The MLP-021 protects the equipment when the voltage on the pair rises above 21V. It also opens the line when the current goes over 150ma. When the voltage or current goes back below the threshold, the MLP-021 automatically resets.
If you don't know the voltage on the line (and there is no ring voltage), put a DC volt meter across the pair to determine the voltage that's on the line normally, then choose a protector that has a higher voltage threshold than what you read (by at least a couple of volts).
Our MLP Protectors come in the following thresholds: 14V, 21V, 30V, 56V, 70V, 110V, 123V and 270V.
You'll need one Protector for each pair.
If you're putting this on a line going to another building (OPX), be sure to put them at BOTH ENDS of the cable leaving the buildings!