There are basically two types of contacts, the serial association and the parallel association (2.6b). In the case of association of normally closed contacts, the NF contacts in the same way can be found in series and parallel and the parallel association is called a non-E function. Just as to change a wheel in a car, when the tire drills, it is need to know the tools and their electrical commands, to really understand the operation of a circuit and later to design the circuit, we need to know the appropriate elements.
The big difference is in the fact that in large panels there are high capacity buses that can subject people to risk situations. When dealing with the association of contacts, it is common to set up a table containing all possible combinations between the contacts and it is known as the Truth Table.
It is permissible to note that in the series combination, the load will only be activated only when these two contacts are activated, thus being called function E. In the parallel combination, any of the connected contacts turns on the load and is therefore called the OR function .