Protection Feature Application Summary

While the vast majority of users will only apply Overcurrent, Earth Fault and Sensitive Earth Fault (Ground fault to our US users) features to protect their distribution networks, we often receive questions regarding the applications for the many other features on offer. This exploration of features can highlight the shortcomings of existing protection schemes which may be addressed by enabling existing features within the controller.

Our protection feature summary diagram has become one of the most accessed images in our documentation. While it summarizes the NOJA Power RC’s capabilities, it only enumerates the features. The application of these features is assumed knowledge. To address this knowledge assumption gap, below is a table of all the protection functions in NOJA Power’s RC Control System, and their key applications. That is, why these features are used in practice.


Feature Name ANSI Code Applications


50P, 51P

Detecting Overloads, currents exceeding equipment ratings

Earth Fault

50G, 51G

Detecting a failure of insulation in a circuit, causing a connection to earth. Reduced efficacy in non-effectively earthed networks

Sensitive Earth Fault 50SG, 51SG Detects High Impedance earth faults and insulation failure
Negative Sequence Current 50Q, 51Q Detect Current Imbalance, such as broken conductors or wiring errors.
Broken Conductor 46BC Exploits the ratio between Positive Sequence and Negative Sequence current, allowing detection of broken conductors without needing units of impedance
High Z Fault Detection Not ANSI Defined Detects Very High Impedance earth faults and earth faults on resonant/isolated neutral networks
Neutral Admittance 21YN Detects earth faults on isolated neutral networks.
Fault Locator 21FL Provides location estimates of overcurrent faults in distance from OSM Recloser
Harmonic Relay Not ANSI Defined Detects failing inverter equipment and industrial/generation connections violating connection code
Directional Overcurrent 67P Provides selectivity of overcurrent protection in scenarios of varied load flow
Directional Earth Fault/SEF 67G, 67SG Provides selectivity against sympathetic capacitive earth fault currents in high capacitance networks.
Directional Power 32 Provides anti-motoring and anti-generating features for distribution network connected rotating machines.
Reclosing 79 Provides Automatic Circuit Reclosing to improve network reliability. Ensures quantity of fault current applied to clear faults remains within equipment nameplate constraints. Exploits fault statistics that indicate 80% of faults are momentary and do not necessitate outages.
Circuit Breaker Failure 50BF Provides a breaker state logical output for substation interlock schemes. Used for inter-trip schemes on critical protection path
Under Voltage 27 Protects against brownout conditions, and can be used to prevent back-feed of power in distributed automation schemes. Can also be used for load shedding
Over Voltage 59P Protects against equipment damage from voltage exceeding nameplate ratings
Vector Shift 78VS Applied for non-synchronous generation connection and anti-islanding.
Neutral Overvoltage 59N Used for Earth Fault detection on isolated neutrals. Generally applied in conjunction with current elements such as SEF
Over Frequency 81O Used to detect excess generation
Under Frequency 81U Used to detect generation shortfalls and islanding
ROCOF 81R Addresses the non-detection zone of OF and UF by acting on the rate of change of frequency. Used for distributed generation connection and anti-islanding protection
Sync Check 25 Interlock feature mandating synchronism of load and source side before a close command can be issued
Auto-Synchroniser 25A An automation of Sync Check. Initiation of a close command starts a timer, and a close will occur when synchronization criteria are satisfied. If the timer expires, the close command is discarded

All these protection features are available in NOJA Power’s RC10, RC15 and RC20 controllers.

“Modern reclosers offer far more than the original ball and chain solutions developed in the 1930s and we are proud to be leading the way in terms of functionality available today,” reports NOJA Power Group Managing Director Neil O’Sullivan.


1930’s era Recloser with Ball and Chain Hydraulic Mechanism

1930’s era Recloser with Ball and Chain Hydraulic Mechanism


NOJA Power’s OSM Recloser with RC series control has over 70,000 installations in 95 countries, in varied applications from feeder protection to renewable energy connection. If you have a medium voltage project in mind, visit or contact your local NOJA Power Distributor.