# Teaser Tuesdays (Feb. 4) February 4, 2014 (Sempre Redemption)

“A taste, they called it. Everyone always wanted a taste. They claimed, as a family, that they all worked as one. They said it was a matter of respect. They said it was the honorable thing to do. ”

~ p.157, “Sempre Redemption ” by J.M Darhower

# Define Recovery Voltage

It is the normal frequency (50 Hz) r.m.s. voltage that appears across the contacts of the circuit breaker after final arc extinction. It is approximately equal to the system voltage.

When contacts of circuit breaker are opened, current drops to zero after every half cycle. At some current zero, the contacts are separated sufficiently apart and dielectric strength of the medium between the contacts attains a high value due to the removal of ionised particles. At such an instant, the medium between the contacts is strong enough to prevent the breakdown by the restriking voltage.

Consequently, the final arc extinction takes place and circuit current is interrupted. Immediately after final current interruption, the voltage that appears across the contacts has a transient part. However, these transient oscillations subside rapidly due to the damping effect of system resistance and normal circuit voltage appears across the contacts. The voltage across the contacts is of normal frequency and is known as recovery voltage.

# Define Restriking Voltage

It is the transient voltage that appears across the contacts at or near current zero during arcing period.

At current zero, a high-frequency transient voltage appears across the contacts and is caused by the rapid distribution of energy between the magnetic and electric fields associated with the plant and transmission lines of the system.

The current interruption in the circuit depends upon this voltage.

If the restriking voltage rises more rapidly than the dielectric strength of the medium between the contacts, the arc will persist for another half-cycle. On the other hand, if the dielectric strength of the medium builds up more rapidly than the restriking voltage, the arc fails to restrike and the current will be interrupted.

# Define Arc Voltage

It is the voltage that appears across the contacts of the circuit breaker during the arcing period.

As soon as the contacts of the circuit breaker separate, an arc is formed. The voltage that appears across the contacts during arcing period is called the arc voltage. Its value is low except for the period the fault current is at or near zero current point. At current zero, the arc voltage rises rapidly to peak value and this peak voltage tends to maintain the current flow in the form of arc.

# Engineering Quote week 9

The Top 10 Things Engineering College Didn’t Teach

#10. There are at least 10 types of capacitors.
#9. Theory tells you how a circuit works, not why it does not work.
#8. Not everything works according to the specs in the data book.
#7. Anything practical you learn will be obsolete before you use it, except the complex math, which you will never use.
#6. Always try to fix the hardware with software.
#5. Engineering is like having an 8 a.m. class and a late afternoon lab every day for the rest of your life.
#4. Overtime pay? What overtime pay?
#3. Managers, not engineers, rule the world.
#2. If you like junk food, caffeine and all-nighters, go into software.
#1. Dilbert is a documentary.