04-07-2012, 11:11 AM
Anode and Cathode Arc Root Movement During Contact Opening at High Current
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INTRODUCTION
THIS paper presents experimental results from a test
system design to recreate the current limiting operation of
a miniature circuit breaker (MCB). Miniature circuit breakers
are widely used in domestic, commercial and light industrial
installations. The devices are usually used where the supply
voltage exceeds 200 V ac and are used to protect circuits rated
up to 100 A from overload and short circuit faults (10 –10
A prospective). During a short circuit fault an electric arc
is drawn between opening contacts. The current through the
conductors of the MCB generates a magnetic field in the arc
chamber, which acts to force the arc away from the contact
region along arc runners and into an arc stack. The arc is
then split into a number of series arcs which results in a high
voltage across the circuit breaker. The high voltage counteracts
the supply voltage and limits the peak fault current. The energy
released by the fault is reduced and damage to both the circuit
and the circuit breaker is minimized.
EXPERIMENTAL METHOD
A high speed arc imaging system (AIS) has been used
to record optical data of arc motion at sample rates of 1
MHz, [8]–[10]. The short circuit tests were carried out in a
flexible test apparatus (FTA) designed to simulate the current
limiting operation of a MCB [1]. Figs. 2 and 3 show details
of the arc chambers used to simulate MCB geometry (A)
and (B), respectively. In geometry (A) the fixed contact is
connected to a long straight runner, whereas in (B) the fixed
contact runner diverges at the corner identified in Fig. 3.
The circles over the arc chambers indicate the optical fiber
positions. The contacts are opened with a solenoid mechanism,
which operates independently of the fault current.
RESULTS
The results of Experiment 1 are shown in Fig. 4, for both
the let through energy and the cathode root contact time
for arc chambers, A and B, for the range of experimental
conditions shown in Table III.
The results of Experiment 2 are shown in Figs. 5(a) and
(b), for the Taguchi based experimental method, and show the
Contact times for the Cathode Fig. 5(a) and Anode Fig. 5(b).
The experimental conditions are defined in Table IV.
Figs. 6–8 are specific data plots from Experiment 2 of the
arc root motion. They show the arc voltage, anode and cathode
root movement in the upper figure, with images of the arc at
given points in time identified in the upper figures.