NOJA Power

Technical Article

Published 08/2023

Three Ways to Reduce Fire Risk with the OSM Recloser System

NOJA Power OSM Recloser installation in Victoria, Australia

Fire ignition is a possible outcome from various fault occurrences on the overhead distribution network. However, applying features of the NOJA Power OSM Recloser system can reduce the risk of ignition during faults.

In this article, we explore a few standard features of the NOJA Power OSM Recloser system that can reduce fire ignition risk.

1. Sensitive Earth Fault Protection

Sensitive Earth Fault protection is used to detect breakdowns in insulation or high impedance materials coming into contact with overhead lines.

For example, a failing pole top insulator on a timber pole could cause electrical discharge to earth on the pole cross arm. This fault would likely be a small current, since the impedance of the entire timber pole is reducing the current. This energy over time leads to fires on pole tops, which wouldn’t be detected by other protection features.

NOJA Power’s OSM Recloser system can detect 1 A primary SEF current in a standard Recloser build option, and 200 mA primary in the High Resolution SEF protection build. When combined with the OSM Recloser’s In Cos Phi SEF detection algorithm, the system works to detect real power being delivered to fault sites, rather than capacitive effects that lead to nuisance tripping.

Aside from detecting these high impedance faults, high resolution SEF protection allows the NOJA Power OSM Recloser to be applied in Resonant Earthing and Isolated Neutral distribution systems with greater fault detection capability.

NOJA Power OSM Recloser installation in Victoria, Australia
NOJA Power OSM Recloser installation in Victoria, Australia

2. Broken Conductor Detection

Failing spans of powerlines may leave lines ungrounded, hanging from the pole and swinging in the wind. Alternatively, these broken cables may land on insulating surfaces, such as road tarmac or bitumen. With a high resistance to earth, these faults go undetected by overcurrent or earth fault protection. Fortunately, NOJA Power’s OSM Recloser system has an integrated Broken Conductor Detection capability.

The broken conductor protection feature of the OSM Recloser uses the ANSI 46BC detection method, dividing the measured Negative Sequence Current by the positive sequence current. Negative sequence current is a symmetrical component derived from the three phase current measurement, which is indicative of line imbalance. When a line breaks, the broken phase current goes to zero. It doesn’t introduce an overcurrent, but this imbalance is easily detected through the vector sum of all phases.

The ratio of Negative to Positive sequence current is near zero on most healthy feeders loaded above 10 Amperes. A phase discontinuity causes this ratio to exceed 20%, indicating the broken conductor without needing overcurrent presence.

The ANSI 46 BC feature is standard on all NOJA Power OSM Reclosers, and has customisable responses including a SCADA Alarm or device lockout, and a minimum current for activation to prevent spurious tripping on low loads.

NOJA Power OSM Recloser installation in Victoria, Australia
NOJA Power OSM Recloser installation in Victoria, Australia

3. Arc Fault Venting

Arc faults occur when an insulator failures cause a catastrophic internal fault in electrical apparatus. This may occur due to overloading, lightning strike, or adverse transport and installation practices. However, if an Arc Fault occurs, the equipment can be designed to safely vent the explosion away from operators.

NOJA Power’s OSM Recloser system has a type tested arc fault venting capability, where any internal arc fault results in the safely controlled venting of gas and electrical fault by-products up and away from operators, local flammable materials and the general public. Equipment without this feature that experiences an internal fault to ground would experience a catastrophic explosion.

NOJA Power’s OSM Reclosers are designed to handle this fault case by minimising the risk of fire or injury through type tested arc fault venting.


Fire risk mitigation is an important but complex engineering discipline. However, standard features in the NOJA Power OSM Recloser can provide risk mitigation for operators of overhead electricity distribution networks.

“Overhead power distribution networks are essential to maintain the livelihoods we enjoy today,” says NOJA Power Group Managing Director Neil O’Sullivan, “and they are exposed to the elements. Our electricity utility customers can improve the protection systems in place to help reduce bush fire risk using our recloser products.” says NOJA Power Group Managing Director Neil O'Sullivan.

For more information, visit or contact your local NOJA Power Distributor.

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