Most circuit breakers are thermal devices that when excess
current run through them they heat up and trip (open up).
They can also be used as a switch to open a circuit.A
contactor is a electric relay that is essentially a
solenoid and a pair of electrical contacts. The solenoid is
activated and it closes the contacts
relay is the device acttached to any circuit which makes it
work automatically.it has less operating voltage but in
contactor it works on the principle of cantact making that
either single phase voltage or 110v ac supply , the plunger
makes the other three phase contact and thus operats on
some high operating voltage
Contactor is a switching device. Unless & until you give
command it wont operate. Contactor is not a sensing device.
on the other hand Relay is the device which sense the
ubnormality & give command accordingly. Relay differentiate
normal & ubnormal condition which contactor can not.
In short if only contactor is connected in circuit.. in
case of ubnormal like OV or OC it will burn.. but if it is
connectedin coordination with relay.. relay will sense the
fault & give signal or command to contactor to trip
Relays and Contactors are electromagnetic switches. The only
difference is that relays are usually used for low voltage
applications while contactors are used for higher voltage
application. Contactors usually have Overload protection.
Some examples that I can give (at least in Manufacturing of
Industrial Control panels):
-Relays are use as electromagnetic switches to control
certain parts of a circuit (low voltage application). A
better example would be such as if you want your circuit to
sound a horn when a certain abnormality is happening at a
certain period of time, then depending how you set up your
circuit, a specific relay will either open or close and you
horn will start making the sound.
-Contactors can be used for example, to start motors or a
number of motors that require a higher voltage in order for
the motor or motors start working. In the example that I
just gave, contactors work as starters.
First Clear Relay Means
1. Protective relay
2. Control Relay.
If talking about Protective relay (Example: over current
Relay, Electronic motor protection relay..etc)
so protective relay is for Protection of circuit
and functionally different from contactor.
they have only potential free contact at output which can
be used in control circuit.
if talking about control relay
1. Power relay (which have Change over contact with high
2. Solid state relay
3. Simple control relay. (Which have Change over contact
with low current capacity call SPDT)
This all relay has functionally similarity with contactor
that they are electromagnetic device (except SSR) but
- relay have changeover contact (NO, NC& Common) while
contactor have power contact NO) & auxiliary contact (NO or
Main difference is
- Contactor is used to Control power Circuit while relay
are used for control circuit (except Power relay which is
now a days introduce for DC/High voltage application).
Relay is an electromagnetic device. There are two types of
relays __ Power relay (or contactor) and Control relay( or
The function of a contactor is to use a relatively small
amount of electrical power to control the switching of a
large amount of power
Control relays are frequently used in the control of low
power circuits or other relays
1) A contactor is an electro-magnetic switching device (a
relay) used for remotely switching a power or control
circuit. A contactor is activated by a control input
which can be a lower or higher voltage / current than that
which the contactor is switching. Contactors come in many
forms with varying capacities and features. Unlike a
circuit breaker a contactor is not intended to interrupt a
short circuit current.
Contactors range from having a breaking current of several
amps and 110 volts to thousands of amps and many kilovolts.
The physical size of contactors ranges from a device small
enough to pick up with one hand, to large devices
approximately a metre (yard) on a side.
Contactors are used to control electric motors, lighting,
heating, capacitor banks, and other electrical loads.
2) A relay is an electrical switch that opens and closes
under the control of another electrical circuit. In the
original form, the switch is operated by an electromagnet
to open or close one or many sets of contacts. It was
invented by Joseph Henry in 1835. Because a relay is able
to control an output circuit of higher power than the input
circuit, it can be considered to be, in a broad sense, a
form of an electrical amplifier...
Any one can explain if there is any harm from humming noise
heard from air circuit breaker : we have 415 switch board
having two incomers from transformers 3.3KV/415V , when bus
coupler closed, and open one ACB incomer to work in one of
the transformers, the current started hunting around 50
Amps, and heard humming noise from the air circuit breaker,
although the load current was only 1200 amps while the ACB
capacity is 2500 Amps, and the transformer 1600 KVA. The
same didn't happen when taken the other transformer offline.
Why this humming noise and current fluctuation? is there
any implication or danger, bearing in mind that we just
returned everything back to normal and stopped the
maintenance of the transformer. Shall we go ahead and
ignore this noise?
Determine the minimum cable size for a 28A single phase
final sub circuit originating from the distribution board
supplied by three phase submain. The route length of the
final sub circuit is 54 m. Assume 1% voltage drop in the
consumer's mains. Submain specification: Cable: 50mm2 4
core V75 copper; Route length 42m; Maximum demand 135A per
We are using Powerica make 100 KVA DG set. Whenever i
connect compressor on DG load (Compressor Make Atlas Copco
5.5 kva) that time we face the lot of fluctuation on Dg
output. Can you please suggest me how can prevent this
i have seen a picture of a power plant. in that pic. i saw
chimney like giant structure which were spreading smoke like
something...i want to know is it chimney or cooling towers, as
my friends use to say.