ground system 7

[Abdoudiallo]

Hi I would like to know if we can omit the motor ground because of the fram that is supposed to be grounded.I work with someone that always say we dont need it.And I always consider bringing a wire from the ground.Maybe someone can clarify me if Im wrong or my coleage.Happy new year to all of you

 

[waross]

Duh, yes the frame is supposed to be grounded. That's why you run a ground wire. To ground the frame like it's supposed to be.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Unotec]

Typically, the motor is grounded through the cables supplying the power, is it not? Don't these come in a conduit with a ground cable?
The frame would have to be grounded regardless if the motor is or isn't.
But I guess we have not answered the OP's main question whether to ground the motor or not if the frame is grounded.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[Abdoudiallo]

Thanks you all for your response maybe I didnt wrotte correctly my quetion.Do we need to bring a fourth wire when we wirring a 3ph motor or we just bring the 3phases and we considere the frame as ground

BEST REGARDS

 

[davidbeach]

If you don't connect the frame to a ground conductor, how is it grounded?

 

[PolyMike]

As well as for equipotentiality of all conductive surfaces, you also have to assure a proper path to the current in the event of an earth fault.

If the motor frame is already earthed properly by means of a earth wire with the right section you can only bring three conductors to the motor.

 

[davidbeach]

PolyMike, that probably wouldn't meet code, depending on where, etc. A ground conductor with the phase conductors will always provide a better path for fault current than some random path through building steel.

 

[rbulsara]

The correct and safest answer is Yes.

By Code (like NEC) you need to bring an equipment grounding conductor with the circuit condutors. Now as far as the code is concerned, the metal raceway enclosing the conductors or the armour ( listed for grounding) of the cable is acceptable. A good design is to have a separate grounding conductor with the circuit conductors.

Now coming to you collegue, if I can hazard a guess, what he may be referring to is that in many industrial facilities the motor may be mounted on a metal frame that is already bonded with building's steel structure. If the service serving the motor is also grounded to the same steel structure, he may think that the motor is adequately grounded and in some cases it may appear to work (fuse/breakers properly opening on a ground fault). BUT this still is not an acceptable grounding conductor per Code or a good design practice.

 

[itsmoked]

Q:
If you have, say, a block long steel printing press. It has a ground cable running from the service entrance to it.

Now you have 10 motors hung/bolted onto it.

Does each motor need a ground run with it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Yes.

 

[itsmoked]

Roger that.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rbulsara]

Keith:

Yes. It is not to say your motors are not adequately grounded. It is not sufficient to meet "equipment grounding requirement" for motors per NEC (assuming NEC is the applicable code).

The conncetion of the frame to service ground meets one of the code requirements of bonding non-current carrying parts to a ground. But motors still need their equipment grounding conductor.

In your particular case if the "press" is factory wired and only one feed is required to an inbox and rest is factory wired, you may only be required to bring the equipment grounding conductor to the inbox and bond to the frame.

You can search NEC article 250. At one place it says that strcutral metal frame of a building shall not be used as equipment grounding conductor for ac equipment.

 

[Abdoudiallo]

I think that Mrs rbulsara and davidbeach has say the word I thanks all off you for your objectif and positif help.As you all say I point out to my colleague that the reason why that specific motor is the only one that having problem and also the only one without ground and I forget to mension that we working in the area wich is always wet.

Best regards to all of you and thanks again for your feedback

 

[rovineye]

Abdoudiallo: Hold on a minute. What is the problem you are having? Not having a ground wouldn't show any symptoms unless there is a fault of some kind, for instance a shock is possible vice a protective device trip.

 

[waross]

To us the "frame"is a part of the motor and not the frame of the machine that it is mounted on. There may have been some confusion with this, but the answer is ground the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[controlsdude]

I have always installed or had motors installed by prints where a ground to the motor frame was run to the main panel or daisy chain to all motors from the main disconnect of the main power drop.

Of course these are conveyor systems. But I have worked at many controls houses and have alwasys seen where this standard was followed.

Of course most believed it was NEC code and I believe it is too.

 

[parlitu]

If you have a motor is absolutly necesary to have a ground conductor conected to it's frame. Having the 3 phases conected to the motor doesn't mean you have also a grounding. The current allways flow to the path with minimum rezistance; if you make a motor visual inspection and put a hand on the motor and in the same time you have a fault on the motor between a phase and the frame then the current will "run" to the groung through you body, and we don't want this to happent. That's why you need a 4 conductor from the motor to grounding grid, to make a minimum rezistance path. Through this path all the ground fault current will flow. There is no place for "supposed to be gounded": if you don't see a conductor from the motor frame to the ground, then you have to consider that you motor is not grounded properly.