John Stark – Russelectric Inc. Overview. Recent changes to the National Electrical Code (NEC) require the selective coordination of overcurrent protective devices at hospitals and other mission-critical facilities. .
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Recent changes to the National Electrical Code (NEC) require the selective coordination of overcurrent protective devices at hospitals and other mission-critical facilities.
Transfer switches with 30-cycle closing and withstand ratings dramatically simplify designing to that requirement.
Air conditioning, Lighting,
Mechanical, Building Loads,etc.
With regard to the emergency back-up and transfer scheme, it is incumbent upon engineers to select the proper equipment for the application. There are many considerations and they are becoming more with each decade.
Article 100 provides the Code definition. Here is another way to describe it:
Selective coordination was first required by the NEC in 1993 for elevator circuits. Amendments to the Code in 2005 and 2008 strengthened the requirements and expanded them to include emergency and legally required standby systems, as well as critical operations power systems. Selective coordination, as defined in the 2008 NEC, is the (as in previous slide)“localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.” It is a complicated process of coordinating the ratings and settings of overcurrent protective devices, such as circuit breakers, fuses, and ground fault protection relays, to limit overcurrent interruption (and the resultant power outages) to the affected circuit or equipment (the smallest possible section of a circuit). In other words, the only overcurrent protective device that should open is the device immediately “upstream” from the circuit/equipment experiencing an overcurrent condition.
Proper Selective Coordination is becoming more and more of an engineering consideration and is being enforced by inspectors more & more often…
Refer to IAEI handout “Selective coordination restricts outages to the circuit or equipment affected,
ensuring reliability of electrical power.”
FusesMore on Selective Coordination
APCB\'s (Air Power Circuit Breaker) are typically 30 cycle withstand devices.
ICCB\'s (Insulated Case Circuit Breaker) are 30 cycle withstand or up to 4 Cycle Instantaneous.
MCCB\'s (Molded Case Circuit Breaker) typically instantaneous or Current Limiting Devices.One-line
An overcurrent event (overload, short circuit, or ground fault) here should trip the 400A MCCB
In the absence of other means to satisfy selective coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.
Fault on load side of ATS could see up to 30 cycles of fault current -depending on the Air Power Circuit Breaker settings that is feeding it- and could travel through the ATS and the ATS contacts.
If the 400A MCCB does not trip/clear…Selective Coordination
Expected July 2010
Expected July 10’
Expected January 2011
S. Carolina Code Council adopted 2009 IRC with 2008 NEC 3/22/10 with implementation effective 1-1-11
State Adopted Unincorporated Areas
2008 NEC – 32 States
HI, basically 2002 NEC but some islands back to 1993 NEC
2005 NEC – 8 States
Local Adoption – (10)
Note: Some local adoption states have earlier than 2005 adoptions in some jurisdictions
Revised April 19, 2010
In the 2008 Code Cycle there were challenges to the selective coordination requirement. Proposal 13-135 proposed the elimination of the selective coordination requirement for 700.27.
The proposal was to remove the selective coordination requirement from the mandatory text and places it in a
non-mandatory in a FPN (fine print note).
But Code Panel 13 rejected this proposal by a vote of 9-4. To follow is their statement:
Then, Code Panel 20, which was responsible for the new Article 708, summed up the need for selective coordination in their statement to Comment 20-13, (which was another proposal for the deletion of the selective coordination requirement).
This comment was rejected 16-0. The actual panel statement to Comment 20-13:
There are numerous proposals being adopted by States and/or City or local governmental bodies which modify the selective coordination requirements.
The most commonly heard proposals fall into two categories:
1. Allow the degree of selective coordination needed to be the responsibility of the qualified person responsible for the project.
(The Commonwealth of Massachusetts was the first State to adopt such a proposal as an exception to the Articles in 700.27, 701.18 and 708.54, which require selective coordination as follows:
Exception No. 2: Where the system design is under the control of a licensed professional engineer engaged in the design or maintenance of electrical installations, the selection of overcurrent protective devices shall be permitted to coordinate to the extent practicable. The design shall be documented, stamped by the professional engineer, and made available for review by the authority having jurisdiction.
2. Proposals to modify the NEC requirement for selective to only be required for above a specific time. The leading proposal is 0.1 seconds (6 cycles) and above. The State of Oregon recently adopted a proposal submitted by the National Electrical Contractors Assoc., Oregon Pacific Cascade Chapter, as Statewide Alternate Method No. OESC 08-04 applying to Articles in 700.27, 701.18 and 708.54. This states the following: “The requirements in NEC 700.27, 701.18 and 708.54 for selective coordination may be demonstrated by providing a selective coordination study utilizing trip-curve data in the range of 0.1 seconds or more.
Substantiation for this proposal included:
1). “…selective coordination is not always possible or practical for all fault current levels when protection is provided by MCCB’s. The requirement for “total” selective coordination means that over current protection devices must be coordinated for all faults, regardless of their magnitude or duration, including the most extreme case, the bolted fault. However, bolted three phase faults which rapidly generate extremely high current in the instantaneous range rarely occur in practice, except at start-up when interruption of power due to a lack of coordination is not likely to compromise safety...” “In order to achieve total short circuit selective coordination, the size of upstream overcurrent protective devices may need to be increased and/or time delay trip characteristics increased, thereby possibly increasing the arc flash hazard.”
“Findings: By omitting the instantaneous range from the requirements for selective coordination, reasonable and affective safety can (still) be achieved. Signing supervisors and engineers can use readily available and published time current curves to determine if a system is selectively coordinated to a substantial degree without having to relay on unregulated manufacturer testing data and inconsistent engineering and design practices.”
This is the other side of the argument regarding the subject of Selective Coordination VS Arc Flash Considerations.
The presenter will not delve into this side of the argument, as he is in the business of providing emergency power
to critical facilities and therefore is in the camp of having a non-sensitive, robust type system, selectively coordinated,
that facility managers want to perform well when called upon. In cases of catastrophic outages, Arc flash
considerations might take a back seat to keeping as much of the facility up and running as possible and only
Tripping CB’s closest to the fault.
For more details on the ARC Flash concerns, and that whole side of the argument, please refer to your handout.
34.1 When tested under the conditions described in 34.2 – 34.15, a transfer switch shall withstand the designated levels of current until the over-current protective devices open or for a time as designated in 34.3. At the conclusion of the test:
36.1 When tested in accordance with 36.2, a transfer switch shall comply with the requirements in 34.1(a) –(f).
36.1 Revised September 18, 1996
36.2 The sample for this test is to be that used for the withstand test. Test procedures and conditions for the closing test are to be as described in 34.3 – 34.19. The switch is to be closed on the circuit.
36.3. The test (for close on) current shall be the same as that used in the withstand
36A.1 A switch marked with a short-time current rating in accordance with 41.20.1 shall be tested under the conditions described in 36A.2 -36A.12 and shall withstand the short-time current for the period specified. At the conclusion of the test:
28.1 Transfer switch equipment shall perform in an acceptable manner, as intended by the manufacturer, when subjected to an overload test consisting of the number of operations specified in Table 28.1, controlling a test current as described in Table 28.2.
Table 28.1 Overload Test
Table 28.2 Method of determining test current for overload tests on transfer switches
28.4 A cycle is defined as making and breaking the required test current on both the normal and alternate contacts. During the test, the alternate source shall be displaced 120 electrical degrees from the normal source for a 3 phase supply or 180 electrical degrees for a single phase supply.
28.6 The minimum on time in each contact position is to be 1/6 second (ten electrical cycles based on a 60Hz source), unless automatic tripping of the over-current device occurs.
30.1 A transfer switch shall perform as intended when subjected to an endurance test controlling a test current as described in Table 30.1 and at a rate and number of cycles described in Tables 30.2 and 30.3.
Method of determining test current for endurance tests
The test cycle is to be 1 second “on” and 59 seconds “off”. A controller may be operated at a rate of more than 1 cycle per minute if synthetic loads are used or if a sufficient number of banks of lamps controlled by a each bank will cool for at least 59 seconds between successive applications of current.
Endurance test cycles for emergency system switches including legally required stand-by systems.
29.1 Transfer switches when tested under the conditions described in 29.2 – 29.12 shall not attain a temperature at any point high enough to constitute a risk of fire or to damage any materials employed in the device, and shall not show temperature rises at specific points greater than those indicated in Table 29.1
29.2 For the temperature test the transfer switch is to be operated under intended use conditions and is to carry its test current continuously at the test potential specified in Table 24.1.
29.3 The test current shall be 100 percent of the rated current.
Short-Time Current Duty Cycle Application. The applicable short-time current duty cycle for unfused circuit breakers consists of two periods of 1/2s current flow, separated by a 15 s interval of zero current.
No overlapping fault current of individual devices.
This is coordinated properly.
In a perfect world this is great.
ATS Feeder Breaker
8 cycles to clear
In this case, since it takes 8 cycles for the upstream breaker to clear the fault, a 3 cycle rated transfer switch is inadequate.
A 30 cycle UL rated Transfer Switch truly gives you complete coordination with any over-current protective device.
-increased thickness to 1.25”
-added strength and stability
Utilizes same mechanics as the 3 cycle switch – Heavier spring
Major Design Changes
Cradle rolls out on Ground – not on rails
- Connected – Bypassed
Shutter Closed (switch in test position or isolated)
Shutter Open (switch racked-in)