Open Circuit Test

Part of Telephony

Diagnosing telephone line faults where a conductor break completely interrupts the circuit.

Why This Matters

An open circuit — a break in the conductor that prevents current from flowing — produces a completely dead telephone line. The subscriber picks up the handset and hears nothing. The exchange shows no loop current and no lamp activation. The line is silent in both directions. Of all telephone line faults, an open circuit is the easiest to confirm (the line is simply dead) but can be the most difficult to locate precisely (the break could be anywhere along the line).

Open circuits are caused by wire breaks at pole attachments, in-line joints that failed mechanically, corrosion that ate through the conductor at a damaged insulation point, or mechanical damage from tree falls, vehicles, or excavation. In areas with wildlife, animals chewing aerial wire cause a significant proportion of open circuits.

Locating the break efficiently without walking the entire line requires systematic testing. This article describes the standard electrical methods for open circuit diagnosis and location on telephone lines.

Confirming an Open Circuit

First distinguish between an open circuit (no conductor path) and other faults that might also produce a dead line (ground fault, cross with another line, exchange equipment failure).

At the exchange end, disconnect the subscriber’s line from all exchange equipment. Connect an ohmmeter between the tip and ring conductors. Short the far end of the line (if accessible) with a wire jumper. With the far end shorted, a good line reads a low resistance equal to the total loop conductor resistance. An open circuit reads infinite resistance regardless of whether the far end is shorted.

Now test each conductor individually against a known good ground. A good conductor shows very low resistance to the far-end ground rod or to the shorted pair mate. An open conductor shows infinite resistance.

If the far end is not accessible (the subscriber premises are unavailable), test without the short. Measure resistance between tip and ring on the disconnected line. A good line shows very high resistance (the ringer circuit capacitor and instrument insulation resistance in parallel — typically megaohms). An open line may show similarly high resistance but for a different reason: the test battery cannot find any path through the break.

To differentiate: connect a battery (48V or higher) in series with a sensitive galvanometer (or microammeter) between one conductor and ground. On a good line, a very small leakage current flows. On a line with an open ahead of a fault, no current flows at all from the test end to the open. On a line with a fault beyond the open, the reading is the same as without the fault (current cannot reach past the open).

Locating the Open: Loop Test

The Murray loop test and Varley loop test that work for ground faults require current to flow through the fault path — they cannot locate an open circuit because no current flows through an open. A different approach is needed.

The capacitance test exploits the distributed capacitance of telephone cable. Every meter of cable has a small capacitance between the conductors and between each conductor and ground (typically 50-100 picofarads per meter for standard telephone wire). An open circuit does not eliminate this capacitance — it just limits how far the capacitance extends along the line.

Measure the capacitance of the open conductor from your test end. Compare it to the capacitance of a known good conductor of the same length. The ratio of the open conductor’s capacitance to the total line capacitance gives the fraction of the total length before the open. If a 50 km line shows 60% of its expected capacitance, the open is at approximately 30 km.

Capacitance measurement requires a capacitance meter or a known AC signal frequency and a voltage measurement. At 1,000 Hz, 50 km of standard 0.4 mm copper wire has approximately 5,000 pF total capacitance. An open at 30 km gives 3,000 pF. This technique is accurate to ±3-5% — adequate to narrow the search to a specific section of line for visual inspection.

Time Domain Reflectometry

The most accurate open circuit location technique is time-domain reflectometry (TDR). A TDR sends a fast electrical pulse down the line and measures the time for an echo to return. At an open circuit, the pulse reaches the break and reflects back with the same polarity (unlike a short circuit, which inverts the echo). The time for the round-trip tells you the distance to the open.

Distance = (echo time × propagation velocity) ÷ 2

The propagation velocity in telephone wire is approximately 60-80% of the speed of light, depending on insulation characteristics. A round-trip echo time of 0.5 microseconds places the open at (0.5 × 10⁻⁶ × 2×10⁸ m/s) ÷ 2 = 50 meters. TDR accuracy is typically ±1-2% of the fault distance.

Improvised TDR using a fast pulse generator and oscilloscope is feasible for a community telephone network. Generate a fast-rise voltage pulse (rise time under 100 ns) from a battery through a small capacitor discharged rapidly. Observe the line voltage with an oscilloscope and measure the time between the transmitted pulse and the reflected echo.

Field Repair

Once located, repair an open circuit by finding the physical break. Aerial wire breaks are often visible — the wire hangs down from the break point. Underground cable breaks may show no surface evidence; excavate at the calculated distance.

Repair the break using the Western Union splice or with a purpose-made line connector. For underground repairs, use a waterproof closure as described in cable splicing procedures. For aerial repairs, splice in a section of new wire if the original wire is too short to rejoin, maintaining proper sag in the repaired span.

After repair, re-test the line resistance and insulation to confirm the repair is good and no secondary fault was introduced by the excavation or splice work. Document the repair location and cause in the cable records.