Frequently Asked Questions

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A protection relay is a device that receives inputs typically from current transformers CT’s (but can be other sources) and then compares these inputs to set levels. If these inputs exceed the set levels a protection relay can then provide that information via display or indication, feed that information to other electronic control devices or it can be connected to a circuit breaker to isolate power.

An arc fault is a type of electrical fault that results from the breakdown of an insulating medium between two conductors where the energy is enough to sustain an arc across the insulator (often air) and can cause extreme amounts of light (arc flash), immense heat upwards of 19,000 degrees, and a resulting explosive pressure wave (arc blast). By using a relay that has optical sensors we can detect the flash of light the emanates from an Arc Flash and within 1 msec send a signal to a breaker to interrupt power. Because of the immense energy being released by an Arc Flash reaction time is critical. It can be seen on the graph that with a relay reaction time of only 1 msec (+ circuit breaker reaction time) we can interrupt power before the Arc Flash can result in critical damage.

An arc-flash happens when electric current flows through an air gap between conductors. There are several ways this can happen but the most common occurrence is when conductor insulation is worn or broken down.

Arc-flash relays are a necessary component in an electrical cabinet because they reduce the amount of incident energy in the system. While an arc-flash relay cannot prevent an arc flash from happening, it will protect critical assets and workers’ safety by reducing the severity of the flash. An arc-flash relay is an integral part of an arc-flash protection scheme that can minimize damage, and save money, time, and lives.

A phase-to-phase fault on a 480-volt system with 20,000 amperes of fault current provides 9,600,000 watts of power. If there is no arc protection and the fault lasts for 200 milliseconds before the overcurrent protection clears it, then nearly 2 megajoules of energy will be released, which corresponds roughly to a stick of dynamite.


The energy formula for an arc flash is:

Energy = voltage × current × time

480 V × 20,000 A × 0.2 s = 1,920,000 J


For a given system voltage, two factors can be adjusted to reduce arc-flash energy: time and current. Time can be reduced by using an arc-flash relay, such as the PGR-8800 or AF0500, to rapidly detect an electric arc flash.

The most important aspects of arc-flash relays are:


  • reaction time
  • trip reliability
  • ease of installation
  • sensor flexibility
  • software
  • sensor design
  • avoidance of nuisance tripping
  • scalability


To gain a deeper understanding of which features to consider when selecting an arc-flash relay please refer to the downloadable application guides.

Arcing and electric arc flashes are uncontrolled, intense, luminous discharges of electrical energy that occur when electric current flows across what is normally an insulating medium. Arc-flash events are dangerous, and potentially fatal, to personnel. According to OSHA, industrial arc-flash events cause about 80% of electricity-related incidents and fatalities among qualified electrical workers. Even if personnel injuries are avoided, an arc flash can destroy equipment, resulting in costly replacement and downtime.

Three phase power systems are most often solidly earthed, however by using a Neutral Earthing Resistor (NER) a power system can be resistance earthed. This is done so that if an earth fault were to occur the amount of fault current will be limited by the NER resistance. One potential problem with a resistance earthed system is; what happens if the NER goes open circuit due to damage? If this were to happen the power system has now changed from a resistance earthed system to a system with no earth reference (floating). Now the NER is no longer restricting the amount of fault current and other devices that rely on an earth reference in order to operate, cannot do so. By using a NER monitoring relay the resistance and integrity of a NER can be continually monitored to detect for any variance in the NER resistance or integrity.

An earth fault can occur as a result of inadvertent contact between and energized conductor and earth or equipment frame. There can also be earth leakage from a system without “inadvertent contact”. This type of fault can be called capacitive earth leakage and it is caused by a capacitive effect between a high power conductor and earth. Small amount of earth leakage current can occur as a result of this capacitive effect. Whether it is smaller amounts current from capacitive earth leakage or larger amounts of current from direct inadvertent contact an earth leakage relay can detect this fault current. In all electrical circuits, current returns to its source. An earth fault relay can use a zero sequence CT to monitor phase current to ensure that all current coming from the source returns on those same conductors or it can monitor current in the connection between transformer neutral and earth. By either method the earth leakage relay detect any lost current to earth.

Yes, currently Fuseco can offer both earth leakage and NER relays that are compliant to AS2081:2011. The relay manufacturer (Littelfuse) is also currently in the final stages of development for an AS2081:2011 compliant continuity relay that we expect to release in the near term.

Yes, all of our relays are covered by a 5 year warranty. Littelfuse relays are manufactured in the United States and undergo rigorous testing at time of manufacture, including a 72 hour “stress” test at 70 degrees C. By design and as a result of this testing they are able to operate effectively in extreme environments such as in the mining industry.

Yes, many of our relays come with various communications protocols to enable integration into SCADA systems. Please contact us for specific communications options.

Yes, in addition to the frequently asked questions in this section, we have provided a FAQ document prepared by Littelfuse that contains a lot of useful information. It can be downloaded from our Knowledge Centre section and is called 'Littelfuse_ProtectionRelay_Arc_Flash_FAQ'.

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Knowledge Centre

Littelfuse Protection Relay and Arc-Flash FAQ
Littelfuse White Paper -Key Considerations for selecting-Arc-Flash
Littelfuse Protection Relays AF0100 Generator Application Guide
Littelfuse PGR-8800 Application Guide
Littelfuse AF-0500 Application Guide
Littelfuse Diagram -Protect Your Wind Turbine From Arc-Flash hazards

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Newcrest WA

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Alpha Coal, Queensland

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