Arcing faults
Arcing faults
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A lamp cord extension run under a desk.
What happens if the chair rolls on the cord repeatedly.
It'll damage the insulation. But if lucky, only a short occurs and blow the fuse.
But if it arcs:
it draws less current than a short
It takes longer to clear
Arc faults then causes fire to start.
Arc faults' charasteristic
Arc faults' charasteristic
Current outside the normal current path.
Current may be less than normal or very
high.
Heat is concentrated in a very small area.
Current changes rapidly during the fault.
Line to neutral
This could be what occurred
under the desk.
Worst case: not enough current to trip the breaker or it
takes a long time to trip.
If the arc draws 10 amps at 120 volts you have 1200 W in a small space.
Line to ground
What if the grounding (bonding) path has high resistance?
Not enough current to trip the breaker or it takes a long time to trip.
What does this say about how we should bond?
Line to line
With big 3-phase systems the amount of energy unleashed by an arcing fault is huge.
These are often 3 phase line-to-line faults.
Line to series
If a circuit opens under load, current may continue to flow by arcing.
This can be a conductor breaking or a switch failing to fully open.
Here, current is limited by the load.
Why is limited current a problem?
These are all parallel arc-faults:
CEC terminology
CEC terminology
26-650 Special terminology
In this Subsection, the following definitions apply:
Arc-fault protection a means of recognizing characteristics unique to both series and parallel arc-faults and de-energizing the circuit when an arc-fault is detected.
Combination-type AFCI
Combination-type AFCI
Combination-type arc-fault circuit interrupter is a device giving both series and parallel arc-fault
protection to the whole branch circuit wiring, including cord sets and power supply cords connected to
the outlets, against unwanted arcing effects.
Diagram
Diagram
the arc fault sensing circuitry
The neutral conductor
The neutral terminal
Outlet branch-circuit-type arc-fault circuit interrupter
Outlet branch-circuit-type arc-fault circuit interrupter
Outlet branch-circuit-type arc-fault circuit interrupter — a device that provides both series and parallel arc-fault protection to downstream branch circuit wiring, cord sets, and power supply cords against the unwanted effects of arcing and also provides series arc-fault protection to upstream branch circuit wiring.
Arc-fault protection
Arc-fault protection
Arc faults are detected by fasting changing current.
Vacuum cleaners' motors and similar appliances have brushes that arc sometimes causing false tripping.
References
[1] Wikipedia
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