Relay Contacts| Operation and Types/Abbreviations

Relay Contacts of Operation

The contacts of an electromagnetic relay are responsible for creating and breaking connections in electrical circuits. These contacts must be able to handle both the inrush current as well as the nominal-load current, which depend on the power they are required to carry and the characteristics of the circuit being used. The overall performance of a relay is determined by its contact arrangement, mechanical construction, and most importantly, the suitability of its contact materials.

Relays have various types of contact arrangements that are used for making and breaking electrical circuits. Each arrangement has different capabilities to allow them to safely transmit electricity through varying conditions.  These contacts are made from materials such as metals or alloys that must be suitable for the conditions, under which they are expected to be used.

The Types/abbreviations commonly used for relay contacts are listed below.

  • SP – Single-Pole
  • SB – Single-Break
  • ST – Single-Throw
  • DB – Double-Break
  • DP – Double-Pole
  • DM – Double-Make
  • DT – Double-Throw
  • NO – Normally Open
  • 3P – Three-Pole
  • NC – Normally Closed
  • 4P – Four-Pole

The National Association of Relay Manufacturers (NARM) has developed codes to simplify the listing of contact arrangements. The table below outlines these codes, such as a 4PDT relay which stands for four-pole, double-throw contact arrangement.

Single-throw contact relays have two contacts that can either be opened or closed depending on the armature position. When the contacts are open in the unoperated position of the relay they are referred to as normally open (NO) contacts; while when they are closed in the unoperated position, they are called normally closed (NC) contacts.

Double-throw contact relays are equipped with three contacts and operate using the break-before-make (Form C) principle as illustrated in Fig. below. When the relay is activated, its contact positions reverse, so that one terminal touches the second but not the third. 

Double-make and double-break contact forms come with two independent contacts which are both connected to a third contact when the relay is in one position. If they are normally open, they are referred to as double-make contacts; if they are normally closed, then they are known as double-break contacts.

Classifications of compound contact arrangements for relays with more than two positions are designated by a symbol in the form MPNT, where M stands for the number of poles and N represents the number of throws. For example, 4PDT indicates that it is a four-pole, double-throw relay.

The coil at the heart of the relay’s electromagnet can open or close its contacts when current passes through it. In general, this requires a certain minimum amount of power to be supplied in order to operate properly; otherwise, the relay will not function correctly. The design of its coil affects various aspects such as sensitivity, operating speed, and power consumption significantly. 

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