Relay Overview 205

2 Form A

An EMR with a normally open, double-pole single-throw switch. 2 Form A contacts are single-make.

2 Form C

An EMR with a double-pole, double-throw switch. 2 Form C relays have single-break and single-make contacts.

Amperage

A measurement of the amount of current flowing in a circuit. Amperage is measured in amperes (A).

Armature

The movable part of an electromechanical relay. The armature in an electromechanical relay moves the pole of a switch.

Automation

The use of self-regulated equipment, processes, or systems that meet manufacturing requirements with limited human intervention. Automation is an efficient means of performing manufacturing processes.

bimetallic strip

A component made of two metals with different expansion temperatures that curls up when exposed to heat. Bimetallic strips are often used to detect temperature and open contacts in circuit breakers and overload relays.

Break

To open an electric circuit. Normally closed contacts break a circuit when a switch is activated.

Break-before-make

Disconnecting from one throw before connecting with another. Form C relays have break-before-make contacts.

Capacitors

An electronic component that stores an electric charge and releases it as needed. Capacitors are used in some timing relays.

circuit breakers

A safety device that opens and de-energizes a circuit when it detects overcurrent conditions. A circuit breaker often contains a bimetallic strip that bends and trips a switch that opens the circuit.

Coil

A length of conducting wire that is wrapped continuously around a magnetic core. Coils are used to create magnetic fields when current passes through them.

Contactors

A relay used to safely control a high-energy circuit. Contactors are also known as power relays.

Contacts

A conductive metal part used to make or break an electrical circuit. Contacts in switches open or close circuits by connecting with or separating from one another.

Control circuit

A relay circuit that uses control devices to open and close an output circuit. Control circuits often have lower voltages than output circuits.

Control relays

A relay that opens or closes a circuit in response to electrical signals from other devices. Control relays are used to perform manufacturing processes automatically or remotely.

Cycle timing relays

A timing relay that closes and opens the output circuit at preset intervals as long as the control circuit is energized. Cycle timing relays are often used in automatic pumps and sprinklers.

Differential relays

A relay that monitors and compares the voltage or amperage at two locations in a circuit. Differential relays activate and open the circuit if there is a difference between the two locations.

Dissipates

To cause to spread out, disperse, and disappear. Double-break and double-make contacts dissipate heat better than single-break and single-make contacts.

Distribution system

A network of electrical components that provides electricity from the utility to specific end destinations. Distribution systems use generators, power lines, and transformers to deliver electric power to buildings and other structures.

Double-break

Opening a circuit at two locations at the same time. Form Y relays have double-break contacts.

Double-make

Closing a circuit at two locations at the same time. Form X relays have double-make contacts.

Double-pole double-throw

DPDT. A switch with two movable contacts that each have two potential circuit paths. Double-pole double-throw switches have two poles that can each connect to two possible circuit paths.

Double-pole single-throw

DPST. A switch with two movable contacts that each connect to one circuit path. Double-pole single-throw switches simultaneously open and close two different circuits.

Electrical faults

Any condition that causes an abnormal flow of current in a circuit. Common types of electrical faults include open circuit faults and short circuit faults.

Electrical isolation

Separation between two electrical circuits or systems. Electrical isolation prevents unwanted or dangerous interaction between different electrical circuits.

Electromagnetic

Operating via a magnetic field created by electric current. Electromagnetic switches are used in electromechanical relays.

Electromechanical relays

EMRs. A relay that uses electric current to create a magnetic field that moves contacts. Electromechanical relays are inexpensive and often used in motor controls.

Emergency stop

e-stop. A button or switch that brings a machine to a safe, rapid stop. Emergency stops are important safety features that are intended to be used in case of emergencies.

EMRs

Electromechanical relays. A relay that uses electric current to create a magnetic field that moves contacts. EMRs are inexpensive and often used in motor controls.

Form A

An EMR with a normally open, single-pole single-throw switch. Form A contacts are single-make.

Form B

An EMR with a normally closed, single-pole single-throw switch. Form B contacts are single-break.

Form C

An EMR with a single-pole double-throw switch containing one normally open and one normally closed throw. Form C contacts are single-make and single-break.

Form D

An EMR with a single-pole double-throw switch that connects to the second throw before disconnecting from the first. Form D relays are similar to Form C relays but have make-before-break contacts.

Form X

An EMR with a normally open, double-make switch. Form X relays are similar to Form A relays but have two pairs of contacts that close at the same time.

Form Y

An EMR with a normally closed, double-break switch. Form Y relays are similar to Form B relays but have two pairs of contacts that open at the same time.

Form Z

An EMR with a switch that has one set of double-break and one set of double-make contacts. Form Z relays are similar to Form C relays but have two pairs of contacts that open or close at the same time.

Gate current

The current applied to the gate terminal of a semiconductor switch that determines whether or not the device triggers. Gate current must be strong enough and applied in the correct direction for a device to turn on.

General-purpose relays

An EMR designed for most ordinary control relay and protective relay applications. General-purpose relays are not designed for high-energy or safety-critical applications.

Generators

A device that converts mechanical energy into electrical energy. Generators operate on the principle of magnetic induction.

heating element

A device that converts electricity into thermal energy. Heating elements emit heat when current passes through them.

Hybrid relays

A relay that contains both SSR and EMR mechanisms. Hybrid relays have the high speed of SSRs and the durability of EMRs but are very expensive.

Internal fault

An electrical fault that occurs inside a piece of equipment. Internal faults inside safety relays must cause the relay to open its output circuit and de-energize any connected equipment.

Interrupt capacity

The maximum amount of overcurrent that a protective relay can safely disconnect. Interrupt capacity is measured in amperes and sometimes varies depending on voltage.

Latching relays

A relay that stays in the activated position even after the control circuit is de-energized. Latching relays may be EMRs or SSRs.

Light curtains

A presence-sensing device consisting of a field of light that stops a machine when the light is blocked by an object. Light curtains prevent mechanical parts from striking employees and equipment.

Light-emitting diode

LED. A semiconductor device that emits a narrow spectrum of light in a forward direction. Light-emitting diodes are used in the control circuits of many solid-state relays.

Machine tool relays

An EMR designed for high-energy applications and harsh operating conditions. Machine tool relays are widely used as control relays and contactors in manufacturing.

Machine tools

A power-driven piece of equipment used to form or cut metals. Common machine tools include lathes, mills, and drill presses.

Magnetic field

The area in and around a magnet in which a magnetic force exists. Magnetic fields exhibit the powers of attraction and repulsion.

Make

To close an electric circuit. Normally open contacts make a circuit when a switch is activated.

Make-before-break

Connecting with a second throw before disconnecting from the first one. Form D relays have make-before-break contacts.

Milliseconds

ms. A unit of measurement that is equal to one thousandth of a second. Milliseconds are used to measure the operation time of relays.

Motor controls

Any device used to start, stop, or change the speed, torque, or direction of a motor. Motor controls are essential for operating manufacturing equipment such as conveyor belts and industrial robots.

Motor starters

A device used to turn a motor on and off. A motor starter consists of a contactor combined with an overload relay.

Multi-function timing relays

A timing relay that can be adjusted to perform various potential functions. Multi-function timing relays are more complex but also more versatile than other timing relays.

NC

Normally closed. An electrical contact that is positioned to allow current flow. NC contacts only open to stop current flow when they are activated.

NO

Normally open. An electrical contact that is positioned to prevent current flow. NO contacts only close to allow current flow when they are activated.

Normally closed

NC. An electrical contact that is positioned to allow current flow. Normally closed contacts only open to stop current flow when they are activated.

Normally open

NO. An electrical contact that is positioned to prevent current flow. Normally open contacts only close to allow current flow when they are activated.

Off-delay timing relay

A timing relay that opens the output circuit a preset amount of time after the control circuit is de-energized. Off-delay timing relays are often used for automatic lights.

On-delay timing relay

A timing relay that closes the output circuit a preset amount of time after the control circuit is energized. On-delay timing relays are often used in motor starters.

One-shot timing relays

A timing relay that immediately closes the output circuit when the control circuit is energized, then de-energizes the output circuit after a preset amount of time. One-shot timing relays are often used for automatic dispensers.

Output circuit

A relay circuit that carries electric current directly to a load. Output circuits are also known as power circuits.

Overcurrent

Excess current flowing through a circuit. Overcurrent conditions can cause circuits to overheat and catch fire.

overload relays

A protective relay that opens a motor circuit when overcurrent conditions persist for too long. Overload relays prevent a motor from overheating due to overloading.

Parallel

Having multiple loads and multiple paths for the flow of electricity. Parallel circuits allow loads to operate independently of one another.

Permanent magnet

A magnet that retains its attractive force after it is removed from a magnetic field. Permanent magnets are used in some latching relays.

Photodiode

A semiconductor device that produces an electric current when exposed to light. Photodiodes are used in the output circuits of many solid-state relays.

Plug-in relays

An EMR that has a separate detachable compartment for its coil and contacts. Plug-in relays allow for easy replacement of coils or contacts that have worn out.

Poles

A movable contact controlled by a switch. The number of poles in a switch indicates the total number of movable contacts.

Power circuit

A relay circuit that carries electrical current directly to a load. Power circuits are also known as output circuits.

Power relays

A relay used to safely control a high-energy circuit. Power relays are also known as contactors.

Presence-sensing devices

A device used to detect the location of people or objects near hazardous areas of a machine. Presence-sensing devices include pressure-sensitive mats and light curtains.

Programmable logic controllers

PLCs. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. Programmable logic controllers can be used to automate processes.

Protective relays

A relay designed to open a circuit when it detects dangerous conditions. Protective relays help prevent hazards caused by overcurrent and electrical faults.

Redundant

A component that duplicates another component's function. Redundant components protect against complete device failure if one component malfunctions.

Reed relays

An electromechanical relay with a set of blade-shaped contacts sealed inside a glass tube. Reed relays have single-pole single-throw contacts.

Relays

A switch controlled by electrical current. Relays may be electromechanical, thermal, or solid-state.

Safety relays

An EMR designed to open output circuits if connected equipment malfunctions or if the relay develops an internal fault. Safety relays are protective relays used with safety equipment such as emergency stop buttons or presence-sensing devices.

Semiconductor switches

An all-electrical control device that can make or break a circuit and is composed of semiconductor materials. Semiconductor switches include diodes, thyristors, transistors, and TRIACs.

Sensors

A device that detects a change in a physical stimulus and turns it into a signal that can be measured or recorded. Sensors can be connected to relays to automate manufacturing processes.

Service life

The length of time a component or device is expected to be in operation before needing to be replaced. Solid-state relays have a longer service life than electromechanical relays.

Single-break

Opening a circuit at a single location. Form B relays have single-break contacts.

Single-make

Closing a circuit at a single location. Form A relays have single-make contacts.

Single-pole double-throw

SPDT. A switch with one movable contact and two potential circuit paths. Single-pole double-throw switches are often selector switches.

Single-pole single-throw

SPST. A switch with one movable contact and one circuit path. Single-pole single-throw switches are typically on/off switches.

Solid-state relays

SSRs. A relay that uses semiconductor devices to open or close circuits. Solid-state relays are fast and contain no moving parts but are more expensive than other relay types.

SPDT

Single-pole double-throw. A switch with one movable contact and two potential circuit paths. SPDT switches are often selector switches.

SPST

Single-pole single-throw. A switch with one movable contact and one circuit path. SPST switches are typically on/off switches.

SSRs

Solid-state relays. A relay that uses semiconductor devices to open or close circuits. SSRs are fast and contain no moving parts but are more expensive than other relay types.

Terminals

A conductive component on an electrical device to which other components can be attached. Terminals generally have a negative or a positive charge.

thermal relays

A relay that uses electric current to create heat that melts or bends components to move contacts. Thermal relays are often used as protective relays.

Throws

A stationary contact connected to a circuit path. The number of throws in a switch indicates the total number of potential circuit paths.

Timing relays

A relay that activates or deactivates on a time delay. Common timing relay types include on-delay, off-delay, one-shot, and cycle timing relays.

Transformers

A device that uses electromagnetic induction to transfer electrical energy from one circuit to another. Transformers can be used to measure amperage and voltage in a circuit.

Voltage

The electrical pressure or potential that forces current through a conductor. Voltage is measured in volts (V) and is also called electromotive force.