Armored cable is also called armor clad cable, or by its trade names BX or Greenfield.
Armored cable is an alternative to running conduit in difficult locations. Instead of running rigid metal pipe that has to be screwed together in twisting or tight locations, it is often easier just to thread flexible armored cable into place. Armored cable comes rolled in spools like regular cable, and is typically available with two, three, or four individually insulated conductors in a variety of sizes1.
Around the conductors is wrapped a layer of paper or clear plastic, and then a grounding wire, which is in contact for the entire length of the cable with the outermost layer, the armor itself. The armor is typically galvanized steel or aluminum and is wrapped in a ribbed spiral around the length of the cable for flexibility, and I assume ease of manufacturing.
Armored cable is cut to length with either a hacksaw or a special tool specifically designed for cutting it. For the hacksaw, one must be very careful not to nick the insulation around the conductors inside, this can eventually if not immediately create a short circuit. Cut the armor most of the way down all the way around and bend it back and forth until it snaps off. Wear gloves, the edges will be sharp.
The special cutting tool is generally designed to hold the armored cable as loaded from the side. A small cutting attachment then comes into contact with the armor, and is rotated around the armor by turning a crank. This produces a cleaner and more easily controlled cut than using a hacksaw, but different sized tools must be used for different sized cables.
Whichever method is used, cut and strip the wires to be longer than the armor, after all they're going to be going into the junction box while the armor will be stopping at the edge of the box. The grounding wire should be bent back over the outside of the armor and wrapped around inside the spiral ribs to keep it well grounded. A plastic fitting is then set in the end of the cable to cover the sharp edges and prevent them from damaging the wires. Finally, any one of a wide variety of couplings can be used to attach the armor to the junction box. They all are designed to grip the armor and the junction box firmly to maintain a good ground bond.
The advantages to armor clad cable are numerous:
- It comes preloaded with wires already in it so you don't have to fish them through yourself.
- It provides its own ground bonding.
- It can be run in places ordinary conduit cannot.
- It provides a great deal of protection from damage by screws or nails as compared to ordinary insulated cable.
Although it does have some disadvantages:
- The freshly cut edges are difficult to work with and very sharp.
- New wires cannot be run through it.
- It is expensive, and a new cutting tool must be bought for each new size.
I wasn't sure which name to node this under, so I ran a Google Fight. "Armored Cable" won with 11,700 hits, with "BX Cable" coming in a distant second with about 3,000.
1. Transitional Man says armored cable with an insulated ground wire is called MX cable. These cables do not have a bare grounding conductor beneath the armor.