Protection of Manual Trasnfer Switches 2

[CuriousElectron]

Hi Guys,
Is there a requirement anywhere in the code/good design practice that would require protection of a manual transfer switch upstream of it? I'm designing a setup where I'll have a MTS directly downstream of a MV step-down distribution transformer. The transformer will have properly selected OCPD on the primary side, with no secondary side protection.

Thoughts?
EE

 

[waross]

Look for a Service Entrance rated Manual Transfer Switch.
The transfer switch should also be rated to withstand the Available Short Circuit Current of the source.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Look for a Service Entrance rated Manual Transfer Switch.
The transfer switch should also be rated to withstand the Available Short Circuit Current of the source.

And what you will find is that non-fused double-throw safety switches such as those often used as "manual transfer switches" are only going to be rated for 10kA SC Withstand, unless protected by specific breakers or fuses. So your plan of using an MTS on the secondary of a transformer that does not have secondary protection will be severely curtailed by that fact. The only MTS I am aware of that can be listed as Service Equipment will be one made from mechanically interlocked circuit breakers.


" 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]

I agree Jeff. I have installed a number of circuit breaker type manual transfer switches.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill, you might get a kick out of this.

40 or so years ago I had to build a 1600A MTS with a SUSE label on it. Klockner Moeller (who I worked for at the time) made a single handle changeover mechanism for their molded case breakers (or did at the time), to where you had one big handle on the outside of the door that operated the rotary shaft mechanisms on the breakers that were an inherent part of the K-M design back then, so it made for a simple assembly (from an engineering standpoint). The exterior handle was about 3ft long with a big round ball on the end. On commissioning at the project site, I demonstrated the changeover and the Project Manager witnessed my cranking on that lever and struggling just a bit. So he called over one of the smaller guys working in maintenance and he could not move the handle enough to make it change over!

I pointed out to them that I was not the one who specified a 1600A MTS that needed to have a SUSE label. I ended up getting a change order to add motor operators to the breakers. Now days I would do it differently because I have more choices at my disposal.


" 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]

I like it Jeff.
I had to build an 800 Amp MTS.
I used two 800 Amp breakers mounted on a 3/8"thick plate with a captive sliding interference bar welded in place to block one or the other breaker handle.
That was over twenty years ago and I believe that it is still in service.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Shanaya]

When designing a power system and defining a transfer switch, the best practise approach is to use circuit breakers for ATS safety. Breaker upstream shielding a transfer switch has a deliberate short time delay of 0.1 such transfer switches would need a 65kA 3 cycle, WCR.