75KVA Padmount transformer feeding 800A service

NewtonLaw

Senior Member
If someone would provide a picture of the transformer installation it would help as would the operating voltage.

If you want to calculate the fault duty yourself all you need to do is come up with the impedance at the point of fault. You can do this by placing a capacitor across the service entrance with no load attached. Measure the open circuit voltage before adding the capacitor, then measure the voltage again after adding the capacitor. The short circuit kVA is approximately found then by:

kVAsc approximately = kVAR / ((V-with cap)-(V-without cap))/V-without cap

This will give you the approximate symmetrical short circuit duty that includes the service conductor, transformers and supply system to the point of fault. (the point where you connected the capacitor).

Also, you did not say if the service is three phase or single phase and at what secondary voltage?

From Eaton, they supply a 45 kVA up to 75 kVA three phase pad-mounted transformer at 208 volts with impedances that range from 2.7% to 5.75% This would give you a range from 3,621 amps to 7,710 amps at the terminals of the transformer assuming an infinite primary bus.

Hope this helps.
 

kwired

Electron manager
Location
NE Nebraska
Nobody willing to go to the trouble to factor in impedances of the MV distribution or even transmission lines that are also a part of the actual results. Plus if they switch lines at any level (whether temporary or permanent) on you the actual number likely changes.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Nobody willing to go to the trouble to factor in impedances of the MV distribution or even transmission lines that are also a part of the actual results. Plus if they switch lines at any level (whether temporary or permanent) on you the actual number likely changes.

The POCOs already have the MV distribution impedance, especially the transmission impedance. I have yet to see a utility feeling confident enough to rely on differential that they can turn off their step distance elements.
 

kwired

Electron manager
Location
NE Nebraska
The POCOs already have the MV distribution impedance, especially the transmission impedance. I have yet to see a utility feeling confident enough to rely on differential that they can turn off their step distance elements.
Well then they are either too lazy to figure out the remaining details, or maybe don't want to give a figure that possibly can change if they change equipment or reroute the supply in any way.

Thing is they can give a "worst case" fault current level, but that may or may not relate to worst case incident energy level when you factor in overcurrent device response time at the "best case" fault current level.

If they change your service transformer or the nearby distribution equipment you may notice and ask for re-evaluation but they could change something further away that changes results and you may never know.

I'd think calculating incident energy is what is typically more critical for needing precision. Max fault current mostly important for withstand/interrupt ratings of equipment.
 

topgone

Senior Member
Well then they are either too lazy to figure out the remaining details, or maybe don't want to give a figure that possibly can change if they change equipment or reroute the supply in any way.

Thing is they can give a "worst case" fault current level, but that may or may not relate to worst case incident energy level when you factor in overcurrent device response time at the "best case" fault current level.

If they change your service transformer or the nearby distribution equipment you may notice and ask for re-evaluation but they could change something further away that changes results and you may never know.

I'd think calculating incident energy is what is typically more critical for needing precision. Max fault current mostly important for withstand/interrupt ratings of equipment.
I agree 100%! There's more than just the single-line diagrams POCO have that can "exactly" tell one the correct figures! They just give the worst figure imaginable with the existing grid setup!
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Well then they are either too lazy to figure out the remaining details, or maybe don't want to give a figure that possibly can change if they change equipment or reroute the supply in any way.

Thing is they can give a "worst case" fault current level, but that may or may not relate to worst case incident energy level when you factor in overcurrent device response time at the "best case" fault current level.

If they change your service transformer or the nearby distribution equipment you may notice and ask for re-evaluation but they could change something further away that changes results and you may never know.

I'd think calculating incident energy is what is typically more critical for needing precision. Max fault current mostly important for withstand/interrupt ratings of equipment.

Or simply not going beyond what is required, which is a shame. Yes system configurations can change, but 99% of the time the system is in steady state with preferred topology in place.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
I agree 100%! There's more than just the single-line diagrams POCO have that can "exactly" tell one the correct figures! They just give the worst figure imaginable with the existing grid setup!
What more is there? Often times the worse case is just an arbitrary number for that "type" of service.
 

kwired

Electron manager
Location
NE Nebraska
Or simply not going beyond what is required, which is a shame. Yes system configurations can change, but 99% of the time the system is in steady state with preferred topology in place.
What more is there? Often times the worse case is just an arbitrary number for that "type" of service.
That worst case is fine when selecting equipment that needs to be able to withstand worst case.

When it comes to determining incident energy for PPE selection purposes, lower available current can mean more incident energy because lower current may result in longer OCPD clearing time. Time is a factor in incident energy level exposure.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Read the very last page of this. With the experience and knowledge you have, this should make sense. https://ep-us.mersen.com/sites/mers...-Arc-Energies-with-Current-Limiting-Fuses.pdf
Thanks for that link. It does a good job of describing how current limiting fuses operate, and also how the timing of the fault can change peak short circuit current.

I also didn't expect the decaying transient, but it makes sense that a short circuit has significant inductive features since the magnetic fields can be so huge.

What I still don't get: if the available fault current is say 24kA but incident energy is higher at lower current, wouldn't the characteristics of the particular fault also need to be considered? Because the fault could limit current to less than its maximum value and thus increase clearing time.

Put another way, when calculating fault energy don't you need to consider faults across the range of 0 to maximum available, not just maximum available?

Put a third way: if you are told that the available short circuit current is 24kA, but the true available current is 19kA, would the 24kA calculations already include 19kA because the fault itself could limit the current that flows to only 19kA?

My understanding is that large 480V systems require ground fault protection because they can get into a state where an arc forms but the arc impedance limits current to less than the OCPD rating of the service. This would be the case of the fault characteristics limiting current, giving an open ended incident energy with the incident current never approaching maximum available.

Jon
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
That worst case is fine when selecting equipment that needs to be able to withstand worst case.

When it comes to determining incident energy for PPE selection purposes, lower available current can mean more incident energy because lower current may result in longer OCPD clearing time. Time is a factor in incident energy level exposure.

Yup. One reason why POCOs should give the fault current. Or you can just measure it yourself.
 

topgone

Senior Member
What more is there? Often times the worse case is just an arbitrary number for that "type" of service.
As power coming from big, selling power companies change (Spot market), the system grid follows. Depending on the proximity and the capacities of the chosen power suppliers' system, the available fault changes drastically. POCOs don't even bother to know if changes occurred! They just provide you the worst case values from the existing tie-ups!
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
As power coming from big, selling power companies change (Spot market), the system grid follows. Depending on the proximity and the capacities of the chosen power suppliers' system, the available fault changes drastically. POCOs don't even bother to know if changes occurred! They just provide you the worst case values from the existing tie-ups!

Any generation additions are run through software first, even at the distribution level. Feeder sequence components, R+Jx, ect are usually known. If not it can be calculated.

No offense, but everyone commenting here doesn't know how ISOs and utilities operate.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
What I still don't get: if the available fault current is say 24kA but incident energy is higher at lower current, wouldn't the characteristics of the particular fault also need to be considered? Because the fault could limit current to less than its maximum value and thus increase clearing time.

Put another way, when calculating fault energy don't you need to consider faults across the range of 0 to maximum available, not just maximum available?

Put a third way: if you are told that the available short circuit current is 24kA, but the true available current is 19kA, would the 24kA calculations already include 19kA because the fault itself could limit the current that flows to only 19kA?

My understanding is that large 480V systems require ground fault protection because they can get into a state where an arc forms but the arc impedance limits current to less than the OCPD rating of the service. This would be the case of the fault characteristics limiting current, giving an open ended incident energy with the incident current never approaching maximum available.

Jon
IEEE 1584-2018 provides equations for the calculation of arcing fault current that are based on five different electrode configurations. The arcing fault current is less than the bolted fault current due to the impedance of the arc. The duration of the arc is based on the overcurrent device's response. For systems above 1000 Volts, the arcing fault current is around 95% of the bolted fault current, whereas systems with voltages below 1000 Volts is around 55% which is one of the reasons why arc flash incident energy levels are worse at lower voltages, because the devices don't operate as fast.
 

kwired

Electron manager
Location
NE Nebraska
Any generation additions are run through software first, even at the distribution level. Feeder sequence components, R+Jx, ect are usually known. If not it can be calculated.

No offense, but everyone commenting here doesn't know how ISOs and utilities operate.
You ever look at how some small town municipal power systems are operated?

Governing board members don't know an amp from a watt, they are only interested in the revenue they can create. The line crew is probably only 2-5 guys in most cases, one with the most experience is sort of the general manager, operations manager, crew chief and "engineer" all in one. System is usually not too large and they probably get away with lots of things design wise they couldn't if it were a bigger system. Outside engineering may come in occasionally when there is to be some major changes and draw up some plans.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
You ever look at how some small town municipal power systems are operated?

Governing board members don't know an amp from a watt, they are only interested in the revenue they can create. The line crew is probably only 2-5 guys in most cases, one with the most experience is sort of the general manager, operations manager, crew chief and "engineer" all in one. System is usually not too large and they probably get away with lots of things design wise they couldn't if it were a bigger system. Outside engineering may come in occasionally when there is to be some major changes and draw up some plans.

Perhaps. But Duke Energy should not have an excuse not to provide a reasonable minimum fault current.
 
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