NFPA 70E – The Safety Standard
NFPA 70E defines the safe parameters for personnel working on electrical equipment. Although adherence is not a legal requirement, the standard provides a benchmark for most industries to demonstrate compliance with OSHA’s General Duty clause. An employer adopting the guidelines offered in NFPA 70E demonstrates a clear commitment to safe working practices and the protection of employees from shock and arc flash hazards.
According to the standard, if personnel will be operating in the presence of energized equipment, then certain safety considerations are applicable. 70E recognizes that there may be the potential for arc flash and arc blast even when conductors are not exposed. Qualified personnel responsible for the work must:
- Conduct an arc flash hazard analysis
- Implement qualified and general worker safety training based on the results
- Establish shock and flash protection boundaries
- Provide protective clothing and personal protective equipment to ANSI standards
- Put warning labels on equipment
- Authorize the job with a ‘live work’ permit
Steps for an Arc Flash Hazard Analysis
Section 4 of IEEE 1584-2002 outlines a 9-step procedure for arc flash hazard analysis. The purpose of this analysis is to “identify the flash protection boundary and the incident energy at assigned working distances…” The nine steps are:
1. Collate system data. Collect system and installation information for a detailed short circuit assessment. You will need to describe the system and the arrangement of its components in a one-line drawing with nameplate specifications for each device and the lengths and cross-sectional areas of interconnecting cables.
2. Consider all modes of operation. Examine the different ways that the system operates and how this may affect the risks and magnitudes of arc hazards.
3. Calculate bolted fault currents. Using the data gathered in the first two steps, calculate the highest bolted fault current expected to flow during any short circuit.
4. Calculate arc fault currents. During an arc fault, the current flow is normally lower than that of a bolted fault in the same equipment because of the added impedance of the arc. For example, for a bolted fault of 40 kA at 480 V the corresponding arc fault would be expected to yield about 20 kA. IEE 1584- 2002 provides formula for estimating arc fault currents.
5. Determine protective device characteristics and arc durations. Estimate how over current protection devices will react during an arc fault. These may react more slowly, extending the duration and power of the arc flash. Through the analysis, it may be possible to reduce the arc flash hazard and lower the PPE requirement by replacing existing circuit breakers. For example, modern, current- limiting fuses may considerably reduce arc flash energies by reacting more rapidly and at lower over current values.
6. Document system voltages. Establish bus gaps and operating voltages.
7. Estimate working distances. Determine the distances from arc fault sources to a worker’s face and chest. Although hands and arms may be closer to any incident, injuries are unlikely to be life-threatening.
8. Determine incident energy. Calculate the incident energy resulting from an arc fault at the working distance.
9. Determine flash protection boundary. Use the same calculations to estimate a ‘safe’ distance from the source of the arc hazard beyond which PPE is not required. This paper describes the Flash Protection Boundary in more detail later.
Shock Hazard Analysis
As the name implies, the determination of the shock hazard is an analysis designed to reduce the risk of electrocution. NFPA 70 recommends the identification of three boundaries to define the safe working limits for personnel working in an area with shock hazards. Each area is associated with a level of training and PPE. NFPA 70E data (contained in Table 130.7(C)(2) allows you to calculate the boundaries using a formula based on the voltage of the equipment.
The Limited Approach Boundary
This is the minimum permitted distance that unqualified and unprotected personnel may approach a live component. Before crossing the limited approach boundary and entering the limited space, a suitably qualified person must use the appropriate PPE and be trained to perform the required work. An unqualified person may enter the limited approach area if they are under the supervision of a qualified person.
The restricted boundary
To cross the restricted boundary and access the restricted space, personnel need to have been trained in shock protection techniques, be wearing the correct PPE and have a written and approved plan for any work in the zone. The plan must make it clear that the worker must not enter the prohibited space or cross the prohibited boundary either personally or by using any equipment or tool.
No worker should cross the prohibited boundary and enter the prohibited area unless: • The responsible authority has carried out a full risk assessment.
• The work has been documented and it has been fully established why it must be carried it out on live equipment.
• The qualified worker has been trained to work on live electrical equipment.
• The worker has been equipped with appropriate PPE. In terms of safety, any worker crossing the boundary must be equipped and protected as they would be for making direct contact with the exposed live equipment.
Establishing these boundaries is an important step in protecting staff from the dangers of electrocution. It ensures that personnel use the correct equipment and procedures when in the proximity of live electrical equipment. However, if the live equipment also poses an arc flash hazard, it is important to establish a ‘safe’ distance for this eventuality: the arc flash protection boundary.
The Flash Protection Boundary
The arc flash protection boundary (FPB) is the minimum ‘safe’ distance from energized equipment that has a potential for an arc fault. It is defined as the distance at which, in the event of an arc flash, a worker would be exposed to a thermal event with incident energy of 1.2 cal/ cm² for 0.1 second. With this exposure, a worker may receive a 2nd degree burn to exposed skin. If it is necessary for workers to cross the flash protection boundary, and potentially be exposed to higher incident energies from any arc flash, they must be wearing appropriate PPE. For example, at an incident energy greater than 1.2 cal/cm², clothing could ignite and bare skin would sustain 2nd degree burns.
An important point here is that the flash protection boundary and the rules governing access within it take precedence over the shock hazard boundaries. So, for example, if the flash protection boundary is greater than the limited approach boundary then no unqualified person can be permitted in the limited approach area and even qualified workers must wear appropriate arc-resistant PPE here.
You can determine the flash protection boundary for an electrical system using the calculating methods contained in NFPA 70E and IEEE Std 1584. The equations are based on the voltage level, fault level and the trip time of the protective device.
The conditional flash protection boundary is 48 inches for low voltage (<600 V) systems where the total fault exposure is less than 100k amperes-seconds (fault current in amperes multiplied by the upstream device clearing time in seconds). On such a system, a qualified person who works closer than 48 inches from the live components must wear PPE for arc-flash protection including flame-resistant (FR) clothing. Of course, further PPE may be necessary for protection against electric shock according to the location of the shock protection boundaries.
Please refer to IEEE 1584 for comprehensive calculation methods for a wide range of electrical systems; the procedures describe calculation methods for equipment with voltages in the range: 208 V to 15 kV.
Choosing the Right PPE
As an option to incident energy analysis to assist in the choice of appropriate personal protection equipment for arc flash hazards, NFPA 70E defines five hazard risk categories (HRCs): 0, 1, 2, 3, and 4.
What is appropriate PPE?
Clearly, the potential injurious effects of an arc flash can be reduced by using a fire flame resistant (FR) suit of a suitable calorie rating to reduce the indecent energy on the body to an extent that the burns suffered are not life threatening. While safety is the paramount concern, it is important to select PPE appropriate for the task. It might seem a good idea to insist on category 4 PPE for all live work, perhaps to avoid a time consuming arc flash hazard analysis, and this may outwardly appear to be a ‘safe’ policy for personnel. However, the use of restrictive or excessive PPE can also be hazardous: an overheating worker struggling with poor visibility and restricted movement is more likely to have an accident. More accidents, more downtime.
In addition, there are scenarios where HRC may not be enough arc-flash protection. The HRC does not account for arc-blast. NFPA 70E provides several tables listing the PPE appropriate for work within the flash protection boundary: clothing and equipment such as gloves, hats or hoods. In addition to the HRC classification, PPE is often described by the arc thermal performance value (APTV). This corresponds to the capability of the garment to withstand a particular incident energy (in cal/cm²).
As we have already discussed, the flash protection boundary defines the distance where an arc flash would produce incident energy of 1.2cal/cm²: a level at which 2nd degree burns could occur. This corresponds to risk category 0. In practice, a worker will need to approach the electrical system much closer than the flash protection boundary. It is therefore important to calculate the likely incident energy for the working position and select PPE accordingly. Most garments are tested and rated for incident radiation at a distance of either 18 inches or 24 inches. This roughly corresponds with the position of the head and chest when working directly on equipment. Of course, during an arc flash incident, the hands and arms may be much closer to the arc fault source and may need protective equipment with a considerably higher rating.
While NFPA 70E and IEEE 1584 cover the PPE requirements for arc flash protection, there are also other considerations and standards to include in any safety appraisal. Workers may require eye protection, insulating gloves, ear and hearing protection, head impact protection and reinforced footwear.
Heeding the Warning Signs
Equipment such as “Switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers in other than dwelling occupancies, which 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. The marking shall be located so as to be clearly visible to qualified persons before examination, adjustment, servicing, or maintenance of the equipment.” Although the basic requirement is for a sign warning of the arc flash hazard (see Figure 6) it is more helpful for engineers if the sign includes other useful information such as:
• Operational voltage
• Fault current
• Flash hazard boundary
• Incident energy at the normal working distance for the arc fault hazard.
• If this information is not present on the warning sign, it should be documented and accessible to all relevant personnel.
It is good working practice to make labeling clear and accurate; too much information is as bad as too little. If a system has several access points and arc flash hazards, label it with the parameters for the greatest hazard.
The “Live Work” Permit
Finally, before any work can commence, the responsible manager must generate and sign an energized electrical work permit. This describes the task and why it must be performed with live energized equipment, along with the shock and flash boundaries plus related PPE.
Next: Electrical Safety in the Workplace Pt 3 – The Electrical Thermographer and IR Windows
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