an EGC and this conductor must be bonded to the motor. If the supply circuit has an EGC of the wire type, it will be bonded in the motor terminal box. If the raceway is being used as the EGC, an external bonding jumper will be required around the flexible connection between the raceway and the motor.

I believe Don has the correct answer. But I have believed things before and been wrong. :)
 

Smart $

Esteemed Member
Location
Ohio
I've attached a couple pages from the shop drawings. Maybe this will help explain what I've described. I've highlighted relevent areas.

We have a ground run from the VFD to the motor (which goes into the peckerhead, but I've not opened it to see what is going on inside) and we have metallic conduit between the VFD and the motor. The VFD is also bonded at its source.

Thanks.

According to the motor documents, there is a "grounding lug" inside of the motor terminal box. I'm assuming that is where the ground wires from the VFD are terminated.

Thx,
Jason
IMO (with emphasis), the drawings indicate the motor frame is to be bonded through an EGC and also a bonding to the Grounding Electrode System (GES).

In my upscale industrial experience (power plants, refineries, and steel mills), most large motors have an EGC run with the feeder conductors, and are also bonded to the ground ring/grid/GES... sometimes twice over.
 

kwired

Electron manager
Location
NE Nebraska
IMO (with emphasis), the drawings indicate the motor frame is to be bonded through an EGC and also a bonding to the Grounding Electrode System (GES).

In my upscale industrial experience (power plants, refineries, and steel mills), most large motors have an EGC run with the feeder conductors, and are also bonded to the ground ring/grid/GES... sometimes twice over.

Can you give us a good reason to do that? Unless you are trying to give lightning a more direct path to earth what is wrong with just EGC? I don't see lightning being much a threat to most motors unless in the path of direct hit, but that results in damage to about anything.
 

lakee911

Senior Member
Location
Columbus, OH
IMO (with emphasis), the drawings indicate the motor frame is to be bonded through an EGC and also a bonding to the Grounding Electrode System (GES).

In my upscale industrial experience (power plants, refineries, and steel mills), most large motors have an EGC run with the feeder conductors, and are also bonded to the ground ring/grid/GES... sometimes twice over.

I think you're right, but we have no ground ring/grid/GES at the equipment ... it's only connected through the EGC. I think in a new install we certainly would have tied it in as such, but I really don't know why other than good practice.
 

Smart $

Esteemed Member
Location
Ohio
Can you give us a good reason to do that? Unless you are trying to give lightning a more direct path to earth what is wrong with just EGC? I don't see lightning being much a threat to most motors unless in the path of direct hit, but that results in damage to about anything.

I think you're right, but we have no ground ring/grid/GES at the equipment ... it's only connected through the EGC. I think in a new install we certainly would have tied it in as such, but I really don't know why other than good practice.
I believe the reason is redundancy and lowering fault path resistance. Obviously the lower the resistance to the grounding system, the safer the installation. As far as Code minimum, just the EGC is required.

The question here is whether grounding as depicted on the drawing is an installation albeit project specification...?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I believe the reason is redundancy and lowering fault path resistance. Obviously the lower the resistance to the grounding system, the safer the installation. As far as Code minimum, just the EGC is required. ...
How much would the remote path for fault current reduce the impedance of the fault return path? The impedance of a remote path is much higher than that of the EGC run with the supply conductors. Yes, I know that it is a parallel path and it will reduce the impedance by some amount, but is this reduction great enough to make any real difference in the safety of the installation?
 

Smart $

Esteemed Member
Location
Ohio
How much would the remote path for fault current reduce the impedance of the fault return path? The impedance of a remote path is much higher than that of the EGC run with the supply conductors. Yes, I know that it is a parallel path and it will reduce the impedance by some amount, but is this reduction great enough to make any real difference in the safety of the installation?
Not much. But the EGC is there for faults, yet what's there for an EGC fault? More for safety through redundancy than anything else... I imagine, as I'm not the one who spec's these installations.
 
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kwired

Electron manager
Location
NE Nebraska
Not much. But the EGC is there for faults, yet what's there for an EGC fault? More for safety through redundancy than anything else... I imagine, as I'm not the one who spec's these installations.

Many motors are mounted on equipment with a lot of steel, or other conductive materials used to build the equipment. There are often times multiple motors each with their own EGC, other conductive piping may be attached, and the whole machine may be fastened somehow to building steel that is alread well grounded. There are of course times where a motor or its driven machinery is fairly isolated from other grounding paths.

I'm willing to bet that the unintentional paths are more reliable than intended paths in the case where there is lots of grounded metal surfaces around if they are not checked and maintained on a regular basis.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
I'm willing to bet that the unintentional paths are more reliable than intended paths in the case where there is lots of grounded metal surfaces around if they are not checked and maintained on a regular basis.
But with high current faults the impedance goes way up as you separate the supply path from the return path. If you lose the intended path, the EGC, the impedance of the remote path could very well be high enough that the OCPD does not quickly trip.
 

kwired

Electron manager
Location
NE Nebraska
But with high current faults the impedance goes way up as you separate the supply path from the return path. If you lose the intended path, the EGC, the impedance of the remote path could very well be high enough that the OCPD does not quickly trip.

And the same is true for a bonding jumper to the GEC or other grounding source, that is why I ask what is the point of installing them, when quite often there is plenty of unintentional path that would serve the same purpose and possibly more reliably. Even in those cases where there is a lot of conductive paths and a good EGC you can not simply tell the fault current that it is supposed to follow the EGC. It is going to follow any path available.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
And the same is true for a bonding jumper to the GEC or other grounding source, that is why I ask what is the point of installing them, when quite often there is plenty of unintentional path that would serve the same purpose and possibly more reliably. Even in those cases where there is a lot of conductive paths and a good EGC you can not simply tell the fault current that it is supposed to follow the EGC. It is going to follow any path available.
Yes, it will follow all of the paths, but almost all of it will follow the EGC. The impedance of any remote path is much higher than that of the EGC that is run with the circuit conductors.
As far as paths to ground, they are of almost no value in the fault clearing process of under 600 volt systems.
 
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