DC Arc Flash

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Safety is paramount within all electrical installations. Safety regulations are put in place by international bodies such as the Occupational Safety and Health Administration (OSHA). One of the factors that endanger the safety of personnel in industries is DC arc flashes. These pose a threat both to workers and equipment leading to the temporary closure of operations to give room for repairs to be made after such an incident occurs.

Our engineers at Mechatronica Engineering have wide experience in conducting DC arc flash studies using analytical techniques as recommended by NFPA 70E and CSA C22.1. With batteries being the main source of fault currents in DC systems, our battery performance studies allow more accurate simulation of batteries under fault conditions and thus obtaining more accurate results of their fault current contribution. 

Definition of an Arc Flash

Arc flashes occur during electrical failures such as wearing out of the insulation, which lead to the flow of electric energy between two conductors. The instantaneous flow of energy occurs in the form of light flash, massive heat waves and explosions which can lead to the evaporation of metallic materials, damaging of electrical and electronic equipment, and a danger to any person who is not at a safe distance. Often, the arc flashes take place unexpectedly.

Definition of an Arc Flash

Calculating DC Arc Flash Parameters

The basic concept for calculating DC arc flash parameters due to a fault at a certain location in the DC network, is to reduce the network to its Thevenin’s equivalent circuit. The bolted fault current is the current passing through the terminals of the Thevenin’s circuit when they are short-circuited.

Arc Flash
The most challenging part in calculating DC arc flash parameters is in the calculation of the arc gap resistance and arcing current. NFPA 70E and CSA C22.1 refer to two methods presented by two different IEEE transaction papers;

1. Maximum Power Method:

This method is based on the maximum power transfer theory that is the maximum arcing current will flow if the arc gap resistance is equal to the Thevenin’s equivalent resistance.

Although this method is known to be conservative as it gives the highest possible arcing current, the resulting incident energy may sometimes be less than the actual due to the short fault clearing time associated with high arcing current.

2. Stokes and Oppenlander Method:

Exhaustive study was done on free-burning vertical and horizontal arcs between electrodes in open air. The study provided suggestion for calculating arc resistance and arcing current through the iterative solution of two equations. The study also suggested corrective formulas for the calculated open air incident energy in case that the arc is enclosed.

This method provide more realistic results for arcing current and therefore is more practical to be used.

Mitigating the Impact of DC Arc Flashes

It is possible to reduce the effects of the arc flashes when they take place. Electric devices such as arc-flash relays as well as circuit breakers, fuses and other fault interrupting devices can be used to detect when the arc flash is about to occur and hence reducing the clearing time and the quantity of energy dissipated when such an electric failure takes place.

1. Arc-flash Relays

The released energy is dependent on time and current. The time factor can be minimized by the installation of arc-flash relays which can detect an electrical arc flash instantly.

The process of developing arc flash starts with light emission. Therefore, if light is detected, an early detection of arc flash can be achieved.

Optical arc flash relays are used to trip protective breakers that are triggered when light is detected.

2. Circuit Breakers and Fuses

Circuit breakers have trip components which could be thermal, magnetic or electronic. These trip components can trip instantaneously at around 15 times the rated continuous current. Fuses comprise of a wire that can melt when the current that exceeds their rated current flows through the circuit. When selecting a fuse, the voltage and interrupting amperage are essential factors to consider. The voltage rating of the chosen fuse should be equal or higher than the system voltage, whereas the interrupting current must be equal to the short-circuit current.

Overall, DC arc flashes pose a high risk to both personnel and equipment in an industry set up. It is essential to put in place control measures such as the use of arc-flash relays, circuit breakers and fuses which minimize the impact of electrical faults to save lives and reduce the cost of repairing or purchasing new industrial equipment.

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