verifying fault current calculation

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dhsvcs

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the maximum available fault current at the transformer secondary terminals is estimated to be 21110
symmetrical amperes at 120/240 volts. The protective device on the line side of the transformer currently in place or to be
installed and serving your property located at the subject location is a 8 amp type K fuse. The primary service voltage is
7.6kV L-G

90 ft in pvc 1/0 aluminum triplex from transformer to meter.....
calculated 5687 amps..
Does that sound correct ?
 
In my opinion, in order to achieve 21.1 kA short-circuit current you need a 50 kVA 1.974% transformer 7.6/0.12 kV.
Z=0.12^2/0.05*1.974/100=0.005685 ohm
Isctr=0.12/0.005685=21.108 kA
Then the Irated=50/.12=416.67A
According to NEC Table 310.15(B)(16) ampacity of Al 1/0 it has to be 135 A for 90 oC.
You need 4 parallel cables, I guess.
 
Don’t expect the primary fuses to do much of anything for the secondary side. This is doubly true for most utility fuses. They are there strictly for primary protection only.
 
paulengr said:
Don’t expect the primary fuses to do much of anything for the secondary side. This is doubly true for most utility fuses. They are there strictly for primary protection only.
Click to expand...
I believe they are there to protect the primary feeder not the transformer primary winding.
 
jim dungar said:
I believe they are there to protect the primary feeder not the transformer primary winding.
Click to expand...

According to an S&C SMD Power Fuse application/selection guide, “The transformer primary fuse should be selected to provide both system and transformer protection”.

paulengr said:
Don’t expect the primary fuses to do much of anything for the secondary side. This is doubly true for most utility fuses. They are there strictly for primary protection only.
Click to expand...

This guide also states, “S&C Type SMD Power Fuses provide full fault-spectrum protection for transformer. These fuses will detect and interrupt all faults - large, medium, and small (even down to their minimum melting currents), regardless of whether the fault is on the primary or secondary side of the transformer or when line-to-line or line-to-ground voltage exists across the fuse”.

I any case, I think the level of protection comes down to the protection engineers philosophy and whether or not that protection is applied correctly.
 
Xptpcrewx said:
According to an S&C SMD Power Fuse application/selection guide, “The transformer primary fuse should be selected to provide both system and transformer protection”.



This guide also states, “S&C Type SMD Power Fuses provide full fault-spectrum protection for transformer. These fuses will detect and interrupt all faults - large, medium, and small (even down to their minimum melting currents), regardless of whether the fault is on the primary or secondary side of the transformer or when line-to-line or line-to-ground voltage exists across the fuse”.

I any case, I think the level of protection comes down to the protection engineers philosophy and whether or not that protection is applied correctly.
Click to expand...
All well and good, but I have had experience with service techs/line workers, from several different POCOs that say they only carry two different fuse sizes on their trucks. The fuses protect the distribution line by removing faulted transformers. It seems the only time they really care about protecting the transformer is when it is an MVA rated unit in one of their substations.
 
jim dungar said:
All well and good, but I have had experience with service techs/line workers, from several different POCOs that say they only carry two different fuse sizes on their trucks. The fuses protect the distribution line by removing faulted transformers. It seems the only time they really care about protecting the transformer is when it is an MVA rated unit in one of their substations.
Click to expand...
Perfect example of how there will always be a difference between what is intended vs. what actually gets done.
 
Xptpcrewx said:
According to an S&C SMD Power Fuse application/selection guide, “The transformer primary fuse should be selected to provide both system and transformer protection”.



This guide also states, “S&C Type SMD Power Fuses provide full fault-spectrum protection for transformer. These fuses will detect and interrupt all faults - large, medium, and small (even down to their minimum melting currents), regardless of whether the fault is on the primary or secondary side of the transformer or when line-to-line or line-to-ground voltage exists across the fuse”.

I any case, I think the level of protection comes down to the protection engineers philosophy and whether or not that protection is applied correctly.
Click to expand...


The SMD fuse selection guide applies to medium size transformers typically located in substations and revolves around power fuses rather than link fuses used in overhead cutouts.

The protection philosophy of medium size power transformers is very different from pole pugs. For example due to the lack of circuit switcher or DTT, the primary fuses must provide secondary busbar protection, even if clearing takes a few seconds vs differential protection which takes cycles. Fusing is typically 125% of 167% of the ONAN rating, and the later part of the selection guide tables do not list certain combinations above that due to secondary clearing being to long.

Pole pigs on the other hand are typically protected at 200 to 300% of the primary FLA, with some POCOs fusing even higher. Rather than protecting a large asset from secondary side faults, the idea is simply to remove a failed disposable from service.
 
jim dungar said:
All well and good, but I have had experience with service techs/line workers, from several different POCOs that say they only carry two different fuse sizes on their trucks. The fuses protect the distribution line by removing faulted transformers. It seems the only time they really care about protecting the transformer is when it is an MVA rated unit in one of their substations.
Click to expand...

This. Small units are meant to be replaced, large units are meant to be tested and re-energized for anything external.

As is fusing of anything larger than 1.5MVA is becoming rarer due to all the down falls associated with fuses. In fact some POCOs never fused power transformers relying on DTT and fault throwers before circuit switchers hit the market.

Xptpcrewx said:
Perfect example of how there will always be a difference between what is intended vs. what actually gets done.
Click to expand...

This is actually intended. More often than not the purpose of a cutout is not to protect anything, rather remove a failed unit from service so the feeder recloser does not trip when the unit expires. Lightning withstand is one reason for larger links, another is the fact POCOs drive pole pigs at 200-300% for short periods of time, or sometimes above 100% continuously on the hopes of a 30 year in service life.
 
jim dungar said:
I believe they are there to protect the primary feeder not the transformer primary winding.
Click to expand...
Tell that to one my local POCO's, particularly on grain storage bin sites. With some automatic aeration control systems I have to stage starting of fans or if too many try to start at same time it blows a primary fuse. These are direct drive centrifugal fans, not extreme high inertia load but still takes a few seconds to accelerate to full speed.
 
mbrooke said:
The SMD fuse selection guide applies to medium size transformers typically located in substations and revolves around power fuses rather than link fuses used in overhead cutouts.
Click to expand...
Taken directly from the S&C SMD Power Fuse Selection Guide:

"This information bulletin is a guide for the selection, application, and coordination of S&C Type SMD® Power Fuses when applied on the primary side of small to medium-sized transformers installed in utility and industrial substations. For the purpose of this guide, transformers having primary voltage ratings between 34.5 kV and 138 kV, with medium-voltage (4.16 kV through 34.5 kV) secondaries, will be covered."

Please be more careful in your responses as they are spreading misinformation.
 
Xptpcrewx said:
Taken directly from the S&C SMD Power Fuse Selection Guide:

"This information bulletin is a guide for the selection, application, and coordination of S&C Type SMD® Power Fuses when applied on the primary side of small to medium-sized transformers installed in utility and industrial substations. For the purpose of this guide, transformers having primary voltage ratings between 34.5 kV and 138 kV, with medium-voltage (4.16 kV through 34.5 kV) secondaries, will be covered."

Please be more careful in your responses as they are spreading misinformation.
Click to expand...


"in utility and industrial substations"



Pole pigs are typically protected by links.
 
Last edited:
Xptpcrewx said:
Taken directly from the S&C SMD Power Fuse Selection Guide:

"This information bulletin is a guide for the selection, application, and coordination of S&C Type SMD® Power Fuses when applied on the primary side of small to medium-sized transformers installed in utility and industrial substations. For the purpose of this guide, transformers having primary voltage ratings between 34.5 kV and 138 kV, with medium-voltage (4.16 kV through 34.5 kV) secondaries, will be covered."

Please be more careful in your responses as they are spreading misinformation.
Click to expand...

To be clear, substations:

 
mbrooke said:
"in utility and industrial substations"



Pole pigs are typically protected by links.
Click to expand...

It’s no different for their fused cut-outs. Everything previously stated holds.

www.sandc.com

Type XS Fuse Cutout

Type XS Fuse Cutouts provide full-fault-spectrum protection for overhead distribution systems rated 4.16 kV through 25 kV.
www.sandc.com www.sandc.com

www.sandc.com

Positrol® Fuse Links

Positrol® Fuse Links are designed to provide reliable protection, and they operate only when a fault occurs within the cutout’s zone of protection. With Positrol Fuse Links, you eliminate unnecessary cutout operations and thereby reduce your maintenance and operation costs.
www.sandc.com www.sandc.com

If you actually spend the time to read what’s in these links you will realize you are actually arguing about nothing.
 
Xptpcrewx said:
It’s no different for their fused cut-outs. Everything previously stated holds.

www.sandc.com

Type XS Fuse Cutout

Type XS Fuse Cutouts provide full-fault-spectrum protection for overhead distribution systems rated 4.16 kV through 25 kV.
www.sandc.com www.sandc.com

www.sandc.com

Positrol® Fuse Links

Positrol® Fuse Links are designed to provide reliable protection, and they operate only when a fault occurs within the cutout’s zone of protection. With Positrol Fuse Links, you eliminate unnecessary cutout operations and thereby reduce your maintenance and operation costs.
www.sandc.com www.sandc.com

If you actually spend the time to read what’s in these links you will realize you are actually arguing about nothing.
Click to expand...

The level of protection stated in their power fuse PDF does not hold true for pole transformers.

One example:


1628647089631.png



50,000 VA / 7200= 6.95 amps, yet asks for a 20 amps fuse. 20/7=2.85 times, which fits the typical minimum 200-300% rule.

You will (should) never find a 10MVA sub transformer protected at 300%+.
 
Xptpcrewx said:
No? Where do you think fused cut-outs are installed? S&C must be really dumb then because that exactly what they say its used for...


Redirecting you to the last sentence of post #14. Please don't respond.
Click to expand...


Look at the tables and graphs. You've got fusing in excess of 300%, and only covering half the damage curve.

The original PDF you cited does not hold relevant to pole transformers. And its not that the linemen are ignoring the EEs. The EEs ask for over fusing. Some POCOs will even stick a 25K link on a 10kva 13.2kv pig and not look back, on purpose.

Substation units on the other hand get a detailed fault study and fusing with the intent to clear secondary fault in the zone of protection.
 
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