Ground Conductor for 12.47 kV System

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timm333

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Electrical Design Engineer
We install TW green insulated conductor as ground conductors on low voltage systems. What should be the insulation of ground conductor on medium voltage (12.47 kV) system? Should the insulation of ground conductor be rated for 7.2 kV (12.47/1.732=12.47 kV), or should a bare conductor be used as ground conductor for 12.47 kV system?
 
timm333 said:
But should we keep the conductor bare, or should we use LV insulation for MV grounding?
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You can use bare (though there are some restrictions), covered, or insulated. Seems odd, but if you do use insulated, it must be rated 600V or greater [200.2(A)].
 
Smart $ said:
You can use bare (though there are some restrictions), covered, or insulated. Seems odd, but if you do use insulated, it must be rated 600V or greater [200.2(A)].
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200 Applies to the grounded conductor
It says for 0--1000V circuit grounded insulation must match circuit voltage or more. (or bare)

250.184 (A)(1) applies to -- ready for this -- NEUTRAL conductors for >1000V
and requires 600V insulation.

[[Now I've been told that articles only cover what their title states so this in 250 may be safely ignored:happyno:]]
 
Sorry there is some confusion. 200.2 deals with grounded conductor (neutral). There are three types of conductors: grounded conductor (neutral), equipment grounding conductor, and grounding electrode conductor. My question was for the insulation voltage level of equipment grounding conductor.
 
The MV cable shield can be utilized as the ground. It must be capable of carrying the fault current for the duration of time that it takes the protection to clear the circuit.

Read Article 250.190, it is very informative in this regard.

Provide the calculation that shows that the insulation shield can handle the fault current. If it can't, than a separate ground is required, and recommended.
 
timm333 said:
Sorry there is some confusion. 200.2 deals with grounded conductor (neutral). There are three types of conductors: grounded conductor (neutral), equipment grounding conductor, and grounding electrode conductor. My question was for the insulation voltage level of equipment grounding conductor.
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The reason 250 (grounding and bonding) refers to the neutral is perhaps because in these systems >1000V there may be no distinction between the neutral and grounding (e.g.: they are the same conductor).
 
250.190 states that "grounding conductors not an integral part of a cable assembly shall not be smaller than 6 AWG copper". So if we use the armor of MV cable as ground, then does it mean that the sum of the cross sections of all the armor wires should be 6 AWG? How to calculate this sum?
 
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