Classify the fault, calculate a distance by Murray loop, conductor resistance, TDR or capacitance, narrow it with a half-split tracker, and combine every estimate into one search window.
Start by classifying the fault from your IR, continuity and CCR readings. Then calculate a distance with one or more methods, narrow it down with the half-split tracker, and read the combined search window along the circuit. Several methods that agree give a narrow, confident window. Every distance is measured from the same end of the circuit.
One-way route length of the cable under test.
Used to name the nearest fixture and set the half-split resolution.
Needed for the conductor resistance method (B).
Placeholder. Use your specification.
From the Circuit Voltage calculator or the as-built load.
At balance. P is the arm connected to the faulty conductor.
Only for a solid fault. Fault and earth resistance add error. Not for an open circuit.
Copper resistance is corrected back to 20 °C.
Placeholder. Take it from the cable datasheet or calibrate on a known length.
Measured on a healthy conductor of the same cable, or nF per km from the datasheet × length.
Open the circuit at a test point and test the section from the source to that point. If it is healthy, the fault is further out. If it is faulty, the fault is nearer the source. The tracker narrows the bracket and gives the next test point.
| Method | Estimate (m) | Nearest fixture | Basis |
|---|
Estimates only. Measure every distance from the same end of the circuit and confirm the fault with a retest before digging.
| Step | Test point (m) | Section source → test point |
|---|
The page classifies the fault first, because the fault type decides which distance methods apply. Each method gives an estimate along the cable, and the combined estimate shows how well they agree.
x is the distance from the test end. L is the cable length, P and Q are the Murray ratio arms, Vp is the velocity of propagation and t is the round-trip echo time.
| Fault type | Methods |
|---|---|
| Earth (insulation) fault | Murray loop with a healthy conductor of the same length, TDR, or conductor resistance for a solid fault. Then half-split. |
| Open circuit | TDR or capacitance, then half-split. Murray and conductor resistance do not apply. |
| Extra series resistance | Half-split, then check joints and connectors in the bracketed section. |
| Missing load or shorted section | Half-split the circuit and check lamps and transformers. |
Murray: L = 1200 m, P = 300, Q = 700 → x = 2 × 1200 × 300 / 1000 = 720 m, about fixture 24 at 30 m spacing.
Resistance: 2.53 Ω at 20 °C on 3.51 Ω/km → x = 2.53 / 0.00351 = 720.8 m.
TDR: 8.2 µs at 170 m/µs → x = 170 × 8.2 / 2 = 697 m.
Capacitance: 70 nF of 120 nF on 1200 m → x = 1200 × 70 / 120 = 700 m.
| 🟢 High | Spread of estimates up to 5% of the circuit length |
| 🟠 Medium | Spread above 5% and up to 15% |
| 🔴 Low | Spread above 15%. Re-check the cable length, ratio arms, velocity, temperature, or whether there is more than one fault |
| ⚪ Single method | Only one estimate. Run a second method to check it |
Open the circuit at the suggested test point and test the section from the source to that point. A healthy result moves the lower end of the bracket out to the test point. A faulty result moves the upper end in. Repeat until the bracket is about one fixture spacing.
The window runs from the lowest to the highest estimate, padded by one fixture spacing or 1% of the circuit length, whichever is larger. The probable location is the average of all estimates, including the half-split mid-point once a result is recorded.
Cable route length, joints, bends and temperature all add error, so use the search window to dig or open the circuit at the marked fixture, and confirm with a retest. Measure every distance from the same end of the circuit.
Murray and conductor resistance read along the conductor. Each isolating transformer primary adds winding resistance that can equal tens of metres of cable, so use them on a disconnected cable section. Always isolate and discharge the circuit before any work.
Quick answers on classifying a cable fault, estimating its distance and narrowing the search.
It classifies the fault, calculates a distance by Murray loop, conductor resistance, TDR or capacitance, narrows it with a half-split tracker, and combines every estimate into one search window along the circuit. Several methods that agree give a narrow, confident window. Every distance is measured from the same end of the circuit.
You enter the insulation resistance to earth and the minimum acceptable value, the conductor continuity, and the CCR output voltage measured and expected. The fault type then decides which distance methods apply. The minimum insulation resistance is a placeholder, so use your specification.
For an earth (insulation) fault, use the Murray loop with a healthy conductor of the same length, TDR, or conductor resistance for a solid fault, then half-split. For an open circuit, use TDR or capacitance, then half-split. Murray and conductor resistance do not apply to an open circuit. For extra series resistance, half-split and then check joints and connectors in the bracketed section. For a missing load or shorted section, half-split the circuit and check lamps and transformers.
Murray loop: x = 2L times P divided by (P plus Q), where P is the ratio arm on the faulty side and Q is the healthy side. Conductor resistance: the resistance corrected to 20 degrees C divided by the resistance per metre. TDR: velocity of propagation times the round-trip echo time, divided by 2. Capacitance: cable length times the capacitance measured to the break, divided by the capacitance of the full length. In each case x is the distance from the test end.
Open the circuit at a test point and test the section from the source to that point. A healthy result moves the lower end of the bracket out to the test point, and a faulty result moves the upper end in. The tracker gives the next test point, and you repeat until the bracket is about one fixture spacing.
The window runs from the lowest to the highest estimate, padded by one fixture spacing or 1 percent of the circuit length, whichever is larger. The probable location is the average of all estimates, including the half-split mid-point once a result is recorded. Agreement is High when the spread is up to 5 percent of the circuit length, Medium up to 15 percent, and Low above 15 percent. With only one method it is shown as a single method.
On a 1,200 m circuit with Murray arms P = 300 and Q = 700, the distance is 2 x 1,200 x 300 / 1,000 = 720 m, about fixture 24 at 30 m spacing. Conductor resistance gives 720.8 m, TDR gives 697 m at 8.2 microseconds and 170 m per microsecond, and capacitance gives 700 m from 70 nF of 120 nF.
No. These calculations give an estimate, and cable route length, joints, bends and temperature all add error. Use the search window to dig or open the circuit at the marked fixture, and confirm with a retest. Always isolate and discharge the circuit before any work.
Please read before relying on any content on this page or on any result or record produced by it.