Double Throw Switch 1

[Thunter78]

Hello,

I am a mechanical engineer by trade and have been learning more and more of the electrical side of things. I'm currently working on a project and hope someone with more experience can make sure I'm at least headed in the right direction.

I have two electrical panels, A and B, that are both capable of providing three phase 460V power. Both of these panels do have adequate motor protection branches. I need to power a 15HP motor from one of these panels, but never able to connect power from both.

We use panel A most of the time, but occasionally need the soft start capabilities of panel B (we cannot use a soft starter when operating with panel A, else I would only use panel B). Right now we have to rewire power to the motor every time we need to change panels, which is slowing us down. I have been trying to find what I need and think something like this will work perfectly:

https://www.se.com/us/en/product/DT...-safety-switches&selected-node-id=12144220094

Before I spend a few thousand on double throw panels, can anyone confirm my idea or think of a better way?

Thanks,

Taylor

 

[RedSnake]

What kind of space to you have in this panels ?
It's a bit difficult to advice without proper electrical drawings.
How far apart are this panels from each other?
Can have two cables in the motor?

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[che12345]

Thunter78 (Mechanical)
Install a 3-pole double throw switch in panel A or in B wherever there is room for it.
1. A a 3-pole double throw switch construction and operation
a)In position A : terminals 1-2, 3-4, 5-6 are close, but terminals 7-8, 9-10, 11-12 are open,
b) In position B : terminals 7-8, 9-10, 11-12 are close, but terminals 1-2, 3-4, 5-6 are open,
c) Link terminal 2-8; 4-10; 6-12.
2. Connect 3-phase from panel A to terminals 1, 3, 5 respectively. Connect 3-phase from panel B to terminals 7, 9, 10 respectively.
3. Connect the 15hp 3-phase to terminals 2, 4, 6 respectively.
4. Attn: a) There shall be only three conductors from the [3-pole double throw switch]. terminals 2, 4, 6 to the motor,
b) earth the motor by another separate earthing conductor,
c) the motor (direction of rotation) shall be the same when switched either to position A or B.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr Thunter78 (Mechanical)
My sincere apology. There is a typo error,
"... Connect 3-phase from panel B to terminals 7, 9, 10 respectively..."
should read ... Connect 3-phase from panel B to terminals 7, 9, 11 respectively..
Che Kuan Yau (Singapore)

 

[IRstuff]

You can just edit your posts

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[waross]

RedSnake and Mr. Che.
The proposed switch is a stand alone switch which will be installed on a wall externally to Panels A and B.
I don't understand how there may be 12 numbered terminals on a 9 terminal device.
My research indicates that the terminals may not be numbered, but are labelled. LINE, LOAD and LINE.
You may be better served by phasing out each source rather than going by assumed phase rotations.
(Ten[sup](b2)[/sup] types of persons, those who have encountered a panel with the phases reversed and those who have not.)
I would just hook up the incoming cables and then check for the zero volt pairs between Line A and line B.
They don't have to match phase to phase, they just have to match rotation.
If the phases are reversed, swap two leads and you are finished.
I have found that the simplest method is often the most dependable and the least susceptible to human error during testing.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[MacGyverS2000]

Perhaps someone is switching neutral...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[RedSnake]

My problem is..
(we cannot use a soft starter when operating with panel A, else I would only use panel B)
If you sometimes need to use a soft starter why not use a soft starter at all times?
And why can't a soft starter be used with panel A?
And in which situations is panel A needed?

A soft starter is usually used with large loads, before it was usually a Y/D start.
But you can as easily start the motor with a low load.

So why can't the motor start and run on the soft starter at all times ?

If you want a cheaper solution then a few thousand.
One extra cable to the motor or even a motor cable between panel A and B.
And a cable for
maneuvering between the panel A and B, that cross locks the motor supply sources should do the job.
But without more info it's hard to tell.

Best Regards A


“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[LionelHutz]

Yes, that switch is the right idea for what you want to do. Also look at manual transfer switches for more options.

 

[jraef]

The double thrown safety switch option is the way I have done similar things numerous times, it's simple and inexpensive (compared to other options).

There is a potential drawback though. If installing it here in the US, non-fused DT safety switches like that are only listed at 10kA SCCR (Short Circuit Current Rating), unless there are other extenuating circumstances. That means you cannot install them on a system where the Available Fault Current is over 10,000A and in many industrial environments, that is going to be the case. On that specific switch, the web page IMPLIES it is rated for 200kA, but that is dependent upon there being specific fuses (Class R,T,J, or L) used up stream from it somewhere. In your case, if Panels A AND B both have those fuses as the main protective devices, then it's fine. If not, and they have SQUARE D circuit breakers of specific types, then it is good for up to 35kA, but if it is any other brand or type than those listed, it defaults back down to 10kA.

Now you know why we EEs get the big bucks...




" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[TugboatEng]

I use cam action rotary switches when I'm building power and control circuits with two inputs.

https://www.electroswitch.com/produ...es/cam-action-rotary-switches/kw40-cam-switch

 

[waross]

Perhaps someone is switching neutral

The switch that the OP is contemplating is a three pole switch.

I use cam action rotary switches when I'm building power and control circuits with two inputs.

I would consider a cam action switch for control circuits, but I would hesitate to use one in a power circuit.
I have seen a lot of 'strange to me' stuff onboard vessels that would not be considered good practice on shore or may be unapproved for the use or considered poor practice.
Not to criticize shipboard practice, it works on board.
But one big difference.
The generators on most smaller vessels have only a small fraction of the Available Short Circuit Current of similar KVA rated transformers.
The cam switch may have a locking option but generally not a door lock feature that may make its use a safety issue with many on-shore codes.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

The cam action switch should be downstream of protective devices. Switches are available that easily exceed the requirements of a 15 hp motor. These switched aren't breaking fault current.

 

[jraef]

Technically, (in the US) any device in the power circuit must have a Short Circuit Current Rating at or above the available fault current of the system, whether they open under a fault or not. Marine systems where the power comes from on-board generators are usually not going to have the kind of AFC that utility based power systems do (nor do they need to meet NEC requirements in the same way).

Most of the power cam switches I am familiar with are limited to 5kA SCCR, with no series listing at higher levels even with fuses. This is why you don't see them used more in things like bypass for VFDs etc., where they would be a nice low cost option to the more complex bypass systems using contactors. The only one I have ever seen that does have a higher rating is from a company called ACI Controls, for their "IBYSS" (Inverter BYpass Selector Switch) line that they say is rated for up to 200kA when used in series with Class CC or J fuses. Given their niche marketing, they seem to have figured out that without this rating, it can't be used in most applications. Still requires the fuses though, no rating in series with breakers alone.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Thanks Jeff, for filling in the details.
That was my thought but you have explained it better than I could.
Yours
Bill.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

Thank you for the explanation. I see your points. I am spoiled with sub-150kW electrical systems.

On that note, could OP use an off the shelf motor reversing contactor provided the input sources can be separated? The ones I have seen are mechanically interlocked. A double pole double throw switch would have to be used on the control circuit to keep that side isolated.

 

[waross]

With close clearances between contacts, there may be an over voltage issue between adjacent contacts.
A condition referred to as opposite polarity.
Some control switch contact blocks are specifically NOT approved for opposite polarity.
A few contact blocks are approved for opposite polarity.
Aside from control circuits does not arise very often EXCEPT in some transfer situations.
Opposite polarity puts double nominal voltage across adjacent contacts.
Beware.
It depends on the construction of the motor reversing switch.
The switch that the OP originally selected is the normal solution for transfer functions.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

Good to know. Those of us on the outside looking in are certainly lacking the detail knowledge.

 

[jraef]

Yep, the devil is in the details sometimes. In addition, things that we got away with for years without thinking about it have only more recently become problematic. The "opposite polarity" issue on contact blocks was never an issue with older NEMA pilot devices, that only started becoming problematic when everyone started switching to IEC style devices that were nice and small, so they took up less space, but had these little "gotchas" that came with them.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[TugboatEng]

I guess I got lucky. McMaster way overcharges for their IEC stuff so I never felt any pressure to move away from NEMA as the prices are similar. The big issue I have noticed with IEC is the contactors go up in smoke if they bounce for any reason such as a motor start on a system with weak generators.