Arc Fault
Contents
Arc Flash History
First official publication on the arc flash hazard was published by Ralph Lee published in "The Other Electrical Hazard, Electric Arc Blast Burns", 1982. In this paper the thermal event associated with an electric arc and its effects on the human body was analysed and described. Value of the 1.2 cal/cm^{2} was defined as the "curable burn level" (defined as the lower limit for a 3rd degree burn) that is still used today, also some calculations to determine the curable burn distance for an electric arc in air. In 1987 Ralph Lee published another paper, "Pressures Developed from Arcs", where the sound and pressure effects of an arc in air were described. Included in this paper were charts to determine the pressure wave forces at various distances based on the fault duties at the location.
Two more papers were published that further defined the energies in arcing faults. The first was the paper "Testing Update on Protective Clothing and Equipment for Electric Arc Exposure", 1997, by Bingham, Doughty, and Neal. In that paper the authors used empirical test data to determine the incident energy at various distances from a low voltage arcing fault. They were the first to express the directional effect of an arc within an enclosure. In 2000, Doughty, Floyd, and Neal published "Predicting Incident Energy to Better Manage the Electric Arc Hazard on 600 V Power Distribution Systems", which defined incident energy based on fault duty, working distance and clearing time for arcs in air or in an enclosure.
This work was used in the NFPA-70E Standard for Electrical Safety Requirements for Employee Workplaces, 2000 Edition, for use in developing safe work practices with regard to arc flash hazards, but was limited to low voltage applications. It also represented the basis for further research that resulted in the publication of the IEEE Std. 1584-2002, "IEEE Guide for Performing Arc-Flash Hazard Calculations".
Arc Flash Definitions
An arc fault is an electrical discharge between two or more conductors, where the insulating atmosphere (air or gas) has been broken down by the electric field between the conductors. Whenever there is an arc fault, the gases and vapours that make up the atmosphere between the conductors become ionised.
The magnitude of an arc fault is highly variable. The instantaneous arc fault current may be high, approaching the bolted short circuit current, or reasonably low, comparable to the load current. An arc will continue until it becomes unstable and extinguishes itself or until it is interrupted by a protection device (i.e. fuse or circuit breaker).
Arc faults are characterised by extreme temperatures that can cause severe burns depending on the distance of the operator to the arc. Neal et al [1] in Table IV determined that a 600 V, 40 kA arc fault with a duration of 0.5 s has enough energy to cause second-degree burns at a distance of 77 inches (1.96 m).
Additionally, arc faults tend to melt terminals that can potentially shower the immediate vicinity with molten metal. The extreme temperatures produced by an arc fault can also lead to fires, causing major damage to equipment.
Annex C of IEEE Std 1584 [2] outlines case histories of real life arc fault incidents. The majority of incidents occurred during energisation and switching operations or live electrical installation work. The potential causes of arc faults include contamination / pollution ingress, equipment failure, rodents / vermin and accidental contact with tools.
Common definitions:
Incident Energy Exposure:the amount of thermal incident energy to which the worker's face and chest could be exposed at working distance during an electrical arc event. Incident energy is measured in joules per centimeter squared (J/cm^{2}) or calories per centimeter squared (cal/cm^{2}).
Flash Protection Boundary: the flash protection boundary is an approach limit at a distance from exposed live parts or enclosed live parts if operation, manipulation, or testing of equipment creates a potential flash hazard, within which a person could receive a second degree burn if an electrical arc flash were to occur.
Incident Energy at Flash Protection Boundary: the arc flash protection boundary (FPB) distance for the specific incident energy, usually provided from the manufactures for the corresponding personal protection equipment (PPE).
Hazard Risk Category: this is the minimum level of the personal protective equipment (PPE) in cal/cm^{2}, as evaluated in the IEEE Standard 1584, with the intent to protect the worker from the thermal effects of the arc flash at 45 cm or 18 inches from the source of the arc.
Grounding Type: according to the IEEE 1584 procedure two grounding classes are applied:
a) ungrounded, which included ungrounded, high-resistance grounding and low-resistance grounding, and
b) solidly grounded.
Gap between Conductors: equipment bus gap in mm. Gaps of 3 to 40 mm were used for low voltage testing to simulate gaps between conductors in low voltage equipment and cables. Gaps 13, 104 and 152 mm. were used in 5 and 15 kV equipment testings. For cases where gap is outside the range of the standard empirically derived model, the theoretically derived Lee method can be applied.
Working Distance: typical working distance is the sum of the distance between the worker standing in front of the equipment, and from the front of the equipment to the potential arc source inside the equipment. Arc-fash protection is always based on the incident energy level on the person's face and body at the working distance, not the incident energy on the hands or arms. The degree of injury in a burn depends on the percentage of a person's skin that is burned. The head and body are a large percentage of total skin surface area and injury to these areas is much more life threatening than burns on the extremities (see Fig.3).
For the Figure 3. following definitions can be applied:
Flash Protection Boundary: An approach limit at a distance from exposed live parts within which a person could receive a second degree burn if an electric arc flash were to occur. Appropriate flash-flame protection equipment must be utilized for persons entering the flash protection region.
Limited Approach Boundary: An approach limit at distance from an exposed live part within which a shock hazard exists. A person crossing the limited approach boundary and entering the limited region must be qualified to perform the job/task.
Restricted Approach Boundary: An approach limit at a distance from an exposed live part within which there is an increase risk of shock, due to electrical arc over combined with inadvertent movement, for personnel working in close proximity to the live part. The person crossing the restricted approach boundary and entering the restricted space must have a documented work plan approved by authorized management, use PPE that is appropriate for the working being performed and is rated for voltage and energy level involved.
Prohibited Approach Boundary: An approach limit at a distance from and exposed live part within which work is considered the same as making contact with the live part. The person entering the prohibited space must have specified training to work on energized conductors or live parts. Any tools used in the prohibited space must be rated for direct contact at the voltage and energy level involved.
Arc Flash Mitigation in Switchgear
Annex ZC6 of AS 3439.1 [3] provides guidelines for the minimisation, detection and containment of internal arc faults in switchgear. These are summarised below:
- Insulation of live conductors (in addition to clearances in air)
- Arrangement of busbars and functional units in separate vented compartments, for more rapid extinguishing of the arc and to contain an arc fault in a single compartment
- Use of protection devices to limit magnitude and duration of arcing current
- Use of devices sensitive to energy radiated from an arc to initiate protection and interrupt arcing current
- Use of earth current detection devices for interruption of arc faults to earth
- Combinations of the above
It should be noted that uncontained arc faults can spread to other parts of the switchboard and develop into larger faults (e.g. functional unit arc fault spreading to busbars).
Arc Flash PPE
As a general guideline, Neal et al [1] recommends the following personal protection equipment to safeguard against arc faults:
- Clothing consisting of outer layer(s) of loose fitting flame-resistant fabric without openings and inner layers of non-meltable fibres
- Switchman’s hood or faceshield with 0.08 inch thick polycarbonate viewing window
- Heavy duty flame-resistant work gloves
- Heavy duty work boots
Annex C of IEEE Std 1584 [2] illustrates a case study (No. 42) of a 2.3 kV switching operation that ultimately ended in an arc fault. The operator was wearing a full arc flash suit, safety glasses and fire resistant shirt and pants. The PPE prevented any burn injuries from the arc flash. Other case studies where the operators were not wearing appropriate PPE resulted in severe burns or death.
Arc flash PPE is normally rated to an Arc Thermal Performance exposure Value (ATPV), which specifies the maximum incident arc fault energy that can protect the wearer (measured in calories per cm^{2}).
By way of example, the results of an arc flash hazard calculation based on IEEE Std 1584 follows to determine the appropriate ATPV rating of PPE. The prospective fault current used was 25 kA. A fault clearing time of 0.5 s was chosen, which is suitably onerous for a worst-case incident.
The calculation concluded that to protect against injury from an arc fault of this magnitude, PPE with an ATPV rating of over 50 cal/cm^{2}). is required. The ATPV rating is typically quoted on commonly available arc flash PPE.
Typical arc fault PPE is available from vendors such as Oberon.
Arc Flash Calculation according to the IEEE Std. 1584-2002
IEEE Std 1584-2002 contains calculation methods developed through testing by several sources to determine boundary distances for unprotected personnel and the incident energy at the working distance for qualified personnel working on energized equipment. The incident energy level can be used to determine the proper PPE required for personnel. The equations developed in the IEEE standard assess the arc flash hazard based on the available (bolted) fault current, voltage, clearing time, equipment type, grounding, and working distance. The working voltage is also used to determine other variables. The equations have other variables that account for grounding, equipment type, and construction. This method can also determine the impact of certain current limiting low voltage fuses as well as certain types of low voltage breakers. It is an improvement over the previous work in that the calculations can be applied over a large range of voltages. The many variables of this method make it the preferred choice for Arc-Flash evaluations, but at the same time requires either a complex spreadsheet or computer program to be used efficiently.
Determining the arc current
For applications under 1 kV:
[math] {\log}({I}_{a}) = {K} + {0.662}\cdot{\log}({I}_{f}) + {0.0966}\cdot{U}_{sys} + {0.000526}\cdot{d}_{G} + {0.5588}\cdot{U}_{sys}\cdot{\log}({I}_{f}) - {0.00304}\cdot{d}_{G}\cdot{\log}({I}_{f}) [/math]
For applications above 1 kV:
[math] {\log}({I}_{a}) = {0.00402} + {0.983}\cdot{\log}({I}_{f}) [/math]
And getting the value from the log_{10}:
[math] {I}_{a} = {10}^{{\log}({I}_{a})}[/math]
where:
I_{a} - the arc fault current (kA),
K - for the open configurations (-0.153), for the closed/boxed configurations (-0.097),
I_{f} - is the bolted fault current for three-phase faults (symmetrical RMS)(kA),
U_{sys} - the system voltage,
d_{G} - the gap between conductors (mm).
The second arc current (I_{2a}) is equal to 85% of the first current, or:
[math] {I}_{2a} = {0.85}\cdot{I}_{a} [/math]
Determine the incident energy
When the arc current is known the incident energy could be calculated. But first the incident energy normalized for the time and distance is calculated:
[math] {\log}({E}_{n}) = {K}_{1} + {K}_{2} + {1.081}\cdot{\log}({I}_{a}) + {0.0011}\cdot{d}_{G} [/math]
[math] {E}_{n} = {10}^{{\log}({E}_{n})}[/math]
Now the incident energy can be calculated as follows, for the systems where the voltage does not exceed 15 kV:
[math] {E} = {C}_{f}\cdot{E}_{n}\cdot\frac{t}{0.2}\cdot\frac{610^x}{D^x} [/math]
And for the location where the voltage exceeds 15 kV the Lee method is used:
[math] {E} = {5.12}\cdot{10^5}\cdot{U}_{sys}\cdot{I}_{f}\cdot\frac{t}{D^2} [/math]
Where:
E_{n} - the incident energy normalized for time and distance (cal/cm^{2}),
K_{1} - for the open configurations (-0.792), for the closed/boxed configurations (-0.555),
K_{2} - for the ungrounded or high resistance grounded systems (0), for grounded systems (-0.113),
d_{G} - the gap between conductors (mm),
E - the incident energy (cal/cm^{2}),
C_{f} - the voltage factor (1.0 for U_{sys} > 1 kV, 1.5 for U_{sys} <= 1 kV),
t - the arc duration time (s),
D - the distance from the possible arc location to the person (mm),
x - the distance factor (check table below),
I_{f} - is the bolted fault current for three-phase faults (symmetrical RMS)(kA),
U_{sys} - the system voltage.
The arc duration time is the clearing time for the source-side protecting device that clears the fault first.
System voltage (kV) | Equipment type | Typical gap between conductors (mm) | Distance factor, x |
0.208 - 1.0 | Open air | 10-40 | 2.0 |
Switchgear | 32 | 1.473 | |
MCC and panels | 25 | 1.641 | |
Cable | 13 | 2.0 | |
> 1.0 - 5.0 | Open air | 10-40 | 2.0 |
Switchgear | 13 - 102 | 0.973 | |
Cable | 13 | 2.0 | |
> 5.0 - 15.0 | Open air | 10-40 | 2.0 |
Switchgear | 153 | 0.973 | |
Cable | 13 | 2.0 |
Determine the flash boundary
The flash boundary is the distance from an arcing fault where the incident energy is equal to 1.2 cal/cm^{2}. For the IEEE Std. 1584 empirically derived model equation is:
[math] {D}_{B} = ({C}_{f}\cdot{E}_{n}\cdot\frac{t}{0.2}\cdot\frac{610^x}{{E}_{B}})^\frac{1}{x} [/math]
For the Lee method:
[math] {D}_{B} = \sqrt{{5.12}\cdot{10^5}\cdot{U}_{sys}\cdot{I}_{f}\cdot\frac{t}{{E}_{B}}} [/math]
Where:
D_{B} - the distance of the boundary from arcing point (mm),
E_{n} - the incident energy (cal/cm^{2}) normalized for time and distance,
C_{f} - the voltage factor (1.0 for U_{sys} > 1 kV, 1.5 for U_{sys} <= 1 kV),
t - the arc duration time (s),
E_{B} - the incident energy in cal/cm^{2} at the boundary distance,
x - the distance factor (check table above),
I_{f} - is the bolted fault current for three-phase faults (symmetrical RMS)(kA),
U_{sys} - the system voltage.
Other calculation approaches for the incident energy
In 2000, Doughty, Floyd, and Neal published "Predicting Incident Energy to Better Manage the Electric Arc Hazard on 600 V Power
Distribution Systems", which defined incident energy based on fault duty, working distance and clearing time for arcs in air or in an enclosure.
For open installations:
[math] {E}_{MA} = {5271}\cdot{D}_{A}^{-1.9593}\cdot{t}_{A}\cdot[{0.0016}\cdot{{I}_{f}^2}-{0.0076}\cdot{I}_{f}+0.8939] [/math]
And for closed/boxed installations:
[math] {E}_{MB} = {1038.7}\cdot{D}_{B}^{-1.4738}\cdot{t}_{A}\cdot[{0.0093}\cdot{{I}_{f}^2}-{0.3453}\cdot{I}_{f}+5.9675] [/math]
Where:
E_{MA} - incident energy for an arc in open air (cal/cm^{2}),
E_{MB} - incident energy for an arc in a box (size 50 cm or 20 inches maximum)(cal/cm^{2}),
D_{A},D_{B} - distance from the arc,
I_{f} - is the bolted fault current for three-phase faults (symmetrical RMS)(kA),
t_{A} - the arc duration time (s).
Criticism of IEEE 1584
In their 2006 paper, Stokes and Sweeting suggested that IEEE 1584 had fundamental errors in the way incident energy was calculated, arguing that it did not adequately capture the physics of the arc plasma cloud [5].
NPFA-70E-2004 Application
In April 2004., the NFPA released an update to NFPA-70E that adopted the IEEE Std. 1584-2002 methods for determining the incident energy. The standard was renamed to NFPA 70E Standard for Employee Safety in the Workplace 2004 Edition. It is different from IEEE Std. 1584 with regard to arc flash in that it is used to determine the appropriate PPE based on the incident energy calculated. PPE is rated by the Arc Thermal Performance Value (ATPV) with units in cal/cm^{2}. The required PPE is determined by comparing the calculated incident energy to the ratings for specific combinations of PPE. An example is given in NPFA 70E as follows in table below:
Hazard/Risk Category | Typical Protective Clothing Systems | Required Minimum Arc Rating of PPE (cal/cm^{2}) |
0 | Non-melting, flammable materials (natural or treated materials with at least 4.5 z/yd^{2}) | N/A (1.2) |
1 | FR pants and FR shirt, or FR coverall | 4 |
2 | Cotton Underwear, plus FR shirt and FR pants | 8 |
3 | Cotton Underwear, plus FR shirt and FR pants and FR coverall | 25 |
4 | Cotton Underwear, plus FR shirt and FR pants and multiple layer flash suit | 40 |
Where FR refers to the flame resistant or flame retardant.
This example should NOT be used for final calculations. For actual applications, the calculated incident energy must be compared to specific PPE combinations used at the facility being evaluated. The exception to this is the upper limit of 40 cal/cm^{2}. While PPE is available in ATPV values of 100 cal/cm^{2} or more, values above 40 are considered prohibited due to the sound, pressure and concussive forces present. Above this level these forces are more significant than the thermal values.
Methods for reducing arc flash hazards
Reducing the Arcing Current: certain protective devices are current limiting in design. By limiting the current available for a fault there is a corresponding reduction in the incident energy for clearing times that are short in duration (1-3 cycles). Fault duties at these devices must be in the current limiting range for them to be effective (typically at least 10-15 times the device rating).
Increasing the Working Distance: since the incident energy is proportional to the square of the distance (in open air), increasing the working distance will significantly reduce the incident energy. Working distance can be increased by using remote racking devices, remote operating devices, and extension tools (i.e. hotsticks).
Reducing the Clearing Time: traditional methods to reduce clearing times include: lowered device settings (permanently or temporarily), bus differential protection, and zone selective interlocking (typically low voltage only). It should be noted that the calculations assume that the protective devices are set in accordance with the study, and that the devices operate properly.
Arc Flash Detection Principles
An arc flash fault typically results in an enormous and nearly instantaneous increase in light intensity in the vicinity of the fault. Light intensity levels often rise to several thousand times normal ambient lighting levels. For this reason most, if not all, arc flash detecting relays rely on optical sensor(s) to detect this rapid increase in light intensity. For security reasons, the optical sensing logic is typically further supervised by instantaneous over-current elements (ANSI device 50) operating as a fault detector. Arc flash detection relays are capable of issuing a trip signal in as little as 2.5 ms after initiation of the arcing fault. Arc flash relaying compliments existing conventional relaying. The arc flash detection relay requires a rapid increase in light intensity to operate and is designed with the single purpose of detecting very dangerous explosive-like conditions resulting from an arc flash fault. It operates independently and does not need to be coordinated with existing relaying schemes.
Responses to Arc Flash Faults
Once the arc flash fault has been detected, there are at least two design options. One option involves directly tripping the upstream bus breaker(s). Since the arc flash detection time is so short, overall clearing time is essentially reduced to the operating time of the upstream breaker. A second option involves creating an intentional three-phase bus fault by energizing a high-speed grounding switch. This approach shunts the arcing energy through the high-speed grounding switch and both faults are then cleared by conventional upstream bus protection. Because the grounding switch typically closes faster than the upstream breaker opens, this approach will result in lower incident energy levels than the first approach. However, it also introduces a second three-phase bolted fault on the system and it requires that a separate high-speed grounding switch be installed and operational. Assuming there is space available for the addition of the grounding switch, there is a significantly higher cost of implementation involved compared to the first approach, and so may not be a practical alternative, especially for existing switch-gear lineups.
Arc Flash Calculator
Also, we have provided a free arc flash calculator for android based smartphones Arc Flash Calculator
References
- Neal, T., Bingham, A., Doughty, R.L, “Protective Clothing for Electric Arc Exposure”, IEEE, July / Aug 1997
- IEEE Std 1584, “Arc Flash Hazard Calculations”, 2002
- AS 3439.1, “Low-voltage switchgear and control gear assemblies – Part 1: Type-tested and partially type tested assemblies”, 2002
- "Arc flash hazard analysis and mitigation", 2004, Christopher Inshaw, Robert A. Wilson
- Stokes, A.D., Sweeting, D.K., "Electric arcing burn hazards", IEEE Transactions on Industry Applications, 2006
- Arc advisor data