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 The scheme of symbols used in the relay

Other related articles:

- The principle of operation of the vacuum relay, general information.
-High voltage vacuum relays. Areas of use.
-High voltage vacuum relays. Basic terms and definitions.

1. Designation of relay contacts.

COM – common contact of the relay, which is movable. Often referred to as BASE or COMMON. The common contact is also called a pole, and those with which it connects are called directions.

NC (Normal Close) - contact with which the common is normally closed (normally closed). This means that the contacts are closed when the relay is de-energized and open when current is applied to the control coil.

NO (normal open) - contact with which the common is normally open (normally open). Those. when the relay is de-energized, the contacts are open, and when the coil is energized, the contacts are closed.

In the circuit with NC, we see that the current flows through the relay when the coil is de-energized, and in order to open the circuit we need to apply voltage to the coil, and in the second case, in the de-energized coil and through the relay contacts, no current flows.

Normal condition - this is the initial state of the relay (with a de-energized coil). But it should be noted that there are types of relays, for example, polarized ones for which there is no concept of a normal state, since it can change, and accordingly the NO contact can become NC and vice versa.

2. Types of switches.

According to the type of switching, all relays can be divided into 2 main types:

- relay opens or closes a contact (SPST). Such a relay has one input and one output, and works like a key. In this case, one such relay may contain several pairs of independent contacts, i.e. have several databases with their own contacts (DPST).

- relay switches between two or more contacts (SPDT). There is one base here, but there may be several exits. Such relays can also have several pairs of contacts (DPDT).

SPDT (Single Pole, Double Throw). One pole, two directions. Those. There is one common contact that can switch with two directions.

DPDT (Double Pole, Double Throw). Two poles for two directions, i.e. 2 groups of switches. In fact, these are two SPDT relays in one, but with a common coil. Sometimes a DPDT type relay is designated as -2SPDT. In this way, relays with a much larger number of switches can also be implemented.

SPST (Single Pole, Single Throw). One pole per direction. Formally, this is a controlled switch, which can be either normally closed (NC) or normally open (NO).


DPST (Double Pole, Single Throw). Two poles in one direction. The double pole DPST relay is equivalent to two SPST switches (NO normally open and NC normally closed) and can be used to switch two different loads.


We have 2 scenarios depending on the relay type

Without coil voltage:

With NO, the loads will be OFF because no current can flow.

With NC the loads will be ON as current can flow

With coil voltage:

With NO, the loads will be ON as current can flow.

With NC the loads will be DISCONNECTED as no current can flow.

4. Designation options.

On complex relay combinations, detailed designations of switch types can be found. As already mentioned above, the DPDT relay can be designated as 2SPDT, although everything is clear here, but in the case of DPST NC-NO, we can not which of the directions is normally closed, but which is normally open, so the type designation 2SPST-1NC- is introduced. 1NO.

We must understand that in this situation DPST NC-NO = 2SPST-1NC-1NO.

5. General table of symbols.

Switch type Switch type (alternative designation) Switching scheme Description
SPST NO A SPST-NO (Single Pole Single Throw - Normally Open)

One contact per close, normally open

SPST-NC B SPST–NC (Single Pole Single Throw - Normally Closed)

One contact per switch, normally closed

SPDT C SPDT (Single Pole Double Throw)

One contact per switch



2A DPST NO (Double Pole Single Throw, Normally Open)

Two closing contacts, normally open



2B DPST NC (Double Pole Single Throw, Normally Closed)

Two closing contacts, normally closed



1A1B DPST-NC-NO (Double Pole Single Throw - Normally Closed - Normally Open)

Two contacts for switching on: one is normally closed, the other is normally open

DPDT 2C DPDT (Double Pole Double Throw)

Two switching contacts