A standard contactor can be fitted with normally open poles (poles open when the contactors is deenergized) and also with normally closed poles assembled on the same axle.

The extreme operational conditions which can be obtained with a continuous adjustability, are the following:

a) when closing the contactor, the N/C pole <<R>> opens before the closing of the normal pole <<N>> and, viceversa, when opening, <<N>> opens before <<R>> closes (type of adjustment suitable for the change-over contactors);
b) when closing the contactor, the pole <<R>> opens after closing of the pole <<N>> and, viceversa when opening, <<R>> closes before <<N>> opens (type of adjustment suitable for the field discharge contactors).

As already mantioned, the main characteristic is the possibility of passing from one to the other of the two extreme setting conditions with no-step, even adjusting within large limits the change-over time, or the delay of one pole in respect of the other.
The poles type <<R>> are produced in standard sizes with rating up to 1000 Amps; for higher ratings poles of the existing sizes have to be connected in parallel.
It is also remarkable that the poles <<R>>, besides with poles <<N>> of the same size, can be combined with poles of different sizes as indicated in the table below. Of course the same combinations can also be made with only poles <<R>> or only poles <<N>> of different sizes.
The poles type <<R>> have the same overall dimensions of the poles <<N>> but for the height.
Therefore the overall dimensions of the change-over contactors correspond exactly to those of the normal contactors of the same size and number of poles, except for those indicated in the table below.

Overall Dimensions of N.C. pole (Table N°12)

Electrical characteristics of the normally closed poles
(table N°13)