What is the true difference between a UL 489 Molded Case switch and a UL 98 switch? Does the UL 489 switch have to provide short circuit protection or is it just a breaker frame with mechanical switch and no thermal or magnetic tripping mechanism?
Also, can someone give me a good example when it would be best to use a molded case switch instead of a breaker or UL 98 switch?
A "Molded Case Switch" is just a Molded Case Circuit Breaker without tripping sensors for over current, or adjustability of the short circuit trip. In most cases the trip mechanism is actually still there along with the instantaneous magnetic trips, but they are fixed at the maximum value that the MCCB would have been UL listed at, typically 10X the
frame rating. That's because in the SC withstand testing done to UL list it as an MCCB (to get the "AIC" rating), it is assumed that the breaker begins to release at some point, and that point is the top end of any possible instantaneous trip system that can be installed in it. So if you have a 200A MCS, it is in a 250A frame, which means at 2,500A, that MCS is going to open, it has to, whether you like it or not.
How that relates to a UL98 switch, is that a similar issue in the switch is that of holding together until the fuse clears, if it is a fusible version. Since fused disconnects hold fuses that can have upward of 200kAIC, that means the UL98 switch mechanism must be tested to be capable of that for the time it takes for the fuse to clear without becoming shrapnel. So even if the switch is not fusible, it is still going to be built to have a withstand rating equating to the let-through current of a fuse that interrupts a 200kA fault because remember, the UL98 switch does NOT have any opening mechanism tied to the fault interruption. So in a similar example to the above, a 200A non fused disconnect is built to be part of a FUSED disconnect that might see 25,000A before the fuse clears. Therefore the internal design for holding together under a fault will be much higher than an equivalently sized MCS, in the above example, 10X the fault current.
If, however, your available fault current is so far below either of these extremes that it makes no difference, there is is no real difference.
As to where one would use an MCS
instead of a UL98 switch, I have had to do it where an equivalent UL98 switch was physically larger than an MCS, but the limitations of the MCS were irrelevant to my application. It's also easier to add something like a Shut Trip or an Under Voltage trip coil to an MCS, which is impossible or very difficult to do on a UL98 switch until you get into the size range of "Bolted Pressure Switches", aka "Pringle Switches".
