Protection
A pilot wire monitor, device 85, supervises the copper pair that links the relays at the two ends of a line. Testing simulates an open, a short, a ground and a loss of supply on the pair and confirms that the monitor alarms or blocks as intended.
A pilot wire monitor, ANSI device 85, checks the health of the communication channel used by a pilot protection scheme. In a pilot wire differential scheme, relays at both ends of a line exchange information over a pair of conductors, and the protection is only as good as that pair. The monitor passes a small supervisory signal over the pilot, watches for an open circuit, a short circuit, a ground or a change in pair resistance, and raises an alarm, or blocks tripping, when the channel is no longer trustworthy. The test confirms that each of these conditions is detected, that the alarm and block outputs operate as designed, and that the monitor resets cleanly. It applies to monitors built into line differential relays, to separate supervision units and to older electromechanical equipment on short feeders, cable circuits and industrial ties.
The pilot pair is a long run of cable that can be cut by excavation, damaged by water, shorted by a failed splice or disturbed by a rise in station ground potential. A protection channel that has failed silently is worse than one that has failed loudly. With the pair open or shorted, the differential scheme may be unable to see a fault on the line and leave it to slower backup, or it may see a spurious difference and trip a healthy feeder. The monitor exists to turn such failures into an alarm, or to put the protection in a safe state, so crews are told while the line is still being protected by something. Because a monitor rarely operates, a defect in its sensing or its output wiring can persist unseen. Testing proves it still catches them.
The monitor behaves as a supervisory circuit. It drives a small signal, direct current in many designs and a coded or alternating signal in others, through the pilot pair and measures how that signal returns. An open pair stops the flow, a short raises it, a ground path changes the balance between the two conductors, and a degraded splice changes the loop resistance. The monitor compares the measured condition with a window around the normal value and, when the measurement leaves the window for long enough, it operates. A time delay keeps brief disturbances, such as induced voltage, from causing an alarm. Pilot circuits are often equipped with isolating transformers and neutralizing reactors to protect the relay from ground potential rise, and the monitor must cope with these elements in the circuit. Outputs include an alarm contact and a contact that blocks or alters the differential protection while the channel is suspect.
The line protection is taken out of service according to the owner's safety plan, the pilot pair is disconnected from the line end and isolated from the protection, and the monitor settings and identification are recorded. The relay circuits are measured for insulation to the frame at the level the standard assigns. The monitor is then connected to a stand-in for the pilot: a resistance or a simulated pair of the right character. With the pair healthy, the monitor is observed in its normal state. An open is simulated by lifting a conductor, a short by bridging the pair, a ground by connecting one conductor to the station ground through a controlled resistance, and a degraded splice by inserting resistance in series. At each condition the technician watches the alarm and block outputs and times their response. Where supervisory voltage or current is adjustable, the sensitivity is probed by changing the simulated fault gradually. The loss of the monitor's own supply is checked next. Finally, each condition is cleared to confirm that outputs reset and that the monitor returns to normal, and the real pair is reconnected and the end-to-end channel is checked when conditions allow.
The record notes the monitor identification and settings, the type of pilot circuit and the stand-in used, each simulated condition and the response observed, the delay before alarm, the state of the block output, the insulation reading, the test instruments and the as-found and as-left condition of the wiring and settings.
A pass means that every simulated condition is recognized, that the alarm and block outputs operate for each of them, and that the monitor resets without latching when the condition is removed. The delays and sensitivities are compared with the manufacturer's data and with the scheme design, and a condition that is recognized late, only partly, or not at all is a finding. Results are compared with the previous test of the same equipment, since a shift in sensitivity can point to aging components. The insulation reading is read against the standard's table for the circuit voltage. The pilot wire monitor is covered among the protective relay tests in ANSI/NETA ATS-2025 for acceptance and ANSI/NETA MTS-2023 for maintenance; refer to your licensed copies of both and to the scheme documentation for the criteria and values.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It protects against an undetected failure of the pilot channel. An open, short or ground on the pair could leave a line differential scheme blind or cause it to trip wrongly. The monitor turns that failure into an alarm or a block so the problem is found while other protection still covers the line.
The monitor is connected to a stand-in for the pair, and faults are created on purpose: lifting a conductor for an open, bridging the pair for a short, connecting a conductor to ground through a controlled resistance and adding series resistance for a poor splice. The outputs are then observed.
Pilot pairs pick up induced disturbances, especially when faults occur near the cable route. A short delay keeps those transients from raising false alarms. The test confirms that real, persistent conditions still produce an alarm and that the delay agrees with the design.
Many designs provide a block output that disables or changes the differential protection when the channel is unhealthy, so a failed pair does not cause a false trip. The test confirms that this output operates for each simulated condition and that it clears when the pair is restored.
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