Steve,
From
http://www.nfpa.org/assets/Files/PDF/ROP/99-A2009-ROP.pdf
99-93 Log #CP309 HEA-ELS
Final Action: Accept
(4.4.2.1.2, 4.5.2.2.2, 4.6.2.2.2 )
_______________________________________________________________
Submitter:
Technical Committee on Electrical Systems,
Recommendation:
Add three new sections (4.4.2.1.2, 4.5.2.2.2 and 4.6.2.2.2)
to read:
4.4.2.1.2 Selective Coordination. Overcurrent protective devices serving the
essential electrical system shall be selectively coordinated down to 0.1 seconds.
4.5.2.2.2 Selective Coordination. Overcurrent protective devices serving the
essential electrical system shall be selectively coordinated down to 0.1 seconds.
4.6.2.2.2 Selective Coordination. Overcurrent protective devices serving the
essential electrical system shall be selectively coordinated down to 0.1 seconds.
Add three new annex sections for each of these sections (A.4.4.2.1.2,
A.4.5.2.2.2, and A.4.6.2.2.2) to read:
It is important that the various overcurrent devices be coordinated, as far as
practicable, to isolate faulted circuits and to protect against cascading operation
on short circuit faults. In many systems, however, full coordination could
compromise safety and system reliability. Primary consideration also should be
given to prevent overloading of equipment by limiting the possibilities of large
current inrushes due to instantaneous reestablishment of connections to heavy
loads.
Substantiation:
Selective coordination is only one of several competing
factors that must be considered in the selection of appropriate overcurrent
protective devices (OCPDs) in health care facilities, Other factors that must be
considered in the selection of overcurrent protective devices include: arc flash
risk hazard, equipment damage, and reduced risk of extended outages; all of
which have direct effects on both staff and patient safety. Mandating selective
coordination as the sole determining factor in OCPD selection will result in
diminished reliability of the essential electrical system.
The method of application of selective coordination directly affects the
performance of the essential electrical system in a health care facility.
Establishment and management of this type of performance criterion
traditionally belongs under the purview of this committee.
Committee Meeting Action: Accept
Number Eligible to Vote: 23
Ballot Results:
Affirmative: 21 Negative: 1 Abstain: 1
Explanation of Negative:
LIPSTER, S.: It is ironic that all of the factors included in the committee?s
substantiation statement
(arc flash risk, equipment damage and reduced risk of
extended outage) are negatively impacted by this proposal. One tenth of second
allows six full cycles of excessive overcurrent to provide energy at the fault
location, which is not acceptable. The substantiation provides no
documentation as to the necessity of this change.
Explanation of Abstention:
KLEIN, B.: 1. I am not qualified to comment on the subject of selective
coordination.
2. I agree that T/C on Electrical System can and needs to require selective
coordination to avoid some fault tripping out a major portion of a facility. I
only question whether it is within the scope of this T/C to select the timing (0.1
seconds). Is this an NEC responsibility?
Comment on Affirmative:
BEY, L.: The concept of selective coordination that requires complete
separation of the overcurrent device time-overcurrent curves for all possible
values of fault current is too restrictive without some qualification. Using time
(above 0.1s) as the qualifier ignores the instantaneous response of molded case
circuit breakers. Using multiples of rated current as the qualifier achieves a
reasonable level of selectivity under fault conditions for reliability reasons. A
possible alternative wording would be as follows - Selective Coordination.
Overcurrent devices serving the essential electrical system shall be selectively
coordinated down to 0.1 seconds for available rms fault current up to twenty
times (20x) rated current of the device(s). Selective coordination of overcurrent
devices above twenty times (20x) rated current shall not be required. Using
multiples of rated current as the qualifier instead of time would allow both
molded case circuit breakers and fuses to be used. Twenty times rated current is
suggested as high enough to avoid cascading overcurrent device operation for
all but seriously faulted circuits or equipment. Using multiples of rated current
simplifies the use of selective coordination tables published by overcurrent
device manufacturers. A coordination study would still be required.
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