Arc Flash

Description
Arc Flash is the result of a rapid release of energy due to an arcing fault between a phase bus bar and another phase bus bar, neutral or a ground. During an arc fault the air is the conductor. Arc faults are generally limited to systems where the bus voltage is in excess of 120 volts. The air itself provides a highly resistive path for the arc, which makes it more difficult to identify and clear quickly than a 3 phase bolted fault with overcurrent elements.

There are two components to an arc flash, a physical and radiative. During an arc flash event the heat is so intense that it causes an explosion (arc blast) as nearby metal expands 50,000+ times its volume as it changes from a solid to gas or plasma. The metal plasma also radiates an incredible amount of infrared and ultraviolet light, which is absorbed by anything around it.

Arc flash studies try to determine the worst case amount of energy release, measured in cal/cm^2. This is used to determine the personal protective equipment (PPE) that is required.



Causes
Inadvertent contact

Loose connections

Insulation failure

Poorly maintained equipment

Voltage transients

Unsuccessful Short Circuit Interruption

Animals (squirrels, snakes .. Etc)

Personal Protection Equipment(PPE)
Here is a table from NFPA 70E on the different levels of PPE protection. It should be noted that the PPE is only designed to prevent serious burn injury but not injury all together during an arc flash event. At higher energy levels (40+ cal/cm^2), the PPE will be unable able to protect personal due to the physical intensity of the arc blast, the rapid expansion of copper and other metals as they change state from a solid to gas or plasma.



Denergize Equipment
Denerergize equipment during maintenance and remove any risk of an arc flash event.

Create Space
Use robotics or remote racking equipment to increase the space between personal and the equipment.



Reduce Available Fault Current                                                                                                                                         Operationally reconfigure the system, use current limiting fuses, circuit breakers, or chokes to reduce the available fault current. The severity of an arc flash is measured in the amount of energy released. Depending on how the system is setup, a low impedance arc may release less energy (I^2*T) than a high impedance arc with lower fault current due time to trip being longer. Reducing available fault current doesn't guarantee lower calculated maximum arc flash energies.

Also, a high impedance grounded system can naturally reduce the available ground fault current below what is required to sustain a ground arc flash event. It won't ,though, provided protect for phase-phase and 3 phase arc flash events, which is worse case in the IEEE 1584 calcs. This won't change the required PPE but it would reduce the risk of an arc flash event. Limiting the severity of ground faults would be limiting the most common fault.

Arc Flash Tested Circuit Breaker Time Over Current Curves
The tripping times using standard curves my the manufacture are often conservative and testing or purchasing tested breakers might reduce the calculated PPE required over using standard curves. The IEEE IAS magazine issue quoted below mentions this as well as this Eaton document. The standard curves are based on the slowest breaker clear time for breakers in that class.

Flash Detection
This type of method operates on the principle that if light is picked up inside of the switchgear by a fiber-optic detector and current above the threshold is detected, an arc flash event has occurred and tripping should occur without delay. Operates very fast and in as little as 0.25 cycles.

SEL-751 and SEL-751A Relays

Siemens 7XG3124 - ReyArc24 - Another product that operates the same way SEL's does.

ABB REA system - operates the same as SEL's.

High and Low Impedance Bus Differential
It can operate in less than 1.5 cycles. Requires additional relay, dedicated CTs, wiring installation. Testing more complex. Does not operate for faults outside differential zone, which makes it secure but differential in itself doesn't provide any sort of protection for out of zone faults.

Maintenance Switch
Use a maintenance switch or push buttons on the relay to speed up the tripping at the expense of security. The new coordinated settings should trip instantly for any detected fault. This method can easily be retrofitted to an existing arrangement with electronic relays.

Re-coordinate the system
Review the system and determine if the over-current curves can be tightened up to reduce the maximum amount of energy released before the fault is cleared. If electronics relays are being used, coordination time intervals of 0.25 may be used and lower still with testing and analysis.

Zone interlocking (ZSI)
It can operate in 3-5 cycles. A poor man's differential. It requires communications. This unlike straight up differential can provide out of zone protection.

Arc Flash Resistant Switchgear
Use arc flash resistant switchgear that is designed to limit the exposure and/or redirect the energy through plenums or arc flaps.

Eaton Arc Resistant Switchgear

Siemens Arc Resistant Switchgear

Schneider Arc Resistant Switchgear

Crowbar System
Use a system that crowbars the three phases when it detects light and current. This must be near the switchgear and may damage nearby equipment. Some systems create a parallel arc concealed inside of a drum to crowbar the system.

Current Limiting Fuses and Circuit Breakers
These fuses and breakers can be used to interrupt the fault and limit peak let-through current. These fuses and breakers become very resistive above its threshold, limiting let-through current. The energy released will be proportional to I^2*T so the effect of limiting the let-through current will be a reduction in released energy.

Tested Circuit Breakers
Using arc flash tested circuit breakers will base the clearing times of the breakers on actual test data instead of conservative generic curves. The generic curves are based on the slowest breakers in that class so often will be overly conservative. This Eaton paper "Applying Low Voltage CIrcuit Breakers to Limit Arc Flash Energy" and IEEE-IAS Issue 4 July/Aug Magazine both talk about this.

Excel Calculators
EPowerPlus Calculator

I-Gard Calculator

Mike Holt Calculator

Software
ETAP

SKM

CAPE