Most of the work on an overhead distribution network happens at a distance from the conductors, and much of it involves questions a crew cannot answer by looking: Is this line live? Which section has the fault? Is the conductor high enough above the road? Are these insulators polluted enough to flash over in wet weather? A small group of specialised instruments answers each of these.
Is It Live? Fixed Live Line Indicators
A fixed live line indicator clamps onto a bare conductor, or sits near the earthing point of equipment, and flashes while the conductor is energised. It works by electrostatic induction from the conductor's electric field, so it needs no battery or connection to earth. Its main job is to make voltage visible at the points where people earth lines or operate earthing switches, guarding against applying earths to a live circuit.
The MEWOI-DXW110 is built for 10–36 kV bare conductors. The supplier specifies a strong flash at 20–60 flashes per minute, legibility within 10 m, operation from −25°C to +85°C and a design life of over 10 years. A fixed indicator is an extra warning, not a replacement for proving dead with an approved voltage detector before work.
Where Is the Fault? Fault Passage Indicators
When a feeder trips, locating the faulted section is often the slowest part of restoring supply. Fault passage indicators (FPIs) installed along the line show whether fault current passed their location: an indicator upstream of the fault trips, and one downstream does not. Patrols can then go straight to the section between the last tripped indicator and the first untripped one.
The Xtester-TQ-J4 flips a red flag that glows at night when it detects a short circuit or earth fault, and resets automatically after a set time. The supplier describes its short-circuit criterion as a sudden current step of at least 80 A, a rise of at least half the pre-fault load current, lasting between 0.02 s and 3 s. For earth faults it compares the phase of the first half-cycle of capacitive current and voltage at the moment of the fault. These criteria help it ignore load changes and inrush.
For networks where earth faults do not trip the feeder immediately, a portable ground fault inspection instrument such as the MEWOI2400 lets crews trace the earth fault along the line without a complete shutdown.
Is It High Enough? Ultrasonic Cable Height Measurement
Conductor height above roads, buildings and vegetation must stay within the clearances set by local regulations, and conductor sag changes with temperature and load. An ultrasonic cable height meter, held at ground level and pointed upward, times the echo from each conductor overhead and reports its height without anyone approaching the line.
The MEWOI-AR600E measures from 3 to 23 m vertically and can report up to six cables at once, lowest-first or highest-first. The supplier specifies ±5 mm resolution below 10 m and a minimum separation between wires of 150 mm.
Is It Polluted? Insulator Salt Deposit Density
Salt and industrial dust on insulators form a conductive layer when wetted by fog or light rain, and that layer can lead to flashover. IEC 60815 uses equivalent salt deposit density (ESDD) and non-soluble deposit density (NSDD) to classify site pollution severity and select insulator creepage distance. ESDD is found by washing the deposit off a known insulator surface area with a known volume of distilled water, measuring the conductivity of the washings, and expressing the result as the mass of sodium chloride per cm² that would give the same conductivity.
The MEWOI-YM3200 automates the conversion. It compensates the solution's conductivity to 20°C, subtracts the background conductivity of the wash water, and reads ESDD directly. The supplier gives a range of 0.0001–9.9999 mg/cm².
Operating at a Distance: Hot Sticks
Insulated operating rods let crews operate drop-out fuses, disconnectors and similar devices, and install clip-on indicators, from a safe distance. MEWOI's triangle-section telescopic hot sticks are listed for system voltages from 10 kV to 500 kV. Insulated tools need regular cleaning, inspection and periodic electrical testing under the user's live-work procedures.
Frequently Asked Questions
Do fault indicators need a power supply? Clip-on overhead FPIs such as the TQ-J4 install live on the conductor and sense the line's own current and field, with no wiring needed.
How often should ESDD be measured? Pollution builds up between rain events and with the seasons, so ESDD is normally measured repeatedly at the same site over at least a year to establish its severity class.