Fault current

[brscott]

In the worst cause scenario with the utility transformer impedance unknown. What impedance would I use for the worst cause fault current calculation?

 

[JoeStillman]

Welcome to the forum!

If you're in SKM, it has typical impedances available for dry type and oil filled transformers. It depends on the size. The POCO in my area recommends using 1.5% to 2.5% for transformers through 500 kVA and 5.5% for transformers 750 kVA and larger.

 

[brscott]

Thanks Utility company gave me info on fault current but no size of transformer or impedance to conduct calculation on my side for available fault current.

 

[jim dungar]

Thanks Utility company gave me info on fault current but no size of transformer or impedance to conduct calculation on my side for available fault current.

The utility has given you fault current for the secondary size of their transformer. You do not need to know anything about their transformer. Use the fault value they gave you as your starting point.

I have never had a utility provide primary side fault data unless the customer was buying power at the MV level.

 

[Jraef]

The utility has given you fault current for the secondary size of their transformer. You do not need to know anything about their transformer. Use the fault value they gave you as your starting point.

I have never had a utility provide primary side fault data unless the customer was buying power at the MV level.

But also, they often base it on”infinite bus” values on their primary, so I have sometimes found it useful to get them to dig in and tell me the real numbers, because they are sometimes lower than what they give you. Had that happen once when their values came up at 44kA and we had an MCC rated for 42kA. Got the real scoop on their transformer (after a bit of a fight) and it came out to 39kA.

 

[jim dungar]

But also, they often base it on”infinite bus” values on their primary, so I have sometimes found it useful to get them to dig in and tell me the real numbers, because they are sometimes lower than what they give you. Had that happen once when their values came up at 44kA and we had an MCC rated for 42kA. Got the real scoop on their transformer (after a bit of a fight) and it came out to 39kA.

Yes, they regularly use infinite bus, which is definitely not acceptable for arc flash studies.But, at the design stage, infinite bus will usually future proof your installation.
For the handful of times when it truly is incompatible with existing equipment, you need to fight with the utility. But even then, you do not need to know anything about their system capacity or transformer information.

 

[Jraef]

Yes, they regularly use infinite bus, which is definitely not acceptable for arc flash studies.But, at the design stage, infinite bus will usually future proof your installation.
For the handful of times when it truly is incompatible with existing equipment, you need to fight with the utility. But even then, you do not need to know anything about their system capacity or transformer information.

Agreed This was a "special case" because the customer had made a mistake in ordering the gear and it was already on site.

 

[brscott]

Correct me if im wrong. You need impedance to do calculation after the secondary value they provided right. Looked at bussman example. Just trying this step by step by step.

 

[don_resqcapt19]

Correct me if im wrong. You need impedance to do calculation after the secondary value they provided right. Looked at bussman example. Just trying this step by step by step.

only of the conductors, if they have given you the fault current at the secondary terminals

 

[jim dungar]

Correct me if im wrong. You need impedance to do calculation after the secondary value they provided right. Looked at bussman example. Just trying this step by step by step.

Not really, as Don said.

All good story circuit spreadsheets/calcultors/software should allow you to input the SCA from the upstream segment as your starting point.

 

[kwired]

But also, they often base it on”infinite bus” values on their primary, so I have sometimes found it useful to get them to dig in and tell me the real numbers, because they are sometimes lower than what they give you. Had that happen once when their values came up at 44kA and we had an MCC rated for 42kA. Got the real scoop on their transformer (after a bit of a fight) and it came out to 39kA.

Agreed This was a "special case" because the customer had made a mistake in ordering the gear and it was already on site.

But do you do when it is kind of close like that and in a couple years or so POCO upgrades things because of growing load in your area and that ends up raising actual available fault current at your site even though you are still on same transformer?

I kind of like the idea of assuming infinite bus when it comes to determining what equipment to use.

I do understand knowing what is actually available is best if trying to determine incident energy for arc flash situations.

 

[ramsy]

POCO upgrades things because of growing load in your area and that ends up raising actual available fault current at your site

Having visions of POCO removing service drop in free air, going from pole-mount to pad mount xfmr, with shorter and larger service latteral.

However, if biger xfmrs with larger design impedance are upgraded on existing-service network, how is fault current increased on same wire?

 

[kwired]

Having visions of POCO removing service drop in free air, going from pole-mount to pad mount xfmr, with shorter and larger service latteral.

However, if biger xfmrs with larger design impedance are upgraded on existing-service network, how is fault current increased on same wire?

I was talking about if they would happen to make upgrades on primary side of a transformer that remains as is. It won't effect max fault current used to determine secondary equipment ratings if you assumed infinite primary bus in the calculation.

 

[ramsy]

upgradese on primary side ..won't effect max fault current….if you assumed infinite primary bus

Roger that