note: Many people read my website, and I dont have a clue as to what
level of expertise the reader has under his or her belt, so to speak.
So with that in mind, I am going to assume that you know absolutely
nothing about relays and/or electricity. For those that do, please bear
with me. Thanks.
If you are ever in search of a relay to buy, and decide to check out
EBay, remember that most of these relays are quite heavy, so there will
be a hefty shipping fee on top of the purchase price! Some sellers
are quite reasonable in their listing price, others can be quite
outrageous, as seen below.
Acknowledgements: I would like to thank Bob Braden for his help
with this section.
As always, if you have something to add or correct, please
over a little signal history before we dig into relays. When railroads
came around in the 1830s, trains only operated during the day, and
there werent many of them. So in the beginning, railroads did not
need signals. Yet, even with daytime operation only, the railroads saw
a need to protect people from the trains where ever the tracks crossed a
road. So the first form of signaling were signalmen placed at the
crossings to stop traffic from crossing the tracks when a train was
approaching. My signal timeline page outlines the significant events in
railroad signaling, and is
Eventually, railroads wanted to run at night, they wanted to run more
trains on the same track, and they wanted to run opposing trains on the
same piece of track. So the railroads were now in a quandary as to how
to effectively manage the trains without them running in to each other.
In the early days, all sorts of mechanical and man powered schemes were
used to regulate trains. Some were more effective than others, and the
ones that didnt do what they promised, fell into the history books
rather quickly. If you have the patience, the U.S. Patent Offices data
base will show you some very interesting railroad devices. Eventually,
the basic electrical
track circuit was developed by
Robinson in 1872, and from
there, a whole host of semi-automatic and fully automatic signals
were developed. Then, once a suitable incandescent light bulb was developed,
reliable night time
signals could now be made (and seen). But none of it would be
possible if it were not for the development of the relay. I will say
development, as opposed to the invention of, because the basics of
the relay existed in the form of the telegraph, and may have existed in
other devices prior to that, so there was a progression in adaptation and
technology which gave birth to the railroad relay that were are familiar
without relays, modern railroad signaling would not be where it is
today. The relay forms part of a simple (basically) three element
circuit, consisting of the relay, a battery, and the track.This simple circuit forms the basis
of a signal system, and is called a
track circuit. A track circuit is responsible for detecting
the presence of a train, engine, car, or anything else in its protection
zone, usually called a block.
A track circuit by itself, in all but the most basic signal systems,
only forms half of the complete signal system. In order for the signal
system to display more than just green or red, additional relays will be
needed to perform some sort of simple logic. These extra relays will
allow the signal system to display the advanced aspects needed in modern
operation, and include things like interlockings, grade crossings,
sidings, crossings with other rail lines or tracks, bridges, etc into
the decision making capability of the relay logic.
Todays modern signaling systems have progressed beyond using track
circuit relays in the traditional sense, being replaced by electronics,
but there are still many, many railroads out there whose signaling
systems rely on the use of relays.
This page, however, will concentrate on the electro-mechanical relay,
or, just plain old relay.
relay is an electrical device, that upon the application of an
becomes magnetized and pulls in a set of contacts. A relay is used
to control, isolate, and/or activate another electrical circuit.
build a relay piece by piece. The first building block
of a relay is the core. Relays
use a magnetic field originating from the core to attract movable
core is made of iron..... stainless steel and aluminum will not work
because they can not be magnetized.
magnetic field is generated by applying an electrical current to a
coil is wound around the core, usually on something that insulates the
coil from the core, sometimes called a bobbin. This assembly is also known
as an electromagnet, or a
solenoid (if the core is hollow).
In contrast to a permanent magnet, like a bar magnet, the things you
stick on the door of your refrigerator, or the one in your speakers
which create a magnetic field all the time, an electromagnet's magnetic
field is controllable.
make a simple
electromagnet by taking a nail and winding wire around it (if you want to try
this, use a larger nail, like a 10 or 12 penny nail, and wind about 200
turns of #22 AWG magnet wire around it, then tape it down with
electrical tape). If you take a battery, like a 6 volt lantern
battery, and connect it to the wires of the coil, the electricity
flowing through the coil creates a magnetic field. If you have electric locks in your car, they are
activated by solenoids. Another common place you will find a
solenoid is in your starter motor - a solenoid pulls a small gear out to
engage the starter motor to the flywheel of your engine... OK, enough on
step in building or relay, is to mount the coil assembly to a
frame, and then add something that moves, for without
something that moves, we can never have a relay. We will use
another piece of steel or iron for the coil to attract, we'll call this
piece an armature.
So far, we have what they call in telegraphy, a sounder,
a device, that when energized, will make a click that can be heard.
This is why I said above that relays were developed, and not really
to the armature, we will connect in one fashion or another, a set (or
sets) of contacts. NOW we have a relay. The
contacts can be arranged so they will either open or close when the relay
is energized. As stated
already, the relay gives us a way to electrically control another
circuit, and this is fundamental to the understanding of how systems
built around relays work.
If I can digress for a minute back to the starter motor, there's a thing
hanging off the side of it called a solenoid, but it also doubles as a
relay, controlling the battery voltage to the starter motor in addition
to pulling out that pinion gear. BTW - In some cars, mostly older ones,
the relay is separate from the starter motor and solenoid. A
nicely illustrated primer on relays used in cars is at:
shows a simple (non-railroad type) relay in its un-energized position.
shows it in the energized position, notice how the armature has been
pulled down to the core of the relay, and that the contacts on the right
side of the photo, are in the up position in the left photo, and the
down position in the right photo.
Picture 1Picture 2
For every field, there are oodles of specialized relays
developed to meet a specific need. For instance, in the field of
telecommunications, the following pictures show a few relays from the
is a line finder relay.
This relay looks for a ground on an incoming subscriber circuit when you
pick up the phone. Once found, it will place a dial tone on the
line and wait for the dial pulses, which will get passed on to
additional Strowger relays. Picture 3B
Strowger (or "step by step") switch
for a rotary telephone system, one that could increment one notch for
each pulse coming from the old rotary telephone dials. This relay
performed in two axis': up, and then around, so it could accommodate two
of the three, four, five, or seven numbers of a telephone number (yes,
going back to a three digit phone number goes back a long, long ways!).
Strowger switches were used as early as the 1920's.
shows a relay that could be used in a pinball machine (it's
another telephone relay tho), and would
typically count the number of times the pinball hit a particular bumper,
and then control the appropriate lights on the playing board, or maybe a
score counter, and/or provide a feed to another relay. I mention these
relays only to illustrate the difference between them and railroad
relays when I get to that point in this discussion. We will talk more about
More info on telephone type switching systems can be found
come in many varieties, the most simple of which are DC and AC
versions. DC relays run off of the voltage that comes out of a battery,
which only flows in one direction, and the DC stands for Direct
Current. AC relays will operate on Alternating Current, which is what
you have coming out of the wall in your standard 120VAC receptacle.
relays are designed to operate off of a low voltage, like 12VDC, and
some, like track relays, are designed to operate from far less,
typically the voltage coming out of one cell, which is normally one and
a half volts. More on why track relays operate on such low voltage
later. Some relays designed to work with railroads powered by
overhead lines, such as the New Haven and
operated off of 110VAC.
terms you will hear in connection with relays is Neutral,Polar, and
A Neutral Relay is one that is
either energized or not. When energized, it pulls in, or picks the relay. When
un-energized, it releases and the contacts return to their un-energized
state. Relay contacts are usually shown on schematics in the
un-energized state unless otherwise stated.
other hand, a Polar Relay has
three states. It can be un-energized, it can be energized by a polarity
in one direction, or it can be energized by a
current of the opposite direction. Equate it to taking the
battery and flipping it around so the current flows in the opposite
direction. We will see later on how a relay
like this can save the railroads a considerable sum of money when
running wires over long distances, because we only need two wires
instead of three to get a control signal from one place to another.
Polar relays can also operate on AC, believe it or not. This is
accomplished by connecting the main coil to the power source so it is
always hooked up the same way, every time the relay is energized. Then, on the
armature, they place a second coil of which they can change the
the way the coil is connected to the power source, effectively changing
the "polarity" of the AC current energizing it - this will make it rotate
one way or the other.
Latching Relay is one where the relay
has two positions, but power is only needed to get it into the other
state, but NOT needed to maintain it in that position. You can see
where this would save electricity, and therefore, money for the
railroads. There are latching relays that use two coils to move
the armature into position, one coil for moving the armature in each
direction (and then it is held in place mechanically),
and there are relays that use one coil where the designers cleverly
designed the mechanism to take advantage of the residual magnetism to
keep the armature in place.
terms I should touch on here are Heel
and Front Contacts, since these
terms are frequently used when talking about railroad relays. A
Back Contact is a normally closed set
of contacts, or those which have continuity with the armature when the
relay is unenergized.
A Front Contact is a normally open
set of contacts, or those which have continuity when the relay is
energized. The movable part, or armature, is referred to as the
list of relay related terms is
at the bottom of the page.
most simple form, a track circuit consist of a relay and a battery,
interconnected by wires and the track. In normal operation,
without a train in the block, the battery energizes the relay via the
current flowing through the rails. This is shown in Figure 1.
If a train enters the block,
what happens? The wheels of the train shorts out, or shunts the current
going to the relay, and the relay is no longer energized. Figure 2 shows us the current
flow when a set of wheels has entered the block.
start off with a very simple two color signal that displays only red and
green, when the relay is energized, the relay will make the signal
display a green. When the trains appears in the block, and the relay
becomes un-energized, the signal turns to red.
didnt signal designers just make the relay energize the red as the
train entered the block, then we wouldnt have to power the relay all
the time a train is not present (thereby shortening battery life, which
was important in the early days of signaling when there was NO local source
of AC power to keep the batteries charged)? This is done to give us
what we call a failsafe circuit.
means that if something in the circuit "goes wrong", it fails to the safest mode it
was designed for. In the case of railroad signaling, the safest aspect
that can be displayed for a train is STOP. This way, the train wont
advance and risk the possibility of running into another train or some
other unsafe problem/situation. After
all, thats the biggest reason why signal system came about, to make
travelling by trains safe.
the circuit failsafe for a number of reasons, such as (but not limited
to): 1) if the battery is too weak
to power the relay, the signal will always display red (as long as the
power source for the signals is also good). 2) If a wire breaks, the
signal will display red, 3) If something happens to the track, like a
bad bond, or the rail breaks, the signal will turn to red, 4) if
something happens to the relay itself, it is also designed to fail so
that the signal will turn to red.
section deals with relays, we will hit on more track circuits on another
This is just to wet your appetite! :-)
Most of the pictures featured
here have been grabbed from elsewhere on the internet, mostly from Ebay
listings, if you have pictures you would like to contribute, contact
In searching for information on
relays, I came across another nice collection of relay photos by Jeffrey
He also has a very nice collection of railroad signals,
traffic lights, and all sorts of other interesting things!
GRS Thermal Relay
This relay can also be
called a timer relay. You apply a voltage, and at some
pre-determined time later, the contact(s) open or close depending on the
relay's configuration. This particular one is designed to close
the contact in 5 seconds. The 11.3 ohm coil is rated for 8VDC.
Thermal relays operate by the interaction of two different metals called
a "bi-metal" strip. The two metals have a different coefficient of
expansion, so when a voltage is applied, the metal strip heats up, and
because of the difference, the strip actually bends. The timing is
controlled by the length of the metal strip, the metals used to make the
strip, the applied voltage, and the spacing of the contacts. This
relay was selling for 20 bucks, much more reasonable.
GRS Type W A.C.
Relay, Size 2
An AC operated relay
designed for 10VAC on 60 cycles. The contact arrangement is 2
poles, double-throw (DPDT). This was another relay that was listed
for a quite reasonable $20.
Models ST-1 and ST-2 are compact vitals relays, typical of the majority
of relay in use today, having replaced the large bulky relays picture
elsewhere in this section. These "snag" came from their website,
and the specs shown below are just a small sampling of what they offer.
Safetran is now part of Invensys.
US&S DM Style Code
This relay was designed to
operate from 8-12VDC, and had a rate of 180 per minute. It has a
set of 4PDT contacts. From the 50's. This relay had a
starting price of 60 bucks on EBay, with a buy it now price of $225, I
didn't follow the auction to see if it sold or not. Personally, I
think the prices were high.
Neutral D.C. Relay
This DC relay has a coil
resistance of 1000 ohms, picks-up at 3.8 volts and drops away at 1.9
volts. It has a set of DPDT contacts. The top set of photos
is of an "A" version, the bottom set an "L" version.
D.C. Neutral Relay
This relay is just about as
simple as they come, with a single set of SPDT contacts. If I read
the specs correctly, it has a pickup voltage of 0.054 volts, and a
drop-out voltage of 0.025 volts. The plate says the coil
resistance is 0.14 ohms.
WABCO (US&S) DN-11
Neutral D.C. Relay
A typical neutral relay,
with a 4PDT configuration. According to the listing
these pictures came from, the relay has a drop-out voltage of 2.25V at
3.3mA, a pick-up voltage of 4.62V at 6.4mA, an operating voltage of 6.7V
at 10mA, and the coil has a resistance of 670 ohms. This is
typical of the relays the Pennsy used on the Northern Central between
Baltimore MD and Harrisburg PA.
FN-16A Flasher D.C. Relay
Flasher relays came about
before the advent of solid state electronics, so the designers of these
relays came up with clever mechanical ways to make the armature rock
back and forth magnetically without the need for anything external to
the relay. Working voltage is 6.8 to 8VDC, with a minimum of 6V.
This relay has a four sets of double-throw contacts (4PDT). This
one was set up for 40-45 flashed per minute, and the rate can be changed
by added or removing washers behind a thing called the backstrap.
I've seen prices for these on EBay in the $80-100 range, phew!
The terms used on this list are terms suggested by the IEEE
and the Association of Relay Manufacturers
. It is a fairly long list. This list covers all aspects of relays,
not just for the railroad field. Also, I have corrected some obvious mistakes that
appeared in the original list
Air Gap Sometimes used for Contact Separation or for Magnetic Air Gap.
Ampere-Turns The product of the number of turns in a magnetic coil and the
RMS current in amperes passing through the coil.
A hinged or pivoted moving part of the magnetic circuit in an
electromagnetic relay. Sometimes used in general sense to mean
any moving part which actuates contacts in response to a change
in coil current.
Armature Contact Sometimes used for Movable Contact.
Armature Relay A relay operated by an electromagnet which,
when energized, causes an armature to be attracted to a pole (or
Auxiliary Relay A relay which operates in response to opening
and closing of its operating circuit to assist another relay or
device in performance of a function.
Back Contacts Same as Normally Closed Contacts.
Sometimes used for the stationary contact of single-pole normally closed contacts.
Backstop The part of a relay which limits movement of the
armature away from the pole piece or core.
Backup Relaying Supplementary relaying designed to operate
if a primary relay should malfunction or a circuit breaker fail
to operate. Back-up relaying usually disconnects more of the
power system than just the part with the faulty element as this
is necessary in order to remove the abnormal condition and to
minimize effect on the remainder of the system.
A relay so designed that a bar actuates several contacts simultaneously.
Break-before-make-Contacts Contacts which interrupt
one circuit before establishing, or making another circuit.
Break Contact Same as Back Contact.
Break Delay Sometimes used for Release Time.
Bridging Bridging is a term used to describe a contact
transfer in which the movable contact touches the normally open
contact before leaving the normally closed contact during the
transfer action, thus never completely opening the circuit of
the movable contact.
Brush Sometimes used for Wiper.
Chatter A sustained rapid opening and closing of contacts
caused by variations in the coil current, mechanical vibration
and shock or other causes.
Clapper Relay Sometimes used for Armature Relay.
Close-Differential Relay Sometimes used for
Todd Sestero 2/2011
Coil A magnetic or thermal winding to which
energy is supplied to activate the relay.
Contact Arrangement Contact arrangement referred to the
combination of different basic contact forms to make up the
entire relay switching structure.
Contact Bounce Uncontrolled making and breaking of a
contact when relay contacts are moved to the closed position.
Contact Follow The distance two contacts travel together
after just touching.
Same as Contact Separation.
Contact Nomenclature Each movable contact of a relay constitutes a pole of the relay.
A combination of stationary contact and a movable contact which are engaged when the coil
is unenergized is referred to as back, break,
form B, or normally closed contacts and
is abbreviated as NC.
A combination of stationary contact and movable contact, which are engaged when the
coil is energized is referred to as front, form A, or normally open contacts and is abbreviated as NO.
A combination of two stationary contacts and a movable contact which engages one
of them when the coil is energized and engages the other when the coil is unenergized is
called transfer, form C, or double-throw
contacts and is abbreviated as DT.
Contrasted with double-throw contacts, NO and NC contacts are called single-throw
contacts, abbreviated as ST.
A combination in which a movable contact simultaneously makes and simultaneously breaks
connection between two stationary contacts is called double-break contacts and
abbreviated DB. For normally open contacts, this combination may be called double-
Relay contact notions are given in the flowing order:
3. Normal Position
4. DB, if double-break or double-break contacts
Example: SPST NO DB designates a single-pole, single-throw, normally-open, double break contacts.
All contacts are single break except when noted as double-break (DB). Relays having several sets of
differently functioning contacts will have the contact forms
listed in alphabetical order of their letter symbols.
Example: 1A2B refers to SPST NO contacts and DPST NC contacts.
For a relay on which the moving contact engages more than two stationary contacts during its cycle of
operation, the contact arrangement is described as MPNT, where
M is the number of poles, and N is the number of throws, e.g., 8P20T
Contact Overtravel Sometimes used for Contact Follow.
Contact Separation Maximum distance
between mating relay contacts when the contacts are in the open position.
Contact Spring A current carrying
spring to which contacts are fastened. Todd
Current carrying parts of a relay which engage or disengage to
make or break electrical circuits.
Sometimes used for a relay with heavy-duty contacts.
Same as Make-Before-Break contacts.
Continuous Duty Relay A relay which
may be energized with rated coil voltage or current at rated
contact load for a period of three (3) hours or more without
failure and without exceeding specified temperature
Current Balance Relay A relay
that allows tripping whenever there is an abnormal change in the
division of current between two circuits.
Current Rating See Rated
Coil Current and Rated Contact Current.
Current Relay A relay which is
designed to operate at a particular rated coil current rather
than at a rated coil voltage.
A controlling mechanism which opens or closes contacts
according to a preset cycle.
To deenergize a relay is to disconnect the relay coil from its
Definite-Purpose Relay A
readily available relay which has some electrical or mechanical
feature which distinguishes it from a general purpose relay.
Types of definite purpose relays are interlock, selector,
stepping, sequence, latch-in, and time-delay.
A relay that is intentionally designed for a time delay
between the energizing or deenergizing instant and the time that
the relay contacts open or close.
Differential Relay A relay having
multiple windings which functions when the voltage, current, or
power difference between windings reaches a predetermined value.
Directional Relay A relay that allows
for tripping for current flow in one direction only.
Double-Break Contacts See Contact Nomenclature.
Double-Make Contacts See Contact Nomenclature.
Double Throw Contacts See Contact Nomenclature.
Double-Wound Coil A double-wound coil
is a coil winding consisting of two parts wound on the same core.
Drop-Out Values Drop-out current,
voltage, or power is the maximum value for which contacts of a
previously energized relay will always assume their unenergized
Rated working time of a device compared to its idle time.
Electric Reset A qualify term
applied to a relay indicating that following an operation its
contacts must be reset electrically to their original positions.
Electromagnetic Relay A
relay whose operation involves the use of a magnetic field which
is produced by an electromagnet.
Electrostatic Spring Shields
Metallic shields between two relay springs to minimize the
capacitance between them.
Enclosed Relay A relay which has
both the coil and contacts protected from the surrounding medium
by a cover that is not airtight. Todd Sestero 2/2011
To energize a relay is to apply a rated voltage to its coil.
Extension Spring Same as a
Fast-Operate Relay A high speed relay
specifically designed for short operate time but not short
Fast-Operate, Fast-Release Relay
A high speed relay specifically designed for both short
operate time and short release time.
Fast-Operate, Slow-Release Relay
A relay designed specifically for short operate time, but not
for a short release time.
Fast-Release Relay A high speed relay
specifically designed for short release time but not for a short
Fixed Contacts Stationary
contacts of a relay which are engaged and disengaged by moving
contacts to make or break circuits.
Sometimes used for Transfer Time.
Follow-Through Contacts Contacts which have contact follow.
The structure on which the coil and contact assembly are mounted.
Front Contacts Sometimes used
for the moveable contact of a single-pole normally open
contacts (also see Contact Nomenclature).
Front Contacts Same as
Normally Open Contacts. Todd Sestero 2/2011
Gasket-Sealed Relay An
airtight relay, the sealing of which involves the use of a
gasket which is not bonded to the other sealing material.
General-Purpose Relay A
readily available relay which has the design, construction,
operational characteristics, and ratings such that it is
acceptable to a wide variety of uses.
A qualifying term applied to a relay indicating that following
an operation, the contacts must be manually reset to their
The part of a hermetically sealed relay through which the
electrical terminals pass.
Hermitically Sealed Relay An airtight
relay, the sealing of which involves fusing or soldering but
does not use a gasket.
High-Speed Relay A relay
specifically designed for a short operate time, release time, or both.
The hold current, voltage, or power is the minimum value for
which contacts of a previously energized relay will always
maintain their energized position.
Homing Homing is a qualifying
term applied to a stepping relay indicating that wipers, upon
completion of an operational cycle, are stepped around or back
to the start position.
Hum Hum, as applied to relays, is the sound caused by mechanical
vibration resulting from alternating current flowing through the coil.
Impregnated Coils Coils which have
been permeated with phenolic or similar varnish to protect them
from mechanical vibration, handing, fungus, and moisture.
Inductive Winding An inductive
winding, as contrasted with a noninductive winding, is a coil
Instrument Relay A relay, the
operation of which depends upon principles employed in
electrical measuring instruments such as the electrodynamometer,
iron-vane, and DArsonval.
Interlock Relay A relay composed of
two or more coils with their armatures and associated contacts
so arranged that the freedom of one armature to move or its coil
to be energized is dependent upon the position of the armature.
Intermittent-Duty Relay A relay which must be deenergized at occasional or periodic intervals
to avoid excessive temperature.
Latch-In Relay A relay having
contacts which lock in either the energized or deenergized
position until reset manually or electrically.
As applied to a stepping relay, the term level is used to
denote one bank or series of contacts.
Level Contact Sometimes used for Movable Contact.
Locking Relay Sometimes used for Latch-In Relay.
A type of contact construction providing low inter-contact capacitance.
Magnetic Air Gap A magnetic air gap is
a nonmagnetic portion of a magnetic circuit.
Magnetic Freezing The sticking of
a relay armature to the core, after deenergization, due to
residual magnetism of the core.
Magnetic Switch Sometimes used for Relay.
Double throw contacts so arranged that the moving contacts
establishes a new circuit before disrupting the old one.
Make Delay Sometimes used for Operate Time.
Marginal Relay A relay which
functions in response to predetermined changes in the value of a
coil current or voltage.
Mercury-Contact Relay A
relay in which the contact medium is mercury.
Motor-Driven Relay A relay which is
actuated by rotation of the shaft of some type of motor, for
example, a shaded-pole, induction-disk, or hysteresis motor.
Movable Contact A contact which,
when the relay is energized or deenergized, is mechanically
displaced to engage or disengage one or more stationary contacts.
Multiple-Break Contacts Contacts so arranged that, when they open, the circuit is interrupted
in two or more places.
Multiple Pile-up An arrangement of contact springs
which is composed of two or more separate pile-ups.
Multiple Stack Same as Multiple Pile-up. Todd
Neutral Relay A neutral relay, in contrast to a
polarized relay, is a relay in which the movement of the
armature is independent of direction of flow of current through
the relay coil.
Non-Bridging A term used to describe a
contact transfer in which the movable contact leaves one contact
before touching the next.
Non-Homing A qualifying term applied to
a stepping relay indicating that the wipers, upon completion of
an operational cycle, do not return to the home position, but
are at rest on the last used set of contracts.
Non-Inductive Windings A type of winding in which
the magnetic fields produced by two parts of the winding cancel
each other and provide a non-inductive resistance.
Non-Magnetic Shim A non-magnetic material attached to
the armature or core of a relay to prevent iron-to-iron contact
in an energized relay.
Non-Operate Value The non-operate voltage, current, or
power is the maximum value for which contacts of a previously
deenergized relay will always maintain their deenergized positions.
Normal Position Deenergized position, open or
closed, of contacts due to spring tension or gravity.
Normal Sequence of Operation The sequence in which all
normally closed contacts open before closure of normally open
contacts of the assembly.
Normal-Speed Relay A relay in which no attempt has been
made either to increase or decrease the operate time or the
Normally Closed Contacts A combination of a stationary
contact and movable contact which are engaged when the coil is
Normally Open Contacts A combination of a stationary
contact and a movable contact which are not engaged when the
coil is deenergized.
Off-Limit Contacts > Contacts on a stepping relay used to
indicate when the wiper has reached the limiting position on its
arc and must be returned to normal before the circuit can
Off-Normal Contacts Stationary contacts on a homing
stepping relay used to indicate when the wiper is not in the
Operate Time If a relay has only normally
closed contacts, its operate time is the longest time interval
given by definition (a) below. If a relay has normally open
contacts (regardless of whether or not it has normally closed
contacts), its operate time is the longest time interval given
by definition (b).
Operate Time For Normally Closed Contacts Operate time for NC contacts
is the total elapsed time from the instant the coil is energized
until the contacts have opened; i.e., contact current is zero.
Operate Time For Normally Open Circuits Operate time for normally
open contacts is the total elapsed time from the instant the
coil is energized until the contacts are closed and all contact
bounce has ceased.
Operate Values Same as Pick-up Values.
Operating Frequency The rated A-C frequency of the
supply voltage at which the relay coil is designed to operate.
Overload Relay A relay which is specifically
designed to operate when its coil current reaches a
predetermined value above normal.
Overvoltage Relay A relay which is specifically
designed to operate when its coli voltage reaches a
predetermined value above normal.
Todd Sestero 2/2011
Partially Enclose Relay A relay which has either contacts
or coil (but not both) protected from the surrounding medium by
a cover that is not airtight.
Partially Sealed Relay A relay which has either contacts of
coil (but not both) sealed.
Pick-Up Values Pick-up voltage, current, or power
is the minimum value for which contacts of a previously
deenergized relay will assume their energized position.
Pile-Up A set of contact arms,
assemblies, or springs placed one on top of the other with
insulation between them.
Plunger Relay A relay operated by energizing an
electromagnetic coil, which in turn operates a movable core or
plunger by solenoid action.
Polarized Relay A relay which is dependent upon the
polarity of the energizing current to operate.
Pole See Contact Nomenclature.
The pole face is the part of the magnetic structure on the end
of the core nearest the armature.
Pull-In Values Same as Pick-Up Values.
Pull-On Values Sometimes used for
Ratchet Relay A stepping relay
actuated by an armature-driven ratchet.
Rated Coil Current Steady-state coil
current at which the relay is designed to operate.
Rated Coil Voltage Coil voltage at
which the relay is designed to operate.
Rated Contact Current Current
which the contacts are designed to carry for their rated life.
Relay A device which is operated by
variation in conditions of one electric circuit to affect the
operation of other devices in the same or other electric
circuits by either opening circuits, closing circuits, or both.
Release Factor Is a ratio,
expressed in percent, of drop-out current to rated current or
the analogous voltage ratio.
If a relay has only NO contacts, the release time is the
longest time interval given by definition (a) below. If a relay
has NC contacts (regardless of whether or not it has NO
contacts) its operate time is the longest time interval given by
Release Time For Normally Open Contacts Release time for NO contacts
is the total elapsed time from the instant the coil current
starts to drop from its rated value, until the contacts have
opened; i.e., contact current is zero.
Release Time For Normally Closed Circuits Release time for NC contacts
is the total time from the instant the coil current starts to
drop from its rated value until the contacts are closed and all
contact bounce has ceased.
Release Values Same as
Drop-Out Values. Todd
Repeating Timer A timing device
which upon completion of one operating cycle, continues to
repeat automatically until excitation is removed.
The length of the magnetic air gap between the pole-face
center and the nearest point on the armature when the armature
is in the energized position.
Residual Pins or Screws Nonmagnetic pins or screws attached to either the armature or core of
a relay to prevent the armature from directly contacting the magnetic core.
Residual Setting Value of the
residual gap obtained by the use of an adjustable residual screw.
Residual Shim Same as
Restoring Spring A spring which
moves the armature to and holds it in the normal position when
the relay is deenergized.
Retractile Spring Sometimes used for
Sometimes used for Motor-Driven Relay.
Rotary Stepping Relay Same as Stepping Relay.
Rotary Stepping Switch Same as Stepping Relay.
A relay which has both coil and contacts enclosed in an airtight cover.
Sometimes used for Wiping Contacts.
Selector Relay A relay capable of
automatically selecting one or more circuits from a number of circuits.
Sequence Control Automatic control
of a series of operations in a predetermined order.
Sequence Relay A relay which
controls two or more sets of contacts in a definite
Sometimes used for Shading Ring.
A shorted turn surrounding a portion of a pole of an
alternating current magnet, causing a delay of change of
magnetic flux in that part, thereby preventing contact chatter.
Sometimes used for Auxiliary Relay.
Slow-Operate, Fast-Release Relay
A relay specifically designed for long operate time and short release time.
Slow-Operate Relay A slow speed relay
which has been specifically designed for long operate time but
not for long release time.
Slow-Operate, Slow Release Relay
- A relay specifically designed for both long operate time and
long release time.
Slow-Release Relay - A slow speed relay
which has been specifically designed for long release time but
not for long operate time.
Slow-Speed Relay A relay
specifically designed for long operate time, release time, or
A highly conductive sleeve placed over the core to aid in
retarding the establishing or decay of Flux within the magnetic path.
Solenoid Relay Sometimes used for
a Plunger Relay.
Solid State Relay Relays that use
various low-power components diodes, transistors, and
thyristors, and associated resistor and capacitors. These
components are designed into logic units used in many ways.
Special Purpose Relays A relay
which has an application that requires special features which
are not characteristic of conventional general-purpose or
Specified Duty Relay A relay which
is designed to function with a specified duty cycle, but which
might not be suitable for other duty cycles.
Spring Buffer A bearing member
made of insulating material which transmits motion of the
armature to the movable contact and from one movable contact to
another in the same pile-up.
Spring Pile-Up Same as Pile-Up.
Spring Stud same as Spring Buffer.
Stack Same as Pile-up.
Stationary Contact A contact member
which is rigidly fastened to the relay frame and which is not
moved as a direct result of energizing or deenergizing the relay.
Stepping Relay A relay whose
contacts are stepped to successive positions as the coil is
energized in pulses. Some stepping relays may be stepped in
either direction. (The Stepping Relay is also called a
Rotary Stepping Switch or a Rotary Stepping Relay).
Sometimes used for an armature relay with an end-mounted coil
and spring pile-up contacts mounted parallel to the long axis of
the relay coil.
Tension Spring Sometimes used for
Thermal Relay A relay which is
operated by the heating effect caused by electric current flow.
See Contact Nomenclature.
Time-Delay Relay A relay in which a
delayed action is purposely introduced.
A motor driven time-delay relay.
Transfer Time Total elapsed time
between the breaking one set of contacts and the making of
another set of contacts.
Transfer Time on Operate Transfer
time on operate is the total elapsed time from the instant the
NC contacts start to open until the NO contacts are closed and
all contact bounce has ceased.
Transfer Time on Release Transfer time
on release is the total elapsed time from the instant the
NO contacts start to open until the NC contacts are closed and
all contact bounce has ceased.
Transit Time Same as Transfer Time.
Trip values Trip voltage, current, or power is rated value at which a
bistable polarized relay will transfer from one contact to another.
Undercurrent Relay A relay
specifically designed to function when its coil current falls below a predetermined value.
Undervoltage Relay A relay
specifically designed to function when its coil voltage falls
below a predetermined value.
Unenclosed Relay A relay which does
not have its contacts or coil protected from the surrounding medium by a cover.
Winding Same as Coil. Todd Sestero 2/2011
Wiper A moving contact on a stepping relay.
Wiping Contacts Contacts
designed to have some relative motion during the interval from
the instant of touching until completion of the closing motion.
New FEB07/2011, JUN15/2013
Last Updated: 11/18/2019