Set 2
When a particular type of relay senses a fault, what does it do?
When a relay is energized, what does it do?
It brings together two pieces of metal, that is a pair of contacts.
It can also separate two pieces of metal, that is a pair of contacts.
These contacts complete a path to allow tripping/opening of a circuit
breaker.
NOTE: The word contact is singular. However, when we see two
contacts together we really should call it a “set of contacts”. But we do
not --- we just call it a contact. You will know which we mean just by
the use or context.
But there is more to it!!
1
Contacts are closed (or opened) other ways.
An energized relay brings together two pieces of metal, that is a
pair of contacts.
Push Buttons can bring together two pieces of metal (a Start Push
Button).
Push Buttons can separate two pieces of metal (a Stop Push
Button).
We want to spend one session looking at some typical, useful
contact circuits used in the power systems area.
Contact circuits are also called logic circuits.
2
A relay can have two contact types
An “a” contact – is Open when
the relay is deenergized (shelf
state), closed when the relay is
energized.
[Called “normally open” NO]
This is the path C-NO
The symbol for an “a” contact is:
A “b” contact – is Closed when
the relay is deenergized (shelf
state), open when the relay is
energized
[Called “normally closed” NC].
NOTE: The contacts on the armature This is the path C-NC.
itself are/must be insulated from The symbol for a “b” contact is:
each other.
3
And a relay can have multiple contacts – all moved by
energizing one coil
Imagine that the armature has an extension on it to become much
longer.. There could be multiple sets of a and b contacts on the
armature all moved by energizing one coil
4
Association of coils and contacts
CAREFULLY NOTE: We always have to clearly identify the coil
and the associated contact which it moves
5
Housekeeping - some relay logic
With contact relay logic:
Logic can be AND means contacts in series
accomplished by OR means contacts in parallel
contacts (original
way, still seen) Indicator = CR1 AND CR2
Logic can be accomplished
by gates (original way as I = CR1 . CR2
transistor devices, now
digital logic)
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Simple motor starting – with seal in contact
Start Push Button - Spring loaded open, push to close
Stop Push Button - Spring loaded closed, push to open.
● Actuating coils often have many contacts associated with them, that is
they often move many contacts when actuated.
Here coil CR1 has two contacts associated with it.
CR1 (upper) – “seal in” contact, no longer have to hold Start PB
CR1 (lower) – contacts to connect motor (Mtr) to DC source
Mtr – Motor to be started or stopped
(You must understand “seal-in contacts”)
7
Three phase motor control circuit – with contactors
You must understand:
“a-contacts” [closed
when associated
coil/device is
energized)
“b-contacts” [open
when associated
coil/device is
energized, or closed
when associated
coil/device is
deenergized]
MC = NOT Overloaded AND (Start PB OR MC-1) AND Stop PB (Not Pushed)
MC = (49-1).(START PB Pushed + MC-1).STOP PB (Not pushed)
8
Three phase AC motor reversal
Three phase motor
reversal - interchange any
two phases
Has overload protection
(could be called a 49
relay)
Pressing both the Forward
and Reverse PBs at the
same time will not short
circuit the source because
of M1 b-contact and M2 b-
contact in the logic control
circuit.
9
Circuit breaker control
Circuit breaker
symbol and
“rackable”
symbol: double
arrows (can be
racked
out/pulled out)
600 volt or 12.47 kV circuit breakers are widely used in Motor Control Centres (MCC).
138, 230, 345 and 500 kV circuit breakers are used on the transmission system
10
Circuit breaker control
Energize Trip Coil = (Breaker Closed) AND (Open PB OR Logic Trip OR Fault Trip)
52T = (La).(OPENPB + LOGIC + FAULT)
Energize Close Coil Aux Relay = (Breaker Not Closed) AND (Close PB OR Logic Close)
52X-1 Coil = Lb.(CLOSEPB + LOGIC)
Energize Main Close Coil = Auxiliary Contact Closed
52C = 52X-1 Contact
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Bigger picture
As shown the CB connects the
AC supply to a motor. Suppose
the motor is driving a pump to
fill a vessel. When the vessel
reaches its maximum, the pump
should stop. So the Logic Trip
could be a Level Contact (or a
Pressure Switch in a closed
vessel).
Likewise for a Logic Close.
The Fault Trip might actually be 5 contacts in parallel – the CB should trip on Over
Current OR Under Voltage OR Over Fluxing (Volts/Hz) OR Thermal Overload OR
Phase Imbalance.
Fault Trip = 50 relay + 27 relay + 24 relay + 49 relay + 47 relay
12
Bigger picture - more
Be aware of the different
processes within circuit
breakers.
In 600 volt and 12.47 kV
circuit breakers, when the 52C
close coil is energized it closes
the CB contacts but it ALSO
compresses a spring
So energizing the 52T Trip Coil
simply releases the spring which
quickly parts the Circuit Breaker
Main Contacts
1n 345 kV circuit breakers out in the switchyard:
Energizing the 52T trip coil releases an Air Blast which parts the Circuit Breaker Main
Contacts in one or two cycles.
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