Selective Coordination, Arcflash and Bay-O-Net Transformer Primary Fusing

[Snowjob]

I just finished a Selective Coordination and Arcflash study.

Power runs to the site some 5 miles from the substation on overhead poles through 2/0 Okonite conductors. At the site there is a 225 KVA transformer with 25 A 12470 V Bay O Net fuses on the PRIMARY.

Both my Selective Coordination and Arcflash calculations change when I include these PRIMARY fuses in my study.

Without the primary side Bay O Net fuses in the study the site has high Arcflash levels and poor coordination. With the primary side Bay O Net fuses in the study , the site has acceptable Arcflash levels and acceptable coordination.

Should I include these Fuses in my Selective Coordination and Arcflash study?

 

[jdsmith]

Are the fuses in the current path and do they affect the downstream incident energy? Yes, so they should be included in your study. I would go as far to say that not including them and providing artificially high incident energy levels to your client is doing a disservice to the folks that will use the labels. My plant has a 138 kV utility substation on site that we get power from. We received available fault current values from the utility and carried the calculation down to the 480V motor control center level. There were still some assumptions made in the original study that were not correct, but we are working on updating the model as we add some new substations.

 

[bob]

Did you include the impedance of the primary conductors in the study?

 

[Snowjob]

Thanks for yur input

Thanks for yur input

Did you include the impedance of the primary conductors in the study?

I used EDSA and I think it does. Thanks for yur input

 

[ohmhead]

Well ive got a question before we had computers to figure out coordination studys i guess we did it with pencil and paper . Can anyone explain the reason & theory of length of conduit in the process and type of conduit or raceway its effects on fault in a system meaning pvc to rigid or emt why is type of conduit needed please understand iam just a simple electrician and have never been able to understand its effects in detail.

We think induction and grounding is the main issue but help me understand the real total picture if i may ask ? WHY

 

[jdsmith]

Well ive got a question before we had computers to figure out coordination studys i guess we did it with pencil and paper . Can anyone explain the reason & theory of length of conduit in the process and type of conduit or raceway its effects on fault in a system meaning pvc to rigid or emt why is type of conduit needed please understand iam just a simple electrician and have never been able to understand its effects in detail.

We think induction and grounding is the main issue but help me understand the real total picture if i may ask ? WHY

It is fairly simple math - all these programs are doing is calculating currents in large networks of hundreds or thousands of impedances connected together. The impedances may be transformers, overhead lines, conductors in conduit or cable, motors, or other loads. The length of conductors, size of conductor, type of raceway it's in, and spacing between the conductors are all variables that affect the impedance of the line. The spacing between conductors matters because the magnetic fields of the three phases interact and the spacing determines how much they interact. A metallic conduit around the conductors acts as a choke and will increase the impedance slightly compared to conductors in PVC conduit or overhead triplex.

I even have one voltage drop calculator spreadsheet that takes into account whether the circuit is in metallic conduit or not - I think that is a bit excessive, but somebody had some extra time on their hands I suppose.

 

[mivey]

Nevermind, jdsmith answered it already.

 

[__dan]

I^2*t

I^2*t

Can anyone explain the reason & theory of length of conduit in the process and type of conduit or raceway its effects on fault in a system meaning pvc to rigid or emt why is type of conduit needed
...

We think induction and grounding is the main issue but help me understand the real total picture if i may ask ? WHY

Comparing RGS conduit and cable tray in a fault, the ground path impedance with RGS is less, RGS performs measurably better. How the software takes this into account, the level of sophistication, I have no idea.

Incident energy is I^2*T. So I is bigger with RGS because of the lower impedance of the ground fault path but T could be less, faster breaker clearing time.

The software is probably only looking at path impedance and spitting out what the breaker clearing time "should" be.

 

[Cold Fusion]

...Can anyone explain the reason & theory of length of conduit in the process and type of conduit or raceway its effects on fault in a system meaning pvc to rigid or emt why is type of conduit needed ...

...I even have one voltage drop calculator spreadsheet that takes into account whether the circuit is in metallic conduit or not - I think that is a bit excessive, but somebody had some extra time on their hands I suppose.

JD's explanation is pretty good. Take a look at T9. It gives the impedance for conductors in a raceway. It gives both the inductive impedance (X) and the resistance. Notice the values vary by conductor size, conduit material, and conductor length.

Under a short circuit, a bolted fault, the voltage drop across the fault is zero. That means the system voltage is all dropped across the transformer impedance and the cable impedances - and that is all that limits the current.

cf