Typically the network protector is set to close when the voltage difference and phase angle are such that the transformer will supply power to the secondary grid, and is set to open when the secondary grid would back-feed through the transformer and supply power to the primary circuit.
The top side is fed from multiple protectors and is always energized unless all units on a spot network are in the open position.
The components are enclosed in a protective housing; some network protectors are installed on transformers below grade and must be in water-resistant enclosures.
The controlling relay monitors voltage and current in the transformer, and opens or closes the contact mechanism through electrical signals.
The network protector is installed to protect the stability and reliability of the secondary grid by preventing power flow away from the customers and into the primary feeders.
In some cases, networks are designed with cable limiters (like fuses) to melt and disconnect the secondary fault under the right conditions.