wewire2;The Bussman Literature is very helpful, and will assist you greatly in covering the Basics per SCA Rocket Sciences.
(SCA = Short Circuit Amperes)
Even though Bussman is quite biased towards Fuses, the Documents contain lots of good data - including time-Trip Current Curves, and of course, SCA Calcs.
The SCA Document describes three methods of Fault Level Calcs:
Ohmic, Per-Unit, and Point-To-Point.
Point-To-Point method is least accurate, but is simple to use, and accepted by most DBS (Building Departments).
Abridged description for any Fault calcs...
- Obtain Fault Quote from Utility Service Engineers. This will give you the Base level of SCA available at the Service Equipment - or, more precisely, at the Line Side of the Service Disconnect.
(more on this later). - Motor Contributions to Faults... Add all Motors' LRA (Locked Rotor Amperes) to the available SCA at the Service Equipment.
This adjusted level is what you use as the "Total Available SCA" on the "Load Side" of the Service Disconnect.
Designate the Switchboard directly connected to the Service Section as the "Main Switchboard", or "MSB". - Using the Total SCA Available (per line #2 above), Fault Levels "Downstream" from the Main Switchboard are figured per run types, lengths and Conductor Reactance (X) / Resistance (R).
- For Transformers (Separately Derived Systems), the SCA available at the Primary is figured; then that value is used to compile the SCA available at the Secondary.
Afterward, the new SCA (available at the Secondary) is used to figure "Downstream" Fault Levels, similar to that described in #3 above.
Notes per Line #1:
a: Optional SCA from Utility - if no quote may be obtained, obtain Transformer Capacity + %Z (Impedance) per Nameplate Data, determine SCA at Secondary Terminals, apply this to the Service Feeders' data (Conductor type, length, X/R).
Best to request Fault Quote from the Utility Service Engineers / Planning Staff.
b: Motor LRA Contributions may be added in where their Circuits derive (at Panelboards, Transformer Secondary Feeders, etc.), or at the MSB.
c: The AIC (Interrupting Capacity) of all devices in the Service Section + MSB needs to be no less than the available SCA.
For example, if the available SCA is 10,100 Amps, the minimum AIC for the Devices will be 14,000 Amps.
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When compiling a Series Rated System:
All Devices will be listed per the Manufacture's Series Rating Compatibility listings.
Combinations will be given per "Upstream" device type, protecting "Downstream" device types, with a maximum Interrupting Rating (AIR) -vs- System Voltage and Phases.
This allows a Downstream Device with an AIC rating lower than what is available, to be protected by an Upstream device with an AIC rating above the available SCA at the Upstream device.
Fully Rated Systems use devices with AIC / AIR ratings, which are greater than or equal to the available SCA at the device.
Good luck, and feel free to ask any questions that surface!
Scott
