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System Grounding

System Grounding

System Grounding

(OP)
In an ungrounded 480V distribution system do the cables used have to be 133% voltage rated (assumin any sigle line-to-ground fault shall be cleared within an hour , but not within a minute) , that is 480x1.33 = 638.4V min? Will 600V rated cables do which is close?

Does anyone know of mfrs. that make 600V cables with both 100% insulation thickness, and 133% insulation thickness; I could not find any.

Otherwise i have to use 1000V rated cables.

In the ungrounded 480V system if one phase is grounded, the other two phase will be at 480V to groung - their insulation will be stressed to 480V. But in case of arching, intermitten fault, the other to phases can experience much higer voltage due to capacitance charging.
Are we addressing that too?

Is there any comprehensive treatment on this. I am trying to get GET 3548 - System Grounding for Low Voltage Power System which i misplaced.

THANKS

RE: System Grounding

Grounded - you go with line to neutral.

Ungrounded - you go with line to line.

This applies to the requirements for cables and other equipment.

The logic is this.  If I have an ungrounded 480 V circuit and then get a line to ground fault, the maximum voltage on the 2 unfaulted phases is 480 V.

So 600 V cable should be adequate for your application.

RE: System Grounding

No. The maximum voltage to ground is 480 so 600V insulation is fine.

Alan
----
"It's always fun to do the impossible." - Walt Disney

RE: System Grounding

Didn't I just say that?

RE: System Grounding

Sorry magoo, I started a response and was interrupted. You replied before I finished mine.

Alan
----
"It's always fun to do the impossible." - Walt Disney

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