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 Post subject: Utility Data ClarificationPosted: Thu Jul 15, 2010 6:33 am
 Sparks Level

Joined: Tue May 18, 2010 7:03 am
Posts: 53
A contractor has hired a large equipment mfg. company to do a power study for his job and has forwarded to me the spreadsheet the company has asked him to fill out for utility data. I am a little confused about the info requested as opposed to what I get from the utility. For example, the form requests 3 ph fault current - max, min, actual. The utility provides me with 3phLLL and 3phGnd fault currents.

The form requests X/R Ratio of system - max, min, actual. The utility provides one value only for X/R.

The form requests %Z of transformer - max, min, nominal. The utility provides one value for the transformer.

Any explanation for what is being requested vs what is being given? The software being used only requires two fault currents, one 3 phase, one ph to gnd. It also only accepts one X/R for system, one %Z for transformer.

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 Post subject: Posted: Thu Jul 15, 2010 7:20 am
 Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 544
Location: Wisconsin
Utility fault values will absolutely change over time.
By comparing Low, Middle, and High values the study engineer will be able to determine which condition will generate the highest arc flash incident energy levels. Effectively the study engineer should perform a little 'sensitivity' analysis to improve the confidence level that a change in the utility value will not impact the study results.

Real information from one utility - which value do you think will be the worse case?:
The actual values represent the bolted short circuit values, which exist for the normal incoming lines during normal operating conditions as of 1/26/09.
Actual Short Circuit Current: 3ph: 3,900 Amps, X/R = 4.48 1 L-G: 2,550 Amps

The maximum design short circuit values represent a worst case maximum fault current which could exist for any future design or operational change.
Maximum Design:10,000 Amps, X/R = 20 (all fault types)

Not all utilities are this cooperative.

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