On Blue's interior wiring, just as on Nine of Cups, stranded copper wire is much preferable to a single solid conductor. Stranded wire has superior flexibility, a longer flex life and is much more resistant to vibration. Once it's stripped of its insulation, however, stranded wire loses a great deal of its strength and resilience. As it begins to unravel, it's subject to breakage and corrosion, which can lead to overheating and short circuits. One way to overcome this when making electrical connections is to use a crimped terminal, such as a butt connector to join two wires or a crimped ring terminal to connect a wire to a terminal strip.
A lot of gear, however, incorporates screw terminal blocks. This is the type of connector used on the 120 VAC selector switch, as well as some of the electronic gear on Blue. While for most part it is acceptable to simply insert the stranded wire into the terminal and lock it into place with the screw, a better method is to use a ferrule.
A basic ferrule is a short tube made of tin plated copper that is slipped onto the end of a stripped wire and crimped in place. The crimped ferrule holds the individual strands of wire together and forms a gas tight seal. To make assembly easier and to increase wire durability, most ferrules also have a conical shaped plastic collar added to one end of the copper tube. Ferrules come in a host of different sizes designed to fit wires from AWG 28 to AWG 1. The plastic collars are color coded to make it easy to identify the size of the ferrule.
There are two significant advantages that ferrules provide:
No stray strands. It's often difficult to push the stranded wire into a screw terminal without a stray strand or two of wire separating from the bundle. Since the ferrule collar is funnel shaped, it is easier to push the entire wire bundle into it. In addition, the terminal block on 120 VAC selector switch is located in a place that makes it hard to see a stray strand, whereas it is easier to spot a problem with the ferrule.
Better long term connection. Weidmuller, a company that manufactures electrical components including ferrules, did a long term study of wire connections using solid wire, stranded wire and stranded wire terminated in ferrules. They measured the electrical resistance of the connections in a standard environment, as well as a salty environment over a period of years. The solid wire performed the best, showing little change in the electrical resistance of the connection over a four-year period, whereas the stranded wire showed a significant increase in the electrical resistance with time. The addition of ferrules to the stranded wire resulted in a connection that performed almost as well as the solid wire.
A special tool is required to properly crimp a ferrule. Unless you are crimping hundreds of connections, however, a very inexpensive tool can be purchased online. I bought a crimper and a large assortment of ferrules that will suffice for most of the electrical work on Blue for less than $30.
Link to Weidmuller_Ferrules_White_Paper