Arc Flash Study
The following Arc Flash study defines what an arc flash is and why it is dangerous. It also outlines best practices for limiting the probability of Arc Flashes occurring and harming workers interacting with the equipment. Because Arc Flashes create such hazardous conditions, warning labels, protective clothing, and special field measures have to be taken to mitigate risk. The ways in which workers and equipment can be protected from Arc Flashes are specified below.
WHAT IS AN ARC FLASH?
A flashover event where electric current leaves its intended path and travels through the air from one conductor to another, or to ground. An Arc Flash can occur because of environmental conditions (dust, condensation), normal/abnormal wear-and-tear (material failure, corrosion), or personnel (dropping tools, accidental touching).
Temperatures can reach as great as 20,000-35,000˚C, with volumetric expansion as great as 40,000-67,000 times. Light intensity as much as 2000 times normal office lighting.
WHAT IS AN ARC FLASH HAZARD?
“An Arc Flash hazard may exist when energized electrical conductors or circuit parts are exposed or when they are within equipment in a guarded or enclosed condition, provided a person is interacting with the equipment in such a manner that could cause an electric arc. Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard.”
It’s noted that equipment that needs trouble shooting is no longer “normally operating”.
WHAT IS INCIDENT ENERGY?
The incident energy associated with an Arc Flash event is the thermal energy (measured in cal/cm2) at a working distance from the arc fault. The thermal energy is a function of a) the amount of current associated with the arcing fault and b) the time it takes protective relaying to open breakers and remove the fault.
In practical terms, it’s similar to holding your hand over an open flame. The amount of absorbed heat (incident energy) is directly related to how close your hand is (working distance), how big the flame is (arcing fault current), and how long you hold your hand over it (arcing time).
What Clothing Is Appropriate?
Arc-rated (AR) clothing is tested for exposure to an electric arc, which is different than how flame-resistant (FR) clothing is tested. Arc-rated clothing is FR rated, but not the other way around. (Material must be flame-resistant before it can even be tested to determine its arc-rating.)
Arc-rated clothing will carry an Arc Thermal Performance Value (ATPV) rating, which is the maximum performance capability in cal/cm2.
Clothing must not only carry an arc rating but also be labeled correctly, to indicate compliance with national standards – ASTM F1506 (material) and ASTM F1959 (test method). NFPA 70E Table 130.7(C)(14) lists the ASTM and ANSI standards to which PPE must adhere, but labeling in accordance with NFPA 70E alone is irrelevant because the NFPA doesn’t approve equipment/clothing.
As with FR clothing, launder arc-rated clothing in accordance with labels.
What Do the Labels Say? What Do They Mean?
Arc Flash labels must include the following information according to NFPA 70E.
- Nominal system voltage, AND
- Arc flash boundary, AND EITHER
- Available incident energy and corresponding working distance OR
- Minimum arc rating of clothing OR
- Site specific level of PPE
All labels will say Warning, whether the Arc Flash Hazard is low (0) or high (4). DON’T GET COMPLACENT! Level 0 and Level 4 stickers look the same! Review all WARNING labels closely to determine what the Hazard is and what PPE to wear.
“Flash Hazard at” – most labels will be either 1 ft. 6 in. or 3 ft., which is the working distance associated with the calculation – in general, the energy is worse if you’re closer than this distance, and the energy is less if you’re further away than this distance.
“Min. Arc Rating” – this number describes how much “incident energy” is available during an arc flash event. Remember that 2nd degree burns occur at 1.2 cal/cm2.
“Flash Protection Boundary” – if you are within this boundary when an Arc Flash occurs, and not wearing the appropriate PPE, you could receive a 2nd degree burn during an arc flash event (incident energy is 1.2 cal/cm2)
As a practical point, put CAUTION tape up at the further of the “Limited Approach” and “Flash Protection Boundary” distances. (In most cases it will be the “Flash Protection Boundary”.)
“Glove Class” – self-explanatory – glove classes are related to voltage levels and electrical shock rather than Arc Flash
“PPE Level” – the Category/Level (0 thru 4) of the PPE that must be worn, and a listing of the PPE. The PPE is directly associated with the “Min. Arc Rating”.
“Bus” – identifies the electrical bus from the system study
“Prot.” – identifies the upstream protective device; adjustments to this device will affect the Arc Flash
These stickers directly relate to the Arc Flash Hazard – No Safe PPE Exists – Do Not Work on Live!
DO NOT OPEN THESE COMPARTMENTS OR EXPOSE LIVE PARTS UNDER ANY CIRCUMSTANCES!
How Does This Affect Day-to-Day Operations?
That depends on what the customer considers normal operation and daily tasks performed.
“If it has a label on it, then don’t open it!”
It Has a Label on It and I Opened It – Now What?
Hopefully nothing, because you first placed the equipment in an “electrically safe” condition, by de-energizing, LOTO’ing, testing for the absence of voltage, and grounding as necessary.
It’s always best to try and mitigate the risk by other means – engineering controls, work procedures, etc. Working on live parts should be a last resort.
What Exactly Did You Open?
Transformer secondaries are especially dangerous, because they rely on upstream protection devices to remove faults at those locations. There can be a lot of time (many cycles to 2 seconds) that passes before the fault is removed.
480 V equipment can be especially dangerous, because of the high currents and delayed trip times associated with this equipment.
At one customer facility – 300 MW generating plant – most equipment had a PPE Level of 2 or less. Some equipment with a PPE Level of 3 or more included:
- 4160 V Switchgear and the secondary windings of the upstream 13.8 kV-4.16 kV transformers.
- 480 V Switchgear and the secondary windings of the upstream 4160-480 V transformers.
- Secondary windings of several 30 kVA and 45 kVA transformers that were fed from MCCs.
- Primary winding of a 45 kVA CPT that was fed from a panelboard instead of an MCC.
- Fire Pump Controller and its upstream transformer.
I Don’t Want to Wear a Moon Suit. What Do I Do?
Anything that can be done to reduce the fault current and fault duration has a direct impact on the incident energy.
SETTINGS ADJUSTMENTS – LITTLE CHANGES HAVE BIG IMPACTS
480 V Load Center – MAIN 1 – Short Time Delay Setting – I^2t value is IN/ON – resulted in longer trip time during a fault condition, resulting in an incident energy of 52 cal/cm^2 “DANGEROUS”.
Consider OEM specific options as appropriate.
- GE offers “Reduced Energy Let Thru” (RELT) system, which allows the user to select a lower instantaneous pickup setting for its electronic trip units. Available with EntelliGuard Trip Units.
- Eaton offers Network Protector Arc Reduction Maintenance System (NPARMS), which overrides the time delay function of overcurrent relays with a preset instantaneous trip setting.
- Others?? Siemens? ABB?
Arc Flash Detection products
- GE offers Multilin A60 “Advanced Light & Pressure Arc Flash Detection System”, which detects both light and sound (pressure wave) via LEDs, bare fiber and diaphragm/membrane, and determines if an arcing fault condition exists. It is a smaller, DIN rail mountable device, typically use one A60 per vertical section of switchgear, daisy chain as needed. Output contacts rated 6A continuous, 30A make and carry – use to trip breakers, LORs, signaling, etc.
- SEL offers the SEL-751 and SEL-751A feeder protection relays, which include Arc Flash detection. The relays use bare fiber sensors to detect the light flash associated with an arcing event coupled with increased phase current to determine if an arcing fault condition exists. Traditional panel-mount or rack-mount relay that provides Arc Flash detection in addition to traditional overcurrent protection.
- ABB offers REA arc detection/mitigation system – arc fault protection is based solely on light intensity and/or combination of light intensity and overcurrent, depending on modules/system configuration
Zone Selective Interlocking (ZSI)
- For use on systems where upstream (main) breakers are set with intentional time delay to coordinate with downstream (feeder) breakers. Establish communications between protective devices, and allow fast tripping of upstream breakers with no intentional time delay.
- If gear is protected by multi-function, microprocessor relays, consider the use of alternate settings during maintenance.
- Remote racking – relocates the operator to a safer location
- Bus differential relaying – high speed, can detect faults and operate within milliseconds
GRAY AREAS – LIMITATIONS
The IEEE 1584 equations are valid for voltages between 208 V and 15 kV.
For other voltages, the Lee Equation is used to determine incident energy, but the Lee Equation is applicable to “open air arcs” (not “arcs-in-a-box”).
What about 25kV or 35kV metal-clad switchgear?
ASSET recommends that DANGER labels be placed on gear regardless of calculated incident energy. The label should include descriptive information such as “Flash Hazard is Indeterminate – Consider Dangerous” and then either state that “No Energized Work is Allowed”, or make reference to site specific work procedures.
Consider the Lee Equation for open air arcs at any voltage.
The Lee Equation uses the bolted fault current to determine incident energy.
Is this practical for live line work? Yes? No? Maybe?
Reference OSHA 1910.269, especially Tables 6 and 7 – these tables provide incident energy levels for open air, phase-to-ground arcs for typical overhead systems.
Table 6 – glove work for voltages 4.0 to 46.0 kV
Table 7 – live-line tool work for voltages 4.0 to 800 kV
The goal of this Arc Flash study has been to limit the probability of Arc Flashes occurring and causing harm. Part of limiting the possibility of the Arc Flash harming someone is through regular maintenance of the equipment. But it is also necessary to educate workers on how to behave safely in the field so that they do not harm themselves, others, or their work environment. An Arc Flash study, such as this one, is one way to do that.