2X voltage rating between two insulated conductors

[electrofelon]

Is there any good reason, other than because thats what the code says, or because there are few system configurations that would utilize it, why we can't use a L-L voltage of twice the conductor rating on a grounded center tapped system, or 1.73 times the conductor rating on a grounded wye system? Are there any other sectors where this is common?

 

[Carultch]

Is there any good reason, other than because thats what the code says, or because there are few system configurations that would utilize it, why we can't use a L-L voltage of twice the conductor rating on a grounded center tapped system, or 1.73 times the conductor rating on a grounded wye system? Are there any other sectors where this is common?

I've wondered that myself. Because you would think that 600V line to ground would cover you for up to 1200V line to line, given that the insulations are in series under ordinary circumstances.

I suppose one explanation is that if the black line faults to ground and doesn't shut off from a trip of an OCPD, then everything that is grounded is now at the full voltage from the opposite red line. So the red line needs to be protected by its own insulation in case any of the other insulation fails.

 

[Sahib]

Is there any good reason, other than because thats what the code says, or because there are few system configurations that would utilize it, why we can't use a L-L voltage of twice the conductor rating on a grounded center tapped system, or 1.73 times the conductor rating on a grounded wye system? Are there any other sectors where this is common?

If I understand your question correctly, the wire is tested by manufacturer at a power frequency over voltage. In India a 230V working voltage PVC insulated wire is also marked with 1100V tested voltage for its insulation withstand capacity.

 

[Carultch]

If I understand your question correctly, the wire is tested by manufacturer at a power frequency over voltage. In India a 230V working voltage PVC insulated wire is also marked with 1100V tested voltage for its insulation withstand capacity.

I don't think the OP is talking about safety factors in general, on voltage withstand of insulation. But rather why you cannot take credit for "adding up" the insulation voltage rating of multiple line wires in the same conduit, to withstand the line-to-line voltage.

 

[electrofelon]

I don't think the OP is talking about safety factors in general, on voltage withstand of insulation. But rather why you cannot take credit for "adding up" the insulation voltage rating of multiple line wires in the same conduit, to withstand the line-to-line voltage.

Correct. Ignore issues such as safety factors, peak vs rms. Talking about having twice the insulation, thus twice the voltage capability.

We can do this with bipolar pv systend (with restrictions), but that seems to be the only situation per nec

 

[JFletcher]

Correct. Ignore issues such as safety factors, peak vs rms. Talking about having twice the insulation, thus twice the voltage capability.

We can do this with bipolar pv systend (with restrictions), but that seems to be the only situation per nec

Is insulation a linear function? If you put 600V on a conductor insulated for 300V, it may not short next to another identical conductor, but what about to ground?

I think you got it right with safety factor... i.e., liability. It's all about who gets sued when something goes wrong.

 

[Carultch]

Is insulation a linear function? If you put 600V on a conductor insulated for 300V, it may not short next to another identical conductor, but what about to ground?

I think you got it right with safety factor... i.e., liability. It's all about who gets sued when something goes wrong.

The two wires worth of insulation are a series circuit, because current has to flow through both of them to fault. Therefore, the combined insulation of both wires together takes twice the voltage rating to exceed its allowable design limit.


The system is specified with volts to ground not exceeding the wire voltage rating, so the only voltage driving a short to ground would not cause the failure.

As I said earlier, the plausible explanation is preparing for a fault that goes undetected, where L1 shorts to ground undetected, and L2 would then become 2X volts relative to ground.

 

[electrofelon]

Is insulation a linear function

I don't know. I would think so, but why are mv cables always shielded?

 

[GoldDigger]

I don't know. I would think so, but why are mv cables always shielded?

Possibly because a surface charge on the outside of the insulation could produce hazardous current? Or because even the capacitance through the insulation could produce hazardous current?

 

[big john]

Is insulation a linear function?

When you do direct current step-voltage testing on an insulator the increase in leakage current should be linear: Seeing an increasing curve is an indicator of failing insulation. That said, in MV insulation construction is often not linear: A conductor designed to handle 15kV will only be about 150% thicker than a conductor designed to handle 8.6kV, even though it's 173% of the voltage.

But why are mv cables always shielded?

Because the shielding prevents uneven electrical stress that causes accelerated insulation breakdown. Is there more focused stress on 8mils of insulation at 1200V than there is on 4mils of insulation at 600V? I don't know.

But 600V when new will perform well above its nameplate rating. I've hipotted THHN at 8kV per the manufacturer's directions and it passed with flying colors.

I'd guess you shorten life of the conductor by increasing the voltage on it, but I honestly can't really envision a scenario where a grounded system would be at unusual risk of failure by running it at 1200V L-L. I've seen seen 600V corner-deltas installed with THHN that had performed fine for years, even though the wire was often energized above it's rated voltage.

 

[Carultch]

Is insulation a linear function?

It's actually a logarithmic function, considering the cylindrical shape. Because there is more surface area at the edge of the insulation than in the core. But microscopically, it still is linear, and still is linear when it is a rectangular shape like an insulated busbar, it is a linear function.

In heat transfer, there is a similar formula for the thermal resistance of a hollow cylinder.
https://en.wikipedia.org/wiki/Thermal_resistance

 

[coop3339]

Is there any good reason, other than because thats what the code says, or because there are few system configurations that would utilize it, why we can't use a L-L voltage of twice the conductor rating on a grounded center tapped system, or 1.73 times the conductor rating on a grounded wye system? Are there any other sectors where this is common?

It maybe just be to prevent confusion on un-grounded, corner grounded and impedance ground systems. Conductors in these systems can see full line to line voltage to ground in a ground fault condition or normally in the case of corner grounded.

 

[Ingenieur]

Insulation ~ high resistance

R = p L/A
p = resistivity Ohm/ft, etc
L length
A area

so for a point basically linear
but the larger the contact area the lower the value

 

[Sahib]

Insulation ~ high resistance

R = p L/A
p = resistivity Ohm/ft, etc
L length
A area

so for a point basically linear
but the larger the contact area the lower the value

But a study of insulation breakdown voltage versus insulation thickness would show it is a nonlinear function.

 

[drcampbell]

... why are mv cables always shielded?

Is the shielding on medium-voltage cables robust enough to serve as a grounding conductor and open the OCPD without further incident after an insulation breakdown?

As for the original question:
Physics says it's probably OK. Code says it's not. If you can't accept that, you're probably in the wrong business; it will come up again.

 

[Sahib]

Is the shielding on medium-voltage cables robust enough to serve as a grounding conductor and open the OCPD without further incident after an insulation breakdown?

More precisely, to prevent local insulation breakdowns of cable insulation due to any non uniform potential gradient along the insulation surface.

 

[Tony S]

Is the shielding on medium-voltage cables robust enough to serve as a grounding conductor and open the OCPD without further incident after an insulation breakdown?

No, the shield is for stress relief only. I’ve got a drawing of how dielectric stresses affect the individual cores, I’ll try and find it.

 

[electrofelon]

I've wondered that myself. Because you would think that 600V line to ground would cover you for up to 1200V line to line, given that the insulations are in series under ordinary circumstances.

I suppose one explanation is that if the black line faults to ground and doesn't shut off from a trip of an OCPD, then everything that is grounded is now at the full voltage from the opposite red line. So the red line needs to be protected by its own insulation in case any of the other insulation fails.

I guess that makes sense. Even with proper function bonding, the metal parts will still rise in voltage toward the faulted conductor, thus potentially exposing the other conductor to more than its rated insulation (for a brief period).

 

[electrofelon]

Because the shielding prevents uneven electrical stress that causes accelerated insulation breakdown.

So that makes sense to me, however why is this not an issue with LV conductors? Why cant a MV conductor just have X times the insulation thickness as a unshielded LV conductor?

 

[Tony S]

Screening isn’t required until you get to about 5kV. I terminated a couple of 3.3kV 3 core cables recently, the only real difference with LV cables was thicker insulation.