Sectionalizer / Fault Passage Indicator for Feeders
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When a feeder fault occurs and a section needs to be isolated quickly, a fault passage indicator (FPI) is often the simplest way to locate the disturbed segment without running a full protection relay or SCADA logic. This page collects all the sensing, power and communication options to build a reliable sectionalizer unit for distribution lines.
What this page solves
Many MV feeders still rely on a single protection relay and recloser at the source end, with minimal intelligence along the line. When a permanent fault occurs, the whole feeder may trip, and field crews need to drive many kilometres, opening and closing switches just to locate the faulty section.
Sectionalizers and fault passage indicators (FPI) provide simple “eyes on the line” at low-cost locations where full IEDs or FTUs are not practical. These devices detect fault current or magnetic disturbances, remember the event, and show which span or branch has seen the fault so that patrol time and switching operations are reduced.
This page focuses on how to design a robust FPI or sectionalizer unit using current or magnetic sensing SoCs, an ultra-low-power MCU, NB-IoT or LoRa backhaul, and energy harvesting. The goal is a device that can sit on a feeder for ten or more years, withstand harsh outdoor conditions, and still give clear guidance about where the fault travelled.
The following sections explain where such devices sit in a typical smart grid architecture and what constraints drive sensing, power, storage and communication choices, without overlapping detailed protection relay or SCADA design topics.
Figure H2-1 — Sectionalizers and FPIs placed along a medium-voltage feeder to narrow down fault location between the substation and multiple branches.
Where it sits in the system
In a typical smart distribution network, protection relays and reclosers at the substation act as the primary “brain” for fault clearing. Feeder automation units such as FTUs or IEDs sit at selected switches and reclosers further down the line, where there is space, power and budget for full measurement and communication.
Sectionalizers and fault passage indicators occupy a different role. These units are mounted at intermediate poles, underground cable joints or ring main units that do not warrant a full IED. They observe the passing fault current locally and provide a simple indication or wireless report about whether a given span has seen the fault, without executing protection algorithms or issuing trip commands.
Information from FPIs or sectionalizers can be read by line crews on site, collected by nearby FTUs, or forwarded by a concentrator to a SCADA or distribution management system. This way, the system preserves a clear layering: relays and IEDs perform protection and control, while FPIs and sectionalizers act as low-power sensors and markers that shrink the search area when locating faults.
The following sections use this system position as a fixed reference, so that current or magnetic sensing, ultra-low-power MCU design and NB-IoT or LoRa backhaul can be sized around the reality of an isolated, often line-powered device at the edge of the feeder.
Figure H2-2 — Layered view of a smart distribution system, where sectionalizers and fault passage indicators form a low-power sensing layer between feeder automation devices and SCADA.
Minimal operating conditions
A sectionalizer or fault passage indicator is expected to sit on a feeder for many years with no maintenance visit. The device must survive wide ambient temperatures, repeated fault events and long periods without any attention, while drawing only microamps from a lithium cell or small energy store between rare wake-ups.
Minimal operating conditions are therefore defined around four current buckets. Deep-sleep current needs to stay in the low microamp range, periodic measurement wake-ups must be very short, fault processing must complete before the supply collapses and radio or visual indication bursts must be rare and well budgeted. The average current across these modes has to remain in the single-digit microamp range to support a ten-year service target from a practical cell size.
Non-volatile storage is equally critical. When a fault is detected and confirmed, the event must be logged in FRAM or EEPROM so that the indication survives brownouts, battery droop or temporary loss of harvested power. The design should reach a safe state before the energy store empties and should restart in a predictable way when power returns.
Energy harvesting adds another dimension. A current transformer, electric-field coupler or small PV panel may recharge a supercapacitor during load current or fault current, offloading peak demand from the primary cell. The sizing of battery capacity, supercapacitor storage, wake-up rate and radio reporting frequency can then be turned into a simple battery life calculation flow, ensuring the design really meets the multi-year lifetime goal.
Figure H2-3 — Battery life and current budget flow, showing how sleep, measurement, fault processing and radio bursts combine into an average current that must support a multi-year service target.
Sensing & trigger options
A sectionalizer or fault passage indicator does not need precise metering accuracy. The sensing chain only has to answer whether fault-level current has passed a location and whether the current direction and duration match the expected pattern for a feeder fault. Three sensing approaches are commonly used: magnetic field pick-up, current transformers and Hall-effect or integrated current sensor SoCs.
Magnetic sensing options rely on coils or magnetic sensors mounted close to the conductor, often on overhead lines or near shielded cables. They are attractive where installation without breaking the line is important and where only a clear distinction between normal and fault current is needed. The front-end typically rectifies and filters the signal before a low-power comparator decides whether a configured threshold has been exceeded.
Current transformers offer a more traditional path. A CT wrapped around the conductor can provide both an energy harvesting source and a measurement signal. The secondary current, converted to voltage across a burden resistor, feeds into rectifier and conditioning stages before a comparator or ADC. CT sizing, burden selection and saturation limits have to be chosen so that both rated and fault currents can be handled without losing the ability to distinguish a genuine fault from inrush or overload.
Hall-effect and integrated current sensor SoCs add convenience and integration. These parts can provide analog or digital outputs with built-in isolation and internal signal conditioning. Direction information, programmable thresholds or integrated comparators may be available, so that the MCU only wakes when a fault-level event occurs. Supply current from these sensors must be managed carefully, often by duty-cycling the device instead of running it continuously.
In all three cases, the sensing chain ends in a simple trigger: a pulse, latch or status line that feeds an ultra-low-power MCU or event counter. Thresholds, filters and timing windows are tuned so that fault current, not normal load or switching transients, is what raises the flag for this particular location on the feeder.
Figure H2-4 — Sensing and trigger chain for a sectionalizer or fault passage indicator, from magnetic or current-based detection through signal conditioning and thresholding into an ultra-low-power MCU and communication or indication outputs.
ULP MCU/SoC & logging
Inside a sectionalizer or fault passage indicator, the ultra-low-power MCU or SoC acts as an event secretary rather than a full protection brain. Its tasks are to react to fault triggers from the sensing chain, apply simple decision rules, update local indication and log events in non-volatile memory so that the device can withstand brownouts and multi-year sleep periods.
A practical architecture relies on hardware wake-up sources instead of continuous polling. Edge-triggered lines from comparators or sensor SoCs bring the MCU out of deep sleep when fault-level current is detected. An RTC supplies periodic wake-ups for self-tests, health checks and rare housekeeping tasks. Each wake-up must be short and deterministic so that the average current remains within the microamp budget defined by the battery life target.
Event data is best stored in FRAM or high-endurance EEPROM, using a fixed-size record format and a circular log. Records typically include a timestamp from the RTC, a fault type or cause code, direction or phase information if available and a status flag indicating whether the event has been reported upstream. Configuration data and thresholds can sit in internal flash or a separate EEPROM partition as they change far less frequently than fault logs.
The clock tree must support deep sleep with a very low-power oscillator for the RTC and a main clock that starts quickly when active processing is needed. Firmware structure should keep interrupt handlers short, push work into a compact main loop and shut the high-frequency clock down again immediately after logging and any indication or communication actions are complete. The result is a predictable data-logging flow that respects both battery energy and non-volatile memory endurance limits.
Figure H2-5 — Event logging flow for a sectionalizer or fault passage indicator, from wake-up sources through ULP MCU handling and non-volatile logging back to deep sleep.
Wireless / wired comms
A sectionalizer or fault passage indicator can be a completely stand-alone node or a simple extension of an existing automation device. Communication choices therefore range from long-range cellular links such as NB-IoT or LTE-M, through LoRa and other sub-GHz radios, to wired interfaces such as RS-485, IO-Link or dry-contact inputs into an FTU or IED. Each option carries a distinct trade-off in coverage, energy usage, integration effort and ownership of the communication infrastructure.
NB-IoT and LTE-M suit isolated locations where utility or carrier networks are available but local gateways are not. These links can deliver alarms and status directly to a cloud platform or control center, but each connection attempt, attach cycle and burst of data transfer consumes significant energy. Device design must limit the number of lifetime messages, use deep sleep modes such as PSM and avoid frequent heartbeats if a ten-year service life from a small cell is required.
LoRa and other sub-GHz radios suit local feeder segments, industrial parks or substations where a concentrator or RTU can be installed. A group of FPIs or sectionalizers can report to a single collector, which then forwards data via Ethernet, fibre or existing SCADA links. Radio power budgets depend on spreading factor, data rate and retry policy, but are typically easier to tune because the network is under utility control and can be scaled for low message volumes and generous link budgets.
Wired interfaces become attractive whenever a nearby FTU, IED, PLC or RTU is present. A dry-contact or optocoupler output allows a very simple integration as a digital input. RS-485 with Modbus or a lightweight proprietary frame can expose richer event counters and diagnostic data. IO-Link offers a structured way to treat the device as an intelligent sensor in panel or ring main unit applications, with the IO-Link master handling diagnostics and power.
Short-range peer-to-peer or mesh links can also be used between sectionalizers and local controllers on the same structure, but long-range and network-facing communication is usually left to LoRa, NB-IoT or the installed SCADA infrastructure. A comparison across range, power profile and integration model helps select the link type that matches each deployment zone along the feeder.
Figure H2-6 — Communication options for sectionalizers and fault passage indicators, comparing wired connections to nearby FTUs or IEDs with LoRa or sub-GHz networks and NB-IoT or LTE-M wide-area links.
Energy harvesting & battery
A sectionalizer or fault passage indicator depends on a carefully planned power system rather than a single battery. A lithium thionyl chloride cell typically provides the long-term baseline energy, while a supercapacitor bank handles short, high-current peaks such as radio bursts, LED flashes or mechanical indicators. Current transformers, small PV panels or other couplers can harvest energy when load or fault current flows, topping up the supercapacitor and reducing stress on the primary cell.
The lithium thionyl chloride cell sets the hard limit for average current over the intended service life. Its capacity, self-discharge and temperature behaviour must be matched to a ten-year or longer deployment, with a safety margin for ageing. Supercapacitors then absorb the short-term power demands that cannot be drawn directly from the cell, especially at low temperatures where internal resistance rises. Charge paths from CT or PV sources feed the supercapacitors through controlled converters and current limits to avoid disturbing the sensing function or overstressing the harvesting devices.
Power management logic supervises cell and supercapacitor voltages and enforces energy-aware behaviour. Fault detection and event logging remain the highest priority loads, followed by local indication and finally communication. Thresholds on energy storage levels can temporarily disable radio reporting, lengthen heart-beat intervals or restrict non-critical functions when reserves fall below defined limits. A simple BMS-style timer scheme translates these thresholds into wake-up intervals and permitted actions.
A power budget chart that groups consumption into sleep baseline, sensing, processing and communication provides a practical tool for design reviews. Annual energy drawn by each function can be compared directly with the energy available from the lithium cell and any harvesting sources. This comparison highlights whether lifetime targets are realistic and which functions must be optimised or re-timed before hardware is frozen.
Figure H2-7 — Power budget chart linking lithium thionyl chloride cell, supercapacitors and harvested energy sources to functional power buckets and annual consumption.
Mechanical & enclosure
Mechanical design determines whether a sectionalizer or fault passage indicator survives the field environment for its full intended lifetime. The enclosure must meet appropriate IP protection levels for the installation point, tolerate UV exposure, temperature extremes and contamination and provide sufficient creepage and clearance for the voltage level and pollution class. Material and coating choices drive resistance to salt spray and other corrosive agents in coastal or industrial areas.
Internal layout must support clean separation between high-voltage sensing elements and low- voltage electronics. CTs, magnetic sensors or Hall devices near the conductor require robust insulation barriers and well-defined creepage paths to the electronics bay and battery pack. PCB layout, plastic partitions and terminal blocks all contribute to the effective insulation system and must be aligned with the targeted insulation coordination strategy for the feeder.
Mounting and enclosure geometry depend strongly on whether the unit is installed on an overhead pole, in a cable pit or inside a ring main unit. Pole-mounted designs must cope with wind, vibration and ice loads, while still presenting indication windows or mechanical flags clearly to line crews on the ground. Underground installations require compact forms, secure fixation and reliable seals against moisture and occasional standing water. Panel or RMU installations emphasise safe access for maintenance staff and compatibility with door or side-wall mounting patterns.
Condensation management and breathing are also critical. Pressure equalisation vents and carefully located drain features can limit moisture accumulation without sacrificing IP rating. Battery compartments and electronics bays should be shielded from direct water paths and designed for safe servicing, with clear mechanical separation from any high-voltage regions. An enclosure layout that makes these zones and interfaces explicit simplifies both compliance checks and on-site installation work.
Figure H2-8 — Enclosure layout showing indicator window, electronics bay, lithium battery compartment, sensing zone near the conductor, cable entry, insulation barrier, breather vent and pole-mount brackets for a sectionalizer or fault passage indicator.
Design checklist & supplier mapping
This checklist condenses the key decisions from the previous sections into a table that can be used to review any sectionalizer or fault passage indicator proposal. Each row highlights a specific design topic, explains why it matters, suggests a typical target and maps the topic to IC or subsystem choices. The final column is left for supplier answers or comparison notes when multiple vendors are evaluated.
The table is intended for practical design reviews: start with the rows that are critical for a given project, verify that datasheets and application notes answer those points and convert any blank or unclear cells into follow-up questions for suppliers. A proposal that aligns well with this checklist is much more likely to survive real distribution line conditions and lifetime expectations.
| Checklist item | Why it matters | Typical target / example | IC / subsystem mapping | Supplier answer / notes |
|---|---|---|---|---|
| Application & environment / line conditions | ||||
| Line voltage and installation type defined | Voltage level and mounting style drive insulation coordination, sensing method and enclosure design; unclear scope makes lifetime claims unreliable. | Example: “12–24 kV overhead line, pole-mounted” or “Medium-voltage cable pit / RMU internal panel” explicitly stated in the specification. | HV sensing elements, insulation barriers, enclosure insulation system and mechanical mounting hardware families. | |
| Environmental category and IP rating stated | IP rating, pollution degree and environmental category determine sealing, material choices and corrosion protection for a decade or more of exposure. | Example: “Outdoor pole-mount, pollution degree III, IP66 or IP67; salt spray rating for coastal deployment where needed”. | Housing material and coatings, gasket system, venting components and fastener material specifications. | |
| Temperature range aligned with battery chemistry | Battery internal resistance and usable capacity vary strongly with temperature; mismatch between rating and chemistry shortens lifetime or limits radio operation in cold weather. | Example: “−40…+70 °C operation; Li-SOCl₂ battery derating curves for low temperature are documented and used in power budget”. | Li-SOCl₂ pack, energy budget calculations, BMS thresholds, radio TX current limits at low temperature. | |
| Fault sensing & trigger chain | ||||
| Sensing method and location clearly defined | Sensing technology and mechanical placement determine sensitivity, selectivity and integration effort with the conductor or cable system. | Example: “Split-core CT around cable”, “Magnetic pickup on overhead conductor” or “Hall sensor in molded core” with installation details provided. | CT or magnetic SoC, Hall current sensor IC, front-end amplifier, comparators and mechanical interface to the line or cable. | |
| Fault and load current ranges covered without saturation | The sensing chain must handle minimum load currents yet avoid saturation or damage at maximum fault levels to maintain reliable indication and logging. | Example: “Rated load 100–400 A, fault 2–10 kA; CT and front-end verified not to saturate at maximum fault current”. | CT core selection, burden network, clamp circuitry, input protection components and ADC / comparator input range planning. | |
| Trigger thresholds and debounce timing specified | Thresholds and timing windows affect discrimination between inrush, overload and true faults; unclear settings lead to nuisance or missed indications. | Example: “Trigger set at 2–3 × In for 1–3 cycles; configurable through MCU parameters or factory options”. | Comparator ICs, gain stages, MCU firmware thresholds and any digitally programmable amplifiers used in the chain. | |
| Direction or phase discrimination (if required) | Direction-aware sectionalizers need reliable discrimination between forward and reverse faults to avoid incorrect operations under back-feed conditions. | Example: “Directional detection via dual sensors or phase comparison; validated with back-feed and reclose scenarios in test reports”. | Additional sensing channels, phase measurement algorithms in the MCU and supporting ADC / amplifier resources. | |
| Fail-safe indication on sensing chain fault | Sensor or front-end failure must not be mistaken for healthy operation; clear diagnostics avoid dangerous false confidence in the device. | Example: “Self-test routines and sensor health flags; distinct indication or status code for sensing failure”. | MCU diagnostics, sensor self-test pins, watchdog logic and error logging entries. | |
| MCU, logging & time base | ||||
| Deep-sleep current including RTC within budget | Sleep current dominates lifetime consumption; an oversized baseline current makes a ten-year specification unrealistic even with harvesting support. | Example: “Total sleep current ≤ 5 µA at nominal voltage, including MCU, RTC and leakage paths; verified across temperature”. | ULP MCU selection, RTC implementation, leakage control in GPIOs, regulators and protection networks. | |
| Non-volatile event log with adequate endurance | Event logging must survive the full lifetime without wearing out memory cells or corrupting records during repeated fault sequences. | Example: “FRAM or high-endurance EEPROM; circular buffer with 100+ records; write endurance ≥ 10⁶–10⁷ cycles on event area”. | FRAM / EEPROM device, MCU NVM driver, event record structure and wear-levelling strategy if needed. | |
| Timestamp resolution and drift characterised | Time stamping enables correlation between FPI events and upstream relay actions; poor drift or resolution reduces diagnostic value for fault analysis. | Example: “Timestamp resolution ≤ 1 s; drift ≤ ±50 ppm, or periodic time synchronisation via gateway or control center”. | RTC block, 32.768 kHz crystal or calibrated RC, time-sync protocol implementation in communication stack. | |
| Firmware update and configuration pathway defined | Access to configuration and firmware updates allows correction of issues and tuning of thresholds without replacing field units. | Example: “Local service port or over-the-air via gateway; secure boot and signed images for update process”. | MCU bootloader, secure element or HSM, configuration storage strategy and access tools for maintenance staff. | |
| Power-up and brown-out behaviour documented | Uncontrolled resets and half-written records during voltage dips can corrupt logs or present incorrect indication to crews and control systems. | Example: “Brown-out reset threshold fixed; event logging completes before supply collapse; clear start-up state machine”. | PMIC reset circuits, MCU brown-out detector, firmware state management around power transitions. | |
| Energy source, supercapacitor & power budget | ||||
| Battery capacity and lifetime target linked by budget | Declared lifetime must be backed by a quantitative power budget that uses the actual cell type, capacity and temperature profile for the project. | Example: “Li-SOCl₂ 19 Ah, design lifetime 10+ years at ≤ 5 µA average, margin included for ageing and low-temperature effects”. | Battery pack selection, power budget calculation, duty-cycle assumptions for sensing, logging and communication functions. | |
| Supercapacitor sized for peak load profile | Supercapacitors must hold enough energy for worst-case sequences of radio bursts and indication actions without forcing the battery into damaging peak currents. | Example: “Supercap supports multiple alarm transmissions and indicator events under cold-start conditions before recharge from line current”. | Supercapacitor bank, DC/DC converter, charge-limiter networks and peak-load modelling for communication and actuators. | |
| Harvested energy contribution quantified (if used) | CT or PV harvesting should be treated as a measurable reduction in net battery drain, not as an undefined bonus that hides a weak power budget. | Example: “CT or PV harvesting yields x mAh/year under typical feeder loading, reducing battery consumption by y % in the budget”. | CT / PV front-end, rectifier or converter ICs, charging path into supercap or battery and associated control logic. | |
| Documented power budget table available | A structured power budget with separate entries for each function is essential for verifying lifetime claims and negotiating design trade-offs with suppliers. | Example: “Power budget table listing sleep, sensing, logging and communication currents, duty cycles and total annual drain vs capacity”. | System-level documentation, spreadsheet or design note; parameters in MCU firmware, radio configuration and timer settings. | |
| Communication link & reporting strategy | ||||
| Primary link type and coverage defined | Communication technology and coverage assumptions decide whether the device suits remote, semi-urban or substation environments and what infrastructure is required. | Example: “LoRa to local concentrator”, “NB-IoT / LTE-M to cloud”, or “RS-485 / dry contact into existing FTU” with target range and gateway role described. | NB-IoT / LTE-M modem, LoRa or sub-GHz SoC, RS-485 transceiver, optocoupler inputs and associated protocol stacks. | |
| Reporting policy vs battery life analysed | Heartbeat and alarm behaviour directly drive radio energy consumption; without clear limits the average current quickly exceeds budget for long life. | Example: “Monthly heartbeat in normal state; immediate alarm on fault with limited retries; policy included in power budget assumptions”. | MCU firmware, modem power-control logic, retry and back-off algorithms in the communication stack configuration. | |
| Behaviour when communication link unavailable defined | Loss of connectivity must not lead to uncontrolled retries or silent failure; the device should preserve events and show a clear comms status to technicians. | Example: “Events buffered locally; indication for communication lost; retry window limited to prevent excessive battery drain”. | MCU state machine, modem watchdog handling, FRAM log design and any comms-status indicators or flags. | |
| Integration options with FTU / IED documented | Local integration with existing RTUs, FTUs or IEDs can reduce network complexity and provide additional supervision channels for the device. | Example: “Dry-contact output mapped to FPI trip state” or “Modbus / simple frame over RS-485 with documented register map”. | Optocouplers or relay drivers, RS-485 transceivers, IO-Link or other short-range interface IP blocks and documentation of mapping to system signals. | |
| Enclosure, mounting & serviceability | ||||
| IP, mechanical strength and corrosion performance specified | Mechanical robustness and corrosion resistance determine whether the device survives wind, vibration, pollution and salt exposure over the target life. | Example: “IP66 / IP67 housing, UV-stable plastic or coated metal, salt spray rating suitable for coastal or industrial use as specified”. | Enclosure materials, coating systems, gasket design and mechanical testing or certification reports referenced in documentation. | |
| Separation of HV sensing and LV compartments defined | Clear separation gives safer maintenance, reduces contamination risk and simplifies insulation coordination for certification and utility approvals. | Example: “Dedicated low-voltage battery bay and electronics area accessible without exposing high-voltage sensing components or terminals”. | Mechanical partitions, creepage and clearance design, terminal arrangement and access procedures described in the mechanical section. | |
| Mounting options and viewing geometry documented | Crews must be able to read indication safely and quickly from typical working positions; mounting also affects mechanical loads and sensor coupling geometry. | Example: “Pole-mount brackets included; indication visible from ground at specified distance; cable entry and strain relief orientations shown”. | Mounting kits, indicator window placement, mechanical drawings and installation guidelines for overhead, pit and RMU installations. | |
| Condensation and venting concept described | Moisture build-up inside the enclosure can lead to corrosion, leakage currents and premature battery or PCB failure if not managed correctly. | Example: “Breather vent location and rating specified; drainage paths avoid direct water flow into low-voltage compartments”. | Venting components, seal design, internal layout around battery and electronics and any anti-condensation treatments used. | |
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FAQs about sectionalizers and fault passage indicators
These questions reflect the topics that usually come up when utilities review sectionalizer and fault passage indicator designs. The answers focus on practical selection and integration hints, so a project team can check whether a proposed device really matches feeder conditions, lifetime expectations and communication or mechanical constraints.
Each answer can be read on its own, but together they form a compact guide to applying the checklist and design considerations discussed in the rest of this page.
