Line isolation is required for the controller to be able to break a fault.
http://www.schneider-electric.us/en...cale=en_US&searchid=1478888260641#__highlight
How often do the motors start/stop? I believe the ATS48 has 10 starts per hour rating in standard duty which goes down to 2 starts per hour when used with a shorting contactor. I do not understand the reasoning behind this but it is in their manual.
Hmmm... I think their use of the word "required" is a bit disengenuous, given that they are essentially "requiring" you to buy their contactor too.
The concept behind this is somewhat valid in this way; when SCRs fail, they fail shorted, as a conductor. So yes, if one SCR fails the line now goes to the load on that one phase. But there is no path for damaging motor current if only one device fails, you must have conduction in TWO of the phases (two SCRs in the same phase also has no current flow path). When you have 2 SCRs fail in separate phases, now you have single phase power going to the motor with no way to stop it and it can damage the motor. It's EXACTLY the same as having two contacts weld in a contactor, and in my opinion, has the same level of risk associated with it. So should we then make sure we put TWO contactors in series with every FVNR starter just to be sure? The contactor mfrs would love that, but how often in your career have you seen a contactor weld, and how may of those involved TWO contacts welding at the same time? "Safety" contactors are required to do this, because even a tiny risk is too much for some hogh safet category machinery. But every day stuff? Over kill.
In my 30+ years of dealing with RVSS starters, I have only once seen a situation where two SCRs failed at the same time on a standard RVSS connected "in-line" (more on that below). It's extremely rare, but from their standpoint, sells a lot more contactors... What typically happens is that an SCR fails while the motor is running, but you actually don't know that it happened yet, until the RVSS goes to turn it OFF and there is no voltage drop on one of the phases (that's how a shorted SCR is detected). So the control board of the RVSS will then disable the Start command from that point on until the shorted SCR is replaced. That effectively prevents you from shorting another one and being unable to stop the current flow.
Now there are three situations where this is no longer the case:
1) CHEAP starters that only use SCRs in 2 of the phases and the third phase is a piece of bus bar. I vehemently object to this practice, but EU suppliers have been pushing it for years now because it makes the RVSS cheaper 9only 2/3 of the expensive parts). it's REALLY bad for the motor if there is any extended ramp time (i.e. over 5 seconds) and/or a high duty cycle. Bottom lione, you can get away with not damaging the motor if it's a centrifugal pump, and they know that a LOT of AC motors go on centrifugal pumps. But the second risk is related to what I said above, that you need TWO phases conducting to have current flow. In this design, one phase is ALWAYS able to conduct, it's just a bus bar. So now even ONE shorted SCR and you can lose the motor.
2) The second situation is what's called "inside the delta" connection of the RVSS when you have a Wye-Delta wound motor. You bring the 6 leads out of the motor and the SCRs are put in SERIES with the two halves of the Delta windings inside of the motor. So again, since one side of each winding is hot all the time, one shorted SCR and you lose the motor.
3) The 3rd situation is now so rare I'm not sure anyone still makes them, but there have been RVSS manufacturers that use an SCR + a diode on each phase to save money. Again, bad for the motor and risky because if one SCR shorts, you have 1/2 wave power flow to the motor that can't be controlled. Payne soft starters were notorious for that, they may have stopped marketing them now.
In all 3 of these, you MUST add the line isolation contactor that is tied to a fault detection circuit in the starter that drops out that contactor if there is one shorted SCR. The irony is, the only reason any of these things are done is to same money on the RVSS, yet by requiring the line isolation contactor, it ends up costing the same, sometimes more! It's a shell game for capturing the attention of buyers who buy on price alone...
