Options for Addressing Arc Flash Risks in Medium & Low Voltage Power Distribution and Control Equipment in System Design

As covered in a previous post, the updated Section 240.87, “Arc Energy Reduction” in the 2014 edition of the National Electrical Code (NEC) applies specifically to systems in which a Low Voltage Circuit Breaker’s Over Current Device can set or adjusted to 1200 amps or higher. But this doesn’t mean arc-energy hazards don’t exist in lower-amperage projects or Medium Voltage applications. This most recent code change has brought the subject of Arc Flash Hazards and Mitigation Techniques to the forefront of Power System design, operation, and maintenance discussions. To choose the most appropriate means for addressing potential arc-flash incidents in such applications, Specifying Engineers need to understand client goals as well as the particular hazards onsite workers might face.

Medium & Low Voltage electrical equipment is a common presence in a range of industrial settings, from chemical plants to wastewater facilities. Owners have varying needs and goals for reducing arc-flash incident levels, from lowering the level of personal protective equipment (PPE) required under the National Fire Protection Association’s NFPA 70E, “Standard for Electrical Safety in the Workplace,” to a need to reduce all risk of a hazard to zero.

Clients may be hoping to achieve any of three general goals, each of which can imply a different solution:

• Remove employees from a possible danger zone.
• Reduce the risk and/or reduce the potential level of arc-flash incident energy.
• Completely contain any possible blast.

Starting with an up to date Arc Flash Study of your power distribution system as required by section 130.3 of the NFPA 70E 2012.

Remove employees from a possible danger zone

The majority of arc-flash incidents occur during maintenance, when cabinets are open and breakers are being moved in and out of the enclosure. An obvious method for reducing employee exposure to possible arc-flash risks is to conduct racking and unracking of breakers remotely – while this approach doesn’t reduce potential incident energy, it does keep employees out of the Arc Flash Boundry during high-risk operations. Shown here in Figure 1, as the personnel is moved further away from the arc flash the energy they are exposed to drops, An example of such a solution is Square D’s Masterpact™ NW Remote Racking device, which allows operation from up to 30 feet. There are standard remote racking options for Medium Voltage Switchgear, Low Voltage Switchgear, Low Voltage Switchboards, and Low Voltage MCC Units.

Figure 1-Employee working distance and arc flash energy exposure.

Reduce the risk

Per the equations in IEEE Std. 1584-2002, arc flash incident energy varies linearly with time – i.e., double the duration of the arcing fault and the available energy doubles; halve the duration and you cut the energy in half. A number of methods can be employed to reduce the arc flash duration including incorporating Zone-Selective Interlocking Schemes, Arc Flash Maintenance Switches, Low Arc Flash Circuit Breakers, or Arc-Flash Protective Relays. These options can clear faults faster than the over current protective device normal operating trip settings reducing the clearing time and arc flash incident energy released. Schneider Electric has white papers, application guides, product literature, and technical consulting specialist that can help you decide which solution is right for you.

Contain the blast

In some industrial or critical power environments, containing arc flash energy is especially important. In these settings, arc-resistant equipment may be required. Such products, including Square D’s Medium & Low Voltage Arc Resistant Switchgear as well as Arc Resistant Low Voltage Motor Control Centers, are able to contain the rated values of arc flash incident energy that are not able to be reduced through relays or other methods. Per the C37.20.7 testing standard, all of the exhaust eases and thermal energies are directed away from the front, sides and rear of the Equipment protecting employees and preventing to proliferation of an Arc Flash to other systems. This includes preventing the Arc Flash from:

• Causing doors or covers to open or blow off during the event
• Fragment and eject parts within the protected area
• Allowing the arcing fault to burn through the enclosure
• Allowing cotton indicators spaced about the gear to ignite
• Have any of its grounding connections become ineffective

For more Arc Flash resources, please visit this page or register here for access to tools and information built specifically for consulting engineers. Otherwise, whatever your need, ask your local Square D sales representative for more information.

References:

1.NFPA 70E-2012, Standard for Electrical Safety in the Workplace, National Fire Protection Association.
2.  IEEE 1584-2002, IEEE Guide for Performing Arc Flash Hazard Calculations, Institute of Electrical and Electronics Engineers, Inc.
3. “Arc Flash Mitigation”, Antony Parsons, PhD, Schneider Electric, 2013

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