Circulating current

[circ]

In an offshore platform, if a 200A conductor is connected from the generator house to the switch board. Assuming earthing is done at both ends. How can circulating current arise in the cable?

 

[rbulsara]

Need more info or clarifications:

Is the generator the only source of power? You need two sources for "circulating" current.

Does the current you referring to actually exist or this is a hypothetical question?

Grounding at multiple places can mess up ground fault detection scheme. Is that what you have in mind?

 

[Skogsgurra]

Sounds like the classical sparky saying "only one ground allowed". There are reasons for this rule. The problem is in the interpretation and how it is applied to the kind of grid you have.

But, the question - as put - has one answer: There will not be any circulating current in a cable that is connected at both ends to the steel frame of an oil rig.

Of course, there are a few prerequisites for this to be true: 1 the steel frame shall be sloidly welded together and 2 there shall not be any appreciable magnetic fields in the loop formed by cable and steel.
Both these prerequisites are fulfilled in your installation.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jghrist]

A circulating current is induced in the shield by the mutual inductance between the phase conductor and the shield. If the shield is grounded at only one end, then a voltage will be induced at the ungrounded end.

 

[waross]

Not all unwanted currents are circulating currents. At 200 amps on a cable that is grounded at both ends you may get unwanted current sharing. Part of the current can be expected to flow in the ground, which in this case is the metal structure. It is not good practice to share currents on a ground connection. Either use the ground as the current carrying conductor, or use a conductor, but do not share current between a conductor and ground. You can develop possible issues with protection, corrosion, and/or magnetic fields in the structure.
In addition, there are most likely codes that apply to your installation.
Notwithstanding, there are special exceptions to this rule, the most common being multiple protective grounding used on distribution systems.

Bill
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"Why not the best?"
Jimmy Carter