Size of a spark
william chan
Posts: 1,326
When you close an electrical contact, inevitably a spark will occur across the switch gap before the switch connects completely.
Today's question is,
What determines the size of the spark and it's level of destruction?
Is it the voltage across the switch, or is it the potential current that is about to flow?
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www.fd.com.my
www.mercedes.com.my
Today's question is,
What determines the size of the spark and it's level of destruction?
Is it the voltage across the switch, or is it the potential current that is about to flow?
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.fd.com.my
www.mercedes.com.my
Comments
The voltage and the material (including physical construction) of the switch and the atmosphere around the contacts determine the characteristics of the first moments of the spark (if any).
The amount of current that flows depends on the power source and the source impedance as well as the material and construction of the contacts and the atmosphere and affects the amount of contact vaporization that occurs. If a lot of vaporization occurs, that could make a difference in the next spark or even whether this one will "quench". Even the physical movement of the contacts (speed / pressure) makes a large difference.
Polarity causes migration of the metal in only one direction. So, one might say that AC is somewhat less destructive as the metal goes back and forth. I suppose this is why we have mercury wetted AC relays and switches. Also the AC duty cycle allows some reduction in the build up off heat.
The voltage really is what makes sparks jump, but arc welding is all about current.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
The sparks can be from several sources. 1. When the contacts mate, molten metal may disipate. 2. When the contact open, air may ionize. Ionized air is a good condutor. An arc welder for instance.
All good switches are engineered for specific applications and a minimum number of cycles. Make and break cycles, bounce period, contact resistance, atmosphere.
1. Inductive loads. Inductive loads will cause the the air to inoize when the swithes opens. Collapsing magnetic field. The contacts become an arc welder. Voltage/current distruction. 2. Tungstin lamps. Tungstin lamps will cause the contacts to disapte moltin metal when the contacts close. Inrush currents. Current distruction.
Contact materials and plating all have an affect on the life of a switch. Copper, brass, bronze for the raw materials, Silver, gold, and other exotic matals for the plating and finishing. There are techniques that are used to draw the arcs away from the contacts to improve performance.
I have worked on swithes in a vacuum. They somtimes behave like vacuum tubes. Not a good thing.
Never ever close a 1000 AMP breaker if it has opend to interupt the flow of current. The contact resistance will be the highes in the circuit. All the power and heat will be disipated by the circuit braker contacts. BOOM. Metal vaporizes, the lab fills with smoke, the exhaust fans turn on and all the engineers go to lunch/home early.
By the way. All the switches under test are operated remotly and·behind 6 in thick glass.
I hope this helps.
SJW
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PLEASE CONSIDER the following:
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Post Edited (Kramer) : 4/3/2008 6:27:02 PM GMT
Terry
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www.fd.com.my
www.mercedes.com.my
Airplanes somewhat avoided the problem by using magnetos to create ignition spark. And airplane engines always required a second magneto in case the first one failed during flight.
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PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······