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By: Monte Fry, P.E. Senior Electrical Engineer at TechSite
In Part 1 of this discussion we covered the increasing emphasis on Arc Flash hazards in relation to working on your facility’s electrical distribution systems. We also emphasized the importance having an Arc Flash Hazard study performed if you do not yet understand the hazard ratings of your gear. Now we want to discuss mitigation techniques that can be considered.
The incident energy of an arc flash event is a function of available fault current squared multiplied by the duration of the arcing current. Thus solutions to reducing the hazard to personnel include:
The first of these two methods addresses the current part of the equation. The second reduces the duration part of the equation. A common way to address available fault current levels is the specification of transformer impedances. This value has a direct affect on the level of fault currents available to a particular system.
The second bullet item is the reduction of clearing time, thus addressing the duration component of the equation. The 2014 NEC addresses this in Article 240.87 for feeders with breakers 1200 A or greater. In this case, the NEC requires one of the following:
The purpose of this discussion is not to review the details of these methods, but to make owners aware that these are engineering/design issues that have great potential to reduce the arc flash hazard levels in their facilities. Another item that can reduce the duration is the specification of the type of molded-case circuit breakers. Often, breakers with fixed instantaneous settings are specified in PDU sub-feed breakers and/or panelboard mains. If these breakers are specified with adjustable instantaneous settings, it provides an opportunity for the engineer to provide breaker settings that reduce the Arc Flash PPE Category. TechSite recently performed a study for a client that has PDUs with sub-feed breakers that supply remote distribution centers. By changing the sub-feed breakers from a GE type THQD to a GE type TFJ the Arc Flash PPE Category went from 3 to 0.
Another design issue that can affect arc flash hazards addresses moving people further away from the source of incident energy. Methods that address this are locating breaker controls away from switchgear/switchboards where breakers are located and utilizing remote racking for breaker removal from switchgear. These methods get personnel out of and beyond the arc flash boundary.
Redirecting arc blast energy is also a technique that can be implemented. This is the concept with specifying arc-resistant switchgear. Arc resistant switchgear, designed to IEEE standards, can contain a fault within the switchgear and re-direct the blast energy through designed ducts. Meeting the IEEE standards assures that an internal arcing fault will not: cause doors or covers to blow open during the event, fragment and eject parts within the protected area, allow the arcing fault to burn through the enclosure, or have any of its grounding connection become ineffective.
The key point is that the application of engineering controls are a very effective way to enhance safety for electrical workers and anyone who interfaces with a facilities electrical equipment. Ideally, arc flash safety is addressed when first designing the electrical distribution for a facility. However many of the techniques discussed here can be retrofitted into existing systems.
As more and more industries and data center operations address arc flash electrical safety concerns, the high risk associated with what used to be normal maintenance and operations task is becoming apparent to owners of these facilities. Often the existing arc flash incident energy levels make it necessary for work on equipment to be done de-energized in order to be in compliance with OSHA regulations. For data centers, de-energizing equipment can often have a significant impact on operations. To this end, implementing arc flash mitigation should seriously considered by personnel responsible for operating data facilities.
Many times incident energy levels can be reduced to allow performance of energized work (with the use of proper Personal Protective Equipment or PPE). While PPE, administrative controls, and warnings are required for every facility and make up key components of an electrical safety program, they are the least effective approach to mitigating arc flash hazards.
What are some of the ways to help minimize risk to personnel with regards to arc flash hazards? The most effective arc flash safety programs are those that do not rely on worker training, warning signs, PPE, and administrative controls, but look to incorporate “safety by design” in the specification of equipment and design of electrical power systems. It is important to realize that typically there is no single hazard reduction solution. The entire electrical distribution system needs to be evaluated holistically to effectively minimize the risk of an arc flash event.
For facility operators that are still unaware of the equipment hazard ratings at their facility, the first step is to have an arc flash analysis and short circuit coordination performed. This is process requires a thorough analysis of your electrical systems by a qualified consultant. After gathering very detailed data on your existing electrical systems, the consultant will utilize specialized software to calculate the hazard levels on your distribution gear. The results of this study will give you understanding of the risks inherent to your equipment. Labels will be created to identify each piece of gear as to the hazard rating. You will then be able to follow well-defined guidelines on how to safely operate, modify and maintain your systems.
Part 2 of this blog will discuss the characteristics of an arc flash and a variety of mitigation techniques.
by: Ron Partin, Manager of Construction Operations at TechSite
You have work planned in your data center over the weekend. You feel sure that nothing will go wrong. But just how confident are you? Are you willing to have a Monday morning meeting with your company executives to discuss who is to blame for an outage? A well thought out Method of Procedure (MOP) and a Risk Management Plan will help prevent those Monday morning meetings.
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