Since arc temperature can easily reach the melting temperature of steel, arcs in air at atmospheric pressure are useful for welding and for light. Arcs for gas filled lamps at low pressure produce light such as in neon signs, fluorescent lamps, and lasers.
To make arcs useful, series impedances or special generators are provided to limit the arc current. For lamps, a series inductance is called a ballast.
An arc short circuiting a utility power system could expand until it consumed the entire capacity of the system, which might be many megawatts. Therefore circuit breakers or fuzes are provided which detect excessive currents and quickly disconnect the portion of the system in which an arc has started. Circuit breakers are sometimes given short time delays in opening to permit transient currents in motors and transformers without undesireable opening.
When a circuit breaker either opens or closes its own contacts, it generates an arc. Therefore very sophisticated means are used to extinguish arcs within circuit breakers themselves.
Domestic circuit breakers are sometimes provided with ground fault detectors which detect current throug a short circut to ground, including a small shock current through a person, and quickly open without waiting for a large fault current.
Circuit breakers are primarily safety devices but they can also perform the function of on-off switches.
Lightning strikes are arcs through air which has been ionized by large voltage gradients between clouds and earth or between clouds. Lightning rods provide paths to earth which de-ionize air and thus prevent lightning strikes in their neighborhood. A lightning strike is limited in time because it discharges the cloud which initiated the strike. However such strikes can initiate arcs in power systems; circuit breakers interrupt such arcs.
A defect in an electrical device can initiate an arc, so circuit breakers and fuzes are provided at many locations in an electrical system.
Fuses are sometimes used instead of circuit breakers. Fuses are reliable, can be made quite fast, and are quite cheap. However a circuit breaker can be quickly re-closed by remote control while a fuse must be repalced by a person, which process requires both time and labor cost.
Other essays in this series deal with electro-mechanical actuators, amplifiers, and other devices.
For a full treatment, please see my book:
"Understanding Electro-Mechanical Engineering" published by IEEE Press and John Wiley. Click here.
My other books are:
"Designing Cost-Efficient Mechanisms" published by SAE Press. This is the book which introduced and explains Minimum Constraint Design. Click here.
"Real-World Engineering" Published by IEEE Press. How to be a successful engineer. Click here.
Questions? Telephone me at 619-224-3494 or e-mail ljkamm@ljkamm.com. No charge!
Lawrence Kamm, Consulting Electrical and Mechanical Engineer and Expert Witness (California License E 5897) e-mail:ljkamm@ljkamm.com