Impedance value of .....

[RonShap]

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.

 

[jghrist]

The resistance of connectors and breaker contacts will be negligible compared to the rest of the system impedances.

 

[RonShap]

I appreciate you comment and almost whole heartedly agree. I am looking to save some expensive equipment, and wish to prove some theoretical assumptions.
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.

 

[jghrist]

Typical MV breaker contact resistance is around 100 micro-ohms. Connectors are required by standards to operate no hotter than the maximum conductor for which they are designed, so I would not figure that their resistance would be more than an equivalent length of conductor. Say for 4/0 AWG copper conductor, about 60 micro-ohms per foot. If you have 12000A fault current at 4.16 kV, the total impedance is 200,000 micro-ohms. I'd say you would need a lot of 100 micro-ohm contacts or connectors to make a significant change in this. Also consider that the contact or connector impedance is resistive and will not add arithmetically to the system impedance which is mostly inductive.

 

[RonShap]

jghrist,
Not as dramatic as I had hoped.
Thanks

 

[peterb]

In either case, the subject impedances are essentially resistive and would have potentially minor impact on the system impedance, which is primarily inductive. This is a problem that many face, where electrical systems have been added to over the years and have outgrown their rating.
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.

 

[DanDel]

If your study calculations are anywhere near the ratings of the switchgear(within 10%), the acceptability of the equipment should be called into question. The basis of a Short Circuit Study is to calculate the 'worst-case' fault, and one accepted method is to disregard the impedances of switchgear and switchboard buses. When you start looking for extra impedances to support keeping existing switchgear, you may not be able to comfortably assume that there is no danger to the equipment, or the personnel that are trusting you with their lives who have to operate it.

 

[RonShap]

Points taken. Generally, I believe that the calculation need to be taken in the bigger picture and evaluated, not just blindly comparing numbers. When evaluating an existing installation I geneally will permit the calcs to go as high as the value of the equipment plus 1% or so. The calcs do not include impedances that we know are in there, such as poor but acceptable terminations/splices.
When designing new equipment, I specify equipment that exceed the calculated values by 20% or so.

 

[jbartos]

Suggestion: NFPA 70-1999 NEC lists impedances of conductors in table 9, three conductors in conduits at 75C 167F

 

[jbartos]

Suggestion: Contact the equipment manufacturers: e.g. T&B, Burndy, etc. for lug resistances, and
GE, Siemens, etc. for circuit breaker resistances. E.g. 3000A/4000A circuit breaker has a contact resistance about 18micro-ohms.

 

[peterb]

RonShap -
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.

 

[PowerSim]

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.
May be this will help...

 

[peterb]

PowerSim -
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.

 

[cuky2000]

RonShap,

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.

 

[RonShap]

cuky2000,
The intent of my question was to calculate in the effect of other impedances that have not been included in the calculation.

 

[jbartos]

Suggestion to PowerSim (Electrical) Jun 21, 2003 marked ///\\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.\\

 

[Bung]

The variation in system resistances betwen a cold winter's night and a hot summer's day will more than swamp any variations due to contact resistances!


Bung
Life is non-linear...