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do motors require seperate earth in steel fin fan structures

do motors require seperate earth in steel fin fan structures

do motors require seperate earth in steel fin fan structures

(OP)
In a structuarl steel fin fan unit do the vertical motors that are bolted to the steel require a seperate earthing connection?  The structural steel is earthed at several locations.  Currently there is only a 3 core SWA cable feeding the motor, the motors are 690Vac motors

RE: do motors require seperate earth in steel fin fan structures

I believe it is an API code requirement. There are good reasons for it. There are a number of scenarios that may develop, some corrosion related, that may allow a dangerous touch potential to develop on the frame of a motor, grounded structure notwithstanding.
 

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

RE: do motors require seperate earth in steel fin fan structures

(OP)
Waross, thank you for your reply this will give me something to go back to our design office with to help justify the cost.  This will include about 300 motors.

RE: do motors require seperate earth in steel fin fan structures

300 motors is 299 more chances for a problem.
You may consider a short heavy ground jumper from the motor frame to the nearest structural steel. This would be about 3 AWG sizes larger than the motor conductors.
It is often difficult to verify the structural grounds, and several years down the road, with an accumulation of dirt and corrosion, the structural grounds may be destroyed or lost without the electrical department being aware of the lost ground. The jumper to adjacent steel will avoid lethal touch potentials developing on the surface of a faulty motor. It is also good practice to run a common ground conductor from the MCC to the fin fans. The NEC is a minimum. You may consider doing a voltage drop calculation to ensure fast tripping if long distances are involved.
I normally see a code mandated equipment ground in the motor feed cable and in addition a heavy gauge jumper from the motor frame to the adjacent structure to protect against lethal touch potentials.
Two issues;
1> A code ground to ensure breaker tripping in the event of a ground fault in the motor.
2> A local jumper to protect the electrician who is checking a failed motor.
 

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

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