Short Circuit Study Requirements

Status
Not open for further replies.

mull982

Senior Member
What are the requirement (if any) for how often a short circuit study/coordination study must be performed. I know it is required by the NEC anytime a new system is installed, but what about for system upgrades on on some sort of repetative basis?

Besides any NEC requirements, what are requirements from saftey organizations like OSHA and MSHA?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
What are the requirement (if any) for how often a short circuit study/coordination study must be performed. I know it is required by the NEC anytime a new system is installed, but what about for system upgrades on on some sort of repetative basis?

Besides any NEC requirements, what are requirements from saftey organizations like OSHA and MSHA?

I know of no NEC requirement for a study, ever.
110.9 and 110.10 require you to install equipment rated for the available SCA, but there is no study requirement.

OSHA cites NFPA70E (Electrical Safety in the Workplace), which says no analysis results are valid after 5 years.
 

mull982

Senior Member
I know of no NEC requirement for a study, ever.
110.9 and 110.10 require you to install equipment rated for the available SCA, but there is no study requirement.

OSHA cites NFPA70E (Electrical Safety in the Workplace), which says no analysis results are valid after 5 years.

Jim

Do you know where in NFPA70E this is referenced?

Does anybody know if this is required by MSHA?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Do you know where in NFPA70E this is referenced?

NFPA70E-2009 - 130.3
The arc flash hazard analysis "shall be reviewed periodically, not to exceed 5 years" to account for changes that affect its results (i.e. available fault current).
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
. . . but what about for system upgrades or on some sort of repetative basis?
If you add a new panel, then you must be able to show that the fault current available at its location does not exceed the panel's rating. You may be able to do that without revising the model used to create the original short circuit study, or you may not. If you add a new motor of a "significant" size, then that will certainly impact the available short circuit current throughout the facility. That would require revisiting the short circuit analysis.

 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
If passed in the next NEC version, the calculated short circuit current will have to be listed on the equipment.
I thought the proposed short circuit current label would be able to be based on a 'design' value rather than an actually calculated one.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
If passed in the next NEC version, the calculated short circuit current will have to be listed on the equipment.
Who would put that information on the equipment, and when? The manufacturer would not know the value; all they know is the limit that the panel can handle. As the design engineer, I can tell you the calculated short circuit current at any point in the distribution system, but I don't know which of the 25 identical panels that the EC purchased for the project is going to be installed in which physical location. :confused:

 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Continuing my earlier bout of amazement, one year after a building first opens for business, when they install a new large motor, that project will cause every value of "short circuit current available" to change throughout the facility. Who is going to go around and revise the labels on all the panels? :confused:
 

ron

Senior Member
Jim and Charlie,
I sent in a comment to the ROP indicating that it shouldn't be added because an electrical worker would use those artificially high calculated value to determine incident energy and chose an artificially low PPE to wear.
The ROC essentially told me to get lost.
 

ron

Senior Member
_______________________________________________________________
1-116 Log #1222 NEC-P01​
Final Action: Reject
(110.24)

_______________________________________________________________​
Submitter:​
Ron

Comment on Proposal No:​
1-183

Recommendation: Delete text as follows:
110.24 Available Fault Current
(A) Field Marking. Service equipment in other than dwelling units shall be
legibly marked in the field with the available fault current. The field
marking(s) shall include the installation date and be of sufficient durability
to withstand the environment involved.
(B) Modifications. When modifications to the electrical installation occur,
that affect the available fault current at the service, the available fault
current shall be verified or recalculated as necessary to ensure the service
equipment interrupting ratings are sufficient for the available fault
current at the line terminals of the equipment. The required field
marking(s) in (A) above shall be adjusted to reflect the new level of
available fault current.
Substantiation:​
If available fault current is marked on equipment, it will
almost always be established conservatively, so that the value is artificially
high. Often it will be calculated using an infinite primary transformer
calculation. This is satisfactory for interrupting capacity and withstand
comparisons for equipment selection, but could result in death to an electrical
worker that uses this information to determine incident energy values. When
marked fault current is artificially high, it will often result in theoretically
faster acting inverse time overcurrent protective device; lower incident energy
and eventually a lower PPE requirement will result. An electrical worker
performing testing or other energized work may be hurt or killed from
inadequate PPE selection stemming from the use of artificially high fault
current values.

Panel Meeting Action: Reject
Panel Statement:​
The panel continues to support the concepts in Proposal

1-183 as evidenced by its action on Comment 1-115.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Jim and Charlie,
I sent in a comment to the ROP indicating that it shouldn't be added because an electrical worker would use those artificially high calculated value to determine incident energy and chose an artificially low PPE to wear.
The ROC essentially told me to get lost.
Ron,

Your request was to delete the entire requirement.
I do not believe the ROP addressed whether their 'marked' fault current was to be a design or actual level.
 

ron

Senior Member
Jim,
The design level is printed on the equipment nameplate already 22kA, 42kA, 65kA, etc.
The field marking implies that it is information not available to the manufacturer, so it would be the calculated value.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
There were a number of comments that called for the rejection of this change and most were based on the concern that Ron stated in his comment. Some of these were from utilities. There was even on that called for a note on the label that would say that these short circuit currents are not to be used for the calculations that are used to select PPE. I think the panel is way off base here. There is no safety benefit to posting this information on the equipment and it could, as stated in the comments, even be a safety hazard.
 
What are the requirement (if any) for how often a short circuit study/coordination study must be performed. I know it is required by the NEC anytime a new system is installed, but what about for system upgrades on on some sort of repetative basis?

Besides any NEC requirements, what are requirements from saftey organizations like OSHA and MSHA?

Our assesment of the regulations:
...................................
1910.303(b)(4)
OSHA Code:
(b) Examination, installation, and use of equipment?
?
(4) Interrupting rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.

Federal Register:
Changes included addition of requirements for equipment intended to interrupt current to have adequate interrupting ratings.

2005 NEC: 110.9

Comments/Discussion:

This requirement applies to all electrical installations and utilization equipment, regardless of when they were designed or installed.

A system wide short circuit study is required including the comparison of equipment ratings and available fault levels.
..............................................
1910.303(b)(5)
OSHA Code:

(b) Examination, installation, and use of equipment?
?
(5) Circuit impedance and other characteristics. The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without the occurrence of extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors, or between any circuit conductor and the grounding conductor or enclosing metal raceway.

Federal Register:
Changes included addition of requirements for the coordination of overcurrent protection for circuits and equipment.

2005 NEC: 110.10

Comments/Discussion:

This requirement applies to all electrical installations and utilization equipment, regardless of when they were designed or installed.

A system wide phase and ground coordination study is required.
 

jghrist

Senior Member
Maybe the idea of field labelling is so you can determine if the NFPA 70E task tables 130.7(C)(9) are applicable. The notes to the task tables restrict their use by maximum available fault current and fault clearing time. If the maximum available fault current exceeds the limit, then calculation of arc incident energy is required.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
The design level is printed on the equipment nameplate already 22kA, 42kA, 65kA, etc.
This is from the manufacturer and is usually tempered by a phrase like "or not to exceed the value of the lowest installed device"

The field marking implies that it is information not available to the manufacturer, so it would be the calculated value.
The field marking is required to give the inspector, or anyone, a value to be used to compare to the equipment label. It makes no difference if this is a design level, an infinite bus value, or an actual calculation, without some number a comparison cannot be made.

Maybe we need to clarify some terms, like Design level:
Consultants - often put a design level on their drawings based solely on voltage levels, like 480V shall be rated 65kAIC.
POCOs - usually have design level fault currents that are meant to never be exceeded regardless of their systems variables, these often exceed infinite bus values.
Manufacturers - these are often tested values, although in the case of control panels, article 409 and UL508, they may not be.

In any case, I agree with your premise that the field value has a high potential to be misused.
 
Last edited by a moderator:
Status
Not open for further replies.
Top