SC calcs and current limiting fuses and IEEE 551

lielec11

Senior Member
Location
Charlotte, NC
I remember reading/hearing in a meeting that IEEE 551 does not allow you to include the effect of current limiting fuses in your calculations but I cannot seem to find it looking now. Did I remember correctly, or is this total nonsense? I know SKM does not have any way of calculating the SC results if you have a CL fuse. Any insight is much appreciated.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
This comes up a LOT, and there are differences of opinion. The most recent discussion is here:

I, and most others, are in the camp of "No, you cannot do this in the FIELD", even though it can be done by device manufacturers, UL panel shops or a registered PE (if you can find one that will do it).
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
I remember reading/hearing in a meeting that IEEE 551 does not allow you to include the effect of current limiting fuses in your calculations but I cannot seem to find it looking now. Did I remember correctly, or is this total nonsense? I know SKM does not have any way of calculating the SC results if you have a CL fuse. Any insight is much appreciated.
I don't recall ever seeing anything in the Violet Book suggesting you cannot include the effects of current limiting fuses, but the question is how exactly would you do it in SKM? Standard short-circuit calculations do not take this into consideration, but it would be allowed when applying series ratings through some other evaluation method. As mentioned in post #2, this would need to be done by a licensed PE (whether or not that individual has a valid evaluation method) or even a manufacturer through tested combinations.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
where in the code does it say a PE can do this?

there is a provision to allow PEs to take CL fuse or CB characteristics into account for circuit breakers in existing installations but not for anything else as far as I can tell.

240.86 Series Ratings. Where a circuit breaker is used on a
circuit having an available fault current higher than the
marked interrupting rating by being connected on the load
side of an acceptable overcurrent protective device having a
higher rating, the circuit breaker shall meet the requirements
specified in (A) or (B), and (C).
(A) Selected Under Engineering Supervision in Existing Installations.
The series rated combination devices shall be selected
by a licensed professional engineer engaged primarily in the
design or maintenance of electrical installations. The selection
shall be documented and stamped by the professional engineer.
This documentation shall be available to those authorized
to design, install, inspect, maintain, and operate the system.
This series combination rating, including identification of the
upstream device, shall be field marked on the end use equipment.
For calculated applications, the engineer shall ensure that
the downstream circuit breaker(s) that are part of the series
combination remain passive during the interruption period of
the line side fully rated, current-limiting device.


note that this provision specifies a single evaluation method that has to be used to apply this provision using a calculation.
 
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xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
where in the code does it say a PE can do this?

there is a provision to allow PEs to take CL fuse characteristics into account for circuit breakers in existing installations but not for anythinmg else as far as I can tell.
Section 240.86(A) for existing installations. It seems like only tested combinations are allowed in new construction... but if a PE claims an equivalent safety standard is met or exceeded for new construction, I am sure it would be permitted.

Also, outside of the NEC, state law generally requires a licensed engineer anytime professional service involves the application of engineering principles and data, such as consultation, investigation, evaluation, planning and design, etc. etc...
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Section 240.86(A) for existing installations. It seems like only tested combinations are allowed in new construction...

Also, outside of the NEC, state law generally requires a licensed engineer anytime professional service involves the application of engineering principles and data, such as consultation, investigation, evaluation, planning and design, etc. etc...
however, the NEC does not allow it so it just does not matter unless state law says a PE can do whatever the heck he wants to regardless of what the NEC says, and I am doubtful that any state law actually says that.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
however, the NEC does not allow it so it just does not matter unless state law says a PE can do whatever the heck he wants to regardless of what the NEC says, and I am doubtful that any state law actually says that.
Everyone is supposed to follow building codes, but there may be instances where a PE's professional judgement, license and insurance could allow them to deviate from the code. From what I understand this is common in the civil/structural realm with the advent of new building materials which have not yet been accepted in local codes.

Within the bounds of public safety and reason, a PE can pretty much do whatever they want when it comes to engineering/design (without discrimination, restraint or limitation) - it will be on them if something goes wrong. If you pull this card, the building department will most likely want something from you in writing stating an equivalent safety standard is being met. At the end of the day its the board of professional engineers as the only regulatory authority having jurisdiction of the engineer.
 
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jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
however, the NEC does not allow it so it just does not matter unless state law says a PE can do whatever the heck he wants to regardless of what the NEC says, and I am doubtful that any state law actually says that.
Nor does the NEC prohibit it.
It is up to the AHJ to decide if using the PE is an acceptable method.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
That is only true if state law allows the authority having jurisdiction to do so.
In most states, if an action is not prohibited by the NEC it is up to the AHJ to make a decision. What law might prevent an AHJ from considering the input from a PE?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
In most states, if an action is not prohibited by the NEC it is up to the AHJ to make a decision. What law might prevent an AHJ from considering the input from a PE?
The way these laws are written requires compliance with certain codes. The AHJ is generally also given some leeway for special cases where a code does not deal with a particular situation to accept an engineered solution, but if it is covered by a code, the code generally has to be followed.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
The way these laws are written requires compliance with certain codes. The AHJ is generally also given some leeway for special cases where a code does not deal with a particular situation to accept an engineered solution, but if it is covered by a code, the code generally has to be followed.
It's really only the assurance that equivalent safety objectives are achieved/maintained which must be met. (See section 90.4)
Requesting special permission is something anyone can do; but the thing that makes a design profession special is they are uniquely qualified and can self-certify (by affixing their seal - which is the highest level of assurance) that an equivalent safety objective has been met. At this point, what legal authority does the Building Official have over a licensed design professional to challenge their expert opinion (and or any evidence) when the scope of the building department is limited to code compliance review (which you are clearly intending to deviate from through the special permission process)? Moreover, what AHJ wouldn't be ready to transfer all liability away from itself?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
... but if it is covered by a code, the code generally has to be followed.
Yes, but this SCCR issue, for other than series rated overcurrent protective devices, is not addressed by the NEC so what is the AHJ to follow?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
My understanding is approvale of requests for variance from building codes follow the lines of this draft proposal for the city of chicago.

Review Criteria 1. Pursuant to Section 13-8-032(a) of the Municipal Code, The Department may approve an alternative method for complying with any requirement of the building code for individual cases where:
a. Specifically authorized by a provision of the building code;
b. There are practical difficulties in carrying out the strict requirements of the building code, specific individual reasons make the strict application of the building code impractical, the alternative to be approved will comply with the intent and purpose of the building code, and the alternative to be approved will provide equivalent or greater health protection, accessibility, life and fire safety, and structural performance; or
c. A material, design, or method of construction not specifically allowed by the building code complies with the intent of the building code and will, in the specific application, result in a building or structure providing equal or greater quality, strength, effectiveness, fire resistance, durability, and safety as one meeting the strict requirements of the building code.

If it does not meet one of these criteria they won't even consider your request (unless it is accompanied by a wad of cash, it being Chicago).
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
My understanding is approvale of requests for variance from building codes follow the lines of this draft proposal for the city of chicago.

Review Criteria 1. Pursuant to Section 13-8-032(a) of the Municipal Code, The Department may approve an alternative method for complying with any requirement of the building code for individual cases where:
a. Specifically authorized by a provision of the building code;
b. There are practical difficulties in carrying out the strict requirements of the building code, specific individual reasons make the strict application of the building code impractical, the alternative to be approved will comply with the intent and purpose of the building code, and the alternative to be approved will provide equivalent or greater health protection, accessibility, life and fire safety, and structural performance; or
c. A material, design, or method of construction not specifically allowed by the building code complies with the intent of the building code and will, in the specific application, result in a building or structure providing equal or greater quality, strength, effectiveness, fire resistance, durability, and safety as one meeting the strict requirements of the building code.

If it does not meet one of these criteria they won't even consider your request (unless it is accompanied by a wad of cash, it being Chicago).
Note that items b and c are about complying with the "intent" of the original requirement (within the bounds of public safety and reason).
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Note that items b and c are about complying with the "intent" of the original requirement (within the bounds of public safety and reason).
Note the first phrase of both provisions b and c. The intent of the code is secondary to it either being impractical to follow the code (b) or impossible (c). If it is not impractical or impossible to meet the code, the intent of the code never comes into play.

How this is interpreted in Chicago probably depends mostly on how connected the person who wants the variance is.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
Note the first phrase of both provisions b and c. The intent of the code is secondary to it either being impractical to follow the code (b) or impossible (c). If it is not impractical or impossible to meet the code, the intent of the code never comes into play.

How this is interpreted in Chicago probably depends mostly on how connected the person who wants the variance is.
I don't see a requirement that it must be impractical or impossible in item c.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't see a requirement that it must be impractical or impossible in item c.
Item c allows for use of things that are not in the code, so they would be impossible. For instance, suppose someone came up with a practical room temperature superconductor. would not be allowed because it is not in the current NEC. So it would otherwise be impossible without some kind of special permission.
 
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