ELEC5205 Chap.11 Switchgear, Fuses and Breaker Ratings
Switchgear, Fuses and Breaker Ratings
Interruption technology and rating are separate decisions
Air, oil, vacuum and SF6 breakers differ in arc control, voltage application, maintenance, safety and environmental burden. Regardless of medium, the selected breaker must carry normal load and interrupt the fault contributed by grid, generators and motors at the relevant clearing time.
Fuses add a melting interval and an arcing interval; AC current zeros, magnetic drive and arc chutes determine whether current is successfully interrupted.
Compare interrupters through the arc
A breaker must force an arc to lose conductivity while its contacts separate and recovery voltage appears.
Air-blast and air-break arrangements lengthen, cool or divide the arc; oil uses decomposition and cooling around the contact region; vacuum controls a metal-vapour arc with rapid dielectric recovery; SF6 combines strong attachment with effective heat removal.
A technology comparison should cover voltage range, arc extinction, maintenance, safety, environmental handling and the consequences of leakage or contamination.
Assemble fault duty from every active source
The utility grid, synchronous generators and motors can all feed a bus fault. Convert each source to a common voltage base and consistent equivalent impedance before summing contributions.
Machine current changes with time as subtransient and transient effects decay, so initial symmetrical duty and later current are not interchangeable. The peak making duty also contains a DC offset factor.
Label rms, peak, symmetrical and asymmetrical quantities explicitly before matching them to a device rating.
Match more than interrupting current
A breaker selection includes rated voltage, continuous current, short-circuit breaking current, making current, short-time withstand and operating sequence.
The calculated value should be rounded up to an available standard rating, not reported as the equipment nameplate. Protection clearing time must fit thermal and mechanical withstand, while transient recovery voltage can challenge dielectric recovery even after a current zero.
State the governing rating and the margin rather than concluding simply that a breaker is large enough.
Read a fuse trace in two intervals
Fault current first heats the element until it melts; that is pre-arcing or melting time. Current then continues through an arc until deionisation at a suitable current zero.
Magnetic extension and arc chutes increase arc length, cool the plasma and raise arc voltage so current is limited and interrupted. From a waveform, identify prospective current, actual limited current, melting instant, arc duration and clearing time. This sequence connects the laboratory observation to protection coordination.
What this chapter covers
- 01
Breaker Media
- 02
Fault MVA
- 03
Symmetrical Current
- 04
Peak Duty
- 05
Fuse Melting
- 06
Arc Interruption
Worked example · free
Build a breaker duty
- 3Convert every source to fault MVA on its own rating basis.
- 3Convert total fault MVA to common-bus rms current.
- 2Apply the stated peak factor and round up to standard duties.
Key terms
- Breaking Current
- Breaking Current is the fault-current duty a breaker can interrupt at the specified voltage and test conditions.
- Making Current
- Making Current is the peak asymmetric duty a breaker can close onto without mechanical or thermal failure.
- Arcing Time
- Arcing Time is the interval after a fuse element melts while current continues through an arc before interruption.
Switchgear, Fuses and Breaker Ratings FAQ
Why do motors contribute to a fault?
Stored rotational energy keeps the machines turning immediately after bus voltage collapses. For the first cycles they behave as generators and feed current back into the fault through their subtransient reactance.
Why is peak duty larger than symmetrical rms current?
Fault inception can include a decaying DC offset whose magnitude depends on switching angle and network X/R ratio. The first peak therefore exceeds the sinusoidal rms-based value used for symmetrical breaking calculations.
Why can an AC fuse be unsuitable for DC?
AC current naturally crosses zero each half-cycle, helping the arc deionise. DC has no natural zero, so the device must create enough arc voltage and cooling to force current to zero by another mechanism.
Exam move
Compare breaker media in a table, then calculate duties independently. Rework one fault-MVA problem and label initial, steady, rms and peak quantities. Review the fuse experiment as a waveform problem: identify melting time, arcing time, arc voltage and the effect of magnetic extension or a deionising chute.
Maintain separate rows for rated voltage, continuous current, symmetrical breaking current, peak making current, short-time withstand and operating sequence. Add each grid, generator and motor contribution on one voltage base, then match the relevant time regime to protection clearing. Finish by checking the next available standard device rather than naming the exact calculated duty as a rating.
Overlay protection timing with equipment withstand so selectivity and backup clearing remain consistent with the current contribution used in the rating calculation.
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