NEC 240.67 COMPLIANCE & NFPA 70E ELECTRICAL SAFETY
The points below offer guidance for NEC compliance and electrical safety. Each point has individual merit.
Point 1: Fuses have different characteristics.
Class L fuses are the same. Fuse manufacturers offer
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.
Some of the more popular versions and their
accompanying data sheets are listed below.
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 can be used at 100% of their rating and are uniquely designed so downsizing can almost always be done without special adaptors. It can be argued that when fuses are rated less than the current rating of the switch, an “energy-reducing active arc flash mitigation system” is created.
If after searching for an appropriate fuse it is determined that none are available that reduce the clearing time to the .07 second requirement, installing an appropriate arc energy mitigation system should be pursued. Arc flash detection relays and sensors are readily available from several manufacturers. Install the components in the sections or cabinets that the fuses feed. Wire the trip signal of the arc flash detection components to the shunt trip mechanism where the fuses are mounted.
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.
Mersen Arc Flash Note 5: Reduce Arc Flash Energies by Reducing Fuse Ampere Rating