Heart Failure...

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I have been asking for fault currents from the POCO for several years, more so since the latest code. Rarely do I find anything over 10K. About had heart failure when the POCO came back with 24,000 at the transformer on a service change we did a few weeks ago.

One service is 3 ph 4 wire 120/240v with a high leg, closed delta. The other is single phase derived from the same bank. Less expensive and easier to run the 2 - 200 amp services this way vs one 400. A spare raceway had been buried by yours truly several years ago, just in case.

Using MHs fault current spreadsheet, the single phase drops wll below the 10k when details of the secondary lateral are included.

Our choices do not include that for a high leg delta? Solution?
 
May or may not be practicable:
Replacing the overhead lines by power cable and having several loops of the cable at both ends may reduce the fault level at the service equipment within acceptable limit.
 
I think T.M. is on the right track, find out how much conductor length you need to reduce the available fault current to an acceptable level and see if there is a reasonable way to install that much length, or maybe aluminum instead of copper if that works, otherwise you probably need equipment with higher ratings.
 
Not sure what you are installing on the service end, but 22k gear is readily accessible and not expensive.
(especially series-rated) Most any run is going to get you to that point.
 
ptonsparky said:
My calcs are L-L 8971 and 3557 L-N on the single phase. I was wondering how to calculate the fault current when I have a 3 phase 240v High leg Delta. That option is not available with MHs spreadsheet.
Click to expand...

I think it will be similiar to what you would get with wye system.

Remember most of the time when doing these calculations you are assuming infinite supply capabilities, when in reality you must also know the amount of power that can be supplied by the primary source also. If you have motors they will contribute to the available fault current, magnetic vs non magnetic raceways make some difference too.
 
ptonsparky said:
I have been asking for fault currents from the POCO for several years, more so since the latest code. Rarely do I find anything over 10K. About had heart failure when the POCO came back with 24,000 at the transformer on a service change we did a few weeks ago.

One service is 3 ph 4 wire 120/240v with a high leg, closed delta. The other is single phase derived from the same bank. Less expensive and easier to run the 2 - 200 amp services this way vs one 400. A spare raceway had been buried by yours truly several years ago, just in case.

Using MHs fault current spreadsheet, the single phase drops wll below the 10k when details of the secondary lateral are included.

Our choices do not include that for a high leg delta? Solution?
Click to expand...

Worst case with series rating, use a fused disconnect for your service disconnecting means and feed a main lug only panel that has 10K branch breakers series rated with the fuses.
 
jim dungar said:
Then simply post the values the utility gave you. Why do you need to do anything 'manipulations'.
Click to expand...

With all due respect and taking into account that I may be missing the point (fairly common)..
If calculations allow him to install gear with a rating less than the POCO 24k available, and he posts the higher number it seems that would be waving a red flag at anyone inspecting or adding to the system at a later date.
 
jim dungar said:
Then simply post the values the utility gave you. Why do you need to do anything 'manipulations'.
Click to expand...

I believe POCO gave him available fault current at transformer output, he still needs to find what it is at the other end of the service lateral.
 
augie47 said:
With all due respect and taking into account that I may be missing the point (fairly common)..
If calculations allow him to install gear with a rating less than the POCO 24k available, and he posts the higher number it seems that would be waving a red flag at anyone inspecting or adding to the system at a later date.
Click to expand...

I absolutely agree, when you are trying to justify the use of equipment with a lower AIC, then you would want to do the calculations and post the lower results.

The worst case would be to ignore the center-tap and just treat this like any other delta circuit.
 
kwired said:
I believe POCO gave him available fault current at transformer output, he still needs to find what it is at the other end of the service lateral.
Click to expand...
You assumed one thing, I assumed something else.
The OP needs to provide more information.
 
jim dungar said:
You assumed one thing, I assumed something else.
The OP needs to provide more information.
Click to expand...
I did not assume that he said the POCO told him 24,000 at the transformer, everything else he is after is finding values at some point beyond the transformer.

ptonsparky said:
I have been asking for fault currents from the POCO for several years, more so since the latest code. Rarely do I find anything over 10K. About had heart failure when the POCO came back with 24,000 at the transformer on a service change we did a few weeks ago.

One service is 3 ph 4 wire 120/240v with a high leg, closed delta. The other is single phase derived from the same bank. Less expensive and easier to run the 2 - 200 amp services this way vs one 400. A spare raceway had been buried by yours truly several years ago, just in case.

Using MHs fault current spreadsheet, the single phase drops wll below the 10k when details of the secondary lateral are included.

Our choices do not include that for a high leg delta? Solution?
Click to expand...
 
kwired said:
I did not assume that he said the POCO told him 24,000 at the transformer, everything else he is after is finding values at some point beyond the transformer.
Click to expand...

You, and Augie, both assumed the OP was trying to determine a fault current that would allow a lower AIC at the end of the service lateral, i did not make that assumption.

Hopefully the utility provided more information than a single 3-phase value of 24kA.
Generally, it is a mistake to treat a center-tapped delta like it was a wye connection instead.
 
ptonsparky said:
I have been asking for fault currents from the POCO for several years, more so since the latest code. Rarely do I find anything over 10K. About had heart failure when the POCO came back with 24,000 at the transformer on a service change we did a few weeks ago.
Click to expand...

FWIW, I had to do an ark flash calculation for our test area earlier this year to confirm what level of PPE would be required.
The fault level at the transformer in our yard is about 27kA. However, there is a cable run of 100 metres (about 110 yards) to the switchboard in our test bay. At that point, the fault level is below that 10kA figure.

I had contemplated the need for puting in line reactors before I'd taken the cable run into account.
 
jim dungar said:
You, and Augie, both assumed the OP was trying to determine a fault current that would allow a lower AIC at the end of the service lateral, i did not make that assumption.

Hopefully the utility provided more information than a single 3-phase value of 24kA.
Generally, it is a mistake to treat a center-tapped delta like it was a wye connection instead.
Click to expand...
The POCO gave him the fault current at the transformer, that is not changing no matter what the OP connects to it. The other end of the conductors that are connected to it will not have the same fault current available unless he can achieve zero impedance in those conductors, so assuming he has to run at least 10 feet of conductor or so there should be some significant difference in available fault current at the load end of said conductors.

I do agree the POCO maybe should have given him more than just 24kA. They should have probably given him 3 phase fault (which is likely the 24kA), a line to line single phase current, and a line to ground fault current. And in the case with a high leg probably a high leg to ground fault current.
 
ptonsparky said:
I have been asking for fault currents from the POCO for several years, more so since the latest code. Rarely do I find anything over 10K. About had heart failure when the POCO came back with 24,000 at the transformer on a service change we did a few weeks ago.

One service is 3 ph 4 wire 120/240v with a high leg, closed delta. The other is single phase derived from the same bank. Less expensive and easier to run the 2 - 200 amp services this way vs one 400. A spare raceway had been buried by yours truly several years ago, just in case.
Click to expand...
Would you provide us with the details of the utility installation?
 
kwired said:
I do agree the POCO maybe should have given him more than just 24kA. They should have probably given him 3 phase fault (which is likely the 24kA), a line to line single phase current, and a line to ground fault current. And in the case with a high leg probably a high leg to ground fault current.
Click to expand...

It is extremely unlikely that the utility calculated the secondary faults for this specific installation. in most cases the utility uses lookup tables, for some utilities the value is a 'design level that will never be seen in real life (this is great for selecting the ratings of service equipment). There are some utliites that will provide L-G and L-L values for MV customers, although soemtimes we get them for 'secondary service' customers.

For equipment selection, based on SCA, there is no need to treat a center-tapped system any different than other delta. The big issue is open versus closed delta.
 
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