Series Rating

[Gene Kelly]

I Have A Note On A Set Of Plans That Sayes The System Is To Be Series Rated. I Have Mcb That Is 65kaic And Feeder Breakers At 42kaic And Then Panalboards With Main Curcuit Breakers At At 18kaic And Finally 10kaic Branch Breakers. Is This Series Rating? Or Do I Have To Maintain The Same Kaic Rating Throuout The System?:

 

[Jraef]

Series ratings are a way of doing what you describe, but difficult to do it on your own. Either the equipment has been series rated by the manufacturer or it has not. It technically can be done in the field but the let-through of each level must be coordinated with the withstand of the next one down stream, etc. etc. Usually something done by a PE.

 

[Dave_PE]

It sounds like the engineer designed the system to be fully rated after calculating the available fault current through out the system. I would ask the engineer if the intention is to allow for a series rated system or a fully rated system before you bid the project.

To answer your question, the system would be "Series Rated at 65K" and the downstream circuit breakers selected by the manufacturer could be rated something less than what the plans say and still be series rated with the main.

 

[jim dungar]

I Have A Note On A Set Of Plans That Sayes The System Is To Be Series Rated. I Have Mcb That Is 65kaic And Feeder Breakers At 42kaic And Then Panalboards With Main Curcuit Breakers At At 18kaic And Finally 10kaic Branch Breakers. Is This Series Rating? Or Do I Have To Maintain The Same Kaic Rating Throuout The System?:

There are very very very few series ratings that involve three breakers in series. I have never heard of 4 breakers in series.

 

[Gustav]

There are very very very few series ratings that involve three breakers in series. I have never heard of 4 breakers in series.

Jim,
this is very similar to my topic "3-level series rating"
And believe me, there will come up more and more of this topics.
I see the reason in the new UL508A - Marking of the SCCR value.
Why - because up to now every one did it by some sort of calculation, i.e. looking of the let through of the upstream device and compare this let through with the capability of the downstream device. If the let through of the upstream device is not higher than the capability of the downstream we would call it okay.
We are also aware that this may not be a well coordinated system in terms of selectivity.
But it is the way as many of the IEC Maschine building companies are working
In the IEC we even do also have specific breaker data, which is called I((sub))cm, this is a peak value specified by the manufacturer of the breaker, able to be compared with the peak let through of upstream devices.
It is called "making capacity" and tells us at which level of peak value the contacts are starting to drive apart.

AND NOW:
UL508A trells us that we have to have an SCCR value but gives us only limited chances to get to higher values.

Gustav

 

[jim dungar]

Why - because up to now every one did it by some sort of calculation, i.e. looking of the let through of the upstream device and compare this let through with the capability of the downstream device. If the let through of the upstream device is not higher than the capability of the downstream we would call it okay.

I know of no manufacturer of protective devices that has condoned using upstream let through values to protect downstream molded case/miniature circuit breakers, since the early 1980s.

The continued misapplication of let through values with devices that exhibit "dynamic impedance" is one reason that NEC 240.86(A) requires an engineer be "engaged primarily" in electical power systems. Also, I see nothing in 240.86(A) that leads me to believe that engineered, as opposed to tested, series ratings are aceptable for installations of new equipment.

 

[Jraef]

Actually, in re-reading the original post it isn't clear that this is totally a series rating issue. It could be that the engineer had also calculated the available fault current at each location as it dropped due to wire lengths, transformer impedances etc. in each of the circuits. We just don't know without seeing the SLD.

 

[Gustav]

It could be that the engineer had also calculated the available fault current at each location as it dropped due to wire lengths, transformer impedances etc. in each of the circuits.

Which engineer is allowed to do such calculations?
An engineer by the user or is it necessary to have an licensed professionally engineer?

Sorry for asking such question, because here in Europe the user himself is responsible for having the correct values and if he thinks that his own engineer or technicians are good enough to calculate this - well fine - it is his responsibility.
For the US, I am not quite sure whether this would be acceptable too.

Thanks
Gustav

 

[steve66]

It would probably vary depending on where you are in the US. Where I am, either an electrician or engineer could do the calculations. Although, I have always done them for any projects I design.

Steve

 

[winnie]

This is a topic that I have been learning about, but only through discussions (here and at ECN), as well as a bit of reading on the internet. So I am by no means properly educated on the subject.

My understanding is that the use of calculations for available short circuit current to determine the required KAIC is totally acceptable. Calculations of transformer secondary fault current, or the resistance of feeders limiting fault current, are considered acceptable.

My understanding is that the use of 'let through' or 'up-over-down' is entirely acceptable to determine the fault currents seen by devices which don't show dynamic impedance, eg. for conductors, bus bar bracing, switches that are not OCPD, etc.

My understanding is that calculations are not acceptable when you have downstream devices which experience 'dynamic impedance'. An OCPD is a device that intentionally has dynamic impedance: low impedance during normal conditions, but automatically increasing impedance if a fault is detected. For these devices, a series rating is required.

-Jon