Series Rating

Status
Not open for further replies.
N

nhee

Member
If an MCC has 22KAIC bracing, but the available fault current at the line terminals is higher, can a current limiting fuse / breaker be used to protect the bussing?

In the case of overcurrent protection devices, I know that the upstream/downstream devices have to be listed as a series system. Is there a similar listing required when protecting downstream busbars?
 
Re: Series Rating

I would consult the manufacturer. Typically they have several tested combinations, which can be found in the booklet that is supposed to be shipped with the gear.
 
Re: Series Rating

MCC is 1970s vintage. Original data sheets indicate busbar bracing is 22KAIC.
 
Re: Series Rating

To answer your first question, NO.

All of the equipment must be rated for the available fault current at the terminals of the equipment, including the panelboard and the panelboard busing. It is only the downstream overcurrent devices that are series rated that may be derated.

Current limiting devices may be chosen to protect equipment but not other overcurrent devices. :D
 
Re: Series Rating

Charlie,

If a Current Limiting device is installed on the line side (separate, not integral to MCC), doesn't that then lower the fault current at the line terminals of the MCC, protecting the bussing?

I recognize that I'd still have issues with any 22KAIC breakers that are within the starters, but for the moment I'm just questioning the busbar protection.
 
Re: Series Rating

Just out of curiosity, what options are out there in order to protect your equipment bus as well as downstream overcurrent devices?
 
Re: Series Rating

Nhee, you are correct. However, the current limiting device will be in series with your downstream overcurrent protection and that is where you will have trouble. Under fault conditions, the current limiting device will be racing with the downstream device and the dynamic impedance will slow down the current limiting device. This will permit additional fault current flow through to the MCC bus and could have disastrous results. :)
 
Re: Series Rating

Jkim, your choices are to either use series rated and tested devices or to replace the equipment with equipment that is fully rated. :D
 
Re: Series Rating

Charlie and Nhee: I wonder if Nhee were to talk with the AHJ and discuss whether or not the allowance 240.86 in the 2005 could be used under special permission and engineering supervision (if the designer so desires). Would this apply, Charlie?
 
Re: Series Rating

Oh, I agree with both of you, but isn't it situations like this got it into the code? I mean, if the designer can't get the AIC rating, he/she has two choices...ignore it ( :( ), or use the allowance of the 2005.
 
Re: Series Rating

Ryan, you are correct. Take a hard look at the requirements of 240.86 and the hoops that must be jumped through to use the new provisions. There are very few places where this can be used. Either the old air frame circuit breakers have to have been used so that they will remain passive for the 1st ? cycle or latching type circuit breakers must be used to ensure passivity for the first ? cycle. Then, an engineer with a PE license must design the installation. :D
 
Re: Series Rating

Ryan, Charlie, thanks for your comments.

I'm in contact w/manufacturer,customer, AHJ to see what options we have.
 
Status
Not open for further replies.
Top