Bolted Fault Current

[mikecj]

Everyone,

Please be gentle, I'm new to this. I'm trying to do the incident energy and arc flash calculations found in NFPA 70E.

One term that keeps creeping up is "Bolted Fault Current"
1. What exactly is the "Bolted Fault Current"?
2. How do I determine it? In most of the examples, it is given not calculated.

Any help will be appreciated.

Thanks,

MikeCJ

 

[bkludecke]

I'm no engineer but as I understand it 'bolted fault' refers to a dead short in two conductors that are joined together (bolted) so well that they can't arc or burn away. Therefore the OCPD must be relied on to clear the fault without destroying the equipment.

 

[bphgravity]

Many faults start out as intermittent, arcing faults with relatively low magnitude currents. A bolted fault on the other hand is one that creates tremendous amounts of current and will sustain this current until the circuit is opened by some means.

A bolted fault current is the maximum available fault current at some point in the electrical system. For example, the bolted fault current of a transformer is the maximum output power of the transformer divided by the transformer impedance and transformer voltage rating.

This value can be used to determine what AIC and SCCR ratings equipment and materials need in order to safely carry and interrupt these faults currents under the worse of conditions.

 

[kingpb]

The reason bolted fault current is used is because any impedance in the faulted current path will reduce the amount of current that will flow. Therefore, the bolted fault current is considered to be the maximum fault current you would have at that point in the system. Although the maximum current is needed, so is minimum current.

When doing these calculations keep in mind that anything with impedance reduces the current flow, including cable. Results may be scued if some impedances are left out of the model, ultimately leading to longer tripping times, which could make your arc flash values worse.

 

[steve66]

Your best bet is to buy commercial software to calculate arc flash. It will be easier, and you will get more accurate results. Commercial software will usually calcuate the bolted fault current for you.

If you want to calculate the bolted fault current yourself, look under "3 phase short circuit calculations" of this link:

http://www.bussmann.com/library/docs/Spd.pdf

 

[ramsy]

I thought "Bolted fault" was (line-to-line) vs "Ground fault" (line-to-ground), since impedance is usually known with branch circuits, leaving differences in running voltage to change the ASCC.

My understanding was conductor (impedance) size & length, along with circuit voltage was the last factors to limit ASCC = (Er / Z). Can't we apply this simple rule up stream, as long as voltage, and feeder, svc., or distrubution-cable impedance is known?

 

[jojo]

there are many types of fault current: single line-to-ground fault, line-to-line fault, double line-to-ground fault, and the three phase fault also known as "three phase bolted fault." the three phase fault calculations is used in selection of protective devices because this type of fault has the largest value of short circuit in computations; momentary rating (first 1-1.5cycle) and interrupting rating (1.5-3cycles). the said three phase fault calculations is used in industrial and commercial power systems...sources of short circuits are: 1. generators, 2.synchronous motors, 3.induction motors, and 4. utility supply (transformers).

 

[zog]

Also keep in mind that while "bolted fault current" is your highest magnitude fault, it will not in many cases be your worst case senario for your arc flash analysis.

Trust me, you are alot better off finding a consultant to do your arc flash analysis. The time you will spend, and the money you will need to invest in the software will exceed the cost of hiring a pro to do it, unless your company is about the size of General Motors.