Pressure reducing regulators are purely mechanical devices capable of failing closed, partially closed, or open. Most pilot-operated regulators have main valves with springs that push them closed which makes them more likely to fail in a closed or partially closed position. To ensure that a malfunctioning regulator would not shut down the gas feed, many stations utilize a duplicate regulator run on standby. The standby run could have an isolation valve keeping it from flowing while the primary run is functional, but more frequently it stays shut by staggering the pressure setpoints. By setting the standby run at a slightly lower pressure than the primary run, the standby run will stay closed but prepared to open if the downstream pressure decreases for any reason.
In the above example, the regulator on the primary run will reduce outlet pressure to its 440 psi setpoint. The regulator in the standby run will stay closed unless the downstream pressure decreases to 430 psi at which time it will open and begin throttling to maintain its 430 psi setpoint.
In addition to protection from underpressure events, another benefit of a duplicate run is that the station can continue service even while performing maintenance on the primary run.