What is an Arc Flash?
An arc flash is like a bolt of lightening that occurs around energized electrical equipment. It can occur spontaneously and is often triggered simply by the movement of air when an electrical enclosure is opened. The NFPA has recognized the significant hazard of arc flash and is attempting to protect workers via the latest implementation of NFPA 70E-The Standard for Employee Safety in the Workplace.
About 10-15 serious arc flash incidents occur in the US each day. Most causes of arc flash are operator induced.
Most technicians who routinely work around energized electrical equipment are familiar with arc flash-having seen it first hand. It is thought of like a major automobile accident: no one really expects it to happen to them, so people have a tendency to drive with significantly less caution than they should. So it is with arc flash, only worse. Similar to driving you can make a mistake, or you can be doing everything right when someone slams into you.
Specifically, what is an arc flash?
An arc flash is electric current flowing through an arc outside its normal path where air becomes the conductor of high thermal energy (5000ºC %2B) and generates highly-conductive plasma. An arc flash will conduct all available energy and generate an explosive volumetric increase of gases which blows electrical system doors off and potentially generates shrapnel.
What are the causes of Arc Flash?
An arc flash occurs when the gap between conductors or conductors and ground is momentarily bridged. There is always a trigger event which almost always involves human intervention. Typical causes and contributing factors include:
- Accidental contact with energized parts
- Inadequate short circuit ratings
- Tracking across insulation surfaces
- Tools dropped on energized parts
- Wiring errors
- Contamination, such as dust on insulating surfaces
- Corrosion of equipment parts and contacts
- Improper work procedures
An arc flash is electric current flowing in an arc outside its normal path where air becomes the conductor.
The vast majority of arc flash faults occur when the door is open or being opened. The National Fire Protection Agency (NFPA) is the author of NFPA 70, also known as the National Electric Code (NEC). This paper is not intended to provide a comprehensive review of the information available in the code, but merely to highlight some of the information that may be related to thermography.
NFPA 70E is the standard for safe electrical work practices.
The NEC is an electrical design, installation and inspection standard. It does not specifically address topics like electrical maintenance and safe work practices. A national consensus was needed for safety while working around live electrical equipment. NFPA 70E is the standard for safe electrical work practices. NFPA 70E addresses four specific topics: safety related work practices, safety related maintenance requirements, safety requirements for special equipment and installation safety requirements. NFPA 70 suggests that a Hazard/Risk analysis must be conducted prior to working on electrical equipment. The core of the analysis is based on shock and arc flash boundaries which must be done by a qualified electrical engineer.
Shock Hazards, Flash Hazards and Personal Protective Equipment (PPE) Selection
Prior to beginning work around live electrical components, an Energized Electrical Work Permit must be obtained and should include but not be limited to the following:
- A description of the circuit, the equipment to be worked on and the location
- Justification for why the work must be performed in an energized condition
- Description of the safe work practices to be performed
- Results of the Shock Hazard Analysis
- Determination of the Shock Protection Boundaries
- Results of the Flash Hazard Analysis
- The Flash Protection Boundary
- Identify the necessary Personal Protective Equipment (PPE) required to safely perform the assigned task
- Means employed to restrict unqualified personnel from entering the work area
- Evidence of completion of a job briefing
- Energized work approval from responsible management, safety officer and owner
Prior to working with live components, the correct Personal Protective Equipment and safe working practice must be determined.
NFPA 70E allows for an exemption to the safe work permit for qualified personnel who are performing tasks such as testing, troubleshooting, voltage measuring, etc. so long as they utilize safe work practices and the proper PPE. Prior to working with live components, the correct personal protective equipment and safe working practice must be determined by carrying out a Shock Hazard and a Flash Hazard Analysis. A Shock Hazard Analysis will determine the voltage to which personnel are exposed, boundary requirements and the proper PPE necessary to minimize the possibility of shock to personnel. The shock protection boundaries are identified as limited, restricted, and prohibited for the distances associated with various voltages.
Unqualified personnel should be notified and warned of hazards by qualified personnel when working at or near the limited approach boundary. When an unqualified person must work inside the restricted boundary, it is important that they be further notified of the risks and hazards and continuously escorted by a qualified person. Under no circumstances should they be allowed inside the prohibited boundary. It is important that a Flash Hazard Analysis be conducted in order to protect personnel from being injured by an arc flash. The analysis will determine the Flash protection boundary and determine the proper PPE. In so doing, the Flash protection boundary is calculated at the distance from energized parts where a burn will be “recoverable” (2nd Degree) and “incurable” (3rd Degree). The guidelines dictate that the Flash protection boundary for systems that are 600 volts or less be 4′ for clearing times of 6 cycles (0.1 second) and available bolted fault current of 50kA or any combination not exceeding 300kA cycles. For all other clearing times and bolted fault currents, the flash protection boundary is normally determined based on the calculated incident energy of an arc fault taking into account system voltage, available current, and clearing time (where incident energy is the measure of thermal energy at a specific distance from the fault). Where it is not possible to perform these analyses (or they have not been performed), NFPA 70 provides guidelines (NFPA 70 Table 130.7-C9a) that can be used to determine the required PPE based on the task conducted. In lieu of a Flash Hazard study, selection of PPE by task is normally allowed. However, for tasks not listed in the table and for clearing times different then those listed there, a complete Flash Hazard Analysis is required. Using Flash Hazard Analysis or Task Risk Assessment, the following table can be used to identify the correct PPE:
Thermography Inspection Practices Infrared cameras have been used to identify problems in electrical systems for many years. Problems in electrical systems manifest themselves by connections and conductors becoming overheated as the result of increased resistance, the result of loose or corroded connections, or load imbalances. An infrared camera can readily identify these problems in a thermal image and is an excellent method for identifying failing or problem components prior to a failure. A failure can disable an electrical system and cause significant lost production, equipment damage and bodily injury. Insurance companies use infrared electrical inspection to help determine risk profiles and rates for industrial customers. More recently, thermographers have found that they can use IR to prevent and predict failures to help further reduce down time equipment failure and increase overall safety.
Often, during thermography inspections, panel covers are removed and subsequently replaced, a method that conflicts with the requirements of NFPA70E.
Like visible cameras, infrared cameras require a direct-line-of-site view of an object. In most cases surveys are hampered by cabinet designs that obscure the target components being inspected and thermographers are put at risk by having to open cabinets or doors in an attempt to gain access to the internal components. IR surveys of electrical systems are best conducted when the system is under heavy if not peak electrical load, which requires the thermographer to perform the inspection in and around live electrical components. Typically, electrical system covers are removed during thermography inspections and subsequently replaced. This working method conflicts with the requirements of NFPA 70E.
Recommendations of NFPA70E as they relate to Thermography Inspection
NFPA 70E recommends that only “qualified” personnel be allowed to perform work inside the flash protection boundary. Thermographers must be accompanied by “qualified” individuals if they intend to have panel covers removed. Both the thermographer and the additional person should be in full PPE. One way NFPA 70E determines Hazard and Risk and the required PPE is based on the activity that you are conducting around the equipment. Risk potentials are determined on a scale from 0-4, where 4 indicates the highest risk potential. For example, removal of a bolted cover on 600V equipment carries a hazard/risk classification of 3 and that goes up to a rating of 4 on voltages greater than 600V. As this work occurs within the Flash Protection boundary, the appropriate PPE must be worn. The required minimum PPE for Hazard/Risk Classification 3 work is to withstand 104.6 J/cm², and the required minimum PPE for Hazard/Risk Classification 4 work is to withstand 167.36 J/cm². As much of the work performed for an IR inspection requires removal of bolted covers, this would be the PPE that is required.
Infrared Windows: Eliminate the Controllable Risk
The first rule in any risk assessment is to eliminate the risk if possible. Infrared Windows eliminate many of the risks associated with live inspections since they enable an infrared camera direct view of live electrical components without the need to open electrical enclosures. They provide an excellent means of accessing electrical equipment efficiently and safely. In addition, a second qualified technician is not required to open and unbolt enclosures. An IR viewing window is basically an infrared transparent material with a holder/mounting body. Thermographers may even decide to not use a window when inspecting energized components at some distance from the cover and use a protective grill in place of a window. The grill must be IP2X certified (the grill size must offer protection against foreign objects with diameters larger than 12mm). This method can significantly reduce the window cost and also has the additional benefit of allowing ultra sound inspections of the electrical switchgear. However when using grills, operators will be exposed to live electrical components and they must wear the appropriate level of PPE identified from the Arc Flash Hazard Analysis of the switchgear. Infrared Windows eliminate many of the risks associated with live infrared inspections since they enable an infrared camera direct view of live electrical components without the need to open electrical enclosures. The optics holder design depends upon a number of parameters: the field of view, equipment lens and window size are all functions of the design and must meet all the parameters that the thermographer requires before a holder is manufactured. Also, a protective cover should be included in the design as crystals are very expensive and in some cases, extremely fragile. Infrared Windows are available in multiple sizes and can be custom made to retrofit dead fronts on distribution and isolator boards. The larger the size of the window, the greater the field of view one can see with their IR camera.
Considerations in Installing Infrared Windows
To correctly install infrared windows, the targets that require inspection must be identified. Typically, traditional surveys only look at the bolted connections within the switchgear. These are generally considered to be the “weakest points” or “points most likely to fail.” These may include:
- Cable connections
- Bus Bar Connections
- Isolator or Circuit breaker connections
The formula for calculating the field visible through an Infrared Window is: FoV = 2 x tan (angle/2) x D, where FoV is the width of the object area that will be viewed, the “angle” is the angular field-of-view of the camera, and “D” is the distance from the camera (ostensibly the window) to the objects being viewed. Once a decision has been made about what objects are to be inspected through the infrared window, the number of windows and appropriate size must be determined as well as where they need to be installed to ensure best coverage (and therefore maximum efficiency). The size of the infrared window will depend on several factors, including the infrared camera’s clear aperture, its ability to focus on close objects, its ability to be placed as close as possible to the window, the camera’s angular field-of-view and the amount of manipulation is possible with the camera when viewing through the window. An important consideration is how the infrared camera can be manipulated when looking through an infrared window. A high degree of manipulation can have the effect of increasing the size of the inspection area by up to a factor of 3. This means that if the object under observation is 12 inches across, depending on several factors, it is possible that a window diameter of 4 inches (for IR window size calculation purposes) can still be used if the operator manipulates the camera from left to right or up and down.
The required size of the window will depend on the following:
- the size of the objects to be viewed and their distance from the panel cover;
- the infrared camera’s angular field-of-view and clear aperture;
- the camera’s ability to focus on close objects and to be placed close to the window.
Typically, infrared cameras have a horizontal field of view of 25°. Those infrared cameras that offer a wide-angle lens option (for example 50°) permit the user to have a substantially wider field of view, resulting in an increase in viewing area through the same infrared window size. This can be a great advantage in certain situations, reducing the size and possibly the number of windows. Other useful infrared camera features are close focus capability, small lens diameter resulting in a small clear aperture, motorized focus (eliminating the need to get fingers on the lens focus ring and moving the camera away from the window) and a chassis design that facilitates movement at the window such as an articulating camera head that allows the user to look into windows above eye level or at near floor level.
The View through an Infrared Window
An infrared window allows a camera operator to inspect the inside of an electrical cabinet to check the physical condition of the components that you have chosen to inspect. As with traditional thermographic inspections we can see temperature differences very clearly. You need to have the confidence in the infrared windows that you are using. They are designed to allow infrared energy to transmit through them at a known transmission rate; therefore, if there is even a slight temperature difference you will be able to see that with your IR camera, and be able to record images for the IR inspection program.
Considerations for Installing Infrared Windows
Installing an infrared window requires cutting holes into very expensive switchgear. Therefore, it is very important to be very sure that they are installed in the correct location and that the switchgear ratings are not degraded in any way. Before installation, the following factors need to be considered:
- NEMA or IP rating of the switchgear and IR windows: Remember to never install an IR window of a lower rating than the rating of the switchgear.
- Test Certifications: Ensure that the IR windows have been tested and approved by the certification bodies as the switchgear for which they are intended (i.e. UL, IEEE. Lloyds).
- Internal obstacles: Before removing internal Perspex/Plexiglas covers or cables, ensure that the local safety manager’s approval is sought first. In some cases you may not be able to totally remove the covers and may only be able to modify them by drilling or punching holes to retain the IP2X requirement for some switchgear.
- Explosion Ratings (if applicable): Some panels are positioned in intrinsically safe areas and as such can never be modified in the field.
- Dielectric Clearances: Where IR windows use grills or inspection orifices, they must comply with IP2X (13mm 0.5″), and clients must be made aware of the safe dielectric clearances for the type of switchgear that they intend to install the window into. The table shown at the left (from IEEE C37.20.2 table A.3) specifies minimum distances from live components, and it is recommended that these be considered as a standard for grills/inspection orifices.
When using Infrared Windows, it is important to correct for the transmission specification of the window and the emissivity of the component that is to be inspected through the IR window. One way of correcting for the effects of the window is by adjusting the camera’s emissivity value for an object of known temperature until the camera’s reading is correct. For objects at the same ambient temperature and emissivity, the new emissivity value can be used.
When using Infrared Windows, it is important to correct for the transmission loss of the window and the emissivity of the component that is to be inspected through the IR window.
Another way of using IR windows is to prepare all components that are to be inspected so that they have the same emissivity (for example, with electrical tape, emissivity paint, IR-ID Labels). In this case, all components being inspected will have the same transmission rate and emissivity readings; consequently, the results gathered will be far easier to compare.
Can IR Windows Carry a Generic Arc Rating?
Electrical switchgear takes on many different shapes and sizes. The surface areas and volumetric elements of the cabinets are different with each model, type and rating. Each cabinet is subjected to the testing that is laid down by the certification bodies such as UL, IEEE, etc. This test is completed on the cabinet assemblies and not the components that make up the assembly. Electrical cabinet designs and dimensions are infinite, and we therefore CANNOT or MUST NOT use the data from one cabinet design for another design unless they are identical in every way. This is why components never carry a generic arc rating and must be subjected to industry standard tests to confirm that they conform to the minimum required level of mechanical strength and environmental properties for the electrical cabinets and assemblies which they are going to be fitted into.
Conclusion
Because of the frequent occurrence of arc flash in industry, it is extremely important to be aware of the risks associated with inspection of high voltage switchgear and related items. Concerns about operator safety due to an arc-flash event are causing inspectors to adopt new practices in accordance with NFPA 70E, the standard for safe electrical work practices. Shock and Flash Hazard analyses are required in many situations. Personal Protective Equipment recommendations are also available. One new common safety practice involves the use of infrared transparent windows which eliminate many of the risks associated with live infrared inspections since they enable an infrared camera to have a direct view of live electrical components without the need to open electrical enclosures.