The need to protect workers from the hazards of arc flash has been understood for decades – the National Fire Protection Association’s Standard NFPA 70E first addressed the issue in its guidelines for safety boundaries and maintenance practices (including personal protective equipment) in 1995. More recently, though, the role electrical equipment, itself, can play in minimizing arc flash hazards has become an important concern. The type of equipment specified, and where in an electrical system it’s installed, can significantly reduce the duration of an arc flash incident, which is the key factor in reducing potential hazards, a fact the National Electrical Code (NEC) now is targeting directly in Section 240.87.
This section was first introduced in the 2011 NEC, following a proposal seeking to address hazards that can arise in some selective-coordination plans incorporating short-time delay. Short-time delay slows the opening of an upstream circuit breaker to provide time for a downstream overcurrent device to clear a short-circuit. A dangerous situation can arise, though, when a short occurs between the two overcurrent devices, because the delayed tripping of the upstream circuit breaker will enable more let-through energy than would have occurred if that circuit breaker utilized an instantaneous trip.
So, to address these hazards, the 2011 NEC incorporated the new section “240.87 Non-instantaneous Trip” to deal specifically with those situations in which a circuit breaker without an instantaneous trip function was used. In this initial iteration, documentation was required to point out where those circuit breakers were located, along with the use of one of the following (or an approved equivalent means):
- Zone-selective interlocking
- Differential relaying
- Energy-reducing maintenance switching with a local-status indicator
The 2014 NEC has taken these concerns a step further, as reflected in Section 240.87’s new name – “Arc Energy Reduction.” Now, arc energy reduction is required where the trip unit in a circuit breaker is rated or can be adjusted to 1200 amps or higher (the same level at which Section 230.95 requires ground-fault protection of equipment) – and not just where non-instantaneous trips are involved. This means that even though an electronic trip circuit breaker with a 1200 A sensor has its current rating switch set to, for example, 0.75 (or 900 A), it will still require an arc energy-reduction means. A new allowable arc energy-reduction method was added to the 2011 to the 2011 list, an energy-reducing active arc flash mitigation system.
Now, an even broader range of installations will be required to incorporate arc energy reduction strategies. If you’re involved in designing and installing affected electrical systems, you’ll need to acquaint yourself with the technicalities of the allowable solutions in this section – and this white paper can help. Additionally, Schneider Electric professionals are active participants in code development and can help with your project planning to help ensure both compliance and safety. More comprehensive information and tools are also available on our website.