LATTC HVAC&R Program
RAC 125 and 159
A summary of information regarding relays used in the HVAC&R industry:
1) The number of poles that a relay has reflects the number of power input
connections that it has.
2) The number of throws that a relay has reflects the number of ON or power
output positions that it has.
3) A single pole, single throw (SPST) relay has one power input connection and one
power output connection and only sends power out when the relay coil is
energized. Its contact is referred to as Normally Open. A single pole, single
throw relay might be used to energize or de-energize a 120 volt device by making
or breaking the Hot (120V) wire. This type of relay has 4 electrical connections:
two for the relay coil plus one for the power input and one for the power output.
4) A double pole, single throw (DPST) relay has two power input connections and
two power output connections and only sends power out when the relay coil is
energized. Its contacts are referred to as Normally Open. A double pole, single
throw relay (contactor) might be used to energize or de-energize a 240 volt device
by making or breaking both Hot wires. This type of relay has 6 electrical
connections: two for the relay coil plus two for the power input and two for the
power output.
5) A triple pole, single throw (TPST) relay has three power input connections and
three power output connections and only sends power out when the relay coil is
energized. Its contacts are referred to as Normally Open. A triple pole, single
throw relay (contactor) might be used to energize or de-energize a three-phase
240 volt or 480 volt device by making or breaking all three Hot wires. This type
of relay has 8 electrical connections: two for the relay coil plus three for the
power input and three for the power output.
6) A single pole, double throw (SPDT) relay has one power input connection
(common) and two power output connections (ON positions) and sends power
out when the relay coil is either energized or de-energized. One of its contacts is
referred to as Normally Open and the other contact is referred to as Normally
Closed. A single pole, double throw relay might be used to energize a 120 volt
furnace blower motor in low speed (heating mode) or high speed (cooling mode).
This type of relay has 5 electrical connections: two for the relay coil plus one for
the power input (common) and one for each power output (N.C. and N.O.). The
common black plastic covered “Fan Relay” is an example of this configuration.
7) A double pole, double throw (DPDT) relay has two power input connections
(commons) and four power output connections (ON positions) and sends power
out when the relay coil is either energized or de-energized. Two of its contacts
are referred to as Normally Open and the other two contacts are referred to as
Normally Closed. A double pole, double throw relay might be used to energize
two separate 120 volt devices or one 240 volt device. This type of relay has 8
electrical connections: two for the relay coil plus two for power input (commons)
and four for each power output (2 N.C. and 2 N.O.). The common brown plastic
covered “Multi-Purpose Relay” is an example of this configuration. A DPDT
relay is actually two SPDT relays that are mechanically linked so that they
function at the same time with one coil.
8) A triple pole, double throw (TPDT) relay has three power input connections
(commons) and a total of six power output connections (ON positions) and sends
power out when the relay coil is either energized or de-energized. Three of its
contacts are referred to as Normally Open and the other three contacts are
referred to as Normally Closed. A triple pole, double throw relay might be used
to energize three separate 24 volt or three separate 120 volt devices. This type of
relay has 11 electrical connections: two for the relay coil plus three for power
input (commons) and six for the power outputs (3 N.C. and 3 N.O.). The
common transparent plastic covered “Ice Cube Relay” is an example of this
configuration.
9) A relay or switching device should NEVER be used on the neutral wire by itself!
Break the hot AND the neutral at the same time.
10) Always determine the coil voltage of the relay and the VA rating of the relay
contacts before using the device in a circuit.
11) Perform an actual resistance check on the relay coil to determine whether it is in
operating condition.
12) A relay that has two states – energized coil or de-energized coil, cannot be
classified as triple throw!
13) Although relays and contactors are functionally the same, relays typically handle
loads of 20 amps or less and contactors handle loads of 25 amps or greater.
14) Magnetic starters are relays or contactors that have over-current protection built
into them.
Compiled by Craig J. Barnett (2007)