Contrary to popular belief the thermoelectric EMF is not created at the junction of two different metals. The effect arises when a conductor has different temperatures at each end, a voltage difference then appears across the conductor caused by the Seebeck Effect , Peltier effect and the Thomson effect.
For users of switching systems it can be of concern since it can create low voltage offsets across a connection. If a system used the same metals throughout it is connection path and the temperature at the two terminals was the same then no voltage would be seen. However, most systems have a variety of metals in their conduction path, including combinations of copper, tin, beryllium copper, gold and reed relay materials. That can then lead to the creation of thermoelectric effects.
In the example shown if the conduction path has the same metals throughout then even though the wires can create a voltage offset the effect cancels out. If however one wire is different then a voltage can appear.
The change of materials and temperature also occurs in relays. Different metals are used in the relay construction (including precious metals used for contacts), and heat sources such as coils creates temperature profiles.
The effects may also be present on the connections used to connect relays to the user connector in a switching system.