⚠ What Happened — Port Coquitlam, Nov 2019
A residential 120/240V overhead service experienced a broken neutral conductor in the triplex cable near the utility pole.
- Root cause: Wind-induced swaying caused mechanical fatigue and abrasion of the bare triplex neutral at its compression connector.
- Failure: The neutral wire severed, opening the return current path to the utility transformer.
- Consequence: The 20 A imbalance current was forced through the cable TV bonding sheath — the only remaining metallic path back to the pole ground.
- Result: The coaxial cable sheath overheated at the cable TV box. Fire broke out, damaging the wooden siding, ceiling, and cable equipment.
⚡ Why 20 A Through the Comm Sheath?
- Line 1 carries
65 A, Line 2 carries 85 A.
- The neutral carries only the imbalance:
85 − 65 = 20 A.
- When the neutral opens, this 20 A must find another path back to the transformer.
- The comm cable sheath is bonded at both the house and the pole — creating a parallel return path.
- With the neutral severed, it becomes the only return path, carrying the full 20 A.
⚠ The Voltage Swing Problem
When the neutral opens, the two 120V legs become a series voltage divider:
- The lightly loaded leg (L1, 65 A) sees voltage rise toward 160 V.
- The heavily loaded leg (L2, 85 A) sees voltage drop toward 80 V.
- L1-to-L2 remains 240 V — the total doesn't change, just the split.
- Overvoltage destroys electronics and bulbs. Undervoltage causes motors to overheat.
How to Catch It Before Fire
- 1. Voltage check: Measure L1-N and L2-N. Readings that shift when loads change indicate a neutral issue.
- 2. Clamp the comm sheath: Any measurable current on cable TV or phone sheath is a major red flag.
- 3. Inspect the triplex: Look for abrasion or melting near compression connectors and attachment hardware.
- 4. Check for heat: Thermal anomalies at weather head, meter base, or bonding connections.
- 5. Clamp the neutral: At the meter base, if neutral reads near zero while loads are on, the neutral is open.
TSBC Findings & Takeaways
- Fire directly caused by the open service neutral on the overhead triplex.
- Lack of separation between bare neutral and compression connectors allowed abrasion and arcing.
- Comm cable bonding sheath provided unintended alternate path for 20 A imbalance.
- Damage: communication wiring, cable box, wooden siding, carport ceiling.
- Melting observed on overhead service conductors and cable TV wires.
Source: TSBC Incident Summary II-1200345-2021 (#22189)
The Series Circuit Math
- Total voltage = 240 V (L1 to L2, unchanged).
- Loads on each leg form a voltage divider.
V₁ = 240 × Z₁/(Z₁+Z₂) and V₂ = 240 × Z₂/(Z₁+Z₂)- Lighter load (higher Z) → more than 120 V.
- Heavier load (lower Z) → less than 120 V.
- With 65 A vs 85 A: V₁ ≈ 136 V, V₂ ≈ 104 V. Actual swing can be much worse with highly unequal loads.