F3: Switches and Relays Flashcards
What is the heart of operational systems?
Switches and relays
What is a switch?
a device that makes or breaks (aka closes or opens) a path in an electrical circuit to allow loads.
What is an example of a manual switch?
a light switch for a room
What happens when a switch is manually moved (aka thrown)?
The pole (moving part) changes
How are switches referred?
by the number of poles / throws they have
What is this?
Single pole, single throw switch
What is this?
Single pole, double throw switch
What is this?
Double pole, single throw switch
What is this?
Double pole, double throw switch
What is this?
7 pole (rotary) switch
What is a momentary switch?
a switch that must be pushed or tuned / held in order for the contacts inside to close.
Are momentary switches spring loaded?
Yes, snap open when released.
What type of switch is used to sense, measure / react to conditions (temp, pressure, flow, liquid level / position)
Automatic switches
How are automatic switches classified?
1) what they measure
2)condition that causes them to react
What type of switch is a thermostat?
open or rise automatic switch
What would a cooling thermostat be classified as?
a close on rise switch
How does a close on rise switch work?
as the temp rises - thermal element expands / closes its contacts to bring on the cooling system
What is an aquastat?
a switch that does for water what thermostat does for air
What is a thermowell?
It is a well that is inserted into an opening in the piping system
What does differential mean for switches?
They have variance between their open and closed positions
What is the fixed differential of an Aquastat?
15F (8.33C)
What is the differential pre-set of a pressure switch?
20 psi
A flow switch is what type?
a single pole, double throw switch
How does a flow switch operate?
When there is flow the paddle moves = causes pole to be pushed from one throw and pulled to another throw
Where is a pressure switch used?
on mid and high efficiency gas equipment
How does a pressure switch operate?
contacts close when the correct pressure is sensed.
When replacing a pressure switch, you should use one that has the same what?
pressure setting
What are float switches used for?
To monitor the level of liquids in a vessel
A float switch will open or close to control what?
a pump supplying water to a cooling tower sump, or to shut off the burner in a steam of hot water boiler whose water level has dropped to an unacceptable point.
What is another name for a float switch?
Low water cutoff
What is this?
Float switch
Where are End switches used?
used if the position of a piece of mechanical equipment has to be proven before the next step in a sequence of operation can take place.
What prevents Dead - heading?
A zone valve that has to be proven to be open before the pump tried to move water through it
What does a 4 wire zone valve have?
end switch wired across 2 of the wires which typically have red insulation
What else is an End switch used for?
operations of the dampers on the inlet to a direct-fired makeup unit (DFMA)
A dimmer switch functions as an?
On / off switch w/ the addition of a variable resistor built into it
Where should a switch be located?
ALWAYS be located in the “hot” leg of the circuit
positioned between the source (power supply) / the load (device that turns electrical energy into work)
What do the volts and amps have to be?
at least as high as those it will be exposed to
What does a relay do?
allows low volts to operate high voltage equipment equipment w/o the two touching each other
What are the 2 varieties of relays?
1) electromagnetic relays (EMRs)
2) solid - state relays (SSRs)
What is the job of the 2 kinds of relays?
use a circuit w/ low power to operate equipment that uses a higher voltage power source.
What is the SSR about?
silent in operation
has no moving parts
Wear and tear isn’t an issue
What is EMR about?
has a moving armature / moving contacts
Can not be used in a volatile environment
What is a downside of a SSR
limited to operating one circuit only
Can EMRs operate multiple sets of contacts?
Yes
Where are SSRs found?
on furnaces / boiler modules, solid state fuse / relay panels in vehicles.
Most relays that are used in hydronic systems will be of what type?
EMR
Most relays that are used in hydronic systems are of what type?
EMR
What do relays consist of?
a Coil = temporary electromagnet
A single or multiple sets of spring loaded electrical contacts
When a relay coil is energized and de-energized what can be heard?
an audible click
What voltage values are relays capable of?
600V
In a nutshell, how does a relay work?
It’s a magnetic switch = uses electromagnetism as the force that operates the switching mechanism
What does SSR stand for?
Solid state relays
What does EMR stand for?
Electromagnetic relays
