Most relays used in industrial circuits have the ability
to have their contacts changed from NO to NC in the field. In other words,
a technician can remove several screws and mount the contacts upside
down and they will change from NO to NC, or vice versa. Ill. 1 shows
an example of this type of contact. This feature makes adding NO or NC
contacts very easy. It also makes it simple to change a set of NO contacts
to NC at the time it's needed. Since these contacts are actually cartridges,
additional sets of contacts can be added to a relay at anytime as long
as there is room. In some cases a second deck is added to the relay so
more contacts can be added.
Above: Ill. 1: Example of contacts that can be changed
from NO to NC in the field.