I started looking today for medium voltage MCC's that contain breakers. I can kind of understand why you would not bother with a breaker over the top of a vacuum contactor (which would basically be the same thing) but I'm trying to understand...why do these things not come with circuit breakers in the incoming sections, similar to low voltage MCC's?

It is a matter of cost. The extra breaker would be $35k. But that still makes it a good design consideration. Rather than putting it in the incoming section, I prefer a breaker away from the lineup attached to the transformer secondary.

Along the same line of thought as Gary B, it would have to be cost. Also, it would eliminate two-high sections, which is fairly standard design. As most installations have real estate as a premium, so space is in high demand.

Also, isn't high interrupting ratings achieved with fuses as well as some current limiting characteristics, which prevent damage?

In general, yes. In fact there is even a paper that Mersen put out which couples some form of current limiting device (fuse or breaker) in conjunction with an electronic trip unit. The current limiter knocks down the AIC so that essentially very small breakers can be used with high speed trip mechanisms. The only trick to this is determining the correct trip setting to use considering that current will be limited.

As a practical matter I have a lot of mining substations. These all have high resistance grounds in them. This means that the overload relay handles all overcurrent/load conditions as well as all line-ground faults of any kind. The fuses are only there to interrupt very high current phase-to-phase faults, essentially bolted faults. Fortunately these are rare so the fuses rarely trip.

The complication of course with this arrangement is that we need bus protection as well on the secondary side. So this can be provided one of three ways:
1. Whatever protection is afforded by the primary side protection (usually almost nil).
2. Insert fuses/breaker on the secondary side. Note that arc flash values are typically high at this location.
3. Insert bushing CT's on the secondary fed to a relay controlling the primary side breaker. This "virtual breaker" arrangement is relatively inexpensive. It eliminates the usual "high arc flash hazard" on the secondary side since there is no physical breaker and since the CT's are on the transformer bushings, the exposure on the secondary side is limited to the inside of the transformer enclosure itself. The only downside is that this works only for a radial feed. It cannot be used where redundancy such as main-tie-main are used.

The AIC (Amps Interrupting Capacity) is a function of the protective device's construction and operation. There is nothing you can do in the field to affect this.

Current limitation will, however, reduce the amount of Short Circuit Amps (SCA).

Got to watch all those TLA

The "virtual breaker" arrangement is a simple and effective concept that is easy for protection non-experts to describe and apply. A similar concept can be applied in main-tie-main and other configurations, but it becomes an engineered project that requires a protection engineer to design. There are relays that are typically sold for transmission bay control applications that have multiple sets of CT and PT inputs and can be programmed to perform a "virtual main" type protection scheme for a main-tie-main.

I can see some ideas which would appear to be the same. But my thought is that "virtual breaker" means that we still have a breaker in the system but the actual physical breaker would be eliminated. The closest analog in an M-T-M arrangement can think of is partial bus differential relaying. If breakers were installed so that when one main is open and the tie is closed, it would certainly be possible to conceive of a M-T-B-M arrangement where the extra breaker (B) serves to provide protection when one of the ties is open. Partial bus differential relaying then operates on either the "left" or "right" bus depending on which mains are open to use the tie breaker as protection for half of the bus without a 4th breaker (M-T-M arrangement) but suggesting that this is a "virtual bus" arrangement is kind of stretching things since there was already a breaker there and we are simply applying multiple relay functions to a single relay. Similarly I don't consider moving the tie relaying functions into the main relays, implying a 2-relay scheme where normally 3 relays would be required, as a "virtual breaker" since again, the breaker is not eliminated.

Partial differential relaying is typically applied when multiple sources are routinely paralleled to feed a bus. A partial diff scheme on a secondary selective sub where the sources are only paralleled during short switching transitions could serve some purpose, but the same protection could be accomplished other ways that would be simpler to understand and operate.

What I was attempting to describe could be characterized as a primary selective scheme. Consider a radial feed substation with a transformer, primary breaker, no secondary main, and several secondary feeders. The primary breaker can be located remotely from the sub if we have breaker status indication and transfer trip capability. If we had two such radial subs we could install a tie breaker between the secondary buses, install CTs on the transformer secondaries, and use those CTs to feed a transfer and protection scheme. The protection and control logic is nearly the same as a secondary selective scheme since we're deriving the current signals from the transformer secondary, we're just using the transformer primary breakers along with the secondary tie to control how the buses are fed. This is probably not a desirable configuration for a new system, but it could occur for an existing system that has been modified to meet changing process requirements.