Impedance value of .....
Impedance value of .....
(OP)
In performing a short circuit calculation for verification of the rating of some installed switchgear, the result reflected equipment with a AIC below the calculated available. I have included the typical impedance elements, but would like to include more, to somewhat validate the equipment so as not to require replacement.
Does anyone have a reference for impedances of mechanical or compression type conductor terminations and bolted bus connections? Also references for the impedance of ANSI power circuit breaker contacts. They are obviously not zero, and are based on the installers capabilities.
With the "non-typical" (although real life) impedances included in the calculation, I believe the equipment will be properly rated per my calcs.
Does anyone have a reference for impedances of mechanical or compression type conductor terminations and bolted bus connections? Also references for the impedance of ANSI power circuit breaker contacts. They are obviously not zero, and are based on the installers capabilities.
With the "non-typical" (although real life) impedances included in the calculation, I believe the equipment will be properly rated per my calcs.






RE: Impedance value of .....
RE: Impedance value of .....
The calculated result is close to the AIC rating of the equipment. I would normally explain this small amount away, and comfortably tell the client that it's ok to keep the existing equipment. In this case, my curiosity is a bit more interested in the proof.
RE: Impedance value of .....
RE: Impedance value of .....
Not as dramatic as I had hoped.
Thanks
RE: Impedance value of .....
Some suggested fixes, other than wholesale replacement -
- Examine the system configuration to determine whether the fault level can be reduced by e.g. operating with bus ties open - easiest and low/no-cost option
- Consider the installation of current limiting reactors at selected locations - may cause voltage drop problems
- Consider the installation of current limiters at selected locations - avoids voltage drop problems, at the expense of added complexity
Note that while it may be theoretically possible to apply switchgear based directly on the results of short circuit studies, I would normally expect to use a safety factor on the results.
RE: Impedance value of .....
RE: Impedance value of .....
When designing new equipment, I specify equipment that exceed the calculated values by 20% or so.
RE: Impedance value of .....
RE: Impedance value of .....
GE, Siemens, etc. for circuit breaker resistances. E.g. 3000A/4000A circuit breaker has a contact resistance about 18micro-ohms.
RE: Impedance value of .....
Please take into account DanDel & my comments re safety factors. "Poor but acceptable terminations/splices" are NOT significant impedances in the overall scheme of things (per Jghrist's post above) and will certainly not constitute an adequate safety margin.
Bottom line - applying switchgear at 100% of the calculated interrupting rating is not a safe or acceptable practice, as I think that your client's insurance provider may agree.
RE: Impedance value of .....
May be this will help...
RE: Impedance value of .....
Hopefully, the LV system switchgear bus is braced to withstand the maximum asymmetrical short circuit current forces. Also, close and latch capability of the breaker used for switching is sufficient to allow closing on to a fault. Interruption isn't the only consideration ...
This stuff isn't just theory - the results of inadequate design in this regard are never pretty, and Murphy's law is also very real.
RE: Impedance value of .....
Still I am not able to understand what are you trying to accomplish insisting in including the protective device impedance in a short circuit calculation.
The SC calculation provides a reasonable accuracy but not an exact result since the process has several acceptable simplifications and approximations.
Trying to add the effect of the impedance of protective devices is so small that do not made any sense to add it in the calculation. If your client does not believe that, he could be free to search in the technical literature or get any qualified third opinion in this matter.
RE: Impedance value of .....
The intent of my question was to calculate in the effect of other impedances that have not been included in the calculation.
RE: Impedance value of .....
I have seen at some substations that Transformer LV side Protections Trip HV side Circuit Breaker as LV side breaker is not capable to Interrupt current at a bolted short circuit on LV Busbar. While LV side CB is only used for Manual Switching, Auto Transfer and Isolation purposes only.
///The emergency power distribution system for fire protection may be designed such that if there are overloads or overcurrents, no trips will happen. This is based on the Fire Protection intent to have equipment, e.g. fire pumps, working to their utmost.\\\
RE: Impedance value of .....
Bung
Life is non-linear...