Available fault current

Status
Not open for further replies.
S

sparks1

Senior Member
Location
Massachusetts
On a 600 amp 3-phase service for example, what method is used to figure the available fault current? Will the power co. provide me this information to determine the correct size interrupt rating of the main breaker?
Refresh my memory...
 
Re: Available fault current

You need to look at the transfromer (assuming that the PoCo won't give you the info). Take the KVA diveded by the voltage divided by the impedance. For example, a three phase 208, 75KVA with 2.4 impedance would be:

75000/.024/ (208*1.732)
 
Re: Available fault current

Thanks Ryan,
In your example, if my math is right, I come up with 8677.3389 amps per leg?
10,000 AIC rating should be ok...

[ June 30, 2005, 01:29 PM: Message edited by: sparks1 ]
 
Re: Available fault current

Ryan is not saying that at all. Your math is correct; however, the transformer size and impedance is not correct for your installation. You have to determine what is there before you do the calculations.

Contact the electric utility first to see what their standard fault current is for the size of transformer that they set. We use the lowest impedance transformer of that type that we have on our system for the standard fault current. That way, if we replace a transformer with another of the same size, the customer will be able to handle the available fault current. :D
 
Re: Available fault current

Charlie,
I realize the AIC rating would be much higher for my installation.

Can a 200 amp single phase breaker for a home carry the available fault current? I know they are rated either 10,000 or 22,000 AIC and are typically fed from a 25kva or a 50kva transformer. I've never asked the poco about the available fault current for a service below 200 amps. At what size service in your opinion do we need to know how much available fault current there is?
 
Re: Available fault current

We post the available fault current at 14 kA for a 50 kVA 1? transformer with the following statement:

"The standard calculated available fault currents are given in amperes, RMS symmetrical, at the secondary bushings of the Company's transformer, assuming an infinite bus and a bolted fault. The intent of these values is to serve as a guide in the selection of the proper service and downstream equipment. These are of no value for the use in determining the proper personal protection equipment since the impedance, fuse size and type, and calculated primary fault current available at the primary bushings of the bank can not be known for any particular installation."

It is up to the user to decide how the information is used. :D
 
Re: Available fault current

sparks1,

Residential load centers are usually "series-rated" for a 22KA SCCR. This allows them to be installed with 10KA branch breakers.

Warning possible thread theft.

Charlie,

How can the customer comply with OSHA "mandated" (and NEC suggested) use of NFPA70E, if your utility refuses to provide actual fault current and protective device operating times?

The tables in NFPA70E (much less any engineered solution) require the knowledge of the actual values at the service entrance. Even if there is never going to be any live work performed on the service entrance equipment, it is not possible to calulate the actual fault current at other locations in the facility if the begining actual value is not given.

Aren't you creating a possibly more unsafe condition by not providing actual values or at least the possible maximumand minimum values and times?
 
Re: Available fault current

Jim
I can understand the utility's hedging on what answer to give when asked for the available fault current. What ever answer is given is likely to change. Sources of power can change from one location to another. The substation capacity can be increased. The feeder can be restrung from #2AL to #795 AL. The answer is dynamic at best. How do you give an answer that
will probably change over time?
 
Re: Available fault current

How do we select anything per NFPA 70E without an answer?

This is a real problem, low fault levels (below current limitation levels) very often, in fact usually, result in high levels of incident energy and therefore require more protection than "worst" case fault levels.

Why can't the utility provide actual (as of a specific date) as well as future (design) values? This way we can do two sets of calculations and pick the higher level of protection.
 
Re: Available fault current

Jim, it can't happen. The problem is that we are always changing the configuration of our lines. Also, just because the fault current goes up doesn't mean that the incident energy goes up since it is based on the amount of time the fault lasts. More fault current may increase the speed in which the overcurrent device operates. Furthermore, the primary fuses we use take a long time to open (we use T rated fuses) and they may never open on a fault. If the fault is in the main bus of the switchgear in front of the main overcurrent device, the switchgear may burn down before our fuse opens, how do you calculate the incident energy?

As a result of the dynamics of our system, we do not give you the exact figures because of the time it takes to calculate them and the fact that they may be out of date even before we give them to you. :D
 
Re: Available fault current

Originally posted by charlie:

As a result of the dynamics of our system, we do not give you the exact figures because of the time it takes to calculate them and the fact that they may be out of date even before we give them to you. :D [/QB]
Click to expand...
How do the breaker makers decide on 22KA for a residential service if they are working with imaginary numbers?
Should I hire an engineer for every job since the poco is reluctant give me any real figures...
 
Re: Available fault current

Simply hiring an engineer is not the answer, because they need the data from the utility to make their calculations.

As far as AIC ratings is required that the equipment be able to handle the maximum current given by the utility. So if they told you 22KA then you need equipment that is series-rated or fully-rated for 22KA.
 
Re: Available fault current

Jim, you need to take into consideration the way we give the information. We give it at the secondary bushings of the Company's transformer so the available fault current needs to be calculated from there. We are also assuming an infinite bus (can never happen) and a bolted fault (not likely to happen).

The infinite bus is where a bolted fault is on the secondary bushings of the transformer and the primary voltage doesn't sag under that load. With a bolted fault, no dynamic impedance is introduced into the circuit. Under those conditions, the only thing left is the impedance of the transformer itself.

By the Bob, you gave a great answer earlier. :D
 
Re: Available fault current

Jim,
The issue with the actual available fault currents for 70E calculations is one that I have posted about a number of times. As it stands now, I don't think that anyone can really select the correct PPE for any system that is supplied by a utility as their systems are so dynamic that the available current is always changing. Their lines are being switched and various forms of generation are coming off and online continuously. How can any utility be expected to provide an actual available fault current. As you said, if the current is lower than expected, the incident energy often increases requiring a higher level of PPE.
I suggested previously, partly in jest, that 70E should be revised as follows:
Delete all existing text.
Replace with: "thou shalt not work on energized systems"
Substantiation: It is not possible to get an actual available fault current for any system served by an utility and without a fault current PPE cannot be properly selected.
Don
 
Re: Available fault current

Don,

I almost agree with you. But it is not possible to never work on "energized" equipment, how do you test voltage to confirm that the equipment is no longer energized?
 
Re: Available fault current

Originally posted by jim dungar:
Don,

I almost agree with you. But it is not possible to never work on "energized" equipment, how do you test voltage to confirm that the equipment is no longer energized?
Click to expand...
Built in voltage indicators? Maybe some lights that will go out when you open the disconnect?
 
Re: Available fault current

Jim,
It appears to be a "catch 22". We can't work hot without the proper PPE. We can't select the proper PPE without the actual fault currents. We can't get the actual fault currents for any system that is supplied by an utility. We can't work on denergized equipment without checking for voltage. We can't check for voltage without the proper PPE. I don't see any possible why of complying with 70E and the OSHA rules that require such compliance.
Don
 
Re: Available fault current

We have 2 issues here:

1) Interrupt current rating of devices;
Interrupt current rating will give us a value that the device can safely interrupt without exploding. Bolted fault calculations are fine for this, as long as final transformer ratings are stable. Transformer impedance will limit the available fault current.

2) Arc fault current level;
This will give us a value for the HRC level and PPE required by our workers. This is the new kid on the block, a learning experience for all of us.

208V or 240V Panelboards connected to transformers less than 125 kVA can generally be ignored (IEEE 1584). These panelboards (IEEE) have a hard time sustaining an arc flash event, incident energies can be high due to long trip times, but the explosive nature of the event is subdued. With proper PPE, a 2 second exit time is sufficient for personnel to escape unharmed.

I cannot provide an answer to all the questions and scenarios on this thread. (NFPA has allocated funding so IEEE can further their research.) But I can pass on my results and management decisions based upon these results.

Number one on managements' list; NOBODY GETS HURT.

Number two on managements' list; HOW DO WE PREVENT ACCIDENTS.

Number three on managements' list; HOW DO WE IMPLEMENT NUMBERS ONE AND TWO COST EFFECTIVELY.

Back to the arc flash portion (interrupt rating has been settled).

The industrial facilities I have performed arc flash hazard studies on have either local generators or utility grid feeds. The facilities have internal, variable switchgear lineups. How does one handle all these variables and establish HRC levels and PPE requirements?

Well . . . Its' been tough!

Local utilities would not specify a range (min / max)of MVASC generation, but they would give me a typical MVASC, X/R at the substation (based on some weird base) and a statement that the substation is the current limiting device, no matter their online generation.

OK . . . So that is settled (assuming infinite trip time) just plug it into the modeling software . . . Hope for the best.

Switchgear lineup provides another layer of questions.

To make a long story short; Establish a STANDARD lineup with operations. All labeling is based upon this STANDARD. Next, for any equipment with trip times greater than 2 seconds (fight or flight instinct), re-run the arc flash calculations using 2 second trip times, label this equipment with a 2 second exit time notice. Then, train all supervisors, permit writers, operations and service technicians how to interpret the labeling.

With multiple generator lineups, run the arc flash calculations using the minimum and maximum STANDARD generators that could be online, adjust labeling for 2 second exit values as required.

Here in Alaska, medical resources are far removed from the work area.

Number one . . . NOBODY GETS HURT!

David Hippe P.E.
 
Re: Available fault current

Charlie, of the assumptions I made, POCO generation / final transformer impedance is the one I do not feel fully comfortable with yet . . . And do not have a solution to. But it is based upon the response of the power companies.

I feel that their current answer is one we have to start with to establish baseline hazard level labeling.

Maybe the POCO's need to understand this issue . . .

Arcing fault current, which is not final transformer limited, presents more hazard to the worker than a higher, transformer limited arcing fault current. This reality should be the driver for establishing arc flash related coordination and refining equipment labeling.

Training can only do so much, worker position affects the 2 second exit along with age and agility . . .

Thanks for the response. I'm looking for ideas to refine the new kid on the block.

Dave
 
Status
Not open for further replies.
Top