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Old October 19th, 2020, 01:28 PM   #1
Engineer_UA
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SPDT Snap Switch

Greetings,
Can a Single Pole Double Throw snap switch be furnished such that it actuates on low level or high level, but remains in "neutral" state when in a dead band region? We need a switch with two normally open contacts, with one contact closing on low oil level condition, and the other contact closing on high oil level condition. We want the switch contacts to remain open when the level is in between the two trip points.
Perhaps I need (2) SPST switches for this arrangement.
This is a Magnetrol switch.


Any thoughts?
Thanks,
EE
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Old October 19th, 2020, 01:42 PM   #2
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This will not answer your question, but I will describe my "best practice".
For critical operations, like oil level, I prefer to have a signal when the level is in a "normal" position. Both low and high level switches would be on and sending a signal to the controller or indicator lamp. If a low or high condition occurs, then that input (or lamp) would turn off. This way, if a wire comes loose or is somehow disconnected, it will alert you that the level is not normal or the switch is not working.
I thought companies like Magnetrol had multiple switch and level options in the same level device. It has been many years since I spec'd one, though.
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Old October 19th, 2020, 01:44 PM   #3
Bob O
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Have you contacted Magnetrol and asked this question?
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Old October 19th, 2020, 02:42 PM   #4
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The traditional definition of a "snap action" switch, while not an official technical term, is that it changes, "snaps" from one side to the other. So that would rule out a 3rd position.
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Old October 19th, 2020, 03:43 PM   #5
Gene Bond
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If you want deadband between the contacts, you'll likely need 2 switches, since most all switches are form C, not 2 independent contacts....
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Old October 20th, 2020, 12:12 AM   #6
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I would be wanting the switch closed between the high and low levels. If your level is good and the cable is cut, you have no feed back and nobody will know.
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Old October 20th, 2020, 09:32 AM   #7
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' I would be wanting the switch closed between the high and low levels. If your level is good and the cable is cut, you have no feed back and nobody will know. '

+1. This is known as "failsafe" method of wiring alarms and statuses. At my last job with an integrator, it was a requirement from most of our customers, and I wired and programmed failsafe even when it wasn't a requirement.
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Old October 20th, 2020, 09:43 AM   #8
danw
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I'm confused. Level switches trip at a given level and have an output to reflect the status of the level.


If you want a low level switch AND a high level switch you have two level switches, one for low level, one for high level and you connect them as needed.


What sort of single level switch detects and signals both high and low level that brings up your question?
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Old October 21st, 2020, 03:41 AM   #9
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Quote:
Originally Posted by drforsythe View Post
This will not answer your question, but I will describe my "best practice".
For critical operations, like oil level, I prefer to have a signal when the level is in a "normal" position. Both low and high level switches would be on and sending a signal to the controller or indicator lamp. If a low or high condition occurs, then that input (or lamp) would turn off. This way, if a wire comes loose or is somehow disconnected, it will alert you that the level is not normal or the switch is not working.
I thought companies like Magnetrol had multiple switch and level options in the same level device. It has been many years since I spec'd one, though.
We use level switches on some of the Navy ships I have designed PLC control systems for that function in exactly this manner. They are a three-wire switch and when in normal operating level both outputs from the switch are "High", if the level goes too high or too low then that side of the switch goes "Low".

We call them "Alarm on Zero" since they are failsafe.

I don't spec the switches but if I remember correctly there made by GEMS.
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Old October 21st, 2020, 07:08 AM   #10
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Quote:
Originally Posted by danw View Post
What sort of single level switch detects and signals both high and low level that brings up your question?
This sounds more like an analog level sensor with multiple switching outputs.
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Old October 21st, 2020, 08:26 AM   #11
drforsythe
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rkicklighter- GEMS is the company I was thinking of, as well. In my response, I stated "Magnetrol", but GEMS is the company I was thinking of. I just did not remember the right company name. Thank you for the info.

If you go to the GEMS website (gemssensors.com), you can search in their Multi-Point Level Switches category.
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