What's an Arc Flash?

An arc flash occurs when a flashover of electric current leaves its intended path and travels through the air from one conductor to another, or to neutral or ground. It often happens unexpectedly and can be explosive but brief, or it can last seconds and be rather visually spectacular. It can cause a little damage or it can disable a system and require the replacement of equipment. An arc flash of any size is quite dangerous because its path is unpredictable; it will be attracted to the nearest item with the greatest conductivity, such as an unsuspecting rodent or house pet, or a person. An arc flash can cause a serious electrical burn or even fatal electrocution.

All electrical systems require a means of disconnection so that the service can be shut down quickly if any dangerous conditions exist. In this section, we will look at the types of disconnects, and the common problems that need to be reported.

Wire types for North American wiring practices are defined by standards issued by Underwriters Laboratories, the Canadian Standards Association, the American Society for Testing and Materials, the National Electrical Manufacturers Association and the Insulated Cable Engineers Association.

Knob-and-Tube

Knob-and-tube wiring is so named because of the porcelain fittings used to support and insulate the conductors from the timber components in the home. The knob holds the wire away from rafters and joists, while the tubes are inserted into holes bored though joists and studs to protect the conductor and its rubber insulation.

Remote Distribution Panels

Although this topic is covered in other areas, because the emphasis is on safety, it's important to review.

What Is Grounding?

Generally speaking, the difference between grounding and bonding is: Grounding is a direct connection to the earth to aid in removing damaging transient over-voltages due to lightning. The purpose of bonding is to ensure the electrical continuity of the fault current path, to provide the capacity and ability to conduct safely any fault current likely to be imposed, and to aid in the operation of the over-current protection device. Properly bonding all metal parts within an electrical system helps ensure a low-impedance fault current path.

The service-entrance cable SEC (blue arrow) is a line of service conductors (white arrows) located between the terminals of the service equipment (main disconnect) and a point usually outside the building, clear of building walls, where they are joined by a tap or splice (orange arrows) to the service drop or overhead service conductors (red arrows).