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Switchgear for DC:: Extinguish arc using contacts in series AC contactor can have longer life-expectancy when switching DC if wired as shown,. This puts 4 sets of contacts on the positive wire. Obviously if all four contacts are disengaged at same moment, it will be more difficult for DC to arc across all four. Or use a 3-pole for better arc extinguish using six contact points Or use 4-pole contactor and 3 poles (six contact points) are used for positive wire, and 4th pole (2 contact points) is used on negative wire. As we can see from this example, DC electrons flow from positive to negative because we are trying to extinguish the arc on the positive wire. Resource: 3-pole contactors Difference between AC and DC How to set up solar array |
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| Electric current is caused
by electrons flowing through a conductor. When electricity is applied to wire, the energy from electricity causes the atoms to heat up. Atoms are made of 'negatively' charged electrons and 'positively' charged protons etc. Electrons rotate around the larger, heavier proton. When atom is heated by electric power, this causes electrons to move into different orbits located farther from the center proton. Protons are heavier and remain stationary relative to the electron. With conductive material, like copper for example, the electrons will easily free themselves from orbit around the neutron when heated. These are called Free Electrons. Free Electrons bounce to the next atom, where they bump more electrons free, and then those electrons bounce to surrounding atoms, and so on until there is a flow of electrons going down the wire. This is electric current. |
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| AC is alternating current. DC is direct current. Current is a measure of heat or power on circuit. The more current flowing along the wire, then the more power or heat is available to do the work of rotating motor shaft etc With AC, the current or flow of electrons reverses 60 times per second in US. This is because the generator rotates 60 times per second. Each rotation has North and South poles of a magnet pass next to a coil of wire. As the N and S poles pass the coil of wire, they reverse the flow of electrons on the wire. The current oscillates or reverses direction. The entire grid energized by the generator(s) DC is different. DC current flows one direction. It does not 'alternate' directions like AC |
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AC electricity oscillates. The voltage rises and falls. 60 times each sectond, the voltages drops to zero. Of course you notice this when grabbing a hot wire, because there is voltage on the line. There is a difference when choosing switchgear. Each time a switch is thrown, there is an arc (or spark) that continues to jump between the contact points of the switch. With AC power, the arc is 'extinguished' more easily because voltage reaches zero at some point and the arc stops. With DC power, the arc is more difficult to extinguish. DC never reaches zero volts, because it doesn't change direction... it doesn't oscillate.... and so electons keep piling against one another, forcing the front ones to jump across the switch contacts. So switchgear must be rated for DC. Using AC switchgear will sometimes work, BUT carful attention must be paid to rating label. Each switch has a rating label or rating specifications. For example a switch might be rated 240 volt 30 amp AC, but only rated 48 volt 3 amp DC. If a device does not show DC rating, then assume it is not sufficient for DC application, and might be fire hazard. Resource: DC arc extinction |
| "the higher is the number
of contacts opening the
circuit, the higher is the breaking capacity of the circuit-breaker. This means that, when the voltage rises, it is necessary to increase the number of current interruptions in series, so that a rise in the arc voltage is obtained and consequently a number of poles for breaking operation suitable to the fault level." So a DC circuit breaker or any DC switchgear must have multiple contacts that disconnect as illustrated in diagram below Resource: DC arc extinction |
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600 volt AC @ 30 amp fuse/ 300 volt DC @ 15 amp 600 volt AC @ 600 amp/ 120 volt DC @ 60 amp 6000 volt AC @ 6000 amp/ 300 volt DC 2000 amp Time delay fuse passes surge currents without sacrifice short circuit limitation AC rating for 200 kA is reduced to 20 kA for DC kA is max surge capability... if too much surge is on the fuse, the fuse will not trip and fuse can melt onto the contacts, feeding high voltage into the wire causing possible fire |
| "The
value of the kA rating determines how much current the circuit breaker
can withstand under fault conditions. The circuit breaker only has to
withstand this for a brief period of time, usually the time it takes
for the circuit breaker to trip. For example, a value of 6kA means that
the circuit breaker can withstand 6,000 amps of current during the
brief time it takes to trip. Why is the kA rating so important? Under fault conditions (such as a short circuit) much more current flows through the circuit than what it was designed for. A circuit that was designed for a maximum of 20A may suddenly be drawing hundreds, if not thousands of amps. The circuit breaker will trip if this occurs. However, what if during a short circuit there is more current flowing through the circuit than the kA rating of the circuit breaker? In this case the circuit breaker will fail, often in either one of two ways. One possibility is that the contacts in the circuit breaker will weld, thus preventing the circuit breaker from tripping. The best case scenario for this is that the cable in the circuit is damaged. The worst case is that a fire is started. Another possibility is that the circuit breaker explodes, as a result of the copper in the circuit breaker overheating and turning into dangerous plasma. This could be very dangerous to people nearby, for example the electrician turning the circuit breaker on after a fault. Conclusion The kA rating of a circuit breaker is a very important safety aspect to consider when designing a circuit. Without it, there is a good chance that a serious accident will occur. It only takes a few minutes to do the calculations when you have the correct tables. The author has personally seen an incident where an incorrect kA rating caused a man to receive burns to 60% of his body. It could happen to you. Be safe and always consider the kA ratings in your design." AC rating for 200 kA is reduced to 20 kA for DC kA is max surge capability... if too much surge is on the fuse, the fuse will not trip and fuse can melt onto the contacts, feeding high voltage into the wire causing possible fire Resources: http://mandrelectrical.com.au/karatings.php http://www.cooperindustries.com/content/ |
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