Utility Cutout and Fuse Selection

[td12345]

Hello,

I’m working on a project with a 34.5 kV three-phase distribution system, but the cutout fuses being installed are 25 kV rated (200A). It seems as though there are many different opinions about whether or not this is an acceptable application.

Obviously, the line-to-ground voltage in a 34.5kV system is around 19.9 kV. Understanding that a single fuse cutout is installed for each phase, the equipment appears to be properly rated. I've been informed by other professionals that this is a standard practice for utilities. However, some engineers argue that the cutout should be rated for 34.5 kV (phase-to-phase voltage).

In case it is helpful, the specific project being worked on is in rural Texas. I've done some digging in the RUS Bulletins for guidance (links below), but they don't specify anything that would clarify this specific situation. I'm also aware of instances like this for other equipment such as insulators.

RUS Bulletin 5-4
RUS Bulletin 1728F-803

Does anyone know of RUS standards or industry guidelines (like NESC or IEEE) that specify the correct fuse voltage rating for this scenario?

Depending on whether or not there are industry guidelines, Is is possible that the utility would be allowed to follow a different set of standards when compared to commercial/industrial installations?

 

[jbchu1371]

This is a huge question - Anyone have any insight?

 

[electrofelon]

I am also curious about this.

 

[JoeStillman]

If everything downstream of the cutouts is grounded-wye connected, you should only ever see L-G volts on a fuse. This gives some insight to that other thread about "why wye?"

 

[td12345]

If everything downstream of the cutouts is grounded-wye connected, you should only ever see L-G volts on a fuse. This gives some insight to that other thread about "why wye?"

I'm assuming a 3-Phase Delta-Wye transformer downstream would not qualify as "everything" being grounded-wye connected.

Why does the downstream connection have any bearing on the fuse cutout? Are you referencing a potential fault?

 

[jbchu1371]

If everything downstream of the cutouts is grounded-wye connected, you should only ever see L-G volts on a fuse. This gives some insight to that other thread about "why wye?"

What if, in a crazy world, an animal connected L-L?

 

[augie47]

Seemingly if we use the same logic as we do on low voltage such as in 240.85. If the primary system is a grounded wye you should be able to use the 25k

 

[jim dungar]

What if, in a crazy world, an animal connected L-L?

You would have 2 devices trying to clear the fault, neither of which would see the full L-L voltage.

 

[td12345]

You would have 2 devices trying to clear the fault, neither of which would see the full L-L voltage.

By what electrical theory principles would this theory be supported? I understand the thought process here, but it seems unlikely in practice. Wouldn't this be contigent on how quickly the fuses are able to clear the fault?

Is there any documentation you can share to help support this theory?

 

[td12345]

Seemingly if we use the same logic as we do on low voltage such as in 240.85. If the primary system is a grounded wye you should be able to use the 25k

As a retired inspector in the LV world, are there any specific code references you can share that would support this?

 

[augie47]

As a retired inspector in the LV world, are there any specific code references you can share that would support this?

240.85 dictates that on anything other than a solidly grounded neutral system you can not use the slash (dual) rated OCP devices. Accordingly you can do so on a SGN system. It would seem the same logic would apply to higher voltages but that is conjecture on my part.

 

[electrofelon]

Seems to me it would come down to who and/or how the voltage rating is determined. I am not that familiar with most MV stuff, what does the manufacturer say about the voltage rating and what systems these cutouts can be used on? I do install load break elbows occasionally and they specifically say that for a 15 kV class grounded system, the max rating of the elbow is the phase to ground voltage so about 8600 volts.

 

[electrofelon]

One other comment relating to this that maybe part of the answer: how come (at least per the NEC) we need full insulation on conductors? Why can't I use 600V conductors on a 1040Y/600 system? Is there any technical reason or is it just because NEC says so?

 

[jim dungar]

By what electrical theory principles would this theory be supported? I understand the thought process here, but it seems unlikely in practice. Wouldn't this be contigent on how quickly the fuses are able to clear the fault?

Is there any documentation you can share to help support this theory?

The fault current must flow from one Line through the animal and back through the other Line. Bother fuses will see the same amount of current.
Yes, there is likely a race between the two elements. Once they begin to open a voltage drop will occur across their resistance, so the L-L voltage will be dropped across three resistances in series.

 

[td12345]

The fault current must flow from one Line through the animal and back through the other Line. Bother fuses will see the same amount of current.
Yes, there is likely a race between the two elements. Once they begin to open a voltage drop will occur across their resistance, so the L-L voltage will be dropped across three resistances in series.

What is the third resistance you are referring to (other than the fuses)?

 

[jim dungar]

What is the third resistance you are referring to (other than the fuses)?

The item causing the fault and any resulting arcing at the point of the fault.