Is this strange?

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anbm

Senior Member
Today, I was looking at one of existing as-built drawing...
there is a electrical wiring diagram detail which shows one 277V
lighting circuit goes through a lighting contractor
(lighting contactor is controlled by a time clock)
then to the wall mounted occupancy sensor
and to the office lights that the occupancy sensor
controls.

I am wondering... why they had occupancy sensor downstream
of a lighting contactor/time clock?
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Today, I was looking at one of existing as-built drawing...
there is a electrical wiring diagram detail which shows one 277V
lighting circuit goes through a lighting contractor
(lighting contactor is controlled by a time clock)
then to the wall mounted occupancy sensor
and to the office lights that the occupancy sensor
controls.

I am wondering... why they had occupancy sensor downstream
of a lighting contactor/time clock?

How many times would the contactor operate if the sensor was upstream?
 

LEO2854

Esteemed Member
Location
Ma
Today, I was looking at one of existing as-built drawing...
there is a electrical wiring diagram detail which shows one 277V
lighting circuit goes through a lighting contractor
(lighting contactor is controlled by a time clock)
then to the wall mounted occupancy sensor
and to the office lights that the occupancy sensor
controls.

I am wondering... why they had occupancy sensor downstream
of a lighting contactor/time clock?

Because the sensor would otherwise shut off the time clock and make it useless.
 

anbm

Senior Member
Because the sensor would otherwise shut off the time clock and make it useless.

What I meant is why we need the timer and occupancy sensor control the same light, is it a waste?
If occupancy sensor or timer does not work, the other is useless.
 

LEO2854

Esteemed Member
Location
Ma
What I meant is why we need the timer and occupancy sensor control the same light, is it a waste?
If occupancy sensor or timer does not work, the other is useless.

I think they want all the lights to just stay off at certain times of day.
 

templdl

Senior Member
Location
Wisconsin
Then they do not need the occupancy sensor!

Oh yes they do. Think about it this way, lets say that you want all the lights out between 8pm and 6am and want them on from 6am to 8pm except that you only want the ones where the areas are occupied. That is where the occupancy sensors come into play. Even though the contactor energizes the lighting circuits only the areas where the occupancy sensors detect motion will go on.
 

anbm

Senior Member
Oh yes they do. Think about it this way, lets say that you want all the lights out between 8pm and 6am and want them on from 6am to 8pm except that you only want the ones where the areas are occupied. That is where the occupancy sensors come into play. Even though the contactor energizes the lighting circuits only the areas where the occupancy sensors detect motion will go on.

Then why we need timer/contactor in this case? Use occupancy sensor throughout can do a better job...maybe cost issue???:?
 

iMuse97

Senior Member
Location
Chicagoland
I like the idea of having both. Since they want it, why not install it? They can set their clock to keep lights off completely during some periods of time, and the occupancy sensors will limit the overuse of the lighting during other periods of time. What templ. said, basically.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Then why we need timer/contactor in this case? Use occupancy sensor throughout can do a better job...maybe cost issue???:?

Because they want to control energy costs and the affected area is not intended to be used night. If it were only occupancy sensors, then anyone could come in and rummage around in the off hours and burn up power. If they come in and the lights don't come on, they tend to leave.

If it were only a timer, then during the work day when nobody is around, lights stay on needlessly.

Having both allows tighter control of energy use. It's common in LEED rated projects.
 

beanland

Senior Member
Location
Vancouver, WA
State Energy Codes

State Energy Codes

Don't blame the designer completely. In some states the energy codes mandate manual switches, occupancy sensors and time clocks to "reduce energy use when the space is unoccupied."
 
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