NEC 240.67 COMPLIANCE & NFPA 70E ELECTRICAL SAFETY
The points below offer guidance for NEC compliance and electrical safety. Each point has individual merit. They are not provided in any specific order of importance.
Point 1: Fuses have different characteristics.
Not all Class L fuses are the same. Each manufacturer offers different fuse types, each with unique characteristics and clearing times. Once you have calculated the available arcing current, look at the clearing times of all available fuses. In many cases, simply installing the appropriate Class L fuse will reduce arc energy to a compliant level and improve safety.
Point 2: Size the fuses according to the continuous load, not the switch size.
In most cases, fusible switches are sized at least 25% larger than the continuous load calculations. Install fuses that are rated for carrying the continuous load current, not based on the size of the switch. Smaller fuses will react faster when an adverse condition occurs. Class L fuses are uniquely designed so downsizing can almost always be done without special adaptors. When fuses are rated less than the current rating of the switch, an “energy-reducing active arc flash mitigation system” is created.
Point 3: Install an Arc Flash Detection System and/or Blown Fuse Detector.
When a fuse opens, it will only open the conductor that the fuse feeds. Other conductors will remain energized. Install an arc flash detection relay downstream where the conductors terminate and send the trip signal back upstream to a shunt trip actuated switch. By doing this, all the conductors become de-energized if an arc flash event is detected. Installation of a blown fuse detector can provide similar benefits.
Point 4: Always avoid work near energized conductors!
This cannot be stressed enough. Always put energized conductors into an electrically safe work condition by turning off the switch. Apply lockout/tagout and approach the conductors as if they are energized. The safest way to work near conductors is when they are de-energized. Even in cases where a maintenance mode is an option, turning the switch OFF provides a safer environment than an activated maintenance mode with the switch ON.