Línia elèctrica caiguda a casa, sostre o tanca metàl·lica¶

Quan les tempestes de vent intenses, l'acumulació de gel, la caiguda de les branques d'arbres o les explosions de transformadors trenquen un cable elèctric aeri i el deixen caure al sostre de la vostra casa, canalons de pluja, revestiment d'alumini o tanca de cadena del pati del darrere, la vostra propietat passa a formar part instantàniament d'un circuit d'alta tensió a terra. A diferència del cablejat interior de la llar, que funciona a 120/240 volts (o 230 volts a Europa) i activa un disjuntor en mil·lisegons després d'un curtcircuit, les línies aèries porten entre 240 volts (caiguda de servei) i 4.000 a 34.500 volts (alimentació de distribució primària). Pitjor encara, els interruptors automàtics de la subestació de serveis públics (anomenats reconectadors automàtics) estan programats específicament per reactivar automàticament una línia caiguda de dues a quatre vegades durant uns quants segons o minuts per cremar fallades temporals de branques d'arbre clares.

Com que les teules de sostre d'asfalt sec, les bigues de fusta, el sòl i el formigó tenen una alta resistència elèctrica, un cable de distribució primària de 13.800 volts que es recolza tranquil·lament al sostre o a la tanca del pati del darrere sovint no treu prou corrent de falla per fer saltar el fusible del pal de serveis. En conseqüència, el cable es manté activat en silenci a una tensió letal mentre dinamitza les canals de pluja metàl·liques, les canonades de coure, els revestiments d'alumini i més de 100 metres (330 peus) de tanca d'enllaç de barri interconnectat**, mentre irradia anells concèntrics de tensió (potencial de pas) a través del sòl circumdant. Aquesta guia completa cobreix les decisions immediates de refugi al lloc en comparació amb les decisions d'evacuació, la mecànica d'incendis de retroalimentació de cable neutre, la letalitat potencial de contacte de la tanca metàl·lica i la tècnica d'evacuació amb peus remenats.

Fes això ara¶

Si un cable, un transformador o un pal de caiguda de servei aèria s'allunya i cau al sostre, revestiment, calçada o tanca del jardí, executeu aquests sis passos immediatament:

1. Traiteu tots els cables caiguts com a vius i letals, fins i tot si no fan espurnes: no suposeu mai que un cable caigut és una línia telefònica, de fibra òptica o de televisió per cable inofensiva, ni suposeu mai que un cable elèctric silenciós està mort. Els reconnectors automàtics d'utilitat poden enviar 13.800 volts de tornada a un cable silenciós sense previ avís.
2. Queda't dins de casa, tret que l'estructura estigui activament en flames: si hi ha una línia elèctrica recolzada al teulat, porxo o calçada i no hi ha fum ni foc a l'interior, queda't a l'interior allunyat de les parets exteriors. Sortir de la porta principal per esglaons de formigó humit o herba humida et situa directament dins de la zona de gradient de voltatge del sòl i sota els canals de pluja energitzats.
3. Truqueu immediatament al 911/112/999 I al servei d'emergència de la vostra empresa elèctrica: truqueu al vostre local números d'emergència (911 als Estats Units i Canadà, 112 a tota la Unió Europea, 999 al Regne Unit) i informa "Una línia elèctrica caiguda descansa directament a casa/la meva tanca." Immediatament després, truca a la línia d'atenció d'emergència de 24 hores de la teva empresa elèctrica perquè el seu centre de control de la xarxa pugui desactivar i posar a terra la línia d'alimentació.
4. NO toqueu el panell elèctric principal, la fontaneria o els aparells metàl·lics si la caiguda de servei està danyada: si una línia primària d'alta tensió va caure a la caiguda de servei de la vostra casa de baixa tensió o si el cable neutre s'ha trencat, la porta de la caixa d'interruptors metàl·lics, les canonades d'aigua de coure i els aparells connectats a terra es poden alimentar a centenars o milers de volts. Manteniu-vos completament allunyat del quadre elèctric, les piques i els marcs de les portes metàl·liques.
5. Manteniu-vos almenys entre 10 i 15 metres (de 35 a 50 peus) de qualsevol tanca, arbre o bassal que toqui una línia caiguda: si un filferro cau a través d'una tanca metàl·lica, una porta de ferro forjat, una tanca de bestiar o una branca d'arbre humida, tota la longitud contínua d'aquesta tanca s'engega el carrer, fins i tot amb tres corrents letals.
6. Si el terrat s'encén i heu d'evacuar, remenar o saltar amb els dos peus junts: si el fum, els arcs crepitants o les flames us obliguen a evacuar la casa mentre hi ha una línia baixa a la propietat, sortiu per una porta al cost oposat de la casa des del cable, no toqueu les baranes metàl·liques ni les aixequeu ** fins que no us aixequeu ** les portes juntes ** almenys a 15 metres (50 peus).

Arbre de decisió tàctica per a una línia elèctrica caiguda en una casa o tanca¶

Utilitzeu aquest arbre de decisions operatives per determinar en qüestió de segons si voleu refugiar-vos dins de la vostra llar o executar una evacuació d'emergència a terra.

flowchart TD
    Start["Línia de serveis públics caiguda al sostre, revestiment, arbre o tanca del jardí"] --> Location{"1. On ets ara mateix?"}

    Location -->|"A l'exterior al pati o al carrer"| OutDist["Retireu-vos immediatament a 15 metres (50 peus) lliure de filferro, tanca i bassals"]
    Location -->|"Dins de la casa"| FireCheck{"2. Olors fum, escoltes un arc de paret o veus foc a l'interior de la casa?"}

    OutDist --> FenceWarn["Perímetre de guàrdia: aviseu els veïns que NO toquin cap tanca metàl·lica o porta connectada"]
    FenceWarn --> CallDispatch["Truqueu al 911 / 112 / 999 i al número d'emergències de la companyia elèctrica"]

    FireCheck -->|"No: la casa és tranquil·la, sense fum ni foc"| ShelterIn["QUEDEU A DINS: traslladeu-vos a l'habitació interior lluny de les parets i finestres exteriors"]
    FireCheck -->|"Sí: fumar al terrat/àtic o espurnes a les preses de paret"| EvacPrep["EVACUACIÓ D'EMERGÈNCIA REQUERIDA: Trieu la porta de sortida més allunyada del cable caigut"]

    ShelterIn --> HandsOffInside["NO toqueu el panell principal, les aixetes metàl·liques, les canonades d'aigua o els aparells endollats."]
    HandsOffInside --> CallDispatch

    EvacPrep --> DoorCheck["Obriu la porta sense tocar el marc de la porta de tempesta metàl·lica, el revestiment d'alumini o els baixants"]
    DoorCheck --> JumpShuffle["Saltar fora del llindar amb els dos peus junts; Barreja els peus a 15 m (50 peus) de distància"]
    JumpShuffle --> CallDispatch

Anatomia dels pals elèctrics: què va caure a casa teva?¶

Comprendre quin cable s'ha trencat del pal de serveis públics davant de casa us ajuda a informar de la gravetat exacta de l'emergència als bombers i als proveïdors de serveis públics. Els pals de serveis públics estan disposats verticalment per tensió, amb els cables de més alta tensió a la part superior i els cables de comunicació de baixa tensió a la part inferior.

UTILITY POLE HIERARCHY & HOUSE SERVICE DROP ANATOMY:
   [Utility Pole]
      (o)---(o)---(o)  <-- LEVEL 1: Primary Distribution (7,200V to 34,500V - LETHAL)
           |
        [=====]        <-- LEVEL 2: Pole Transformer (Steps down to 120/240V)
           | \
           |  \===(Triplex Service Drop: Two 120V Hots + Bare Neutral)===> [Roof Weatherhead Mast]
           |                                                                       ||
           +------(Black Coax / Fiber / Phone Cable)---------------------> [House Wall Box]

Why a downed power line on your house can set walls on fire from the inside¶

When a tree limb falls across the overhead triplex service drop running between the street pole and your roof's steel weatherhead mast, three dangerous electrical faults can occur simultaneously inside your home's framing:

1. Pulled-out meter base and energized rain gutters¶

When a heavy tree limb yanks the overhead triplex cable downward, it rips the steel conduit mast loose from your roof eaves and pulls the exterior electric meter socket box partway off your siding.

• Energizing aluminum siding and rain gutters: As the pulled service entrance wires scrape across the sharp sheet-metal edge of the weatherhead, metal roof flashing, or aluminum rain gutter, the black rubber insulation slices open. Because this fault occurs on the utility street side (line side) of your main circuit breaker panel, your house's 200-amp main breaker cannot trip to stop it.
• The bare 120/240-volt wire feeds hundreds of amps directly into your metal rain gutters, aluminum downspouts, and metal siding. Anyone standing on wet ground outside who touches the downspout or metal storm door completes the circuit to earth and is fatally electrocuted.

2. The "Open Service Neutral" hazard: why plumbing pipes and walls catch fire¶

In North American 120/240-volt split-phase systems (and many international TN-C-S / PME grounding systems), the bare silver steel cable in your overhead service drop is both the physical support rope and the neutral return conductor.

• What happens if the bare neutral wire snaps while the two black 120V hot wires stay connected:
• Wild voltage swings occur instantaneously across your house outlets: half of your 120-volt outlets drop to 30 volts, while the other half surge to 200 to 230 volts, causing surge protectors, microwave ovens, furnace boards, and power strips to pop and burst into flames inside rooms.
• Unbalanced return current flows through your copper water pipes and gas lines: With the overhead neutral wire broken, all electrical current returning from your house tries to find a path back to the pole transformer through the earth via your grounding electrode wire connected to your copper water pipes. Your copper plumbing pipes, cast-iron drain stacks, and flexible steel gas connectors can overheat to glowing temperatures and electrify kitchen and bathroom faucets.
• Neighbors' neutral current flowing through your municipal water pipe: Even if your own breakers are off, if your neighbor's neutral wire is damaged, return current from their house can travel through the shared metallic underground street water main into your basement plumbing. Never touch metal water pipes, indoor faucets, or the main electrical breaker panel when service wires are down.

3. Primary-to-secondary cross-contact (High-voltage intrusion)¶

If a 7,200-volt or 13,800-volt primary wire at the top of the street pole snaps and falls across the lower 120/240-volt secondary service wires feeding your house, thousands of volts surge directly down your service mast into your main breaker panel. Household circuit breakers are only rated to interrupt 240 volts; 7,200 volts jumps straight across an "OFF" breaker gap in a continuous plasma arc, blowing outlet faceplates off bedroom walls and igniting wood studs inside the wall cavities.

Metal fence electrocution: why chain-link and wire fences conduct for blocks¶

One of the deadliest scenarios after a windstorm, hurricane, or ice storm is a downed power line resting on a backyard chain-link fence, wrought-iron perimeter fence, or agricultural barbed-wire / woven-wire fence.

1. Why the fence does not ground out the power line¶

Homeowners naturally assume that because steel chain-link fence posts are set in the dirt every 2.4 to 3 meters (8 to 10 feet), a power line falling on the fence will immediately short out into the ground and trip the utility breaker. In reality:

• Concrete post footings and dry or sandy top-soil have relatively high electrical resistance (often 50 to 500 ohms per post).
• A 7,200-volt line resting on a chain-link top rail may only push 15 to 40 amps into the soil, far below the 100- to 400-amp threshold required to trip a utility substation feeder breaker.
• Consequently, the entire continuous metal fence network, often spanning 5 to 10 connected backyards down an entire residential block, remains energized at thousands of volts for hours. A child, dog, or neighbor three houses away who has no idea a tree fell down the street can walk into their own backyard, grab their chain-link gate latch, and be electrocuted instantly because their wet shoes and body provide a lower-resistance path to damp earth (touch potential).

2. Step potential vs. Touch potential explained¶

When high voltage flows from a downed wire or an energized fence post into the earth, the electricity spreads outward through the soil in concentric circles, exactly like ripples from a stone dropped in a pond. Voltage is highest at the exact point of contact and drops off with every meter of distance.

CONCENTRIC VOLTAGE RINGS (STEP POTENTIAL) AROUND A DOWNED LINE OR FENCE POST:
          [Downed 7,200V Wire on Ground / Fence Post]
                          ||
               ( ( (   7,200 V   ) ) )
            ( ( (      5,000 V      ) ) )
         ( ( (         3,000 V         ) ) )
      ( ( (            1,000 V            ) ) )
                        [Foot A]  [Foot B] <-- Normal stride bridges 3,000V to 1,000V = LETHAL
                        [FootA+B]          <-- Shuffling feet together keeps voltage difference = 0V

How to evacuate a house safely if a downed power line starts a roof or wall fire¶

As long as your house is free of smoke, sparks, and fire, stay inside in an interior room until utility linemen confirm the line is de-energized and grounded. However, if a downed wire on your roof or siding ignites the attic rafters or wall studs and you smell smoke or hear crackling fire inside the house, you must evacuate immediately without getting electrocuted at the doorway:

1. Choose an exit door on the side of the house OPPOSITE the fallen wire: Look out the windows before opening any door. Never exit underneath the roof eave where the wire is hanging, and avoid driveways or patios that have puddles of water touching the foundation.
2. Check the doorframe and avoid touching metal trim: If you have a wood or fiberglass door leading onto a dry wooden porch or concrete step on the far side of the house, open the door using only the interior handle. Do not grab an aluminum storm door frame, metal screen door mesh, or metal porch handrail.
3. Hop clear of the doorway threshold with BOTH feet together: If you suspect the house siding, metal threshold sill, or porch railing is energized, do not step out with one foot inside the house and one foot on the ground outside (which bridges the house's voltage potential to the earth). Stand on the interior floor, tuck your arms against your chest, and hop with both feet pressed tightly together so you land on the ground outside with both feet simultaneously without touching the doorframe.
4. Execute the two-foot shuffle to 15 meters (50 feet):
5. Once outside on the ground, keep your feet pressed together side-by-side so your ankles touch.
6. Slide one foot forward no more than 5 to 8 centimeters (2 to 3 inches) along the ground without lifting your sole off the earth, then slide the other foot up to meet it, keeping both feet in continuous contact with each other and the ground.
7. Alternatively, hop like a bunny with both feet locked tightly together so only one point of ground contact exists at any instant.
8. Continue shuffling or hopping until you are at least 15 meters (50 feet) away from the house, the downed wire, any metal fences, and any wet ground.

Solar panels, backup generators, and post-storm electrical reconnection¶

Before power can be restored after a utility line falls on your house, two additional electrical hazards must be managed: rooftop solar photovoltaic (PV) arrays and portable backup generators.

1. Rooftop solar panels remain live in daylight even when the grid is down¶

If a falling tree limb or utility line crashes onto a roof equipped with solar photovoltaic (PV) panels:

• Even when the utility grid fails and your solar inverter automatically shuts down anti-islanding export (Rapid Shutdown), the individual solar glass modules and DC string wiring on the roof continue generating 30 to 600 volts of direct current (DC) whenever sunlight or bright daylight hits the cracked glass.
• Water from rain or fire hoses contacting shattered solar panels creates an immediate DC arc-fault fire and energizes metal roof racking and rain gutters. Report rooftop solar panels immediately to arriving firefighters.

2. Never backfeed a portable generator into a damaged house system¶

After a storm rips down your overhead service drop, never plug a portable gasoline generator into a dryer outlet or flip a generator transfer switch ON until an electrician verifies your overhead service mast is completely disconnected from the downed street wires:

• If your generator backfeeds power into a damaged meter box or snapped service drop lying in your yard, the utility pole transformer steps your generator's 240-volt output backward up to 7,200 or 13,800 volts, energizing the downed wire in your yard and electrocuting utility linemen or neighbors.
• If your house's service neutral or grounding electrode wire was ripped loose when the mast fell, running a generator bonded through the panel can energize your home's metal appliances and plumbing. Only run portable generators outdoors at least 6 meters (20 feet) from windows using heavy-duty extension cords plugged directly into individual appliances.

3. Underground service laterals, pad-mounted transformers, and energized ground faults¶

Even in modern neighborhoods with no overhead utility poles, severe electrical hazards occur when floodwaters, uprooted tree roots, or digging equipment breach an underground service lateral or a green steel pad-mounted transformer box on your front lawn:

• Never touch or sit on a green pad-mounted transformer cabinet: The green steel box in a residential yard steps 7,200- to 13,800-volt underground primary cables down to 120/240 volts. If a vehicle strikes the box, floodwater submerges it, or internal mineral oil overheats, stay at least 10 to 15 meters (35 to 50 feet) away and shuffle out of the area if the ground is wet.
• Melting snow circles and steaming soil: If an underground service cable shorts out beneath your lawn or driveway after tree root movement, the high-resistance earth fault heats the soil, causing a circular patch of snow to melt rapidly, steam to rise from the grass, or a distinct acrid smell of burning rubber insulation. Keep people and pets completely off that lawn section and call your electric utility immediately.

First aid and rescue rules if someone is shocked outside¶

If you look out your window or arrive home and see a person or pet lying unconscious near a downed wire, a metal fence, or a gutter downspout:

1. STOP and do not run over to grab them: If the person is lying on energized ground or still touching an energized chain-link fence, running up to touch them will immediately kill you via step potential or touch potential.
2. Call 911 / 112 / 999 immediately: Tell the dispatcher a person has been electrocuted by a downed utility line and the line is still live.
3. Only approach once utility crews confirm the grid is de-energized, or if the victim is clearly outside the high-voltage zone: Once the scene is confirmed electrically safe by emergency responders, check the victim for responsiveness and normal breathing. High-voltage alternating current causes immediate ventricular fibrillation (chaotic heart quivering) or respiratory paralysis. Begin Hands-Only CPR (100 to 120 chest compressions per minute, 5 to 6 centimeters / 2 to 2.4 inches deep) and apply an Automated External Defibrillator (AED) as soon as one is available.

Taking a hands-on first aid and CPR course from the Red Cross, St John Ambulance, or a local accredited provider prepares you to perform life-saving resuscitation and treat high-voltage entry and exit burns once emergency crews make the scene safe.

Preguntes freqüents¶

Fonts revisades¶

• Occupational Safety and Health Administration (OSHA, United States): Downed Electrical Power Lines Fact Sheet, Step and Touch Potential Physics, and High-Voltage Clearance Standards.
• Federal Emergency Management Agency (FEMA / Ready.gov, United States): Power Outages and Downed Utility Line Safety After Severe Storms.
• Electrical Safety Foundation International (ESFI, United States): Downed Power Line Safety, Open Service Neutral Hazards, and Post-Storm Meter Base Inspection Guidelines.
• Health and Safety Executive (HSE, United Kingdom) and Energy Networks Association (ENA, United Kingdom): Avoidance of Danger from Overhead Electric Lines (GS6) and Public Emergency Guidance for Fallen Conductors.
• National Fire Protection Association (NFPA, United States): NFPA 70 National Electrical Code (NEC) Service Entrance Grounding Standards and Electrical Fire Response.

Guies relacionades¶

• Downed power lines and vehicle electrocution
• Electric shock first aid and high-voltage safety
• Tree fallen on house or roof
• Arcs i sobreescalfament del quadre elèctric
• Power outage preparation and safety
• Directori de números d'emergència