DT Safety Switch vs Manual Transfer Switch 1

[hockeyrcks9901]

Hello everyone,

First, I'm a new EE graduate and they really don't teach enough in school.

I'm wondering what the differences are between a DT Safety Switch and a Manual Transfer Switch are. For example, the differences between an Eaton DT365UWK and an Eaton MTVXLDA30400XDU.

Thanks for the help everyone.

 

[waross]

A safety switch controls and protects a circuit. It is common for a safety switch to be fused.
A transfer switch allows a load to be fed from either of two sources or allows a supply to feed either one of two loads. It is uncommon for transfer switches to be fused, however interlocked circuit breakers are used in some designs of transfer switch.
Also some designs of transfer switches use molded case switches. A molded case switch is a molded case circuit breaker with no trip unit.
I once installed a circuit breaker based transfer switch and experienced a lot of nuisance tripping. I disabled the the trip units, in effect making the breakers into molded case switches. Note: Such modifications are not allowed in many jurisdictions. Such modifications were not prohibited in the jurisdiction where the switch was installed.

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

 

[hockeyrcks9901]

I understand that most often safety switches are used to disconnect a load. But a double throw safety switch can be used as a manual transfer switch. I'm trying to figure out why I would use that over an actual transfer switch like the Eaton listed. My supplier is quoting a safety switch at a much higher cost and it seems like it should be the other way around. I'm wondering if there is something I'm missing there.

 

[waross]

I can't find a "MTVXLDA30400XDU" Got a link?
A fusible transfer switch may be suitable for service entrance equipment, saving on the cost of a main switch.
A fusible transfer switch that is lockable in the center off position may be suitable as a disconnect and overcurrent protection for a motor load, saving the price of safety switches after the transfer switch. It depends on local codes and the actual rating of the switches.
Do you have a particular application?
Full disclosure gets much better results on this site.

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

 

[hockeyrcks9901]

The transfer switch is going to be used to supply power to switchgear which feeds motors and normal loads. Fed from one of two different feeds on site. The insulated overhead lines are old (40+years) and they've been starting to fail in places and the load is semi-critical to operations. Basically, if we loose the service to the normal side of the transfer switch, we need the capability to switch the alternate feed. Neither of which are backup generators, they are utility feeds.

It's not critical and I would have thought the mechanical switch would be cheaper than a contactor/breaker based xfer switch but that's not the case.

http://www.eaton.com/Eaton/Products...icalDistribution/ATS/Breaker/index.htm#tabs-1

The specific transfer switch I'm looking at is at the link above.

 

[waross]

If cost is a factor you may want to consider "rolling your own".
Use a circuit breaker based manual transfer switch and add motor operators to the breakers. Use a couple of economy phase/voltage loss monotor relays and a cheap PLC for control.
I haven't done transfer switches for a few years but my experience back in the day was that:
Cheapest transfer switches.> Ordered as a package with a standby generator.
Most expensive transfer switches.> Purchased as a stand alone switch.
In the middle.> Roll your own.
But a factor to consider, whether going with the switch that you have chosen or rolling your own;
The switch you have chosen seems to be rated for service entrance duty. That will save you the price of two service disconnect switches.
You may be able to locate a manual switch with service entrance ratings.
Note: I consider a manual transfer switch to be one that must be operated manually each and every time it is switched, not an automatic switch with a manual over-ride circuit.

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

 

[jraef]

It's not a simple issue.

The MTVX is using Molded Case Switches (non-automatic circuit breakers) and a mechanical interlock mechanism. The problem with the part number by the way is the X in the 3rd to the last place, it is supposed to denote the voltage rating, "E" is for 600V, the catalog does not show an X (but it may be special, i.e. 480V?). The difference is, the MTVX is UL listed under UL1008 for Emergency Standby Power Systems and UL489 for circuit breakers, the DT365 is listed under UL98 as a service disconnect. The testing requirements for UL98 are much more stringent than they are for UL1008 and for UL489; UL98 requires a much higher duty cycle at a much higher overload condition, so as a consequence UL98 listed disconnects are over built by comparison. The advantage that the MTVX has is that the Molded Case Switches have an inherent withstand capacity, in this case 65kA, whereas the DT365 must have Class R, J or T fuses ahead of it somewhere to have any rating (100kA), but if it doesn't, it is only 10kA.

So it will depend a lot on what your overall circuit design looks like, at least on the utility feed where the Available Fault Current will likely be much higher than from the generator side. If there is already a Service Entrance Disconnect with fuses (the right kind as per the DT365 listing), then it's fine up to the rating of those fuses, i.e. 100kA. If there is a CB Service Entrance system ahead of this, and the AFC is 65kA or below, then the MTVX is fine. If there is a CB Service Entrance, but the AFC is above 65kA, then you will have to add fuses ahead of the MTVX per their listing as well.


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