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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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Overview l.jpg
Overview

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.


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Transfer Equipment in a Common Scenario

Commercial

Utility Power

UPS

Transfer Switchgear

Emergency

Generators

Network

Computer

Loads

UPS Batteries

Air conditioning, Lighting,

Mechanical, Building Loads,etc.

Generator Paralleling

Control Switchgear

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.


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What is Selective Coordination?

  • Definition (Article 100 – NEC)

    • 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.

Article 100 provides the Code definition. Here is another way to describe it:

  • “For the full range of possible overcurrents, the act of isolating an overloaded or faulted circuit from the remainder of the electrical system, thereby eliminating unnecessary power outages.”

  • The circuit causing the overcurrent is isolated by the selective operation of only that overcurrent protective device which is closest upstream to the overcurrent condition.


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Selective Coordination, History & Requirements

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.


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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.”


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NEC 2008 –Verbiage on Selective Coordination an engineering consideration and is being enforced by inspectors more & more often…

  • NEC(2008) 700.27 Coordination: requires “Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.”

  • NEC(2008) 701.18 Coordination: requires “Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.”

  • NEC(2008) 517.26 Application of other articles: requires “The essential electrical system shall meet the requirements of Article 700.”

  • The overcurrent protective devices may include the following:

    • Molded Case Circuit Breakers

    • Fused devices

    • Insulated Case Circuit Breakers

    • Air Power Circuit breakers


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  • Requirements an engineering consideration and is being enforced by inspectors more & more often…

    • Selective coordination requirements for life safety are not a new concept for the Code. There has been a Code requirement to coordinate selectively the over-current protective devices for elevator circuits since 1993.

    • Most engineers agree this is the simplest way to assure coordination, however…..

  • Breakers

    • Instantaneous circuit breakers will not coordinate properly because typically, they aren’t adjustable.

Fuses

More on Selective Coordination


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G an engineering consideration and is being enforced by inspectors more & more often…

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.

ATS

MCCB's (Molded Case Circuit Breaker) typically instantaneous or Current Limiting Devices.

One-line

Utility

4000A

APCB

1600A

APCB

1600A

APCB

800A

ICCB

400A

MCCB

An overcurrent event (overload, short circuit, or ground fault) here should trip the 400A MCCB


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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.

400A

MCCB

G

Utility

4000A

APCB

1600A

APCB

1600A

APCB

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.

ATS

ATS

800A

ICCB

If the 400A MCCB does not trip/clear…

Selective Coordination


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Review of Code Requirements coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • Article 517 Health Care Facilities

  • 517.26 Application of Other Articles

  • Article 620 Elevators, etc

  • 620.62 Selective Coordination (2008)

  • Article 700 Emergency Systems

  • 700.9 (B)(5)(b), Exception

  • Article 701 Legally Required Standby Systems

  • 701.18 Coordination

  • Article 708 Critical Operations Power Systems

  • 708.54 Selective Coordination


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2005 Code Adoption coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.


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2008 Code Adoption coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

WA

ME

ND

MT

VT

M I

MN

NH

MA

OR

NY

WI

CT

SD

ID

RI

M I

NJ

WY

PA

IA

NB

OH

DE

Expected July 2010

MD

IN

NV

IL

WV

Expected July 10’

UT

VA

CO

MO

KS

KY

CA

NC

Expected January 2011

TN

State Adopted

SC

AR

AZ

NM

OK

S. Carolina Code Council adopted 2009 IRC with 2008 NEC 3/22/10 with implementation effective 1-1-11

TX

GA

AL

MS

LA

State Adopted Unincorporated Areas

AK

FL

AK

HI

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


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Code Rulings coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

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:

  • Panel 13 Statement:

    • “This proposal removes the selective coordination requirement from the mandatory text and places it in a non-mandatory FPN (fine print note). The requirement for selective coordination for emergency system over-current devices should remain in the mandatory text. Selective coordination increases the reliability of the emergency system. The current working of the NEC is adequate. The instantaneous portion of the time-current curve is no less important than the long time portion. Selective coordination is achievable with the equipment available now”.

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:

  • Panel 20 Statement:

  • “The overriding theme of Articles 585 (renumbered to 708) is to keep the power on for vital loads. Selective coordination is obviously essential for the continuity of service required in critical operations power systems. Selective coordination increases the reliability of the system.”


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Exceptions to Code Rulings coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated. Refer to IEEE handout “Selective Coordination versus Arc Flash…” page 12

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.


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Exceptions to Code Rulings (cont.) coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

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.”


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Arc Flash Considerations coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated. Refer to IEEE handout “Selective Coordination versus Arc Flash…” page 10

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.


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UL 1008 Withstand Test coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

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:

  • The switch shall be capable of being operated by its intended means;

  • The fuse mentioned in 34.14 shall not open,

  • There shall be no breakage of the switch base to the extent that the integrity of the mounting of live parts is impaired,

  • The door shall be prevented by its latch, without bolt or lock installed therein, from being blown open, and deformation of the door alone is not determined to be unacceptable;

  • No conductor shall have pulled out of a terminal connector and there is no damage to the conductor insulation or the conductor (see 41.56); and

  • For a plug in or draw out unit, the point of contact is to be the same both mechanically and electrically as before the test.


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UL 1008 Closing Test coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

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

test.


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UL 1008 Short Circuit Test History coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • Around 1989 UL introduced an optional 3 cycle test for any over-

  • current protection device.

    • Prior to this, manufactures could test with any over-current device.

    • If a manufacturer didn’t test to 3 cycles, they would be required provide a label that lists all breakers that the switch was “coordinated with”.

    • This requirement did not take into consideration air power circuit breakers APCB’s. Some of these breakers were 4-5 cycle devices (GE AKR and Westinghouse DS)

  • January 9th, 2002 UL introduced an optional short time current

  • rating test.

    • A withstand and a close and withstand test is required to get a UL short time rating.

  • This requirement did not take into consideration air power circuit breakers APCB’s. Some of these breakers were 4-5 cycle devices (GE AKR and Westinghouse DS)


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UL 1008 Short Time Current Test coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

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:

  • The transfer switch shall be capable of being operated by its intended means,

  • The fuse mentioned in 36A.7 shall not open,

  • There shall not be any damage to the switch base to the extent that the integrity of the mounting of live parts is impaired,

  • The door shall be restricted by its latch, without bolt or lock installed therein, from being blown open. Deformation of the door itself is not reason for rejection,

  • No conductor shall have pulled out of a terminal connector and there shall not be any damage to the conductor insulation or the conductor (see 41.56),

  • For a plug-in or draw-out unit, the point of contact shall be the same both mechanically and electrically as before the test,

  • The Temperature Test, Section 29, shall be performed on the transfer switch at the completion of the tests described in 36A.8 and 36A.9, without maintenance, and the temperature rise shall not exceed the values given in Table 29.1, increased by 10° C or 18° F, and

  • The Dielectric Voltage-Withstand Test (Repeated), Section 36B, shall be performed on the transfer switch at the completion of the tests described in 36A.8 and 36A.9.


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UL 1008 Overload Test coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

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.


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UL 1008 Endurance Test coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

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.

Table 30.1

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.

Table 30.2

Endurance test cycles for emergency system switches including legally required stand-by systems.


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UL 1008 Temperature Test coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

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.


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Overcurrent Protective Devices coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • Molded Case Circuit Breakers –MCCB (UL489)

    • May be Current Limiting to 200KA

    • Long Time Overcurrent

    • Instantaneous Interruption is less than 3 cycles

  • Fuses and Fused Devices

    • Current Limiting

    • Mostly used on 200KA circuits

  • Insulated Case Circuit Breakers -ICCB (UL489)

    • May be Current Limiting to 200KA

    • Instantaneous Interruption is typically less than 4 cycles

    • Short Time delay available (30 cycles) with Instantaneous over-ride

  • Low Voltage Air Power Circuit Breakers -APCB (UL1066)

    • May be Current Limiting to 200KA

    • Instantaneous Interruption is typically less than 4 cycles

    • Short Time delay available (30 cycles) without Instantaneous


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Low Voltage Air Power Circuit Breakers coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • APCB’s are ideal protective devices for the application of selective tripping.

  • Short Circuit Duty Cycle: Oc,15 s – CO (applying fault current

  • to a closed CB for ½ second [30 cycles] separated by 15 seconds

  • of zero current flow, then close on fault current for another ½

  • second [30 cycles] ).

    • This test may be performed with or without an instantaneous override on the closing cycle.

    • The GE AKR was tested without the instantaneous. Note some breakers now have a “Trip Free” feature in which the breaker will still clear a fault without instantaneous trip. Opinions vary on whether this is desirable or not in emergency power systems.

  • Short Time Current

    ANSI C37.13.10.2.1(2)-1990

    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.


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Selective Coordination - Good coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

No overlapping fault current of individual devices.

This is coordinated properly.

In a perfect world this is great.


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Selective Coordination - BAD coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

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.


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Complete Coordination coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

A 30 cycle UL rated Transfer Switch truly gives you complete coordination with any over-current protective device.


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New Russelectric coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated. 30 Cycle Automatic Transfer Switchesand Bypass Isolation Switches


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Back Plate Assembly coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

BACKPLATE:

-increased thickness to 1.25”

-added strength and stability

  • SIDE BARRIER:

  • 5/8” thk glass polyester

  • Greater arc & track resistance

  • Excellent flame resistance

  • Movable contact support


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CROSSARM MECHANISM coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • Made from 1.125 Square Steel Stock

  • Overcenter Spring Mechanism to Latch Contacts Closed and Open

    Utilizes same mechanics as the 3 cycle switch – Heavier spring


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OPERATORS coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • Open Transition Switches with EMO

  • Reliability of Motor Operators


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30 CYCLE BYPASS SWITCHES coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

Major Design Changes

  • Elimination of Isolation Handle

  • Gearbox, Rack-in Mechanism to engage switch

  • Bottom and Side Guiderails to align and Contain Switch

  • Secondary Disconnects accessible on Left side of cabinet

  • Optional Shutter Design

  • 800A Cradle is ‘On the Ground’ Rollout Design

  • Complete Finger Cluster Redesign for all Sizes

  • ATS Contacts Redesign


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  • Single Handle is for Bypass Operation

  • 800A Rollout Switch

Cradle rolls out on Ground – not on rails


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Gearbox Rack-in Mechanism coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • Access Rack-in Shaft through door - only in Bypass Mode

  • Position Indicator Window

    - Connected

    - Connected – Bypassed

    - Test

    - Isolated

  • Gearbox needed for Increased Spring Pressure

  • - Must Pass the “Liz Test”


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Secondary Disconnect coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • Located on left Side of Cubicle for accessibility

  • Allows for Test Position

  • Incorporated into Side Guide-Rail


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Guide Plates coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • Used for Left to Right Alignment

  • Prevents Rollout Switch from jumping or shifting during fault


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Shutter Design (Optional) coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

Shutter Closed (switch in test position or isolated)

Shutter Open (switch racked-in)


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Finger Clusters coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.

  • Added Spring Pressure for Clamping

  • Increased Contact Surface Area

  • Withstood 100KA for 3 Cycles and 85KA for 30 Cycles - without a scratch

2500A Cluster

800A Cluster