700-27 pleas explain
- Thread starter arnettda
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700.27 Coordination.
Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.
Exception: Selective coordination shall not be required in (1) or (2):
(1) Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exists on the transformer secondary,
(2) Between overcurrent protective devices of the same size (ampere rating) in series.
From the HB:
Section 700.27 requires that all emergency system overcurrent devices be selectively coordinated with the overcurrent devices installed on their supply side. The term coordination (selective), as defined in Article 100, indicates that a selectively coordinated system is one where the operation of the overcurrent protective scheme localizes an overcurrent condition to the circuit conductors or equipment in which an overload or fault (short-circuit or ground-fault) has occurred. Because the purpose of an emergency system is to provide power to essential life safety systems in a building or facility, a selectively coordinated overcurrent protection scheme that localizes and minimizes the extent of an interruption of power due to the opening of a protective device is a critical safety element. Continuity of operation of illumination for occupant evacuation or maintaining continuity of operation of essential safety equipment such as smoke evacuation systems is necessary for occupant safety during a fire or other emergency. Simply put, an overcurrent event (overload, short-circuit, or ground-fault) in a 20-ampere branch circuit cannot cause the feeder protective device supplying the branch-circuit panelboard to open. This coordination must be carried through each level of distribution that supplies power to the emergency system.
Design and subsequent verification of electrical system coordination can be achieved only through a coordination study that entails detailed knowledge of electrical supply system fault-current characteristics and a design that integrates overcurrent protective devices that react to overcurrent and interact with each other in such a manner that the objective of minimizing outages by localizing the overcurrent problem and isolating that part of the emergency system can be achieved. Note that modifications to the electrical system subsequent to the initial design and installation can significantly impact the original implementation of the coordinated system. For additional discussion on selective coordination, see the commentary for 620.62 and Exhibit 620.7.
The exception to 700.27 was added to the 2008 Code to recognize series-rated systems where the immediately upstream device will open before the downstream device.
ARTICLE 701 Legally Required Standby Systems
Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.
Exception: Selective coordination shall not be required in (1) or (2):
(1) Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exists on the transformer secondary,
(2) Between overcurrent protective devices of the same size (ampere rating) in series.
From the HB:
Section 700.27 requires that all emergency system overcurrent devices be selectively coordinated with the overcurrent devices installed on their supply side. The term coordination (selective), as defined in Article 100, indicates that a selectively coordinated system is one where the operation of the overcurrent protective scheme localizes an overcurrent condition to the circuit conductors or equipment in which an overload or fault (short-circuit or ground-fault) has occurred. Because the purpose of an emergency system is to provide power to essential life safety systems in a building or facility, a selectively coordinated overcurrent protection scheme that localizes and minimizes the extent of an interruption of power due to the opening of a protective device is a critical safety element. Continuity of operation of illumination for occupant evacuation or maintaining continuity of operation of essential safety equipment such as smoke evacuation systems is necessary for occupant safety during a fire or other emergency. Simply put, an overcurrent event (overload, short-circuit, or ground-fault) in a 20-ampere branch circuit cannot cause the feeder protective device supplying the branch-circuit panelboard to open. This coordination must be carried through each level of distribution that supplies power to the emergency system.
Design and subsequent verification of electrical system coordination can be achieved only through a coordination study that entails detailed knowledge of electrical supply system fault-current characteristics and a design that integrates overcurrent protective devices that react to overcurrent and interact with each other in such a manner that the objective of minimizing outages by localizing the overcurrent problem and isolating that part of the emergency system can be achieved. Note that modifications to the electrical system subsequent to the initial design and installation can significantly impact the original implementation of the coordinated system. For additional discussion on selective coordination, see the commentary for 620.62 and Exhibit 620.7.
The exception to 700.27 was added to the 2008 Code to recognize series-rated systems where the immediately upstream device will open before the downstream device.
ARTICLE 701 Legally Required Standby Systems
Hendrix
Senior Member
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- New England
Google: Cooper bussmann selective coordination. Scroll down to 2008 nec cooridination updates. That gives you more of a laymans explanation
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