Posted By: Redsy
Calling Mr. Neeser - 04/17/02 11:43 AM
Could you please discuss "prospective short circuit current" vs. "available short circuit current" vs. "instantaneous peak current".
Is the multiplier 2.3 a constant on all systems?
Posted By: Dan Neeser
Re: Calling Mr. Neeser - 04/18/02 02:22 AM
Good to hear from you again.
In answer to your question. Prospective short-circuit current and available fault current are basically the same thing. Both reference a bolted fault condition (worst case).
However, when talking about the current-limitation of fuses, the term "prospective short-circuit current" is used when determining the let-through current of the fuse (peak and rms). The word prospective is used because the current never reaches that value since the fuse chops the current before it ever reaches that value.
The let-through charts are based upon a power factor of 15%, which relates to an prospective instantaneous peak current (during the first half cycle) of 2.3 times the prospective rms current, as suggested. This value is used because fuses are tested at a power factor of 15% (provided it is tested over 10kA...at 10kA the power factor is 50% which relates to a lower peak current). This is done to reflect real-life conditions during a short-circuit.
This information is useful becase the reduction of rms current (squared) relates to the reduction of thermal stresses and the reduction of peak current (squared) relates to the reduction of mechanical stresses. Thus if a device reduces the rms current by 10, the reduction of thermal energy is 100. If the fuse reduces the peak current by 10, the reduction of the mechanical stresses is 100.
If a non-current-limiting device was used the entire rms current and peak current could be let-through to the downstream equipment.
See the Bussmann SPD for more information under current-limitation and the let-through charts. Also, see the EDP-1 (especially the last page for peak current versus power factor) for more detailed information. Both documents can be found on our website under application info, pubs and applications.
Just as an fyi, if my memory is correct, circuit breaker are tested at 20% to 25% power factor based upon ampere rating above 10kA and at 50% below 10kA. Again, I would have to check the UL 489 standard to verify this though.