Relay Designations, Symbols, and Diagrams
1.4 Contact Assembly Designations
The contact switching combinations available on a relay are defined in terms of
number of poles, number of throws(single or double), normal position(open or
closed contacts),and the sequence to make and break. The various combinations
have been given form letter symbols in Figure 1.6 to simplify overall
identification. Abbreviations used to define the exact nature of the contacts
are as follows:
Number of poles. The term single pole (SP) contact denotes that all
contacts
in the arrangement connect in one position or another to a common contact. A
double pole (DP) contact consists of two single-pole contact arrangements
actuated by the same mechanical system and operation concurrently. Likewise, a
triple pole (TP) contact consists of three single pole contact systems. Larger
numbers of poles in relays are generally indicated by the number of single pole
contacts followed by a "P" (i.e., a four pole relay is 4P).
Number of throws. Single throw (ST) contact combinations have a pair of
contacts open in one relay position and closed in the other. Double throw (DT)
contact sets have three contacts. The common one is in contact with the second,
but not with the third, in one position of the relay, and reverse this
connection in the other relay, and reverses this connection in the other relay
position. The basic double throw contact combination is the break-make (Form
C).
Normal position of contacts. The combination in which the contacts are
open in
the normal or nonoperated condition of the relay is designated normally open
(NO).
The combination in which the contacts are closed in the de-energized or
unoperated position is designated normally closed (NC).
Double make and double break. These contact combination have two
independent
contacts both connected to a third contact in one position of the relay. They
are designated double make (DM) when normally open and double break (DB) when
normally closed.
Sequence of abbreviations. When abbreviations are used to designate a
contact
assembly, the following order is used: (1) Poles (2) Throws (3) Normal
position (4) Double make or break (if applicable).
Example: SPST NO DM refers to single pole, single throw, normally open, double
make contacts.
NO-Normally Open
B-Break
M-Make
DP-Double pole
DT-Double Throw
NC-Normally closed
DB-Double break
DM-Double make
TP-Triple pole