10-82 Log #3562 NEC-P10
Final Action: Accept in Principle
(240.87 (New) )
_______________________________________________________________
Submitter: James T. Dollard, Jr., IBEW Local 98
Recommendation: Add new text to read as follows:
240.87 Short-time Delay. Where short-time delay is utilized on a circuit
breaker, one of the following shall be provided:
(A) Zone-selective interlocking
(B) Differential relaying
(C) Energy-reducing maintenance switching
FPN: An energy-reducing maintenance switch allows a worker to set a circuit
breaker trip unit to instantaneous while the worker is working within an arc-
flash boundary as defined in NFPA 70E, and then to set the trip unit back to a
short-time delay setting after the potentially hazardous work is complete
Substantiation: Short-time delay is an industry-proven method to achieve
selective coordination of circuit breakers. It delays the opening of an upstream
circuit breaker while the downstream overcurrent device clears a short-circuit.
If however, a short occurs between the two devices, the upstream circuit
breaker will still delay its tripping operation, allowing for more let-through
energy than would have been allowed if the upstream circuit breaker had
utilized an instantaneous trip. This type of installation is typical for electrical
power distribution systems. This extra amount of let-through energy may injure
workers or damage equipment. There are at least three methods to prevent the
increased let-through energy.
Zone-selective interlocking allows the upstream circuit breaker and
downstream circuit breaker to communicate with each other and determine
whether the upstream circuit breaker should open as quickly as possible or after
a pre-set amount of time delay. For shorts on the load side of the downstream
circuit breaker, the downstream circuit breaker signals the upstream circuit
breaker, telling it to “hold off”, not to trip, since the downstream circuit breaker
will trip and take the short off-line. For shorts between the circuit breakers, the
downstream circuit breaker does not “see” the short, and therefore does not
send a signal to the upstream circuit breaker. Without a signal from the
downstream circuit breaker, the upstream circuit breaker will trip as quickly as
possible. This reduces the amount of energy that is released into the system for
shorts between the devices, and allows for selective coordination at the same
time. It offers a higher degree of arc-flash protection for employees and also
greater equipment protection.
With differential relaying, the amount of current coming in to the upstream
circuit breaker is compared to the amount of current leaving the downstream
circuit breakers. Under normal conditions, the difference is zero. If, during a
short circuit condition on the load side of the downstream circuit breaker, the
difference is zero, the upstream circuit breaker waits for the affected
downstream circuit breaker to open. If the difference in current reaches the
setting, due to a short circuit between the circuit breakers, the upstream circuit
breaker opens as quickly as possible. For short circuits between the devices, as
might occur when an employee is working in energized switchgear, the
upstream circuit breaker limits the amount of let-through energy to which the
worker could be exposed.
With an energy reducing maintenance switch, the worker is able to set the
trip unit to instantaneous whenever working within the flash protection
boundary as defined by NFPA 70E, and then turn back to the short-time delay
mode when finished. This system provides let-through energies while energized
work is being performed.
Panel Meeting Action: Accept in Principle
Add new text to read as follows:
240.87 Non-instantaneous Trip. Where a circuit breaker without an
instantaneous trip is utilized, one of the following or approved equivalent
means shall be provided:
(1) Zone-selective interlocking
(2) Differential relaying
(3) Energy-reducing maintenance switching with a local status indicator
FPN: An energy-reducing maintenance switch allows a worker to set a circuit
breaker trip unit to instantaneous while the worker is working within an arc-
flash boundary as defined in NFPA 70E, and then to set the trip unit back to a
normal setting after the potentially hazardous work is complete.
Panel Statement: The modifications meet the intent of the submitter. The
addition of “without an instantaneous trip” more accurately reflects the
applications referred to by the submitter. “Other approved means” was added to
allow for future technology. The title change more accurately reflects the
subject. The addition of “a local status indicator” helps assure that the switch
will be returned to the proper position at the completion of work.
Number Eligible to Vote: 12
Ballot Results: Affirmative: 9 Negative: 3
Explanation of Negative:
DARLING, D.: Primary direction of NFPA 70E is to De-Energize electrical
equipment for worker safety. The proposed features will reduce but will not
eliminate the electric hazards which can only be eliminated by removing all
energy sources.
If any work needs to be performed on energized equipment a hazard risk
analysis must be conducted justifying why the system cannot be de-energized.
A similar requirement for first de-energizing electrical equipment unless it is
infeasible to do so exists in OSHA Subpart `S”.
FREDERICKS, C.: I am voting against the panel action to accept. The
proposal’s intent to protect workers is good, but the language as accepted could
unnecessarily restrict other design and operational options that would provide
for an equally safe system. These options could include application of PPE that
is rated to meet or exceed the available arc flash hazard energy, current
limitation, available fault current reduction, arc resistant equipment, and
de-energized maintenance. Some of these methods are operational rather than
specific to the installation design. As a minimum, these alternate methods
should be recognized in a fine print note.
MANCHE, A.: NEMA supports performance enhancements that provide
increased worker safety, as covered in NFPA 70E.
However, there has been no substantiation presented that this requirement will
always enhance the protection being sought by the submitter. The new
70-234
Report on Proposals A2010 — Copyright, NFPA
NFPA 70
language introduces added equipment in all instances for such devices when
having such functionality on selected devices in the system may require the
same PPE Category protection for the electrical worker in accordance with
NFPA 70E. No data was provided to support that an instantaneous trip function
in all circuit breakers will ensure the enhancement in safety
sought by the submitter. Overcurrent device instantaneous levels can be set
well above any real-world current level that may be induced during an arc flash
event. NEMA interprets the submitters goal as seeking a means to enhance
worker safety, neither the proposal nor the panel action addresses such an
enhancement directly by simply requiring additional product features that may
or may not meet the goal of the submitter. Additionally, any performance
requirement must be the same for all overcurrent devices, and not specific to
circuit breakers.
The TCC should also review this proposal to ensure it resides within the
scope of article 240. The scope, and FPN found in the scope, establishes that
this article has jurisdiction over the overcurrent protection of conductors and
equipment to protect them from excessive or dangerous temperatures. No
where is CMP-10 charged with the scope of protecting personnel from hazards
arising from the maintenance of electrical systems.
Comment on Affirmative:
COOK, D.: I support the submitted effort to provide greater protection of
those required to perform electrical task on energized equipment. I agree with
the Panel actions that add “other approved means” to address future technology
and require the local status indicator with the energy-reducing maintenance
switch. As an enforcement representative I have some concern about the
difficulty of identifying non-instantaneous trip circuit breakers in the field. Text
similar to that in 500.4(A) could be included to read: Where non-instantaneous
trip circuit breakers are installed, documentation shall be available to those
authorized to design, install, inspect, maintain, or operate those devices. I also
have concerns that complex overcurrent protection schemes that involve field
assembly, calibration, and testing to determine functionality. Text to require
commissioning of those installations by an accredited party and documentation
for that commissioning should be provided to the Authority Having Jurisdiction
(AHJ). Typical inspection departments do not have the resources to evaluate
that type equipment.
OCKULY, G.: It is my understanding that this requirement will apply to
power circuit breakers that do not have an instantaneous trio or where the
instantaneous trip is turned to the off position. This requirement would not
apply to molded case and insulated case circuit breakers as listed to UL 489
