US What FLISR is
FLISR (fault location, isolation and service restoration) is a distribution automation function that responds to a fault on a power line automatically. When a fault hits a distribution feeder, the protection trips and everyone downstream can lose supply. FLISR uses data from devices along the feeder to work out which section the fault is in, opens the switches either side of that section to isolate it, and then closes other switches to feed the healthy sections, sometimes from a neighboring feeder.
The result is that most customers are back on supply within a minute or a few minutes, while crews repair only the short section that is still off. The same idea is also called FDIR (fault detection, isolation and restoration) or, more loosely, a self-healing grid. FLISR is the best-known use of feeder automation, which covers the wider set of devices and schemes on an automated line.
How FLISR works, step by step
The example below follows a fault on a feeder that has three remote-controlled switching points and a normally open tie switch to a second feeder. The timings are indicative and depend on the scheme and its settings.
| Step | What happens | Devices involved | Typical time |
|---|---|---|---|
| 1. Detect | Fault current flows; the substation breaker or first recloser trips and may reclose to test for a temporary fault | Breaker or recloser and its protection relay | Under a second to a few seconds |
| 2. Locate | Controllers report which switching points saw fault current; the fault lies between the last one that did and the first one that did not | Fault passage indicators, sensors, controllers | Seconds |
| 3. Isolate | The switches either side of the faulted section open | Remote-controlled switches or sectionalisers | Seconds to a minute |
| 4. Restore upstream | The breaker or recloser closes to re-energise the healthy sections between the substation and the fault | Breaker or recloser | Seconds to a minute |
| 5. Restore downstream | The normally open tie switch closes to feed the sections beyond the fault from the neighboring feeder, after checking it has capacity | Tie switch, DMS or peer controllers | Around one to a few minutes |
| 6. Repair | Crews fix the isolated section; the network is then returned to its normal layout | Field crew, control room | Hours, with most customers already on |
What a FLISR scheme needs
FLISR can only act on what it can see and switch, so the feeder needs the right equipment at enough points along it:
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Sectionalising points. Reclosers or remote-controlled switches that split the feeder into sections. More points mean smaller sections and fewer customers left off.
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Alternative supplies. Normally open tie points to neighboring feeders with spare capacity, so sections beyond the fault can be fed from elsewhere.
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Fault sensing. Fault passage indicators or controllers that measure current, so the scheme can tell which side of each switch the fault is on.
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Communications. Radio, cellular or fiber links fast and reliable enough to report faults and carry switching commands.
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Decision logic. Either a distribution management system (DMS or ADMS) in the control room, or controllers that share data and decide among themselves.
Centralized and distributed FLISR
In centralized FLISR, software in the control room collects data from every device, works out the switching plan and sends the commands. It sees the whole network, so it can choose the best restoration path, but it depends on every communication link working. In distributed FLISR, controllers along the feeder share data peer to peer and act on their own, which is faster and keeps working if the control room link drops, but each scheme covers a limited area. Many operators run both, with distributed schemes on critical feeders and central software overseeing the rest.
Benefits and challenges
Network operators are judged on how many customers lose supply and for how long. In the USA these are customer interruptions and customer minutes lost; elsewhere the usual measures are SAIFI and SAIDI. By restoring most customers in minutes rather than hours, FLISR improves both and reduces the number of trips crews make simply to operate switches.
The challenges are practical. Rooftop solar and other generation on the feeder can confuse fault location, so schemes need settings that account for it. Tie feeders must have capacity at the time of the fault, not only on paper. And because FLISR can operate switches remotely, the controllers and links need strong cyber security, with authenticated protocols and hardware such as a TPM holding the credentials.
Controllers at each switching point
FLISR depends on a reliable controller at every switching point, usually in a small cabinet on the pole beside the switch. It has to run unattended for years, through frost, sun and electrical noise, and restart cleanly after an outage. Fanless DIN rail PCs with serial and Ethernet ports and a cellular modem suit the job. Our distribution automation case study covers the fanless computers that sit in those pole-top cabinets, and our white paper on designing outdoor computer enclosures covers the cabinet itself.
Quick answers on FLISR
FLISR: common questions
What does FLISR stand for?
Fault location, isolation and service restoration. The three parts describe the sequence: find the fault, cut it off, and bring everyone else back on.
Is FLISR the same as FDIR?
Essentially, yes. FDIR stands for fault detection, isolation and restoration. Different utilities and vendors use one name or the other for the same function.
How fast is FLISR?
Typically seconds to a few minutes. Distributed schemes can restore supply in under a minute; centralized schemes usually take one to a few minutes, depending on communications and settings.
Does FLISR prevent power cuts?
No, it shortens them. Customers on the faulted section stay off until it is repaired, and others may see a brief interruption, but most are restored far sooner than with manual switching.
What is ADMS?
Advanced distribution management system. It is the control room software that models the network and often runs centralized FLISR alongside outage management and voltage control.
We supply fanless DIN rail and wide temperature controllers for FLISR schemes with the serial, Ethernet and cellular modules fitted as standard, and test each system before it ships. Tell us the switchgear, protocols and communications, and we will specify the build to match.