Hi Guys,
I don't post here very often but this is a topic I'm interested in and have experience with. As an EE with some experience with switching 300 volts DC at 2500 amps for mining track rectifiers and also with distribution voltages up to 69KV, let me explain a bit about switching DC currents.
As someone above alluded to, switching DC current is problematic in comparison to AC due to the fact that DC current does not have a current zero every half cycle. The AC half cycle interruption allows the arc to extinguish for a short period and usually that's sufficient to allow the ionized gasses to cool and not conduct on the next half cycle. All you need is enough air gap between the switch contacts to develop enough dielectric strength to prevent a restrike of the arc at the begining of the next half cycle. The interruption is then complete.
DC does not do that. It continues to arc and ionized air is a very good conductor of electricity, although not as good as copper or aluminum. Since there is no current zero the arc continues and is sustained by the ionized air (or any other gas) between the contacts. To extinguish a DC arc a number of techniques are used and most of them involve increasing the distance the arc has to travel thus building dielectric stength sufficient to extinguish the arc. It could be compressed air as another poster mentioned but that is mostly used in large distribution AC circuit breakers. Sulphur hexaflouride (SF6) gas is used and so are vacuum interrupters for AC.
In most large DC contactors blow out coils are used to force the arc away from the large, heat resistant contact tips using a magnetic field to force the arc outward. The arc is then forced into a device called an arc chute that further lengthens and cools the arc until it is extinguished. These DC contactors are interesting to watch and look like arc welders until the arc is out, which usually happens in a few hundred milliseconds, as long as the current is below the interrupting capacity of the contactor. They can make some very interesting noises.
The movie posted above is also very interesting. That is an AC arc which has caused a fire since the ionized arcing byproducts are generated in large enough quantities that the arc can't be interrupted. Arcing faults like that are very dangerous since the current is somewhat limited by the electrical resistance of the arc but yet a large amount of energy is released, thus the fire. The current is usually below the trip point of protective relays and fuses so the arc just continues to burn. Eventually something happens and the breakers trip, but only after a lot of damage is done. I've seen entire ends of switchgear steel cabinets burned out by arcing faults.
So, the message is, be CAREFUL! Switching power levels like this can be dangerous. There are ways to do DC switching so talk to a friend that knows about it if you don't happen to. Semiconductors can do it if properly rated. Normally a small switch that is not designed for DC will not unless the current is very low, which generates a low intensity arc that is easier to interrupt.
Hope this is valuable.
Regards to all,
Ed Long
I don't post here very often but this is a topic I'm interested in and have experience with. As an EE with some experience with switching 300 volts DC at 2500 amps for mining track rectifiers and also with distribution voltages up to 69KV, let me explain a bit about switching DC currents.
As someone above alluded to, switching DC current is problematic in comparison to AC due to the fact that DC current does not have a current zero every half cycle. The AC half cycle interruption allows the arc to extinguish for a short period and usually that's sufficient to allow the ionized gasses to cool and not conduct on the next half cycle. All you need is enough air gap between the switch contacts to develop enough dielectric strength to prevent a restrike of the arc at the begining of the next half cycle. The interruption is then complete.
DC does not do that. It continues to arc and ionized air is a very good conductor of electricity, although not as good as copper or aluminum. Since there is no current zero the arc continues and is sustained by the ionized air (or any other gas) between the contacts. To extinguish a DC arc a number of techniques are used and most of them involve increasing the distance the arc has to travel thus building dielectric stength sufficient to extinguish the arc. It could be compressed air as another poster mentioned but that is mostly used in large distribution AC circuit breakers. Sulphur hexaflouride (SF6) gas is used and so are vacuum interrupters for AC.
In most large DC contactors blow out coils are used to force the arc away from the large, heat resistant contact tips using a magnetic field to force the arc outward. The arc is then forced into a device called an arc chute that further lengthens and cools the arc until it is extinguished. These DC contactors are interesting to watch and look like arc welders until the arc is out, which usually happens in a few hundred milliseconds, as long as the current is below the interrupting capacity of the contactor. They can make some very interesting noises.
The movie posted above is also very interesting. That is an AC arc which has caused a fire since the ionized arcing byproducts are generated in large enough quantities that the arc can't be interrupted. Arcing faults like that are very dangerous since the current is somewhat limited by the electrical resistance of the arc but yet a large amount of energy is released, thus the fire. The current is usually below the trip point of protective relays and fuses so the arc just continues to burn. Eventually something happens and the breakers trip, but only after a lot of damage is done. I've seen entire ends of switchgear steel cabinets burned out by arcing faults.
So, the message is, be CAREFUL! Switching power levels like this can be dangerous. There are ways to do DC switching so talk to a friend that knows about it if you don't happen to. Semiconductors can do it if properly rated. Normally a small switch that is not designed for DC will not unless the current is very low, which generates a low intensity arc that is easier to interrupt.
Hope this is valuable.
Regards to all,
Ed Long
N-Channel said:Eva,
You've let the magic smoke out!As a member of the SAE 42V Committee, I have seen, firsthand, the effects of trying to switch large (over 1/2A) DC Currents above 35V. Relay and soleniod contacts trying to switch a few amperes at 42-48VDC usually wear out after about 5-10 cycles (ouch!), and switches have melted in the same amount of time. It was generally agreed that, for anything over 35VDC, conventional switching means would not do.
Two possible solutions were to try and enclose the contact area in some kind of inert gas, like Nitrogen or Argon, to minimize the arcing. The other was to use exotic metals that designed withstand repeated sustained arcing, which has been demonstrated almost 500mS!
Even a new type automotive fuse had to be developed because of the arcing issue. At least you didn't let the magic smoke out of you.
Thank You for sharing this with us. Like Rick, I appreciate the caution, too.
Regards,
Steve
Hi Steve!,
It sounds like you and I travel in the same local circles!
(Slight Brag to follow)
My company (Also in Michigan) is one of the leaders in HV wiring & connectors in the world. We have our components in virtually every Hybrid vehicle sold in the US.
As such, we have been researching "Higher Voltage" systems for motor vehicles for YEARS!
The idea of using a higher voltage system to reduce the size of electromechanical devices, and to boost the available power for automotive systems, has been being battered about here in the Detroit (and other automotive cities) area for some time now.
With a higher voltage system, electromechanical devices, like starters and other motors, Etc., and their wiring can be smaller/lighter for a given load.
The Major setbacks have been in the areas of switching and connector arcing.
When you start using 36vdc or 48vdc +high amp systems, you are basically running an Arc welder! with all of the associated arcing problems.
Tall Shadow
There's hope... if you have money to spare....
Gigavac builds nice vacuum relays capable of handling high DC currents at high voltages.
Tyco (Previously KiloVac) also builds one of my favorites: EV200 Contactor for around 100 Euro's, capable of making and breaking 25A at 600V easily.
Merlin Gerin (Schneider Electric) offers the Compact NS and Masterpact NW circuit breakers. Capable of switching DC op to 750V, breaking currents ranging from 16A to a mindboggling 4000A.... I wonder how much dough the last one costs....
So professional solutions exist.... And no, line switches won't do...
Gigavac builds nice vacuum relays capable of handling high DC currents at high voltages.
Tyco (Previously KiloVac) also builds one of my favorites: EV200 Contactor for around 100 Euro's, capable of making and breaking 25A at 600V easily.
Merlin Gerin (Schneider Electric) offers the Compact NS and Masterpact NW circuit breakers. Capable of switching DC op to 750V, breaking currents ranging from 16A to a mindboggling 4000A.... I wonder how much dough the last one costs....
So professional solutions exist.... And no, line switches won't do...
N-Channel said:
Pity you guys weren't around in time to prevent the Apollo 13 "problem".
(The supply voltage for heaters had been increased from 28V to some 60V in the middle of the project, and nobody had remembered to uprate the thermostat contacts on the O2 tank heater. They promptly welded shut when it would have been time to limit the temperature in the tank.)
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