Faults and safety in distribution networks
The distribution network is the most exposed part of the power system: it is outdoors, above the street, within reach of people who know nothing about electricity. That is why this page has two halves that need each other: how a fault in a low- or medium-voltage network is identified and resolved, and how that network is kept safe for anyone walking along the sidewalk.
01Fault types, and the case of the lost neutral
A distribution network runs for kilometres outdoors or under the sidewalk, and it fails for three kinds of reasons: external (wind, branches, lightning, vehicle impacts, excavations, animals), ageing (dried-out insulation, loose connectors and splices, broken insulators) and excess load. The network’s protection —fuses, reclosers, circuit breakers with relays— clears the fault, but only someone who understands it can say what failed and where. Each kind of fault leaves a different signature in the voltages and currents, and that signature can be measured.
| Fault | What happens | What you notice |
|---|---|---|
| Broken conductor (open phase) | A conductor breaks (wind, branch, impact, a splice that gives way) or a fuse blows. The users on that phase lose voltage. If the loose end touches the ground or an object, it is also a ground fault. | No power on one of the three phases. Three-phase motors keep turning on two phases: they hum, lose torque and overheat. The network may not trip, because there is no overcurrent. |
| Phase to earth | An energized conductor touches the ground, a pole or a branch, or its insulation is punctured. It is the most frequent fault. The current depends on where it occurs and on how the system neutral is earthed (solidly, through an impedance, or isolated). | The earth-fault protection trips if the current reaches its setting. If the contact is resistive (a tree, dry soil), the current may stay below the setting: the fault persists with the conductor energized. |
| Phase to phase | Two or three conductors touch (a long span in wind, branches, dragged objects, a flashover). It is a short circuit with very high current. | Fast tripping of the feeder; a flicker or voltage dip across the whole area supplied before it is cleared. |
| Lost neutral | In low voltage, the neutral is interrupted or comes loose upstream of the users. The phases are still healthy, but there is no longer a common reference point: the voltage is shared out according to each user’s load. | Uneven voltages: some homes with more than 220 V and others with less. Lamps burn out and equipment is damaged. Nothing trips. |
| Loose connection | A loose or corroded connection (connector, splice, meter terminal, fuse holder) has resistance and heats up; as it heats up it loosens further. | Lights that flicker with the load, radio noise, a burning smell, a discoloured terminal or plastic, a hot spot on the thermal image. |
| Overload | More current flows than is allowed for a long time: too many users on a transformer or a cable, or an unforeseen rise in consumption. | Voltage drop at the far ends, hot transformer and cables, thermal tripping, accelerated ageing of the insulation. |
On overhead lines many faults are transient: the branch that brushes the line, the bird, the flashover that jumps the insulator and goes out. That is why reclosers open and reclose several times before staying open: if the cause is gone, service comes back by itself, and if not, the fault is permanent and has to be hunted down. Underground cables are the opposite: almost every fault is permanent, because it means damaged insulation. The principles of protection are in Electrical protection.
A section that lost service can be re-energized at any moment: by a recloser, by a utility switching operation, or because another source (a badly connected generator set, a circuit that back-feeds) supplies it from the other end. Every conductor of a network is treated as energized until someone with authority over that network declares it dead, verifies it with an instrument and earths it.
A loose connection is underestimated because the resistance looks small, but heat grows with the square of the current and is concentrated in a spot the size of a terminal:
For a connector of 0.05 Ω carrying 40 A:
That is 80 W in a spot a few centimetres wide, for hours: enough to carbonize the neighbouring insulation. That heat raises the resistance, and a higher resistance raises the heat. How to detect it with a thermal camera is in Maintenance of electrical components, and how to hunt faults that come and go is in Fault diagnosis and detection.
The lost neutral deserves a case of its own because it does the most damage and gives the least warning. In a 3×380/220 V low-voltage network the neutral is the reference for the three phases: each home connected between a phase and the neutral receives 220 V as long as the neutral holds. If it breaks upstream, the neutrals of the homes stay tied to each other but floating, and loads connected to different phases end up in series between those phases. The line voltage (380 V) is shared between them like a divider, and the load that draws the least takes the largest share.
Home A (phase L1) has 2200 W of heating on, and home B (phase L2) only 440 W of lighting and televisions. The neutral breaks upstream of both. Each load is modelled as a constant resistance (a simplification: real equipment is not purely resistive, but the result is right in order of magnitude). What voltage does each receive?
Resistance of each load, from its power at 220 V:
With the neutral broken, the two resistances are in series across the 380 V between L1 and L2:
Home B receives 317 V, 44 % above its 220 V nominal. Since the power of a resistive load goes with the square of the voltage, its 440 W becomes:
Everything connected in B receives more than twice the power: lamps that burst, power supplies and refrigerator motors that burn out within minutes. Home A, on the other hand, receives 63 V: the heater barely warms and a motor will not start. A third home on L3 with very little load would see, with respect to the floating neutral, about 354 V.
Neither the circuit breaker nor the RCD of the homes acts: there is no overcurrent (the current is barely 2.9 A) and no earth leakage. They are protected only by an overvoltage device, which an ordinary installation does not have.
The general divider rule is:
With three phases and different loads, the neutral point “shifts” inside the triangle of line voltages and the phase voltages become uneven, but the voltages between phases remain 380 V: that is the clue for recognizing it (next section). Usual causes are a loose or corroded neutral connector in a splice, the neutral of a bundled (aerial) cable cut by an impact or a fallen pole, and a neutral stolen or cut during works. The three-phase connection and the role of the neutral are in Three-phase circuits.
Lights that suddenly become very bright or very dim, or that change when another appliance is switched on, a hum or a burning smell from equipment are the sign. The user’s immediate action is to switch off the main breaker of the home and notify the utility. Restoring or repairing the network neutral is a job for qualified utility personnel, and on an energized network it is governed by the rules for live working (Live working).
02Finding the fault: from the complaint to the measurement
The method is that of any diagnosis (see Fault diagnosis and detection: rule out, do not confirm), applied to a network that cannot be switched off as a whole or looked at from a bench. It is done in four steps:
Before leaving, what is known: how many users are affected and where (a block, a branch, a whole neighbourhood?), since when, what happened (a storm, sparks, a bang, a smell, lights going up and down), which protection operated and whether there were automatic reclosures. A single user without power is almost always a problem of their service connection or their own installation; several on the same phase, of the low-voltage branch; a whole neighbourhood, of the feeder or the transformer station.
The network is divided into sections between switching devices and measured at intermediate points: if there is voltage, the fault is downstream; if not, upstream. Each measurement rules out half of what is left.
Voltages between each phase and neutral, and between phases, under load; and, with the section dead, verified and earthed, continuity and insulation. The signature of each fault is in the table below.
With bisection each measurement rules out half of the remaining sections, so the number of measurements is the exponent to which 2 must be raised to reach the number of sections: eight sections, three measurements (); sixteen, four. Checking section after section in order, by contrast, can take eight. In a real network the order is also conditioned by access to the switching points, so the division is made where it is possible to open and measure safely.
What the voltages say. In a 3×380/220 V low-voltage network, measured under load at the users’ connection point:
| Reading | Interpretation |
|---|---|
| Three phase-to-neutral voltages of about 220 V and about 380 V between phases | Normal network at that point. |
| One phase-to-neutral at 0 V (or nearly), the other two normal; between the dead phase and the other two, about 220 V instead of 380 V; between the two healthy ones, 380 V | Open phase upstream: broken conductor or blown fuse. (With motors or loads that back-feed, a residual voltage can appear on the dead phase: that is why you measure with an instrument that loads the circuit, not only a high-impedance one.) |
| Uneven phase-to-neutral voltages (for example 272 / 217 / 181 V), but between phases they are still about 380 V | Lost or loose neutral upstream. |
| All three phase-to-neutral voltages low and even (for example 190 V), and low between phases too | Voltage drop from overload or long conductors; or a badly chosen transformer tap. |
| Values that fluctuate when a load is connected or disconnected | Loose connection upstream of the measuring point. |
Overload is confirmed by measuring current per phase with a clamp at the transformer secondary and comparing it with the rated value: a 160 kVA transformer at 0.4 kV has 231 A rated per phase (Substations and distribution lines explains the calculation).
Continuity is measured with the section dead, verified and earthed: the far ends of the conductors are joined (or a test lead is run out) and the resistance of each one is read from the near end. A broken conductor gives infinite resistance; a bad splice, a resistance higher than that of the other two conductors in the same cable. Insulation between conductors and to earth is measured with an insulation tester, at the test voltage set by the standard or by the cable manufacturer. Insulation and earthing measurement techniques are in Earthing, insulation and generator testing.
The medium-voltage network is operated, switched and measured only by authorized utility personnel, with the protective equipment and procedures set by the regulations. In low voltage you measure with an instrument of the right measurement category (CAT III or IV, depending on the point) and with insulated probes, following the five golden rules and lockout in Occupational health and safety. The technique of working on an energized network and its requirements are in Live working.
03Underground cables: locating what cannot be seen
On an overhead line the fault is visible: a conductor on the ground, a broken insulator. In a buried cable nothing is visible, and opening trenches “to see” along kilometres is not viable. It is located in three stages:
- Classify the fault. With the cable dead, verified, discharged and earthed (a medium-voltage cable is a capacitor and holds charge), the insulation of each conductor against the screen and between conductors is measured with an insulation tester, along with continuity. The result places the fault in one of three classes: open conductor, short circuit or low-resistance fault, or high-resistance fault that only appears at service voltage or at a test voltage.
- Pre-locate (how far from the end it is), by reflectometry or by bridge.
- Pinpoint on the ground, following the real route of the cable.
Reflectometry (TDR). The instrument sends a pulse down the cable and listens for what comes back. Wherever the impedance of the cable changes —an open end, a short circuit, a splice, a fault— part of the pulse is reflected. The time between the pulse sent and the echo, with the propagation speed of the pulse in that cable, gives the distance:
The speed is a property of the cable (it depends on its insulation, and is usually of the order of half to two thirds of the speed of light): it is loaded into the instrument or calibrated by measuring a known length of the same cable. The shape of the echo is informative: an open end or a broken conductor returns an echo of the same polarity as the pulse; a short circuit, an inverted echo. Near the instrument there is a blind zone where the echo merges with the outgoing pulse.
A 1200 m underground cable has a short circuit. The TDR, with the cable’s speed loaded as 160 m/µs (assumed data), shows an inverted echo at 11.25 µs. How far away is the fault?
It is 900 m from the end where the instrument is connected, and therefore 300 m from the other end. That distance is measured along the cable: it has to be carried over onto the route plan, not in a straight line along the street.
Bridge (Murray loop). This is the classic method and is still used as a cross-check. It works when there is a healthy conductor of the same cross-section and material next to the faulty conductor (in a multicore cable, any of the others). The two conductors are joined at the far end and, at the near end, a Wheatstone bridge is set up whose loop is the cable itself: the earthed fault divides it into two stretches, of length x and of length 2L − x. The ratio of the arms is varied until the galvanometer reads zero:
A 1200 m cable, with a healthy conductor joined to the faulty one at the far end. The bridge is balanced with Ra = 300 Ω (arm to the faulty conductor) and Rb = 500 Ω (arm to the healthy one).
It matches the 900 m from the TDR: two independent methods that agree is the best sign that the pre-location is reliable.
Pinpointing. Pre-location gives a distance, not a point on the street. That distance is measured along the route (using the utility’s plans, allowing for bends, crossings and cable reserves at splices) and refined with a surge generator that causes a discharge at the fault and an acoustic or electromagnetic detector carried along the sidewalk: above the fault the thump of the discharge is heard. High-resistance faults, which the ordinary TDR does not show, are studied with this surge equipment, operated by utility personnel or specialized companies.
The cable about to be opened is confirmed dead, discharged and earthed, and identified on site (by route tracing or by a safety cable-spiking tool, never “by colour” or from the memory of whoever laid it). Near the cable, dig by hand. If other cables or pipes show up in the trench, assume they are energized until proven otherwise.
04Resolve, restore and record
Finding the fault is half of it. The intervention is ordered by priorities, and that order prevents the urgency to “get the lights back” from causing a second accident:
- Safety. The section is isolated with the switching devices, locked out, verified dead and earthed with short-circuiting (the five golden rules of Occupational health and safety). The work area is marked and protected for the public.
- Restore the healthy users. Before repairing, whatever has no fault is re-supplied by another path. In a medium-voltage ring network, which is normally operated open at one point, the disconnectors on both sides of the faulty section are opened and the normally open point is closed: the two healthy stretches are fed from opposite sides and only the faulty section stays out of service.
- Repair. Replace the conductor, the insulator or the connector; a splice or terminal on an armoured cable is made by qualified personnel with the manufacturer’s kit. A fuse is replaced by another of the same rating and type: fitting a larger one leaves the network unprotected.
- Verify before energizing. Continuity and insulation of the repaired section are checked again, along with the identification and order of the phases (swapping two phases reverses the rotation of every motor), and that the earthing has been restored and no tools or temporary connections are left behind.
- Re-energize and measure. The switching device is closed from a safe place, and voltages and currents are measured in the affected area to confirm that service came back in good condition, not just that “there is power”.
- Record. It is part of the job, not a formality.
| Data of the intervention | What it is for |
|---|---|
| Time of the complaint, of arrival, of partial and of full restoration | Measures how long the outage lasted and how much of that time went to searching and how much to repairing. |
| Users affected and section, with location | Feeds the service continuity indicators. |
| Type of fault, probable cause and damaged element | Shows whether a cause repeats (branches along a section, the same connector model). |
| Measurements before and after (voltages, currents, insulation) | Proves the repair was done well and serves as a reference for the next one. |
| Switching operations performed, with time and who did them | Avoids energizing a section that someone has locked out. |
| Preventive actions proposed (pruning, replacing insulators, reinforcement) | Moves from repairing to preventing. |
The frequency and duration of outages are precisely what is used to measure the quality of the technical service a utility provides, under the control of the regulator (in Córdoba, the ERSeP): see Power quality and machine protection. And the cause that repeats three times is almost never where it was repaired: it is the same warning given by board maintenance (see Maintenance of electrical components).
05Safety on public streets: siting, heights and distances
All the safety of a line installed in the street comes down to three ideas: that a person cannot touch a live part, that they cannot come so close that current jumps through the air (the higher the voltage, the greater the distance), and that nothing that hangs, rises or moves in the street gets in the way of the line. Siting —where the line is placed— and distances are the concrete way of meeting them.
Overhead lines. They run on poles, on the side of the sidewalk set by the utility, with the conductors higher the higher the voltage. In low voltage, bundled insulated cable is preferred, which tolerates a branch brushing it and reduces the risk of contact; in medium voltage, in tree-lined areas, covered or protected conductor. Poles are placed where an impact is unlikely, and where that is not possible they are protected and marked. Crossings of streets, roads, railways and other lines (telephone, cable TV) are made with the distances and permits that each regulation sets.
Underground lines. They run under the sidewalk, at the depth set by the standard, with mechanical protection above (bricks or slabs) and a warning tape that alerts whoever is digging before they reach the cable. Road crossings are made in ducts, and splices and pull points are grouped in covered chambers. They are separated from other buried services (water, gas, sewers, telecommunications) by the regulatory distances, and before digging on any sidewalk the utility companies’ plans are consulted and the route is located with a detector, not from memory.
Clearance height. The lowest point of the conductor is between two poles, not at the pole. Its height above ground is that of the attachment point minus the sag, and the sag grows with temperature (the conductor expands and hangs lower). That is why it is verified in the worst condition:
The regulation sets Hmin according to voltage and location: a roadway used by trucks, a pedestrian sidewalk, a country road and a railway crossing are not the same. The higher the voltage, the greater the height and distance.
| Where | What is verified |
|---|---|
| Roadway | Clearance under maximum sag, considering trucks, raised dump bodies and machinery. |
| Sidewalk and pedestrian space | Clearance, and that no live part is within reach of a person standing, with a long object or from a ladder. |
| Facades, balconies, windows and roofs | Horizontal and approach distance to any part of the building that a person can reach or touch with an object. |
| Trees | Distance to the branches, also in wind and allowing for the growth expected until the next pruning. |
| Signs, awnings and antennas | That they cannot be installed, moved or leaned at less than the regulatory distance, and that if they fall they do not reach the line. |
| Other lines (telecommunications, another voltage) | Vertical and horizontal separation between them, with the higher-voltage one on the upper level. |
| Underground cable | Depth, mechanical protection, warning tape and separation from other services. |
Height and distance values are not remembered from memory or copied from another page: they are read in the current standard and for the specific case. Exterior underground lines are covered by AEA 95101; exterior overhead medium- and high-voltage lines by AEA 95201; transformer stations by AEA 95401; and overhead low voltage by the AEA regulation and the utility’s requirements, under the control of the ERSeP. The user’s interior installation is in AEA 90364 (see Electrical installations), and lines and transformer stations in general, in Substations and distribution lines.
Access prohibited. Transformer stations, underground chambers, switching cabinets on the street and equipment enclosures are restricted to authorized personnel. They are closed with a door and lock, with ventilation grilles that do not let a hand or an animal through, and with an electrical-hazard sign stating the ban on entry and the utility’s phone number. An open door, a broken padlock or a chamber cover out of place is reported to the utility, and nobody “goes in to have a look” even if the equipment seems off, or leans anything against the fence.
06Work in the street, third parties and a downed wire
Signing and fencing the work. A trench or work on a line over the sidewalk or the roadway is a hazard to anyone who does not know it is there: the distracted pedestrian, the blind person, the child, the driver arriving at night. Signing is set up from the outside in and removed in the reverse order:
- Advance warning far enough ahead of the work: signs and cones that tell traffic what is coming.
- Detour of traffic and a protected pedestrian path with a firm surface, so as not to push people onto the roadway.
- Continuous fence around the work area: tape alone is not enough for an open hole. A trench is covered or fenced with no open stretches.
- Own light at night (flashing beacons) and reflective elements, and personnel in high-visibility clothing.
- Sign of the person in charge of the work, with their phone number, kept up outside working hours as long as there is an open hole or equipment half assembled.
This adds to what Law 19.587 and Decree 351/79 require for the protection of workers (see Safety, hygiene and quality in installation work) and to what the municipality requires for occupying the public road, which is arranged before starting.
Risks to third parties. Almost all accidents with networks in the street involve people outside the trade doing something everyday near a line:
| Who or what | The risk | Prevention |
|---|---|---|
| Trees | A branch that touches the line connects it to earth or between phases, and a wet branch conducts. Pruning done close to the line can drop a branch on it or bring the tool near a conductor. | Pruning near lines is done by the utility or by personnel it authorizes. If a branch touches the wire, report it and do not cut it. |
| Antennas and masts | The classic accident: when mounting, moving or lowering an antenna, the mast touches the line. It can also fall on its own in the wind. | Install it at a distance from the line greater than its length, so that if it falls it cannot reach, and never move it under the line. |
| Signs and metal structures | They are leaned on, come loose or are climbed near the conductors; an energized metal part can electrocute everyone who touches it. | The regulatory distance and the utility are consulted before installing them; they are not leaned on poles or held with cables. |
| Kites | The string, especially if coated with ground glass or carrying metal, or if damp, can conduct. A child pulling a string tangled in the line is in the circuit. | They are flown far from lines. If one gets tangled, let go of the string and report it; do not pull it or climb to get it. |
| Trucks, cranes and machinery | A raised dump body, the boom of a crane or a concrete pump can reach a line without the operator having seen it. | The route is planned, a spotter is used, and the utility is asked to switch off or protect the line if the work must pass close. |
| Ladders, scaffolds and long objects | Pipes, steel rods, poles, metal ladders: if they fall or are raised toward the line, they touch it. | They are worked at a distance; to work beside a line, the utility is asked to de-energize or protect it. |
| Pressurised water | A continuous jet from a hose or a pressure washer close to energized equipment can conduct. | Energized equipment or lines are never washed. |
A downed wire. A conductor that has touched the ground can still be energized: if the fault is resistive, the current is not enough for the protection to trip, and it makes no sparks or noise. And even if the protection opens, a recloser can close again. The current the wire passes into the ground spreads outward from the contact point and the ground takes a voltage that falls with distance, like the contour lines of a map: a person walking through that area has their feet at different voltages and current flows between them. This is the step voltage. In the ideal model of a point contact on uniform soil:
A downed wire passes 100 A into the ground, of resistivity 100 Ω·m (assumed example data). A person approaches and puts the leading foot 1 m from the contact point. At 1 m, the ground voltage is:
If they take a long step of 0.8 m, the other foot lands at 1.8 m:
If they take a short step of 0.2 m, it lands at 1.2 m:
The long step subjects the body to a voltage 2.7 times greater between the feet. How much voltage a person tolerates depends on how long it lasts, and a downed wire can stay energized for a long time: a difference of hundreds of volts between the feet is dangerous. That is why a person should move away with the feet together, shuffling with very short steps without separating them, and not run.
1. Assume it is energized, even if it makes no sparks, no noise and the power on the block is out. 2. Move away, with short steps and feet together, and keep people back: the dangerous area extends several metres around the contact point, and is larger with wet ground. 3. Do not touch or move it, nor with a stick, a broom or any other object: damp wood conducts. 4. Notify the utility (in Córdoba, EPEC or the local electricity cooperative) and the fire brigade, giving the exact address, and wait at a distance for authorized personnel to arrive. 5. If the wire fell on a vehicle with people inside, tell them to stay inside and not touch anything metal outside. Only if there is a fire and they must get out do they jump clear with feet together without touching the vehicle and the ground at the same time, and move away with short steps. 6. A person who is being electrocuted is not touched until it is confirmed that there is no voltage: see “If someone receives a shock” in Occupational health and safety.
07In the workshop and on site
Materials: isolating transformer with adjustable output (variac), two 12 V lamps (one 5 W and one 21 W), AC voltmeter, lamp holders, single-pole switch and leads.
Steps: 1) Set the output to 20.8 V: the lamp voltage multiplied by the square root of 3, the same relation as between line and phase voltage in the network. 2) Connect the two lamps in series across those 20.8 V: this is the equivalent of the two homes with the neutral broken. 3) Before measuring, calculate the voltage each lamp should receive, from the rated power and voltage of each (the resistance is the voltage squared divided by the power). 4) Close the switch and measure both voltages for a few seconds only: the 5 W lamp is overstressed.
Acceptance criterion: the smaller lamp receives the higher voltage; the readings match the calculated ones within 15 % (the filament is not a constant resistance: its hot resistance is about ten times its cold resistance). Even at low voltage, the circuit is built and modified with the source off.
Materials: 100 m of two-conductor cable of 2.5 mm² (joined at the far end), two resistance decade boxes (or a teaching Wheatstone bridge), a 1.5 V cell with a 10 Ω resistor in series (limits the current through the loop), a 100 Ω resistor, an earthing lead and a digital multimeter on the millivolt range instead of the galvanometer.
Steps: 1) The teacher marks, without saying, a point on the cable (for example 36 m from the near end) and connects it to earth through the 100 Ω resistor: this is the “fault”. 2) Build the bridge: Ra to the faulty conductor, Rb to the healthy one, the multimeter between the common point of both arms and earth, and the cell (with its resistor) between the near ends of the two conductors. 3) Keep Ra + Rb = 1000 Ω and vary Ra until the multimeter reads zero. 4) Calculate x and measure it along the cable.
Acceptance criterion: the calculated position differs from the marked one by less than 2 m (2 % of the cable length).
Materials: survey sheet, camera, clinometer (or a tilt app), tape measure, high-visibility vest.
Steps: 1) Walk a block with an overhead line without going closer to it than the eye can reach, and photograph the critical points: trees that touch or approach the conductors, antennas, signs, awnings, balconies and roofs, poles exposed to impact, pillars, signage of the transformer stations. 2) Measure the horizontal distance d to the base of the pole with the tape along the ground, and with the clinometer the angle α to the lowest point of the conductor. 3) Calculate its height by adding the eye height.
Acceptance criterion: a sheet with each critical point located, photographed and classified as compliant or not against the value of the standard that the teacher has looked up (AEA 95201), with the heights calculated to ±0.5 m. Distances are always measured from the ground: a tape or a pole is never extended toward a line.
Materials: cones, barriers, hazard tape, advance-warning and detour signs, beacons, vests; a rope or a disused cable as the “downed wire”.
Steps: 1) In the yard, set up an imaginary 4 m trench with complete signing from the outside in (advance warning, pedestrian detour, continuous fence, sign of the person in charge). A group audits it with the list in section 06. 2) Drill: the rope is laid on the floor as a downed wire; each student, in turn, must say what they do and how they move away (feet together, short steps), and who notifies and what they report. 3) Two students act as passers-by and try to cross the area: the group must give the correct order.
Acceptance criterion: fence with no gaps, advance warning placed before the closure, and the downed-wire actions named in order and without ever touching it.
08Common mistakes
| Mistake | Consequence |
|---|---|
| Working on a section “dead” without verifying with an instrument and earthing | A switching operation, a recloser or back-feed energizes it again. It is one of the main causes of accidents on networks. |
| Believing that a downed wire without sparks is dead | A resistive fault does not trip the protection and the conductor stays energized; getting close or touching it is enough. |
| Running or taking long steps away from a downed wire | The step voltage between the feet increases. You leave with short steps and feet together. |
| Measuring the three phase-to-neutral voltages and not looking at the line voltages | A lost neutral (normal line voltage) cannot be told from a supply problem (low line voltage). |
| Replacing a fuse or a breaker with a larger one “so it doesn’t trip” | The protection is removed from the circuit; the next fault burns the cable instead of being cleared. |
| Reading the distance to the fault from the TDR as a straight line on the street | The cable has bends, crossings and reserves: the excavation ends up metres from the right place. |
| Re-energizing a repaired cable without checking the phase order | Motors turn backward and loads are fed with swapped phases. |
| Leaving a trench with only tape or no light at night | Pedestrians and vehicles fall into a hole that cannot be seen. |
| Copying a minimum height or safety distance from memory | It may be the value for another voltage, another location or a standard already replaced. It is read in the current standard. |
09Self-assessment
In a 3×380/220 V network the neutral breaks upstream. Home A (phase L1) draws 1100 W and home B (phase L2), 2200 W. What voltage does each receive?
Resistances at 220 V: 44 Ω and 22 Ω. They are in series across the 380 V between L1 and L2, with the voltage shared in proportion to resistance.
A, the lighter load, receives 253 V (15 % above nominal) and B, 127 V. The breaker and the RCD notice nothing.
How can a lost neutral be told from an open phase by measuring voltages?
With a lost neutral the phase-to-neutral voltages are uneven (some high, some low) but the voltages between phases are still about 380 V. With an open phase, one phase-to-neutral is almost 0 V and between that phase and the other two there are about 220 V instead of 380 V; between the two healthy phases the 380 V is kept.
A TDR, with the cable speed set to 170 m/µs, shows an echo at 8 µs. How far away is the fault and what does the echo polarity say?
The pulse goes there and back, so you divide by 2:
An inverted echo indicates a short circuit; one of the same polarity, an open conductor.
On a 600 m cable, the Murray bridge balances with Ra = 200 Ω (arm to the faulty conductor) and Rb = 600 Ω. Where is the fault?
300 m from the end where the bridge is. Measured along the cable, not in a straight line: it is carried over onto the route plan.
With the data of the downed-wire example (100 A, 100 Ω·m), what voltage is there between the feet when moving from 2.0 m to 2.5 m from the contact point? Why do you leave with the feet together?
The voltage falls with distance, but the step still crosses a potential difference. With the feet together, the difference between them is nearly zero; with long steps more “steps” of the ground potential are crossed, and the voltage between the feet grows.
You see a wire on the ground in front of a house, with no sparks, and the power on the block is out. What do you do and what don’t you do?
You treat it as energized. You move away with short steps and feet together, keep people back, do not touch or move it with any object, notify the utility and the fire brigade with the exact address and wait at a distance. You do not “cut” it or “push it aside”: a recloser can close again and a resistive fault does not trip the protection.
Why is the clearance height of a line verified with maximum sag and not with the sag on the day it was installed?
With heat the conductor expands and hangs lower: the sag grows and the clearance (attachment point minus sag) falls. The worst condition for the distance to the ground is at maximum temperature, and that is the one that must meet the minimum value set by the regulation.