The follwoing is an explaination of the three wires in the RTD and a description of the six wire dual element.
(1) The three leadwire RTD connections are recommended for the RTDs having low sensing element resistance, where a small leadwire resistance can have a large effect on the readout accuracy. In the two wire RTD, the resitance of the leadwire is in series with the sensing element, and the readout is the sum of resitance of the sensing element and the leadwire. The lead wire resistance can be high and must be compensated for. Compensation is achieved with a three-lead configuration. The three leadwire Copper RTD, for example, has three wires marked R1, W1, W2 or L1, L2, L3. W1 and W2 (L2 & L3) are connected to one side of the element and R1 to the other side. The measurement of the resistance is between R1 to W1, R1 to W2, and W1 to W2. Take the average of R1 to W1 & R1 to W2. Subtact from the average the resitance W1 to W2. By doing so the resultant resistance is close to the sensing element resistance. Platinum RTD is the same with three lead wires marked B1, Y1, Y2 (L1, L2, L3). R is Red, W is White, B is Blue, and Y is Yellow.
(2) The six leadwires ( 3 for the copper & 3 for the platinum) dual element stator winding RTDs provide extra protection and backup (in case one element failed). Dual elements can be one copper and one platinum, or both platinum. Most of the large hydro generators stator windings require a minimum of 12 dual elements with an even number of RTDs in each circuit of the three phases. The elements are placed at two different axial locations in the slot. Assume a 60 inches stator core length and the dual element length is 40 inches long (20 inches Copper RTD & 20 inches Platinum RTD). Half of the copper elements, in the dual element RTD assembly, will be at the top of the stator core while the platinum element is at the center of the stator core. The other RTD assemblies per phase shall have the platinum at the center of the core while the copper element is at the bottom of the core. This means that every other RTD, around the circumference of the stator shall have the copper element at the top. This arrangement will provide a full spectrum of the temperature distribution along the slot from top to middle to bottom.
Hope the above discussion clarifies the termenolgy used.
The question is still why the copper RTDs are not favored anymore and the Platinum RTDs are teking over on the rewind and new winding projects. The price is the same, therfore it is not a factor.