72kVA
high voltage cable with loadbreak elbows
There's very little point in having a distribution transformer if its secondary winding isn't connected to a load! So, to answer your question, a 'secondary system' (by which you, presumably, mean 'load'), isn't 'essential' to a distribution transformer, it could just sit there doing nothing!
120/208VAC
false
200 amps
200 amp/25ft long cable
an interlocking device
Primary electrical distribution carries medium voltage power to distribution transformers near the customers. Secondary electrical distribution transfers the power from the transformer to the households.
the three phase indicator lights will illuminate
It is connected to the secondary side.
This isn't always the case, although it is common throughout Europe. It is because high-voltage transmission and distribution lines are three-wire systems, while low-voltage distribution systems are four-wire systems.
The primary winding is the winding connected to the supply, while the secondary winding is the winding connected to the load. The terms, 'primary' and 'secondary' are unrelated to voltage levels.