Hi everyone!


Currently I'm having a problem. I am doing an arc-flash analysis and I am having this problem:

The main circuit breaker of a bus is rated above the short circuit current of its bus, so it can clear the fault, but the derived circuit breakers are rated under the short circuit current an so they can't clear the fault on their terminals.

So, my question is, can I calculate an arc flash hazard in this bus?

I want to apologize because of my english.


Thank you.


Juan.

Is it possible that the downstream breakers are series rated with the immediate breaker upstream?

Hi Barry

Thanks for your answers.


Attached I send the SL diagram.

The upstream breaker is a 65 kA breaker, meanwhile the downstream are 25 kA breakers.

We need to know the exact upstream and downstream breaker manufacturer and model numbers in order to determine if is a tested series combination between the two. I do not know what Bielomatik is or what some of your other labels are.

If Bielomatik is a manufacturer I am unfamiliar with them and will be unable to help. Now, if the manufacturer is Siemens, Square D/Group Schneider, GE or Eaton/Cutler Hammer/Westinghouse we can look those up easily enough.

Hi,


The upstream protectios is a SCH-Masterpact NW Micrologic 2 A, and the downstream protections are Square D type JD. But my question is if theres is possible to perform an arc flash analysis when, despite the manufacturer, the downstream is no capable of clear the fault, but the upstream can clear the fault.


Thanks

Looks like you're out of luck for a series rating : https://www.schneider-electric.us/en/faqs/FA124584/

I looked at the charts and found nothing for the NW Masterpact and then found that page that says "no".

I don't know what software you're using but in SKM you can choose to include the upstream device in the arc flash analysis.

The arcing fault current is less than the short circuit rating so it may turn out that the combination would work. Otherwise just model the downstream breakers as if they aren't there and then the upstream breaker handles both short circuit and arc flash.

Long term either you need to consider upgrading the breakers or else look at adding current limiting to get the higher rating either by adding line reactors to the line side somewhere or current limiting fuses somewhere in the circuit (usually downstream).

At extremely high currents there is the TCL (triggered current limiter) option but let's just say that this sounds good on paper but not so great in practice. A chemical plant customer of mine has 3 cogen turbine generators and sits right across the street from a power plant. At some point in their past they went to closing both mains and the ties on their two main substations forming a ring due to the lowered impedance which allowed them to do what they are doing...most of the time they are a net power producer harvesting energy off of what is apparently a fairly exothermic reaction. The problem is that on their main buses they are pushing something like 40-50 kA short circuit but the breakers they have are rated in the 20-25 kA range (typical for draw out 15-25 kV medium voltage breakers). With two 8000 HP compressors adding line reactance is not practical due to the reduced voltage conditions it would create. So that left them with TCL's as the best option. They put two of them in to split the two buses between the main subs. The remaining issue is that every time they fire the replacement cartridge (it's like a really expensive fuse) is $8,000 each.

What is the transformer impedance? Changing the transformer to one that is higher impedance might get you there too. Or artificially increasing the transformer secondary conductor length. Either of those may be no-go's for a variety of reasons but may also be possible. Or use Paul's first suggestion and just replace the two 25 KAIC breaker with 42's or 65's. That may be the cheapest and easiest option.

I just quickly looked up the JD breakers and it looks like they only went up to 250A and 65 and 100K JJ and JL versions can be found used under $1000.

Increasing transformer impedance lowers your arcing and short circuit current which is a plus but it will also increase your trip time if you are not instantaneously tripping so your incident energy will be going up which is the negative...really also true for any current limiting/reduction approach.