Selective Coordination Overview of Short Circuit Selective ...
Selective Coordination of Overcurrent Protective Devices A Change is Gonna Come What Happened in the NEC 2014? Big Changes, Big Headaches!
NEC 2014 - Article 100 Definition What You See, Is What You Get Article 100 implies full range = 0.01 sec Change How You Think About Designing
Elevator Circuits Emergency Systems
Health Care Facilities Legally Required Systems Critical Operation Data Centers Critical Operation Systems You Will Have to Change How You Think About
Elevator Circuits - 620.22 Health Care Facilities 517.26
Emergency Systems 700.28 Legally Required Systems 701.28 Critical Operation Data Centers 645.27 Critical Operation Systems 708.28 Elevators Typical Health-Care Facility Electrical System
(Informational Note Figure 517.30) Change Must meet Article 700 per 2014 edition
Changed to branch in 2014 edition Deleted in 2014 edition Must meet Article 700 per 2005-2011 editions Emergency Systems
Emergency Systems Legally Required Systems Critical Operations Power System (COPS) Critical Operations Power System (COPS)
What Does These NEC Code Articles Have in Common? Theses articles all state, selective coordination shall be selected by a licensed professional engineer. You have the power!
Data Centers How Often Does This Happen? 3 - breakers in series No overlapping of TCCs. 60F day in August in Atlanta
If It Was Easy Anyone Could Do it Circuit Breaker Challenges Higher fault currents makes it more difficult to selectively coordinate breakers down to 0.01 seconds. Most breakers have instantaneous trips Only UL 1066 LVPCBs come w/o instantaneous trip, i.e.
turn off the I function in LSI trip. UL 1066 are larger frame sized breakers with solid state trips (800A -6000A). Larger breakers = larger electrical apparatus = less revenue producing space. Not all mfrs trip units function the same. How do you level the mfrs technical differences. Molded case switches have lower instantaneous levels &
lower interrupting ratings. Fusible Equipment Challenges Typically larger than circuit breaker equipment. Larger electrical rooms = less revenue producing space Fusible panelboards & Elevator Modules with CLFs are more expensive than comparable breaker equipment. Stocking and maintenance concerns with fuses.
GFP is difficult on standard switches. GFCI forget about it! Easier to coordinate when using fuses ratio tables. Current Limiting Fuses Easier to selectively coordinate. Selective Ratio Charts
2:1 ratio of upstream to downstream fuse Circuit Breakers have tables, too! Avoid Mixing Fuses & Breakers Avoid using upstream fuses
and downstream circuit breakers. Why? The fault current must be less than the current limiting threshold of the fuse. The CL threshold is below the .01 time band on the fuse
curve. Where is the Current Limiting Threshold Point? Thermal Magnetic Circuit Breaker Thermal (overload) element is fixed. It cannot
be adjusted. Instantaneous can either fixed or adjustable. 20/1 CB Q frames are fixed. 150A frame some adjustability with most mfrs.
So, its really all about the Instantaneous trip. Do they overlap at 0.01 seconds? Solid State or Microprocessor Trip Units Trip Unit Adjustments
TCC Adjustability Methods to eliminate the overlapping effect of the instantaneous trip units Reduce the available fault current Set the instantaneous trip above
the actual fault current Select a breaker will an instantaneous over ride higher than the fault current. Thermal Mag Breakers CAN Be Selectively Coordinated If you can reduce the
fault current level below the instantaneous level of the downstream breaker, then selective coordination is possible. Add transformer
Add impedance Use the 3:1 ratio rule 3:1 Ratio Rule for Breakers The frame size of the up stream breaker should be 3X as large as the downstream breaker. Dont confuse trip with frame size. A 20A breaker is typically a 60A frame.
Example: 40/3 breaker should be fed from a 250A frame breaker. Check with the breaker manufacturer tighter ratios are possible. Solid State Trip Unit 100,000 A rms sym fault current
Instantaneous curve ends at the max fault current point Solid State Trip Unit 60,000 A rms sym fault current Instantaneous curve ends at the max fault current point
Solid State Trip Unit 30,000 A rms sym fault current Instantaneous curve ends at the max fault current point Solid State Trip Unit
25,000 A rms sym fault current Instantaneous curve ends at the max fault current point. What happened? 25kA was less than the instantaneous trip rating of the upstream breaker. Electro-magic!
Solid State Trip Unit 15,000 A rms sym fault current Instantaneous curve ends at the max fault current point Solid State Trip Unit
10,000 A rms sym fault current Instantaneous curve ends at the max fault current point Now, you can slide those 20/1 breakers in and underneath the curves of the two upstream breakers Thats what Im talkin bout, Willis!
Solid State Trip Unit Adjust the upstream breakers instantaneous trip unit above the available fault current. Fault current = 41,638 Instantaneous trip set to 12X Eliminate main breakers in panelboards.
Molded case switches arent an improvement. Use the 3:1 ratio rule with downstream thermal magnetic breakers. Impedance is your friend- it helps with selective coordination. Dry type transformers are your friend it adds impedance & reduces fault current.
Use breakers with adjustable solid state trip units (LSI) turn off the I UL 489 listed breakers (molded case, insulated case) with adjustable solid state trips (LSI) have an instantaneous over ride built in.
Example: Manufacturer S, 1200A frame 50kAIC @ 480V Instantaneous over ride is 24kA. Turn off the I Allow for +/- 10% tolerance, use on circuits with available faults below 22kA.
Generator Main Breakers (UL 489) Fault currents are relatively low unless large sets in parallel. Use breakers with adjustable solid state trip units (LSI). Raise short time delay. Inst off. Maximize the space below and to the left of the gen breaker curve.
In many cases the inst. over ride will be higher than the fault current produced by the genset Generator Main Breaker How Do I Fix This? Use Solid State Breakers on Gensets
Adjustable LSI trip units The Red diagonal line is the thermal damage curve for the alternator. 1.5 seconds is a critical an inflection point. Ideally we want our breaker trip curve between the green and black lines. But ,we also want our I turned off.
Set the STD to about 0.15 seconds. Use Solid State Breakers on Gensets The last thing you ever want to happen is tripping the main breaker on the generator when running in emergency mode.
The idea is to create as much space below and to the left of the alternator damage curve. Turn off the I Set the STD high Use a solid state breaker with the instantaneous over ride higher than the decrement curve fault current >
4000A. Thats easy! Generator Main Breaker How Do I Fix This? Summary Avoid mixing breakers & fuses. Reduce the available fault current.
Add impedance to the circuit. Use breakers with solid state trip units. Set the instantaneous trip above the actual fault current. Select a breaker will an instantaneous override higher than the available fault current. Use the 3:1 Ratio Rule for Breakers Eliminate main breaker panelboards where possible. Check with the breaker manufacturer for hybrid breakers
that improve coordination.
Please list the available short circuit currents on the single line diagram. Thank you!!! Ken Box, P.E. Schneider Electric USA 30000 Mill Creek Avenue, Suite 300 Alpharetta, GA 30022
Phone: 770-792.4839 [email protected] References & Specials thanks to National Fire Protection Association NFPA-70 The National Electrical Code 2014 edition. Robert E. Fuhr, P.E. Power Systems Engineering, Covington, WA Meeting the NEC Requirements for
Selective Coordination. Square D/Schneider Electric Data Bulletins
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