Reducing Utility Fault Levels on LV System

[mivey]

2K is not much. But when it gets that close, I subtract 10% from the transformer impedence since it can vary 10% from the label.

Good idea but an even more conservative is to use the min impedance from the POCO as they have min avg and max values for a particular size. They really should give you the min impedance and maybe even for a bigger transformer if they anticipate growth at the site (like with undeveloped property or buildings).

Its probably pretty safe to assume the POCO will never build a substation that can supply infinite current directly on your clients property.

But you could model a pretty stiff source. For some transformers it won't make much difference.

 

[mivey]

wehave not been advised of the transformer size

I'm guessing around 225-300 kVA based on what we have so far.

 

[bob]

Also, for the few interested...I'm reading an "Electrical Protection Handbook" from Cooper Bussman which has seven pages
of series combinations of line side fuses and load side circuit breakers. Of course Bussman fuses are line side with load side circuit
breakers from Square D, Cutler-Hammer, General Electric and Siemens...
These charts can be used by an electrician without requiring some engineer to sign off as they are "Tested Combinations"....

I doubt the inspector would sign off on that. If you can go back to the utility and get further information we can help.

1. Substation KVA. Is it a 2500 or 23000 kva? %Z
2. Voltage 4 kv or 13 kv?
3. distance from sub to your site in miles.
4. Primary conductor size if possible. If not I can guess large.
5. Your transformer size and %Z

 

[petersonra]

I doubt the inspector would sign off on that. If you can go back to the utility and get further information we can help.

1. Substation KVA. Is it a 2500 or 23000 kva? %Z
2. Voltage 4 kv or 13 kv?
3. distance from sub to your site in miles.
4. Primary conductor size if possible. If not I can guess large.
5. Your transformer size and %Z

as long as they are tested why would the inspector care?

 

[don_resqcapt19]

How did my earlier comment morph into a "series rated combination"?
The OP would like to reduce 24kA to less than 22kA...my comment was somewhat general as we
have not been advised of the transformer size, the impedance, the ampere rating of the main overcurrent device, I recommended
the use of a fuse with a reduction in let through current. The example I gave was a 400 ampere, RK5 fuse which reduces a 24kA
line side short circuit current to less than 10kA...
Assuming the OP's gear is rated at least 10kA (most are) this would enable a fully rated system, not a series combination....

Using an upstream OCPD to limit the current at a downstream OCPD is a series combination.

 

[mivey]

I doubt the inspector would sign off on that. If you can go back to the utility and get further information we can help.

1. Substation KVA. Is it a 2500 or 23000 kva? %Z
2. Voltage 4 kv or 13 kv?
3. distance from sub to your site in miles.
4. Primary conductor size if possible. If not I can guess large.
5. Your transformer size and %Z

1. I don't think it will matter
2. I don't think it will matter
3. Than can be unless close to the station.
4. A factor if #3 is actually in miles
5. Assuming 3-phase, approximately (maybe within one kVA size): Padmount, 300 kVA, 3.47%Z OR (3)75 kVA pots at 2.60%Z

 

[bob]

Originally Posted by SparkyHC Also, for the few interested...I'm reading an "Electrical Protection Handbook" from Cooper Bussman which has seven pages
of series combinations of line side fuses and load side circuit breakers. Of course Bussman fuses are line side with load side circuit
breakers from Square D, Cutler-Hammer, General Electric and Siemens...
These charts can be used by an electrician without requiring some engineer to sign off as they are "Tested Combinations"....

Petersonra "as long as they are tested why would the inspector care?

The post by SparkyHC does not sound as thought there has been a test. He is proposing that he read the information off of a
chart and make a decision. IMO that won't cut it.

 

[bob]

1. I don't think it will matter
2. I don't think it will matter
3. Than can be unless close to the station.
4. A factor if #3 is actually in miles
5. Assuming 3-phase, approximately (maybe within one kVA size): Padmount, 300 kVA, 3.47%Z OR (3)75 kVA pots at 2.60%Z

If you want to make the substation the infinite buss, they all mater. I don't follow your reasoning. If I have the above information I
can supply the secondary fault using the sub as the infinite source. Depending on the magnitude of the primary, there may not be
much difference as to where you choose the infinite buss.

 

[petersonra]

The post by SparkyHC does not sound as thought there has been a test. He is proposing that he read the information off of a
chart and make a decision. IMO that won't cut it.

the phrase series rated combination strongly implies that these particular combinations have been tested under UL auspices and thus are a listed series rated combination, as long as they are applied correctly.

 

[topgone]

Also, for the few interested...I'm reading an "Electrical Protection Handbook" from Cooper Bussman which has seven pages
of series combinations of line side fuses and load side circuit breakers. Of course Bussman fuses are line side with load side circuit
breakers from Square D, Cutler-Hammer, General Electric and Siemens...
These charts can be used by an electrician without requiring some engineer to sign off as they are "Tested Combinations"....

Not to highjack the thread, but if you read again the provisions on 240.86, it allows a Tested Combination "and" the limiting provisions on 240.86(C) which requires you to use series-rated protective devices only if there are: 1) no motor loads connected at the load side of the higher-rated protective equipment or the line side of the lower-rated breaker "and" 2) the sum of the full-load amps of the motor loads do not exceed 1% of the lower-rated breaker's interrupting capacity rating. The idea behind that provision is that motors contribute to the fault current at the very first moments of the fault and the higher-rated protective device of the tested combination cannot possibly protect the lower-rated breaker as the fault contribution path does not pass the higher-rated protective device.

IMO, charts use are okay but please make sure you follow the code on what it allows as well as what it forbids. BTW, when there's nothing you can do about fault current levels in a system that have gone beyond it's initial design due to additions, NEC added a provision in 240.86 after so many proposals for its inclusion in the code-making process. It's not as if the CMP didn't give it much thought and calculations and just allowed "series-tested"combinations.

 

[mivey]

If you want to make the substation the infinite buss, they all mater. I don't follow your reasoning.

My thought was that even if the system is not stiff today, it may very well be tomorrow. Old small substations will be upgraded and if you think of it from a per-unit basis, the substation might as well be infinite bus.

The same would apply for the MV feeder as they may very well put in a tie-lie between stations that is big wire. Unless you are away from the station on a radial tap that has little chance of being upgraded, then you might as well assume zero feeder impedance. Assuming feeder impedance is a big assumption as I just upgraded a line through a residential section to big wire. You would almost have to be on a road to nowhere but only the POCO planners would know for sure so it would be best to assume a stiff feeder as well.

The MV voltage is also a non-argument as they may very well be upped also. 2 kV began to be phased out long ago and has about gone. 4 kV is being phased out now and will eventually be history. Not only for carrying more load but also because the higher voltages are safer for relaying reasons.

The best MV argument is not going to be the substation size or primary voltage but rather the distance to the sub.

For kicks, let's look at some numbers:

A small substation might have a 2-5 kA fault level on the low side but an upgrade would probably be more in the 10 kA range. Using the 300 kVA pad-mount with a 3.47 %Z and 2.96 X/R, we can look at the available source fault at the 208 volt level:
At 2,000A IF_280V = 20.0kA
At 5,000A IF_280V = 22.2kA
At 7,000A IF_280V = 22.7kA
At 10,000A IF_280V = 23.1kA
At 15,000A IF_280V = 23.4kA
At 20,000A IF_280V = 23.5kA
At infinite source IF_280V = 24.0kA

Use #336 ACSR and an infinite source substation:
At 0.25 miles IF_280V = 23.8kA
At 0.50 miles IF_280V = 23.5kA
At 0.75 miles IF_280V = 23.3kA
At 1.00 miles IF_280V = 23.1kA
At 1.50 miles IF_280V = 22.7kA
At 2.00 miles IF_280V = 22.3kA

Use #336 ACSR with a #1/0 ACSR tap and an infinite source substation:
At 0.25 #336 + 0.25 miles #1/0 IF_280V = 23.4kA
At 0.25 #336 + 0.50 miles #1/0 IF_280V = 23.1kA
At 0.25 #336 + 0.75 miles #1/0 IF_280V = 22.8kA
At 0.25 #336 + 1.00 miles #1/0 IF_280V = 22.5kA
At 0.25 #336 + 1.50 miles #1/0 IF_280V = 21.9kA
At 0.25 #336 + 2.00 miles #1/0 IF_280V = 21.3kA

 

[bob]

Good point. however, some of those combination might allow the OP to get below 22ka.

 

[Phil Corso]

PGonski...

How about a section of commercially available busduct?

It will support the newer SC-duty requirements, including the mechanical forces due to any 1st -cycle peak asymmetry! And its ends can be fitted with approprate connectors to interface with original terminations!

Regards, Phil

 

[mivey]

Good point. however, some of those combination might allow the OP to get below 22ka.

That is true but I would concentrate on the site configuration first. If you have to then you could start arguing the substation location relative to the site. It would have to be a really compelling argument to bring in the voltage and substation size.

 

[petersonra]

just out of curiosity, why doesn't the motor contribution matter any if all the breakers involved are rated the same AIC?

you still get a presumably higher SCC if you have a fault than what can be supplied from the source.

 

[jim dungar]

just out of curiosity, why doesn't the motor contribution matter any if all the breakers involved are rated the same AIC?

you still get a presumably higher SCC if you have a fault than what can be supplied from the source.

That is true.
The X/R of the source, versus that used in the device testing, may also impact the breaker AIC rating.

 

[don_resqcapt19]

just out of curiosity, why doesn't the motor contribution matter any if all the breakers involved are rated the same AIC?

you still get a presumably higher SCC if you have a fault than what can be supplied from the source.

You still include the motor contribution for fully rated systems when doing the fault current calculations.

240.86(C) limits the use of series rated systems where there are motor loads.

 

[topgone]

You still include the motor contribution for fully rated systems when doing the fault current calculations.

240.86(C) limits the use of series rated systems where there are motor loads.

That's how I see 240.86(C) too.

But the conjuction/end-word of 240.86(C)(1) is an "and", that 240.86(C)(2) -->"the sum of the full-load currents exceed 1 percent of the interrupting rating of the lower-rated circuit breaker" becomes misleading! 240.86(C)(1) says "no motor loads" and the next paragraph (240.86(C)(2) sets a limit on motor load at the load side of the higher-rated breaker or the line side of the lower-rated breaker! I believe the CMP allows a little motor load, just under 1% of the IC rating of the lower-rated (protected) breaker in a series-rated setup.

 

[don_resqcapt19]

That's how I see 240.86(C) too.

But the conjuction/end-word of 240.86(C)(1) is an "and", that 240.86(C)(2) -->"the sum of the full-load currents exceed 1 percent of the interrupting rating of the lower-rated circuit breaker" becomes misleading! 240.86(C)(1) says "no motor loads" and the next paragraph (240.86(C)(2) sets a limit on motor load at the load side of the higher-rated breaker or the line side of the lower-rated breaker! I believe the CMP allows a little motor load, just under 1% of the IC rating of the lower-rated (protected) breaker in a series-rated setup.

(C)(1) says no motor loads between the higher and lower rated breaker and (C)(2) limits the motor loads on the load side of the lower rated breaker.

Other than a tap, is there any way to have motor loads between the higher and lower rated breakers?

 

[topgone]

(C)(1) says no motor loads between the higher and lower rated breaker and (C)(2) limits the motor loads on the load side of the lower rated breaker.

Other than a tap, is there any way to have motor loads between the higher and lower rated breakers?

Thanks a lot for the heads up. I was looking at that mains-to-feeder breaker junction alone, not thinking it is the "motor loads at the load side of the lower-rated breaker" that 240.86(C)(2) meant.

Making taps unto main breakers is the usual thing. It's just that people forget that the breakers in parallel with their tap get their IC ratings topped due to additional motor fault contributions should their additional tap feeds a motor/s.