H. Kuhnke Ltd, Romsey, UK
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Relay Company Index

A relay

A relay is an electrically operated switch. Please see the "what is a relay" page on this web site for a more lengthy view. Manufacturers tend to categorise relays into applications to make it easier to select a product from extensive ranges The Relay Company uses terms such as "Industrial Relay" for heavy duty applications, "Automotive Relay" for the ardous standards of car manufacturers (mainly high temperature), "Signal Relay" for low currents, "Safety Relays" for applications requiring mechanically guided contacts, "Panel Builder Relays" for products used in control panels. Other terms include "PCB relays": any relay made to be mounted directly onto a circuit board, "Interface Relays": any relay using the output of a plc to switch another circuit, and "Telecoms Relays" another term for low current applications

Approvals CE/UL/CSA/TUV

The following approvals are most frequently applied to relay products. This is not a full list by any means. Most relays on this web site carry these approvals.
CE:European Compliance (with the European EMC directive 89/336/EEC)
EMC = Electromagnetic Compatiblity.
Technically not essential for components such as relays, but often put onto relays or sockets sold in Europe.
UL:Underwriters Laboratories. A widely recognised US standards authority that certifies the stated loads and or materials.
CSA:Canadian Standards Authority, a similar standards authority to UL
TUV: German certification body, mainly for motor industry standards .

Arc suppression

An arc will form as contacts come together and currents flow, and when they break apart. With AC current the condition is seldom a problem in relays, but with high DC loads the arc can be substantial causing contact damage. Arc suppression can be achieved using a blow out magnet. This facility is available on Kuhnke IA industrial relays.

Blow out magnet

See arc suppression. Available as an option on Kuhnke IA relays.

Bounce

Occurs as a moving contact strikes a fixed contact and "bounces" before remaining full at rest. This has to be minimised, and ideally engineered out of the switching process by the relay manufacturer. Bounce creates signal noise and contact wear. Some manufacturers may quote bounce times.

Coil

That part of a relay which is energised to create a magnetic field that attracts a lever that in turn carries out the switching function. The coil voltage( nominal coil voltage) is set at manufacture. Coils can be permanently energised if necessary. The supply to the coil switches the relay. The coil will activate its lever at a ceratin voltage (the pull in voltage) and release it at the drop out voltage. Standard coils are designed to operate from 80-85% to 110% of the nominal coil voltage at -5 to + 55°C in most circumstances. You should alawys check data sheets if you need a product to operate outside these parameters.

Contacts

Contact configuration

Contact configuration is a way of describing the number of contacts, and their function for example "2 pole changeover" or "1 make contact".

contacts:make contact

Two contacts which come together to make an electrical circuit, for example "1 make contact" meaning two contacts coming together into one circuit.

contacts:break contact

Two contacts which move apart to break an electrical circuit. for example "1 break contact" meaning two contacts separating to break a circuit.

contacts:changeover

One changeover contact comprises a set of 3 different contacts. A moving contact is at rest touching one fixed contact but is apart from a 2nd fixed contact. On changeover the moving contact breaks its connection with the first, and then makes its connection with the second. A relay may have 1 (set of) changeover contacts or more. For example 1C/O or 1 Pole changeover; 4C/O or 4 pole changeover.

contacts NO

Contacts Normally Open. A pair of contacts are separated at rest with no electrical connection

contacts NC

Contacts Normally Closed. A pair of contacts are together at rest making an electrical circuit.

contact material

The material used to manufacture the actual contact points. Please see "Materials" on The Relay Company web site. Contact material is critical and varies according to the load being switched.

contacts:twin

Twin contacts may be used for switching small loads ( below 50mA). Two parallel contact blades open and close the circuit, making with a standard fixed contact. This design was commonly used in Kuhnke Industrial and panel builder relays as an optional feature. Recent improvements in contact materials have made this feature largely redundant as lower and lower loads can be switched by standard products.

contacts mechanically guided

Mechanically guided, "mechanically linked" or "positive guided" contacts are used in "safety relays" They may be called "forced contacts". Usually a 2 pole ( 2NC + 2NO) changeover configuration, the normally open and normally closed contacts can never be closed at the same time The NO contacts are normally used for the load, and the NC contacts for monitoring. If an NO contact welds in the closed position then on de-energisation the NC contacts will remain open See IEC 60947-5-1/A2 and EN50205

Drop out voltage

The voltage at which a relay (coil) de-energises sufficiently for the operating lever to move back to its rest position. It is normally expressed as a % of the nominal coil voltage.

Housing

The whole relay assembly is enclosed in a housing, enclosing coil contacts, test buttons etc, with pin connections on the underside The housing may be standard dust proof, flux tight ( for soldering connections) or completely washable. Check individual data sheets.

Indicator

A mechanical display showing the position of the contacts, often found in relays used for panel buiding. Usually an arm attached to the moving contact inside the case with a visible part showing in the top of the relay. Useful for seeing if the contacts move when the coil is energised. Often known as a "status indicator" or a "flag indicator" or a "mechanical indicator"

Inrush

Inrush current is the peak current passing across the contacts of a relay when the contact is first made and is dependent on the load being switched. A relay which has contacts rated for a continuous current, the nominal contact current, may be capable of withstanding much higher currents for short periods, and this will be shown in data sheets, A high inrush capacity may be double the nominal current, but the load may require 10 times! Seek advice if you are unsure.

Load

The electrical circuit which is being switched is measured and defined by (1) current (A= Amps) (2) voltage (V) DC or AC and (3) load type (Inductive or resistive defining how the current flows when the contact is first made). A relay is generally limited by the amount of heat that occurs when an electrical current passes across its contacts. This represents the "load" that a relay can switch and is normally presented in literature as an electrical value This is usually stated as a contact current in A then a voltage often standardised at 250VAC/DC followed by a maximum capacity at a resistive load. This is the result of mutiplying current by voltage, expressed as VA. It is usually the maximum permissible load at any time including starting and stopping.
So a 10A relay with a contact voltage of 250VAC and max switching capacity of 1540VA can only switch 6.16A at 250VAC, but 10A at voltages below 154VAC.
The "type" of load is more important than the nominal current (A) and voltage (V) because the load is defined as the part of the switched circuit which is using electrical power. It could be a motor, a lamp, a transformer, a control device etc etc.
There are standard definitions of loads with categories such as AC-1, AC-2, AC-8b etc and what is most significant is the peak current drawn by those loads at start up A small electrical motor may use 10 times its normal operating current at start up, but that peak or "in rush" may be enough to burn out relay contacts even if it lasts for only milliseconds. Instead of publishing the switching capacity for the wide variety of possible loads, which could be very confusing, relay manufacturers generally give a nominal current A, a nominal voltage V a maximum VA for a resistive load, and a max inrush current.

Modules

A term usually applied to small components that can be purchased separately to plug into a relay socket. Usually to provide a coil indicator light and/or electrical coil protection The socket has to be of a particular design Popular with relay and socket assemblies in control panels

Pin out

The dimensions and configuration of the pins, usually on the underside of a relay which form the connections to the coil and the contacts. Some standardisation applies to relays supplied to the panel builder industry. Expect 2 pins for the coil connections, and 3 pins each per set of change over contacts.

Pull in voltage

The voltage at which a relay (coil) operates and switches. It is normally expresseed as a % of the nominal coil voltage. Sometimes known as threshold voltage. It is affected by temperature.

Safe separation

The practice of keeping the electrical current paths to the operating coil separate from those to the contacts A socket with safe separation will have the coil terminals at one end, and the contact terminals at the other. A standard exists that sets out the basic requirement of insulation between circuits up to 1000VAC or 1500VDC ( DIN EN 61140)

Socket

Relays are either mounted directly into a circuit board ( and soldered) or fitted into a socket. Todays sockets are generally designed to mount directly onto DIN rails. Some relay sockets are only suitable for a specific relay, others are interchangeable between manufacturers. If sockets have coil terminals on one side and contact terminals on another side they are said to have "safe separation" Some modern sockets have a a facility to insert function modules. A socket will be rated for electrical load and insulation properties.

Switching Capacity

Switching capacity is the product of switching voltage and switching current The current which a relay will switch will vary according to the voltage being used but is usually the first value shown in data sheets of relays. Note that the maximum often includes the value occuring at peaks ( see "inrush"). A minimum also applies, because contact materials that can withstand high current loads may be poor at making contact at low current loads. Please see "loads"

Test button

A mechanical push button that operates the switching contacts of a relay without using coil power. May require a tool to avoid accidental use. May have a latching facilty.