Types of Strand Construction
BUNCHED stranding is composed of any number of strands twisted together in the same direction without regard to geometrical arrangement of the individual strands. Normal direction of lay is left-hand.
TRUE CONCENTRIC stranding consists of a central wire surrounded by one or more layers with lay direction reversed for successive layers, and with the length of the lay increased for each successive layer. Normal direction of lay of the outer layer is left-hand.
COMPRESSED stranding contains a conventional conductor manufactured to a diameter not more than 3% below the nominal diameter of a non-compressed conductor of the same cross-sectional area.
UNIDIRECTIONAL CONCENTRIC stranding is the same as true concentric except that lay direction is the same in all layers. Normal direction of lay of the outer layer is left-hand.
UNILAY stranding is the same as true concentric except that lay length is the same in each layer. Normal direction of lay is left-hand.
EQUILAY stranding has both lay length and lay direction the same in each layer. Normal direction of lay is left-hand.
ROPE stranding is composed of cabled groups of any of the above stranded members.
19-WIRE COMBINATION UNILAY conductors are 19 strand constructions with a straight central wire, an inner layer consisting of six wires of the same diameter as the central wire, and an outer layer of six wires of the same diameter as the central wire alternated with six smaller wires. The smaller wires have a diameter of .732 times the diameter of the central wire. The 12 wires in the outer layer and the six wires of the inner layer all have the same length of lay and direction of lay.
Tinned Copper Conductors
Tinned copper conductors are usually specified for use in electrical and electronic equipment. Although they are slightly more expensive than bare copper, they facilitate rapid soldering. Tinned conductors are also more resistant to corrosion.
Types of Tinning
TINNED COPPER has individually tinned strands and is the lowest in cost of all tinned varieties. Acceptable under UL, CSA, ASTM, MIL and most industry standards.
HEAVY TINNED COPPER is similar to tinned copper, but contains substantially greater tin thickness. Used with high frequency induction heaters to bond on the stripped area. The rest of the wire retains its original flexibility. Acceptable under UL, CSA, ASTM, MIL and most industry standards.
PREFUSED or PREBOND COPPER is composed of twisted strands of heavy tinned copper fused along its entire length by heating. While it presents the characteristics of a solid conductor, Prefused or Prebond copper will not work-harden and break like a solid conductor when continually flexed. Acceptable under UL and CSA; not generally acceptable under most Mii-W-16878 types.
OVERCOATED COPPER consists of tinned copper strands twisted together followed by an overall tin coating. Offers the same advantages as Prefused or Prebond copper. Acceptable under UL and CSA; not acceptable under MIL specifications.
TOPCOATED COPPER is the same as Overcoated copper except that bare copper strands are used. Acceptable under UL and CSA; not acceptable under MIL specifications.
Conductor twist (called "pitch") may be short or long. A short pitch will not flare out as readily as a long pitch conductor. When a lead wire is cut on automatic equipment and terminated separately, a flared conductor may result. Today, most automatic cutting equipment strips the insulation off the conductor and applies a terminal in one operation. In this case, the longer pitch conductor is satisfactory.