Manual Transfer switch an a Double Throw switch
- Thread starter wmeek
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- Location
- Wisconsin
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- PE (Retired) - Power Systems
A manual transfer switch is tested to switch one load between two sources, while a double throw switch is tested for one source between two loads.
It may sound like nit-picking, but imagine that you are switching one active load from a live source to another live source, like during re-transfer, wouldn't you like to make sure the arc (from breaking the load) does not bridge the gap between the sources?
ATSman
ATSman
- Location
- San Francisco Bay Area
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- Electrical Engineer/ Electrical Testing & Controls
Nomenclature
Nomenclature
A manual transfer switch has no automatic operation and has to be manually transferred. It transfers the load from a preferred source to an alternate source of power (genset or another utility source.)
A double throw switch is a 3 position switch. A manual transfer switch is an example of a double throw switch.
A single throw switch has 2 positions. An example would be a light switch.
Nomenclature
A manual transfer switch has no automatic operation and has to be manually transferred. It transfers the load from a preferred source to an alternate source of power (genset or another utility source.)
A double throw switch is a 3 position switch. A manual transfer switch is an example of a double throw switch.
A single throw switch has 2 positions. An example would be a light switch.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Close.
A single throw switch has only two positions and only two contacts for each pole. It will either be open in one position and closed in the other or closed in one position and open in the other. (and if all the poles are identical you can choose which just by rotating it 180 degrees>)
A common double throw switch will still have only two positions. But it will have three contacts per pole. One common pole will be connected to one of the remaining two in one position and to the other one in the other position.
A double throw, center-off, switch has three positions. Exact same contacts per pole, but in the center postition the common is not connected to either of the other contacts.
A three-way switch is double throw.
A transfer switch is double throw with lots of care taken to guarantee that it breaks one side of each pole before closing the other. And it may have a center off position for servicing or to let motor and reactive voltages die down before connecting the alternate power source.
They even make center-off switches that force you to pause in the middle instead of going directly from one side to the other. Some ways of interlocking CBs to make a transfer switch will do this because you have to manually move the interlock slider after opening the first breaker.
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- Placerville, CA, USA
- Occupation
- Retired PV System Designer
You can, as long as it is otherwise qualified. Under some circumstances a TS (either ATS or MTS) will have to be service rated. A typical double throw switch will not be. It has to meet all of the requirements for a disconnect, etc. I think.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Not per any manufacturer's data that I have seen.
broadgage
Senior Member
- Location
- London, England
It should work fine in practice, but likely be a code violation if not sold as and listed as a transfer switch.
ptonsparky
Tom
- Occupation
- EC - retired
We installed a SQ D last year that could be used either as, two sources one load, or one source two loads. Field convertible.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The service entrance rating is only part of the problem. The center off position has little to do with it also. A double throw switch is not rated to switch between two live sources. The issue is primarily the management of the source to source arcing.
ptonsparky
Tom
- Occupation
- EC - retired
SqD DTU361 Series F, "UL Listed as suitable for use as Service Equipment." Pag3 3-14 Digest 173 October2003. Dated cataloge but I just bought it for this purpose last year.
Last edited:
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The Digest calls these 'double throw safety switches', which is pretty much a marketing term (UL calls them enclosed switches). It goes on to say that they are suitable for use in accordance with article 702 of the NEC and that they are UL Listed.
UL category WIAX (standard UL98) does call them 'double throw switches'.
So back to the OP, what does 'double throw switch' mean to you; safety switch or wiring device switch?
If it is a UL Listed enclosed switch it will say something like ""Suitable for Use in Accordance with Article 702 of the National Electrical Code".
I will be using a Cutler hammer DT225URK an it is rated for Service Entrance use. It also can be used for one load an two different power sources. I just contacted Eaton for confirmation. I will be installing a 400A Breaker ahead of this switch.
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