2011 proposals for voltage drop

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The part about the DIT breaker was proposed by Paul Keleher of KO Instruments for the 2008 code cycle. I am not sure if he included the voltage drop then. Part of his work with his breaker test instrument shows that in a number of dwelling units, that there is not enough current available to trip a breaker in the instantaneous trip range for the breakers with a higher instantaneous trip. As I recall he said that the trip points range from 8x to 40x the breaker rating and that the UL breaker standard doesn't have any requirements for the instantaneous trip. He did not get a good reception from either CMP 10 or UL. It appears from the url of the link that Pierre posted that UL is now onboard with this breaker requirement. However I just took a quick look at the proposals for CMP10 and did not see any of this there.
The following is the substantion for the proposal 10-55 for the 2008 code from Paul that would have required the DIT breakers.
Substantiation: Overcurrent in a branch circuit can occur as a result of a
current load placed on a circuit that exceeds the rating of the circuit, or from
damage to one or more components or the insulation in the circuit that results
in current flowing where it should not flow, a fault condition. The overcurrent
protection means required by 240.21 in branch circuits should be required to
provide specific protection from the hazardous effects of overcurrent from both
overload conditions and from fault conditions. UL standard No. 489 requires
that standard circuit breakers be tested to respond to decreasing time pick-up
tests of 135 percent, 200 percent and 600 percent of the rated current of the
circuit breaker as protection against excess loading of a circuit 1 ; however, no
protective response to fault-level overcurrent is specified in the requirements
for standard circuit breakers. The submitter suggests that the optimal fault-level
overcurrent protection in a standard circuit breaker requires limiting the energy
delivered from a fault that is capable of causing ignition to the greatest extent
possible in order to mitigate the risk of fire. This can be easily achieved by
incorporating an instantaneous trip setting that is as low as possible without
incurring an undue risk of nuisance tripping from normal inrush and starting
load currents. A UL investigation 2 and the long experience of circuit breaker
manufacturers have demonstrated a sufficient understanding of the threshold
required to eliminate nusiance tripping in residential environments to now
safely require all standard circuit breakers rated 20 amperes or less installed in
dwelling units to provided a defined instantaneous pick-up as protection against
fault conditions in addition to their present requirements for long-time pick-up
and other existing requirements.
The fact that the major manufacturers of standard circuit breakers commonly
installed in dwelling units voluntarily provide an instantaneous response in
their residential product lines, despite the fact that they are not required to do
so, is implicit testimony that circuit breaker manufacturers must believe that an
instantaneous response is necessary. The instantaneous pick-up setting of this
voluntary response is set at the discretion of each manufacturer. Prior to the
mid 1990’s these settings ranged from 10-25 times the rated current of the
circuit breaker resulting in a fault current as high as 450 amperes necessary to
clear the overcurrent immediately. Underwriters Laboratories found in a factfinding
study conducted in 1993 that the average available short circuit current
in 943 15-amp residential branch circuits studied is 300 amperes. 2 Following
publication of this study, several several manufacturers of residential thermal/
magnetic circuit breakers voluntarily reduced the instantaneous pick-up settings
of their residential lines of 15 and 20-amp circuit breakers to self-defined
settings that the UL study suggested would reduce the incidence of fire from
the hazards of overcurrent. In addition, at least two manufacturers have
consistently maintained and successfully implemented similar instantaneous
pick-up settings for many years, reflecting their connviction of the positive
value in keeping the instantaneous pick-up setting as low as possible in
residential environments. The experience of these circuit breaker manufacturers
with millions of circuit breakers in the field over a long period of time has
demonstrated that an instantaneous pick-up setting in the range suggested by
this study will not incur significant incidence of nuisance tripping in typical
residential environments. In addition, the UL study referenced in footnote 2
conducted testing for nuisance tripping and concluded in summary that
nuisance tripping does not occur in any circuit breaker with a nominal
instantaneous trip setting higher than 105 amperes. A nominal instantaneous
trip setting of 11 times the rated current of the breaker should provide ample
margin for tolerance from nuisance tripping. Furthermore, this proposal should
not require a significant deviation from current practice for breaker
manufacturers, but rather will simply make an instantaneous trip setting that is
close to current practice standardized and part of permanent practice. Without
modification, the present standard will not prevent any manufacturer from
reverting to a higher instantaneous pick-up setting or even eliminating it
altogether if they should so choose. This proposal will merely bring the
standards for requirements into line with existing practice. This proposal is
directed toward standard circuit breakers since they make up the vast majority
of overcurrent protection installed in residential circuits. This proposal is not
intended to have any impact on the requirements for AFCI, GFCI or HACR
circuit breakers, which are listed separately from standard circuit breakers.
The requirement for DIT rated circuit breakers is limited to dwelling units
where inrush currents have been studied and similar instantaneous settings have
been in regular use for many years. Non-residenial commercial/industrial
environments and heavier, 240-volts branch circuits ranging from 30A-60A
have, therefore, been excluded from the requirements of this proposed article
since those circuits are likely to contain appliance loads with a wider variety of
inrush and starting currents than are found in typical residential environments.
The purpose of the proposed 240.87(C) is to facilitate the obligation of the
Authority Having Jurisdiction to assure installation of a branch circuit breaker
listed per 240.87 by requiring the manufacturer of DIT circuit breakers to
clearly identify this performance characteristics on the exterior surface of the
breaker where it will be visible after installation.
For a detailed explanation of the specific changes to UL489 that would be
acceptable under this proposal, the submitter refers the code panel to a
proposed revision to UL489 entitled SUPPLEMENT FOR MOLDED CASE
CIRCUIT BREAKERS HAVING SPECIFIED INSTANTANEOUS TRIP
LEVELS, pages A-1 through A-3, which is part of the Underwriters
Laboratories, October 25, 1993 Fact-Finding Report on An evaluation of
Branch Circuit Breaker Instantaneous Trip Levels, file E87837 cited elsewhere
in this proposal and provided hereto by reference. The submitter offers that the
above-cited Supplement to Standard UL489 is acceptable in its entirety with
the following exception: The paragraph entitled, “PERFORMANCE/
Instantaneous Trip Test” be changed to read as follows: “A 15 or 20 ampere
rated circuit breaker having a defined instantaneous trip shall trip in 1/2-cycle
(or less) when tested in accordance with the following test procedure: A fault
current in the range of 130-210 amperes shall be applied for a 1/2-cycle.”
1 Underwriters Laboratories standard: UL 489/10-31-1996; pp 33.35.36:
Tests for Standard Circuit Breakers
2 Underwriters Laboratories, October 25, 1993 Fact-Finding Report on An
evaluation of Branch Circuit Breaker Instantaneous Trip Levels, file E87837,
Summary, pages 14-17
Note: Supporting material is available for review at NFPA Headquarters.
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I plotted a bolted fault at the end of a 500 foot run of 12-2 at various voltages on a QO120 trip curve to show what effect the voltage drop has on the time it takes for the breaker to open:

qo20amptripcurve.jpg
 
iwire said:
Good job Marc, you proved the need, 5-17 seconds clear time at 100 volts is to long in IMHO
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Actually, it's well below the NEMA AB-2 standard, and it's not much different than 3-10 seconds at 120 volts.

I'd rather see the proposal written as a maximum impedance and not a maximum voltage drop. Impedance testing would find more problems than just undersized conductors that could potentially increase the breaker's trip time.
 
mdshunk said:
I plotted a bolted fault at the end of a 500 foot run of 12-2 at various voltages on a QO120 trip curve to show what effect the voltage drop has on the time it takes for the breaker to open:
Click to expand...


I'm having trouble reading that graph. (these darned eyes.) Did you make the plot based on impedance then (500' #12)?
 
peter d said:
I'm having trouble reading that graph. (these darned eyes.) Did you make the plot based on impedance then (500' #12)?
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Yes, I picked 500 feet of 12 gauge because the impedance is basically 2 ohms per 1000 feet to make things easy. Work the ohm's law to find out what the current will be. Do the math to find out how many times the breaker's value that is and plot it on the graph as a white line.
 
The part that gets overlooked is with the AFCI's on almost everything won't they will see a bolted fault as an arc signature ? They seem to see every thing else as an arc fault.:D

NOTE to those with no sense of humor the items highlighted in red are a Joke.
 
mdshunk said:
but still well below the max standard. Admittedly, the max standard is probably what makes FPE's okay to use.
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Well thanks for meeting me part way, your suggestion of impedance testing may well be better but can you hear the outcry from the trade if that was proposed?

Does the NEMA suggestion carry any weight at all?
 
mdshunk said:
Yes, I picked 500 feet of 12 gauge because the impedance is basically 2 ohms per 1000 feet to make things easy. Work the ohm's law to find out what the current will be. Do the math to find out how many times the breaker's value that is and plot it on the graph as a white line.
Click to expand...

Why don't you do a few more plots for #14 and #10? Just for fun. Right, "fun." ;)
 
peter d said:
Why don't you do a few more plots for #14 and #10? Just for fun. Right, "fun." ;)
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Yeah, go ahead. I'll wait on you. It took me a half an hour to make that one. It was really the only way I'd prove to myself, one way or the other, whether the proposal has merit. After I did it, I'm sill undecided.
 
iwire said:
Well thanks for meeting me part way, your suggestion of impedance testing may well be better but can you hear the outcry from the trade if that was proposed?
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Yes, it would be roughly the same type of reaction I used to get when I suggested using a megger to troubleshoot tripping AFCI's and GFCI's. The 100 mile stare, followed by a 5 minute tirade.
 
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