Arc-Flash Protection: Key Considerations for Selecting an Arc-Flash Relay
Arc-Flash Relays are an effective defense against dangerous Arc-Flash events, and the decision to include such a relay in a design is an easy one. Less easy, however, is selecting the optimal relay for an application. According to OSHA, industrial Arc-Flash events cause about 80% of electrically related accidents and fatalities among qualified electrical workers. Even if personnel injuries are avoided, Arc Flash can destroy equipment, resulting in costly replacement and downtime. In response, many designers are adding Arc-Flash relays to electrical systems. These devices greatly mitigate the effects of an Arc Flash by detecting a developing incident and sending a trip signal to a breaker to disconnect the current that feeds it. Arc-Flash Relays are complex devices; an understanding of the technical details of their operation and features is essential. This white paper covers key points of Arc-Flash relay technology so that specifying engineers, OEM designers, and end users can make an informed selection decision.
T. Locker, J. Seedorff Oct 2011
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Ground-Fault Protection and Ground-Conductor Monitoring for Portable Mine-Power Cables
Resistance grounding offers many of the advantages of both solidly grounded and ungrounded systems. This paper describes a practical approach to the selection of a grounding resistor let-through current and the operating value of the ground-fault relays. When portable line-power cables are used, ground conductors must be monitored. It is suggested that ground-conductor monitors should be resistance sensitive and that the resistance trip-level should be determined by the allowable ground-fault voltage and the operating value of the ground-fault relays.
G.E. Paulson, J.J. Dudiak 1995
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Ground-Fault Protection for Solar Applications
Grid-connected commercial photovoltaic (PV) systems are trending towards larger sizes, resulting in systems with higher bus voltage and current levels. Problems caused by ground faults are becoming a bigger concern due to increased energy available at the point of fault; arc-flash and shock hazards, equipment damage, and fires can result.
Tyler Klassen, P.Eng.
February 2012
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Ground-Fault Protection with Variable-Frequency Drives
Ground faults can be dangerous. A short to ground in a solidly grounded system creates large currents that can damage equipment and cause a shutdown. A ground fault can also create an arc flash, which can cause serious injury to nearby personnel as well as damage to equipment. On top of that, an arc flash may not draw enough current to rapidly trip an overcurrent protective device. One way to reduce or eliminate many ground-fault problems, including most arc-flash incidents, is to use a high-resistance-grounded (HRG) system, in which the neutral point of the transformer (either the X0 of a wye- connected transformer or generator, or the artificial neutral of a zigzag transformer) is connected to ground through a neutral-grounding resistor (NGR). Even though high-resistance-grounding offers great benefits in improving safety, there are some important things to consider, including a number of factors that can make it difficult to detect low-level ground faults. Several of these are exacerbated by the use of variable-frequency drives (VFDs). This paper will explain some aspects of how HRG systems work, examine the various challenges with monitoring ground faults in HRG systems, and show how technology can resolve those challenges.
M.J. Savostianik, P.Eng.
February 2012
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Lowering the Limits for Earth-Fault Detection
Current flowing to earth has only two paths—it can flow to earth through an earth fault, and it can flow to earth through distributed capacitance. Current flowing to earth through distributed capacitance can cause sympathetic tripping during an earth fault and it can cause nuisance tripping during normal operation. If the earth-fault trip level is high enough to eliminate sympathetic tripping, nuisance tripping due to unbalanced and harmonic capacitive current is usually not a problem. However, if sympathetic tripping is not a concern and earth-fault trip levels are lowered, nuisance tripping can become a problem that worsens with the increased use of adjustable-speed drives. This paper discusses the sources of current flowing to earth that are not the result of an earth fault, and it shows how a digital filter tuned to the fundamental component of earthfault current can provide lower trip levels without nuisance tripping.
M.J. Savostianik
May 2000
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Monitoring Neutral-Grounding-Resistors - An Update
Many of the problems associated with ungrounded and solidly grounded distribution and utilization systems are overcome with resistance grounding. Resistance grounding can limit point-of-fault damage, eliminate transient overvoltages, reduce the flash hazard, limit voltage exposure to personnel, and provide adequate tripping levels for selective ground-fault detection and coordination. Charging current, ground-fault detection, and ground-fault coordination are reviewed. Reasons for monitoring the neutral-grounding resistor (NGR) are presented. Problems associated with NGR monitoring are discussed and monitor design requirements are summarized. These design requirements, and two decades of experience, guided development of two generations of NGR monitors which detect both resistor faults and ground faults.
G.E. Paulson, M.J. Savostianik
July 2003
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Motor Selection for Belt-Conveyor Drives
Rated power is the motor parameter always specified when motors are selected for a belt conveyor—motor slip is usually ignored. This paper shows how the running and starting characteristics of a belt conveyor are influenced by slip. It shows that high-slip motors improve load sharing between directly coupled motors, and it shows that high-slip motors reduce the effect of belt stretch to improve load sharing between belt-coupled drums. The interaction between stretch and slip is illustrated graphically to show the percentage of connected power available to a conveyor without overloading the motor(s) driving the secondary drum. If the power requirement for the conveyor has been determined correctly and if the power available is inadequate, the stretch-to-slip ratio is too high—probably the result of an inadvertent selection of high-efficiency motors with low slip and poor starting characteristics. With these motors, mechanical devices that introduce slip are required if the conveyor is to operate near design capacity. A preferable solution is to avoid the problem by using directly coupled high-slip motors to improve load sharing and increase starting torque. Examples are given of two motors that eliminate the need to introduce slip mechanically.
G.E. Paulson
September 1998
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