Electrical Engineering Study - Arc Flash Study, Selective Coordination and Protection

Topics :

• Selective Coordination and The NEC
• Selective Coordination
• Arc Flash
• Designing Safe Electrical Systems

Selective Coordination & The National Electrical Code :

Article : 517 Healthcare Facilities
Article : 620 Elevators, Dumbwaiters, Escalators, Moving Walks, Wheel Chair Lifts, and Stairway Chair Lifts
Article : 700 Emergency Systems
Article : 701 Legally Required Standby Systems 701.18
Article : 708 Critical Operations Power Systems (COPS)
NEC Article 100 Definition :

Coordination (Selective)
“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.”

Note: When ONLY the overcurrent device protecting the specific circuit
that has an overload or fault opens to clear it.

What Does This Mean?

• The NEC Has Mandated What Engineers Have Been Doing For Years.
• The Intent Requires A Second Look At Traditional Coordination Practices.

“Panel Statement: The instantaneous portion of the
time current curve is no less important than the long time portion.”

TCC Curves Typically Only Show Breaker Characteristics Down To 0.01
Seconds. The Need Now Is Down To Time Zero.

Selective Coordination - What Current Levels :

• Equipment Is Sized To The “3-Phase Bolted Fault” Calculated Values
• Must Selective Coordination Be Met At These Same “3-Phase Bolted Fault” Calculated Values?

If So, One Must Have No Over Lapping Of Curves Up through And Including The Calculated Available 3-Phase Bolted Fault Current.

Selective Coordination –Off The Charts :

• What happens when these two devices see a current above 50kA?
• Can you tell from the curves?
• Device Changes Can Make The Questions Go Away


TCC Curves Only Provide A Portion Of The Story

Effective Coordination Designs :
• To Perform Effective Coordination Studies
• Design Engineers Need A Better Understanding Of The Protective Devices Being Applied
• Data Beyond The TCC Curves Is Required

Achieving Selective Coordination :

To Achieve Selective Coordination the following is needed:
• Understanding of how Overcurrent Protective Devices Operate
• Short Circuit Study
• Coordination Study
• Manufacturers Test Data
Studies are to be done by a Qualified Person


Preservation of Selective Coordination :

To Preserve Selective Coordination over time, the initial study should be reviewed when changes occur to the distribution system:
• Available short-circuit current is increased/decreased
• Additional loads are installed
• Existing equipment is replaced with higher rated equipment
• Fault condition occurs
Studies are to be done by a Qualified Person

Circuit Breaker Short Delay Time Settings :

Short Delay Time SettingsThe maximum Short Time Delay Setting is determined by the ability of a Circuit Breaker to keep its contacts closed under fault conditions

Molded or Insulated Case Breakers in Series :

Power Circuit Breakers in Series :

Selective Coordination :

Current Limiting Fuses & Current Limiting Circuit Breakers

• Not current limiting for currents less than approximately 10 to 15 times the fuse or CL Circuit

Breaker ampere rating.
• Time-Current Curves can be used to determine Selective Coordination in non-current limiting mode typically 0.01 seconds.
• In the current limiting mode need to use specific manufacturers selectivity ratio tables.

Selective Coordination Discussion – Fuse Devices :

To Achieve Selective Coordination:

• Each upstream overcurrent device must wait longer and longer on fault conditions to clear

• The higher the available fault current the more likely that additional techniques must be used

Selective Coordination :

Solutions – Used when the fault current levels Prohibit total selective coordination:

• Larger overcurrent protective devices
• Increase Impedances (Z) to reduce available fault currents i.e. relocate equipment with longer conductors
• Utilize step down or isolating transformers
• Change single line diagram i.e. Reduce the number of overcurrent devices in series

Selective Coordination & Arc Flash :

• Meeting Total Selective Coordination Up To Available Bolted Three Phase Calculated Fault Currents.

• Requires Upstream Protective Devices To Wait Longer To Enable Downstream Protective Devices To Clear The Fault

Protective Devices Permitting Fault Currents To Flow With Intentional Delay Sets The Stage For Higher Arc Flash Energy

Arc Flash :

An Arc Flash is produced by the flow of electrical current through ionized air after an initial flashover or short circuit.

Releases emissions that can result in major equipment damage and/or serious injury or death to personnel:

• Thermal Heat
• Toxic Fumes
• Pressure Waves
• Blinding Light
• Sound Waves
• Possible Shrapnel

Typical Causes of Arc Flash

• Inadequate short circuit ratings

• Tracking across insulation surfaces
• Contamination such as dust on insulation surfaces
• Corrosion of equipment and contacts

• Accidental contact with energized parts

• Tools dropped on energized parts
• Wiring errors
• Improper work procedures

Arc Flash Impact In Codes And Standards :

• NFPA 70E – Standard for Electrical Safety in the Work Place

• Enforcement by OSHA Is increasing
• NEC Section 110.10 Circuit Impedance and Other

Characteristics: “The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. …”

NEC Section 110.16 Flash Protection: “Electrical equipment, such as switchboards, panelboards, and industrial control panels, meter socket enclosures, and motor control centers, that are in other than dwelling occupancies, and are likely to require examination, adjustment, servicing or maintenance while energized shall be field marked to warn qualified persons of potential electric arc flash hazards….”

NEC Handbook “…Field marking that warns electrical workers potential electrical arc flash hazards is required because significant numbers of electrical workers have been seriously burned or killed by accidental electrical arc flash while working on “hot” (energized) equipment…..”



To Achieve Arc Flash reduction the following is needed:

• Short Circuit Study
• Selective Coordination Study
• Arc Flash Study
• Understanding of how Overcurrent Protective Devices Operate

Studies are to be done by a Qualified Person

Arc Flash energy is dependent on two major factors:

1. Available fault current
2. The time it takes the protective device to open and clear
the fault
[3 phase to phase, single phase to phase or phase to ground].

In addition, to determine proper Personal Protective Equipment (PPE), the distance from the arc flash

point to the person must also be considered.The distance varies based on the class of protective devices involved - typically 18 inches for low voltage devices.

• The amount & duration of Arc Flash energy.

• Total Clearing Time.
- Fault current level
- Short Delay or Instantaneous settings
• For higher available fault currents, fuses and Current Limiting Circuit Breakers, when in the current limiting mode, will be more effective in reducing the percentage of I 2 T and Peak Let Through Currents

Solutions to reduce Arc Flash Energy :1. Use current limiting circuit breakers

2. Use a more current limiting fuse (RK1 versus RK5)
3. Zone Selective Interlocking
4. Arc Reduction Maintenance Systems
5. Reduce available fault current – more cable, transformers, CL reactors.
6. Isolation barriers inside equipment