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ground grid and equipments

ground grid and equipments

ground grid and equipments

(OP)
If I have a motor that its frame is connected to ground through a metalic conduit, which is connected, at its other side, to the CCM wich is connected to the ground grid of a substation, when a Fault occurs at the substation the motor frame is at GPR of the substation. Personel standing at the motor are subject to a potencial diference between the motor frame and the ground where they are standing. I am wrong? How you solve this problem? Thats why you suppose to build an earthing grid at the motor area?.. If this is the case you suppose to build a ground grid for every metalic equipment you install in a plant, no matter what size?. Thanks.

RE: ground grid and equipments

cmelguet,
It looks like you have raised the same question in thread # 237-286792.
If the motor is sitting in a substation where there is a ground grid provided,yes its frame must be bonded to the ground grid to avoid dangerous potential differences.But if your motor is remotely installed there is no ground grid provided.Then how do you bond it?Are you providing a ground grid at the motor location for each motor?No.Therefore,you should provide a seperate ground electrode which is the safety ground with a low electrode resistance.In addition to this safety ground you should also provide a seperate bonding conductor running along the motor cable PROPERLY SIZED for the ground fault return current which is the system ground.(Note:The metal conduit running between the MCC and the motor frame is NOT a very good ground fault return path for very high ground fault current situations eventhough it is metal)  

RE: ground grid and equipments

(OP)
Kiribanda.

So basically we should provide a ground electrode at every location where we install a metallic enclosure? Even if the enclosure is connected to the ground grid of the system through the cable conductors or metallic conduit? For a ground electrode you mean a copper bar installed in an earth pit near the motor, or metalic enclosure area. I no it sound strange, but where i work my boss state that we need to build an earth grid at every motor above 50 kW and 380 Vac.

RE: ground grid and equipments

Check your local regulations. Provision of equipotential bonding ensures that all conductive parts are at a similar potential, certainly it should limit potential differences between adjacent equipment and structures to a non-harmful value. Note that equipotential bonding does not require a connection to local earth to be effective, although the expectation is that metalwork of electrical plant will be connected to the main earth terminal. Of course it may be more economical to provide a local earth electrode than try to establish an equipotential zone.

Also check whether your regulations permit use of structural steel and reinforced concrete re-bar to be used as an earth electrodes.
  

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