High Voltage breakers don't have instantaneous and over current units like low voltage breakers. So when we say a HV breaker is 1200A, does it mean that it can be placed on all circuits which are upto 1200A, and then we can get it tripped separately by using relays like time over current relay and overload relay?
Attached is a line up of a double ended medium voltage switchgear with a differential relay scheme that I sold in the the mid 90's that are installed in the LTV steel plant in East Chicago which may be closed by now. I believe it was 4160V but it may have even been 13.8kv. I can't remember.
For example, on the far left structure you will see (2) breakers using the top one as an example there are (4) NEMA 50/51 (instantaneous - NEMA # 50 and time overcurrent - NEMA # 51 mechanical relays) the top one that is by itself is for the ground, and the (3) in a row are for the 3 phases.
On a schematic the one for the ground would be a device 50/51G and the others just device 50/51. If my memory serves me correctly all 4 are the same devices except the 50/51G is applied differently wired using a zero sequence scheme of CTs from each of the 3 phases.
The breaker itself is stupid and needs to be told what to do, tripped using a shunt trip/spring release mechanism which is commonly powered by a 125v DC source usually a battery bank.
The breaker springs that causes te breaker to close must be mechanically charged by either a hand cranking lever which manually charges the spring or a 125vdc motor which will charge the spring automatically.
All of the relays instantaneous/TC settings must be set as determined by the engineer.
If you look closely there are knob/handles that are uses to operate the breakers manually as well as 2 green and red pilot indication lights powered by 125vdc.
This stuff was fun to sell.