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72A-1898 Rev. A 11/01 A-1
Monday
, November 26, 2001 1:09 pm
Appendix A. TRXField
™
Software and Circuit
Breaker Testing Theory
This appendix discusses several topics useful in understanding circuit
breaker testing. These include:
• Basic Circuit Breaker Interruption Principles
• Understanding Contact Measurement
• TDR9000 TRXField Software and Test Plan Dynamics
• Interpreting Test Results
Basic Circuit Breaker Interruption Principles
When a switch carrying AC current opens, an arc forms. This arc strikes
as the last metal-to-metal contact breaks, as shown in A of Figure A.1.
Figure A.1 Arc Principles
The arc is a conductor. As the contact in motion moves away from the
stationary contact, the arc stretches. This stretching and cooling enables
the arc to quench at a point where the current is zero, t
2
, B in Figure A.1,
subsequent to contact parting. In a sixty cycle sine wave, the voltage and
current cross zero twice each cycle. At these zero crossings the arc
extinguishes. It remains extinguished if the dielectric strength of the
medium between the contacts is greater than the dielectric strength
necessary to withstand the voltage across the contacts.
If the dielectric strength across the contacts is insufficient to withstand
this voltage, a restrike occurs and the arc is re-established. At the next
zero crossing the arc again extinguishes and this process continues until
the dielectric strength becomes sufficient to withstand voltage across the
contacts.
A B
T
1
T
2
T
1
t
1
−
t
2