Any electrician undergoing through a constant exposure with the electrical system can attest that arc flash hazards can be devastating, costly and at worst-case scenario, fatal. In light with the escalating numbers of arc flash events, the electrical committee and the code-making panels in charge with updating the 2017 National Electrical Code (NEC) continuously fine-tune Section 240.87: Arc Energy Reduction to mitigate such hazards and practice workplace safety.
For your better understanding of the changes implemented for arc flash mitigation, here's an overview of the revision history of NEC 240.87 back from its initial discussion in the 2002 edition. In the 2011 version, electrical equipment peculiar to arc flash hazards should be labeled as mandated by code, and this labeling requirement was further improved with the inclusion of arc flash boundary and incident energy in 2015 NFPA 70E. Latest developments for this code requirement in 2017 NEC added methods to reduce the clearing time — further outlining just when the application of arc flash mitigation should be conducted.
Arc Energy Reduction
As referenced by the City of Chicago in its own 2018 electrical code, CEC 240.87 states that:
Where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 A or higher, 240.87 (A) and (B) shall apply.
The product of incident energy is calculated by multiplying current with time. Given that there is a direct relationship between the factors of incident energy — when increased currents are present, fault clearing times with higher values pose a special concern as well. Citing CEC 240.87 as a reference, if a circuit breaker can be set to trip at 1,200 A or higher, it is only fitting that the fault clearing time must be reduced to impose safety.
Although electricians prefer to navigate the electrical field work by employing common safety workplace protocols, such cases occur when de-energizing electrical equipment before examination or inspection is not feasible. Take for example, when a circuit breaker is used without an instantaneous trip, providing a means to reduce the incident energy is a critical measure that needs to be factored in when electricians need to engage with energized equipment. CEC 240.87 provides a comprehensive list of the clearing reduction methods, which will be further discussed in detail below.
Eliminate arc flash hazards through electrical code compliance.
Clearing Reduction Methods
With safety as the principle in practice, there is no one-size-fits-all approach when it comes to the method selection of fault-clearing times. For complete code-compliance on CEC 240.87, match your electrical application with any of the best-suited methodologies below so you can yield the benefits of arc energy reduction.
Zone-Selective Interlocking (ZSI)
Zone-selective interlocking operates by using wired connections between electronic trip units and protective relays to create a protected zone. By detecting a fault, ZSI will block an upstream protective relay in operation until the local protective device clears the fault. In the event that the local protective device fails to do so, the upstream device takes over and attempts to do its part in clearing the fault.
The goal of this method is to isolate the power outage to the fault’s area, and when faults are detected in this protected zone, rapid tripping is activated. In effect, ZSI provides selective-coordination benefits, enabling the local circuit breaker to operate with an instantaneous setting since trip levels are frequently set above the arc-fault current.
Differential Relaying
Differential relaying utilizes current transformers on both line and load sides of the protected equipment to monitor and ensure that current flow is equal on both sides. When the current flow falls out of equilibrium, the circuit breaker’s shunt trip is activated.
In comparison with the other methods for arc energy reduction, differential protection do not prove to be quite as effective when the source is not bad wiring or conductive medium. Often used for medium-voltage and less common at low-voltage, the extensive space requirement needed for relay-class current transformers and wiring complexity adds up to the substantial costs of this method.
Energy-Reducing Maintenance Switch with Local Status Indicator
This methodology allows operator intervention by setting the electronic trip units or motor-protection relays of circuit breaker to faster acting instantaneous trip function when working within a protective arc-flash boundary zone, then reverting it back to a normal setting once the potential hazardous live-work is done. The feature of local status indicator acts as a notification signal for the worker, confirming the on-off switch when protection is engaged and after the work is completed.
Energy-Reducing Active Arc-Flash Mitigation System
With high initial overhead but lower life-cycle costs, this arc energy reduction system reduces arc-fault damage by the early detection of a developing arc, causing an upstream circuit breaker to trip in its instantaneous time which results in less fault-clearing time. Arc-flash relays make selective-coordination plans possible by combining it with “crowbar” device or arc-quenching device to create a low-impedance current path, inducing faster performance of arc-flash relays in most applications.
Instantaneous Trip Setting or Instantaneous Override
Listed as separate clearing reduction methods in CEC 240.87, these two systems operate with the same functionality. An instantaneous trip setting or instantaneous override causes a circuit breaker to trip with no intentional delay in the minimum possible time when arc flash currents exceed the trip level. To be code-compliant and further reduce the arc flash energy levels, instantaneous trip setting or override must remain below the arc fault current. Where selective-coordination is required, such method is impossible to achieve as instantaneous trip levels are frequently set above, or higher, than the arc-fault current.
An Approved Equivalent Means
This refers to advanced technology for arc energy reduction that may be developed in the near future.
Conclusion
Safeguarding against arc flash hazards should be one of your top priorities to prevent occupational injuries in the workplace. Any failure on arc flash mitigation can cause your corporate bottom line to recede, but through strategic measures placed by our proficient electrical engineers, optimum safety management in line with the international standards calls for your decreased liability in incidental personnel hazards.
