Single strings of twisted 3-wire mini-lights help to decorate parties, special events, and holidays. Trying to understand their 3-wire 120-volt-alternating-current (VAC) assemblages can be difficult without foreknowledge. Actually, it is fairly simple. Here is how they are designed and put together.
Mini-light strings manufactured today.
Several lengths of mini-light strings (from 15-to-150-feet) are available to consumers today, each one holding a different number of clear or colored mini-bulbs. However, recent attempts to standardize the manufacture of these light strings has focused on the making of 50-bulb sets that can either be used alone as one string, or can be pre-assembled into longer 2-to-3-set, 100-to-150-bulb strings. Each completed string whatever its length will have an electrical plug at each end like an extension cord. This standardization appears to be true both for the older tubular incandescent lights (with their filaments sealed within their 3-mm-diameter bulbs) and the newer LED types (having light emitting diodes encased within the bulbs).
Currently, the newer LED strings are more expensive than the incandescent ones, but they use less electricity than the filament types. Still, the manufacturing of the shorter light strings of either kind having less than 50-bulbs is still done, and they can be found in the stores. But, they are not a major part of the current mass production of mini-lights.
Thus, each full-length string of any bulb-number will have a male plug at one end and a female plug at the other end. Also, these plugs are made so the individual strings can be stacked (two or three them plugged piggyback into each other at each end to make the total string longer) without causing electrical failure. The male plug of each string also has two small 3-amp buss fuses within it, one for each of its two main 120-volt wires. Yet these fuses will blow if the stacked strings get too lengthy (beyond 5 strings) or if too much wattage is drawn by them for other reasons.
To illustrate the description of a 3-wire single-string of mini-lights, this article uses a common 100-bulb string, which contains two 50-bulb sets pre-manufactured as one string. Each bulb in the two sets is rated at about 2.5-volts; i.e., 120-volts/50-bulbs = 2.4-volts/bulb. Short sections at each end of this string and at its middle between the two 50-bulb sets will show two wires only, the two power wires. The third wires are for the bulbs themselves.
Mini-light-string design and assemblage.
Basically, two of the wires on any three-wire mini-light string carries the household 120-volts of electricity from one end of the string to the other end similar to the way any extension cord does. The so-called third wire of this string holds all of the mini-bulbs and their sockets in succession (series) a few inches apart. Although the bulbs themselves are in series, this so-called third wire holding the bulbs is connected at each of its ends to each of the two 120-volt wires in parallel.
That is, one end of this bulb wire is connected to the first of the two 120-volt wires at one end of the whole string. The other end is connected to the second 120-volt wire at the other end of the set further down the same string. For a 100-bulb string, a second set of 50-bulbs is connected to the two 120-volt wires the same way further down the string from the first set, where it functions independently from the first set. The finished string can be from 20-to-50-feet long, depending on the spacing between the bulbs.
These connections are hard to see by looking at the finished product because all three wires are twisted to form a tight rope-like string, leaving only the bulbs exposed. Yet, each of these two bulb sets is connected parallel to the 120-voltage wires, which allows them to work independently of each other.
Another advantage of these parallel connections is the electricity itself continues to flow through the main two wires from one-end plug to the other end-plug if one or both bulb-sets fail for any reason. As a result, when a 100-bulb light string begins to show unlit bulbs, these failed bulbs could appear in either one or both sets. For example, one 50-bulb set could have all or only a few of its lights not working, while the second set could show all of its bulbs lit okay because each set operates separately from the other one. Since the electricity is still flowing through the two power wires in the latter example (i.e., several lights are still working), then some of the bulbs in the failing set need to be replaced.
To see the diagrams and photos of these mini-light strings, see this site.