Arc Flash

Electrical hazard and Arc Flash Mitigation

Article 80 of D.Lgs 81/08 and the methodology of NFPA 70E and the IEEE 1584 algorithm

GT Engineering performs analyses of Electrical Hazard based on what is required by the Article 80 of D.Lgs 81/08. We consider both the hazard of direct and indirect contacts and the arc flash hazard, using the methodology of NFPA 70E and the IEEE 1584 algorithm.

Insights
Earth Fault Loop Impedance Measurement
NEMA CLASSES Vs ANSI Tripping Curves
NEC and the GFCI
BS 7671: an introduction
Grounding of Industrial Generators in DG Systems
Grounding and Bonding
The Earth Fault Loop includes the Load, the bonding conductor, the earthed (Grounded) neutral point of the tranformer, the source line winding and the live conductor from the source to the load. The earth fault loop impedance verification is performed to prove the correct operation of the protective device in order to protect in caso of indirect contats. The aim is to check that, in case of a ground fault, the related fault current is strong enough to cause the action of the protection device, breacking the power supply. In order to achieve a proper fault corrent value, the impedance of fhe fault loop should be lower than a specific value. According to EN 60204-1 standard, in case of TN system, the measurement should be performed using a loop tester, which must comply the IEC 61557-3. During the test the system should be under normal operating conditions, so each load should be energised. The instrument should be connected between the live conductor and the bondig conductor of the e
Overload relays typically operate on an inverse time curve where the tripping time becomes less as the current increases. They are rated by trip class. Trip class specifies the length of time it will take for the relay to open in an overload condition. In Europe the shape of the overload relays tripping curves are defined by ANSI or IEEE standards. A few examples are: ANSI Extremely Inverse, Very Inverse, Inverse, Short Time Inverse; or IEEE Moderately Inverse, Very Inverse and Extremely Inverse. In North America the NEMA Standard MG-1 defines 4 types of Classes as the most common: 5, 10, 20 & 30. Class 5, 10, 20 & 30 overload relays will trip within 5, 10, 20 & 30 seconds respectively at 600% of motor full load amps. Class 5 is usually used for motors requiring extremely fast tripping. Class 10 is commonly used to protect artificially cooled motors such as submersible pump motors of low thermal capacity. Class 20 is usually sufficient for general purpose applications. Class 30 i
A GFCI (Ground Fault Circuit Interrupter) is the American equivalent of the High Sensitivity Differential protection or "Salva Vita" in Italian. It detects if the current is flowing where it should not (ground fault) and shuts off the power. It triggers at 6mA (and should not trigger below 4mA). Touching a live conductor that is protected by a GFCI will still cause a shock, but the GFCI will open the circuit before it becomes lethal. They are used in the USA and Canada in bathrooms and kitchens at home, where they protect people from electric shock by quickly interrupting the circuit as soon as a ground fault is detected.  Simple and reliable, this technology is credited with cutting residential electrocutions in the USA in half since it was mandated by the NEC in the 70s.  A different story can be told about the workplace. From 1992-2002, there were 3,348 electrocutions on the job in the USA. Despite higher voltages in the workplace that increase the danger of shock, only recently h
BS 7671: 2008 is the National Standard for most electrical installation work undertaken in the United Kingdom. It is also adopted by a number of other Countries in the World. The first edition of BS 7671 was published in 1992 when the British Standards Institution (BSI) adopted the IEE Wiring Regulations, 16th Edition and made it a National Standard. The British Electrotechnical Committee (BEC) is part of the British Standard Institution (BSI) and is the UK member of the International Electrotechnical Commission (IEC) and the European Committee for Electrotechnical Standardisation (CENELEC). The purpose of IEC and CENELEC is to create commonality of Electrical Installation Standards throughout the World and Europe respectively. The standards thus created are known as Harmonised Documents (HDs). BS 7671 is the UK collection of HDs incorporated in a single publication with many other regulations that accommodate specific situations and practices applicable in the UK. A number of countri
Many existing and new industrial facilities include multiple generators operating on plant distribution buses at the medium-voltage level. The fault type to which stator windings are most often subjected is a short circuit to ground. Many incidents of severe damage to bus-connected generators from stator ground faults have been reported in recent years. It has been recognized by recent studies that the generator damage is caused more by the ground fault current contribution from the generator itself than from the system. The significant increase in such incidents has alerted users and insurers. Also, multiple grounding of sources will result in very high fault currents causing severe damage and coordination problems. Therefore, special attention must be given to generator grounding and ground fault protection. It should be noted that the method of ground fault protection is directly related to the method of system grounding used. There are many decisions, considerations and alternat
Currently the NEC in Article 100 defines the terms "ground" or "grounded" as "connected to the earth or to some conducting body that serves in place of the earth." Yet, the NEC often uses the term "ground" when it really means "bond" (connected to an effective ground-fault path to clear a fault) [250.2 and 250.4(A)(5)]. The two concepts have quite different meaning. Grounding - Grounding metal parts to the earth in premises wiring is only useful to provide a path for lightning, shunting high-frequency noise, or reducing static discharge. Bonding - Bonding all metal parts together and then to the system winding is done to provide a low-impedance path to the source (system) to facilitate the opening of the circuit-protection device to remove dangerous voltage on metal parts. In addition, bonding the system to metal parts (typically to the X0 terminal of a transformer) stabilizes the system voltage to the metal parts and it provides a zero system reference (to the metal parts). So far