Bolted fault current

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bthielen

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I have been trying to understand the definition of "bolted fault current". Could someone enlighten me, please?

In particular, how it relates to arc flash calculations.

Thanks,

Bob
 
Re: Bolted fault current

Imagine turning off a panel or switchboard. Then connect a large metal bar (i.e., "large" enough to have essentially no resistance) across the bus bars for all three phases. Connect it by bolting it in place, so that there is no way it will come off. In this "thought experiment," we don't want any strange variables, such as the possibility of reducing the released energy by having the short circuit element vaporize or blow away from the circuit, thus terminating the event too early. Now throw the switch to turn it on. Did I mention that you should be in another room at the time?
DON'T TRY THIS AT HOME!
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This is the theoretical "worst case" fault, in that it will give the highest release of energy while the fault condition persists. A fault from one phase to ground or from one phase to another will not be as bad. That is why the "bolted three phase fault" is used in discussions of worst case conditions.
 
Re: Bolted fault current

Bolted fault current relates to the worst case senario or highest magnitude fault current that would result at a paticular point in a system.

A simple formula is Ibf= (100/%Z)xFLA

Where %Z from transformer nameplate and FLA is full load rated seconday amps from the suppling transformer.

This is a rough estimate and does not take into account motor contribution.

The bolted fault current is used as one of the calculations for determining the flash protection boundry and the Incident Energy level (Expressed in cal/cm2) for a specified distance a worker would be, this info is placed on the Arc Flash label.
 
Re: Bolted fault current

Thanks.

So, with regard to arc flash calculations of a machine in a facility, one would then need to know some of the specifics about the supply conductors, OCPD, transformers, etc., in order to determine how quickly the system will deteriorate and at what current level, correct?

Pursuant to NEC 110.16 we have been placing an arc flash warning label on our machinery enclosures. 110.16 only specifies that a warning label is used but does not seem to specify what information in particular is required on the label (not that I have found anyway). Unfortunately I don't have access to a copy of the NFPA 70E code. Is there a reference there that spells this out? Admittedly, we have not included any specifics. Now, some of our customers are requiring or requesting that we include some of the calculated data such as Flash Hazard Boundaries, Incident Energy Level, PPE level, etc. on the warning label. Without knowing specific details about the supply circuit to our equipment, I don?t fully understand how we could provide that level of detail or am I misunderstanding something?

Bob
 
Re: Bolted fault current

What you are asking for and your customers are asking for can not be known with a dynamic power distribution system. We can give you the calculations but the system may be configured differently by the time you have them in hand. With large transformer banks, the overcurrent protective device (cutout) will hold in for a long time while it is burning down the conduit, switchgear, or bus (BTW, that is why 230.95 exists).

In my opinion, the calculations should not be posted unless you can be guaranteed a static system such as from an industrial substation. The calculations, which will be an estimate, should only be done just prior to live work being performed. :D
 
Re: Bolted fault current

First, there is no arc-flash calculation, chart, or table that is based on "maximum bolted fault" values. Everything requires you to know your
"actual" available current. See FPN No. 1 warning in NFPA 70E 130.7(C)(9)(a).


Charlie,
Is fault current on the secondary of a utility owned "dedicated" transformer really that dynamic(other than on true networks)? Isn't it possible for the utility to provide a range of "as installed" fault values.

The utility X/R ratio is not needed if fault values are given on the secondary of a dedicated transformer. Utility fault clearing times are also not required, if the line side of all service entrance equipment is considered dangerous. Utility clearing times are not considered when calculating arc-flash on the load side of any customer owned service entrance protective devices.
 
Re: Bolted fault current

Jim, if we know what the available fault current is on the primary side of the transformer really is then we would be able to know what the clearing time of our overcurrent device is for the bank and how much fault current will be available on the secondary side of the transformer. You need this information to do your calculations inside the plant and we can't provide it to you without making a lot of assumptions ourselves.

With our SCADA systems, yes, the fault current on the secondary of a utility owned "dedicated" transformer is really that dynamic, especially on hot days when we do a lot of switching between substations to keep our system up. Unless there is an accident or a system failure on lightly loaded days, our distribution system is static. Do you really want to take a chance that our system will be static on the day that someone is working on your system?
Utility clearing times are not considered when calculating arc-flash on the load side of any customer owned service entrance protective devices.
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Agreed, however, if you use our maximum fault current to do your calculations with and that amount of fault current is not there, the amount of incident energy may be a lot higher since it will take longer to open up the current limiting device that was calculated. Jim, I know that you understand that part but I am addressing that for general knowledge. :D
 
Re: Bolted fault current

The NEC article refers you to the ASTM Z535 reference that gives you the labeling requirements. Prior to conducting an arc flash study you need to conduct a short circuit analysis, the info from the s/c study is used for the flash hazard analysis and information is then placed on the labels for FHB, IE 9usually at 18") and the PPE required to operate that equipment.
 
Re: Bolted fault current

There's a lot that goes on with NFPA 70E. The basic calculation given is out of date. The research done by DuPont indicates that the flash hazard increases as the available fault current increases. There might be some graphs in NFPA 70E that show it - I don't remember - but there are some graphs in the original DuPont paper.

The available fault current is a function of the available fault current from the utility (the thevenin impedance), the transformer impedance, the nature of the connected loads (motors contribute to the available fault current), the impedance of the feeder circuits, and the clearing time of the fuse or circuit breaker.

Westinghouse used to have a way to estimate the short current current on the secondary of a transformer at an industrial site. It was:

Isc = IFL * ( 4 + 100 / %Z)

This allowed for motor contributions assuming an infinite bus at the transformer primary.

The fault current will decay slightly until the fuse or circuit breaker interrupts the current - that depends on the time constant of the circuit.

Bolted faults seldom if ever happen. The bolted fault assumes there's no fault impedance. That's seldom the case. A bolted fault would be sort of like putting a screwdrive solidly across the hot an neutral of a circuit.

The petrochemical group of the IEEE has put some work together on flash hazard analysis. It might be worthwhile to see what data they have.
 
Re: Bolted fault current

The NEC (NFPA 70) Article 110.16 simply requires a label that states "Warning, potential arc flash hazard".
No calculations required for NEC compliance.

NFPA 70E which is only enforced by OSHA and your own local safety officer (not the AHJ), requires calculated values shown on the label. You cannot provide this 70E label for your customer unless you do an analysis on their facility.
 
Re: Bolted fault current

The research done by DuPont indicates that the flash hazard increases as the available fault current increases.
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The problem comes in when the available fault current is less that what was used in the calculations and this results in the OCPDs taking longer to clear the fault, and this in turn often results in a higher incident energy.
Don
 
Re: Bolted fault current

Along with what Don said, as the fault current increases, it does not always result in higher incident energy since the inverse time characteristics of the protective device may activate faster (especially if the device is in its current limiting region).
This is why correct fault current values is important, and unfortunately almost impossible to get.
 
Re: Bolted fault current

Thank you all for your input. You have confirmed my suspicions.

Bob
 
Re: Bolted fault current

Research shows that an arc will not sustain itself at less than 38% of bolted fault current so a calculation should be done at 38%BF where as ron mentioned the clearing time is longer and the resultant arc flash hazard may be greater.
 
Re: Bolted fault current

Zog,
I haven't seen the 38%BF value before. Which document recommends this, NFPA 70E or IEEE 1584?
Arc Flash fault current is typically calculated with a pretty involved formula which is dependent on the equipment configuration (K), bolted fault current (IB), voltage (V) and bus gap.
Ia = K + 0.662 lg (IB) + 0.0966 V + 0.000526 G + 0.5588 V lg (IB) - 0.00304 G lg (IB)
Typically the incident energy is calculated at Ia for IB, and then again Ia calculated for 85% of IB. The trip time and incident energy at both the 85% and 100% arcing fault currents are calculated and the larger of the two Incident Energy values is used.
 
Re: Bolted fault current

That's what my understanding is Don. If one considers the cause of the bolted fault is it capable of carrying fault current of high enough magnitude to trip an OCPD before it evolves into disintegrates and evolves into an arcing fault. The instantaneous/magnetic pickup trip value of the common industrial breaker is 10x the breaker rating +-20%
With arcing faults it has been of my understanding that the impedance of and arc can be all over the place and commonly will cause less current to flow than the original cause of the fault. When this occurs we are even less likely to trip a breaker on instantaneous and no are depending upon the thermal element to clear the fault which may take more time than one cares to consider.
From what I have read and evidence that I have seen arcing faults often start as either L-L or L-N faults. As the air ionizes it may quickly involve other phases and/or ground.
When it involves the ground then one could depend upon the GF if one were available at current values far lower than that of the magnetic trip pickup of the breaker.
The bottom line as I see it there are no givens when it comes to a fault as there are too many variables and each is going to be different.
 
Re: Bolted fault current

templdl,

Semantics correction required;

The bottom line as I see it there are no givens when it comes to a fault as there are too many variables and each is going to be different.
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There are givens. They are known, you have stated them.

You seem to have a good grasp of the fundamentals, but the perceived number of variables and their range seem to be clouding the direct answer you require.

A variable, as the name implies, is not a fixed value, it has a range (or limits) within some domain.

The identification of variables, their range and domain, relationships and effects, are the heart of engineering. There are no prescriptive solutions to your problem. You must establish your method of solution within the confines of existing knowledge and stand by your results!

Dave
 
Re: Bolted fault current

Engineering is not a profession of "black and white," as is sometimes its reputation. Rather, it is a profession of "understanding the grays." So long as we employ common engineering methods and apply a reasonable standard of care, we can survive a trip to the witness chair. Just like in the electrical trade (or any other profession, for that matter), it is the ones who work outside their field of expertise, and who act with a disregard for the safety and well being of their clients, who find themselves sweating in a witness chair (or in the defendant's chair).
 
Re: Bolted fault current

Originally posted by charlie b:
Engineering is not a profession of "black and white," as is sometimes its reputation.
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That is enlightening, I thought it was math that produced a definite answer.
:confused:
 
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