Dialing Down the Risk of Arc Flash Incidents with
Energy-Reducing maintenance Switches
December 2018 %7C By Brian Schmalberger
Brian Schmalberger is an OEM segment market leader for ABB.
While there are many causes of arc flash incidents—from worn cable insulation and corroded electronical contacts to a static electricity discharge—maintenance, upgrade or repair work on electrical distribution equipment, by even well-trained personnel, heightens the risk of an arc flash accident.
Combined with a host of preventive practices and systems, one of the strongest ways to improve worker safety is to limit the potential arc flash incident energy that personnel may be exposed to during maintenance or upgrades.
The power and dangerous impact of an arc flash incident is a function of two factors:
- What is the magnitude or level of the energy emitted during an arc fault incident? Understanding the potential extent of released energy is typically part of an arc flash hazard study which includes calculations based on metrics in the Institute of Electrical and Engineers (IEEE) 1584 standard.
- What is the duration that fault current is active before the overcurrent protective device engages to clear the fault? There are many inter-related arc flash mitigation technologies that help to monitor, react and isolate a fault, including our Instantaneous Zone Selective Interlocking (I-ZSI) and Waveform Recognition (WF) technologies. Working in conjunction with these systems is the use of an energy-reducing maintenance switch, such as our Reduced Energy Let-Through, or RELT, technology.
Lowering the Risk Threshold for Arc Flash
RELT protects workers and equipment with a method that lowers arc flash incident energy levels when electrical maintenance personnel are working on or near energized equipment. This function limits the duration of the fault current by lowering the pickup settings on a second instantaneous overcurrent protection device, or maintenance switch.
The use of an energy-reducing maintenance switch, such as our RELT technology, is described in a section of the 2017 National Electric Code® (NEC) 240.87 9 for circuit breaker-protected circuits as a method to reduce arc flash incident energy in circuits 1200A and larger. This requirement specifies that if normal instantaneous protection by a circuit breaker is not able to operate at the estimated arcing current, then one of several methods must be part of the circuit design. The use of an energy-reducing maintenance switch is one of those methods.
The concept behind the RELT function is simple: in addition to a circuit breakers normal protection functions, there is a second instantaneous protection function with a dedicated threshold and algorithm optimized for both speed and sensitivity. Personnel set the threshold as needed, enabling the RELT function when required for maintenance or special monitoring.
By selecting a lower pickup point for the instantaneous trip function of the breaker, the time factor in the calculations is reduced, thus lowering the arc flash energy expended, and the risk or impact of the event on an employee or equipment.
The RELT device can be adjusted from 1.5x to 15x (±10 percent) times the chosen primary current (Ie) in steps of 0.5x. Selecting a faster pickup point for the instantaneous trip function of the circuit breaker allows the time factor in the calculations to be reduced, thereby lowering the arc flash energy workers may be potentially exposed to.
Activation Control and Feedback
Since RELT is an alternative setting for the instantaneous trip pickup level at the trip unit, the maintenance worker must first be able to activate this alternative setting and then, secondly, receive a clear indication that the system is active.
Activation is accomplished by a 24Vdc signal initiated through a switch mounted on the electrical equipment, or a switch located outside of the arc flash boundary for that piece of equipment. Additionally, if the electronic trip unit is equipped with communication capabilities, the command can be activated via serial communications using either Modbus or Profibus. The activation of the RELT can then be done via a human machine interface (HMI) monitoring and control system.
Once the system is activated, the worker needs positive feedback that RELT has been activated. When a hardwired system is used, the trip unit can provide a contact closure that illuminates a lamp indicating that the system is activated. When the system is activated remotely via serial communications, the feedback indicator is engaged via a remote HMI control panel.
After the RELT functioned is activated with protection levels set to a lower pickup threshold, maintenance workers can more safely operate in and around electrical equipment to conduct critical maintenance and repair operations.
Additional Reference and Sources
“RELT – Reduced Energy Let Through” by Rick Schlobohm, P.E. http://library.industrialsolutions.abb.com/publibrary/checkout/RELT?TNR=White%20Papers%7CRELT%7Cgeneric