cbaman
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- Berlin, MA
Article 250.4(A)(5) of the NEC requires the equipment grounding conductor that is run with the circuit conductors to be an effective, low-impedance path for fault current “to facilitate the operation of an overcurrent protective device” when a fault occurs on a circuit. If the available fault current at an outlet is less than the threshold current required to trigger the magnetic response of the breaker protecting the outlet, the magnetic trip in the breaker will not activate on a short-circuit or ground-fault, and the requirements of 250.4(A)(5) will not be met even when an equipment-grounding-conductor is installed in full compliance with all other requirements of Article 250.
Test data recently gathered from short-circuit tests conducted at 120-volt receptacle outlets in residences indicates that more than half of all circuit breakers installed to protect 120-volt outlets on 15 and 20-ampere branch circuits DO NOT provide an instantaneous response to a short-circuit at an outlet on the circuit. The chart below summarizes the results of over 1000 residential receptacle outlets in various parts of the country whose breakers have been tested for their short-circuit response. The breaker response test is all cases is a bolted short-circuit conducted at receptacle outlets on a 15 or 20-amp branch circuits by a field test instrument known as the CBA-1000 Circuit Breaker Analyzer. The CBA-1000 has been certified to conduct this test. More information is available at . . . {MODERATOR'S NOTE: I removed the link to their web site, to avoid the appearanceof advertising. If you wish more information, contact the originator by Private Message, or do an independent on-line search.}
TOTAL NUMBER OF SHORT-CIRCUIT TESTS IN SAMPLE: 1017
THERMAL RESPONSES (SLOWER THAN 1-CYCLE) 57% 579
MAGNETIC RESPONSES (1-CYCLE OR FASTER) 43% 438
If the intent of 250.4(A)(5) is to trigger the magnetic response of a circuit breaker, an equipment-grounding-conductor installed in full compliance with article 250 will not meet the requirement of 250.4(A)(5) at any outlet where the magnetic response current threshold setting of the circuit breaker protecting the branch circuit exceeds the available fault current at the outlet.
Proposal 10-55 to the 2008 NEC is the first of two proposals whose combined intent is to ensure that all outlets on 120/240-volt branch circuits are protected by an effective instantaneous breaker response. The first proposal, #10-55 for 2008, clarifies expectations for the NEC user regarding the response of dwelling unit branch circuit breakers to short-circuits or ground-faults by standardizing the threshold current for the magnetic trip in circuit breakers at 11 times the handle-rating of the breaker and magnetic response clearing times at 1-cycle.
A subsequent proposal to Article 210 in the 2011 NEC will require that the voltage drop at the furthest outlet of a 120-volt branch circuit not exceed 9%, ensuring that the available fault current at the furthest outlet of a branch circuit will be sufficient to trigger the magnetic response of a breaker that meets the requirements of proposal 10-55.
If accepted, these two requirements will establish an impedance-based limitation on the length of a branch circuit of a given size that will ensure that all outlets are protected from short-circuits or ground-faults by an effective instantaneous circuit breaker response that will clear in 1-cycle.
Test data recently gathered from short-circuit tests conducted at 120-volt receptacle outlets in residences indicates that more than half of all circuit breakers installed to protect 120-volt outlets on 15 and 20-ampere branch circuits DO NOT provide an instantaneous response to a short-circuit at an outlet on the circuit. The chart below summarizes the results of over 1000 residential receptacle outlets in various parts of the country whose breakers have been tested for their short-circuit response. The breaker response test is all cases is a bolted short-circuit conducted at receptacle outlets on a 15 or 20-amp branch circuits by a field test instrument known as the CBA-1000 Circuit Breaker Analyzer. The CBA-1000 has been certified to conduct this test. More information is available at . . . {MODERATOR'S NOTE: I removed the link to their web site, to avoid the appearanceof advertising. If you wish more information, contact the originator by Private Message, or do an independent on-line search.}
TOTAL NUMBER OF SHORT-CIRCUIT TESTS IN SAMPLE: 1017
THERMAL RESPONSES (SLOWER THAN 1-CYCLE) 57% 579
MAGNETIC RESPONSES (1-CYCLE OR FASTER) 43% 438
If the intent of 250.4(A)(5) is to trigger the magnetic response of a circuit breaker, an equipment-grounding-conductor installed in full compliance with article 250 will not meet the requirement of 250.4(A)(5) at any outlet where the magnetic response current threshold setting of the circuit breaker protecting the branch circuit exceeds the available fault current at the outlet.
Proposal 10-55 to the 2008 NEC is the first of two proposals whose combined intent is to ensure that all outlets on 120/240-volt branch circuits are protected by an effective instantaneous breaker response. The first proposal, #10-55 for 2008, clarifies expectations for the NEC user regarding the response of dwelling unit branch circuit breakers to short-circuits or ground-faults by standardizing the threshold current for the magnetic trip in circuit breakers at 11 times the handle-rating of the breaker and magnetic response clearing times at 1-cycle.
A subsequent proposal to Article 210 in the 2011 NEC will require that the voltage drop at the furthest outlet of a 120-volt branch circuit not exceed 9%, ensuring that the available fault current at the furthest outlet of a branch circuit will be sufficient to trigger the magnetic response of a breaker that meets the requirements of proposal 10-55.
If accepted, these two requirements will establish an impedance-based limitation on the length of a branch circuit of a given size that will ensure that all outlets are protected from short-circuits or ground-faults by an effective instantaneous circuit breaker response that will clear in 1-cycle.
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