The fused circuit-switcher combines a three-pole switch and three current-limiting fuses to protect transformers. IEC 62271-107 places it between the switch-fuse combination and the circuit breaker: it operates over the whole current range, has no transfer current limit, and its tripping comes from an external device.
Short answer
A fused circuit-switcher is a device made up of a three-pole circuit-switcher and three current-limiting fuses. It can make and break any load or fault current up to its short-circuit breaking current, under the transient recovery voltage and power factor conditions set by IEC 62271-107 (edition 2.0, 2012, clauses 1.1 and 3.4.102).
It is intended for circuits with normal mechanical and electrical endurance requirements. The standard cites as an application the protection of high- and low-voltage transformers. It excludes distribution lines and cables, motor circuits and capacitor bank circuits. It covers rated voltages above 1 kV up to 52 kV.
How it shares the duty
- Low short-circuit currents: up to the rated take-over current, they are cleared by suitably arranged supplementary devices, such as strikers or relays, which trip the circuit-switcher.
- High currents: they are cleared by the fuses, which are included so that the breaking capacity of the assembly exceeds that of the circuit-switcher.
Ith, the maximum thermal current
Clause 4.4.101 defines the rated maximum thermal current (Ith) as the maximum value of the thermal current of the fused circuit-switcher. It is not required to be taken from the R10 series. The actual current depends on the fuses installed. Clause 8.103 adds that it is assigned by the manufacturer, verified by the temperature-rise test, and may have to be reduced if the ambient temperature exceeds the normal one.
The standard does not apply to this equipment the rated short-time withstand current, the rated peak withstand current or the rated duration of short circuit (clauses 4.5, 4.6 and 4.7).
Rated breaking and making currents
- Short-circuit breaking current (Isc): chosen from the series 8 – 10 – 12.5 – 16 – 20 – 25 – 31.5 – 40 – 50 – 63 – 80 – 100 kA (clause 4.101). The breaking capacity depends on the fuses: only fuses whose maximum breaking current is equal to or greater than Isc may be listed in the manufacturer’s manual.
- Short-circuit making current: 2.5 times Isc at 50 Hz or 2.6 times at 60 Hz (clause 4.103).
- Rated take-over current: the maximum RMS value of take-over current the circuit-switcher can interrupt. The declared value must be greater than the take-over current given by the fuses with which the maximum thermal current is demonstrated (clause 4.104).
What happens between Ith and I3
I3 is the minimum breaking current of the fuse, defined in IEC 60282-1. With a current between the thermal current and I3, protection is provided only by an external trip, such as an overcurrent or over-temperature relay. A striker, actuated by overcurrent or by temperature, can give the order, and the circuit-switcher clears the current (clause 8.104).
The standard warns of the risk of overheating and unpredictable behavior if that condition persists, with a current above the thermal one and below I3. In those cases a fuse with thermal limitation is advisable (clause 8.101).
Take-over
The take-over current is the current above which the fuses take over clearing from the tripping device and the circuit-switcher. It depends on the minimum opening time of the circuit-switcher and on the time-current curve of the fuse (clause 8.106). The maximum melting curve of the selected fuses must lie to the left of the point defined by the rated take-over current and the minimum opening time. That way, with external protection, currents greater than the rated take-over current are cleared by the fuses alone.
The type test TDIlow (clause 6.101.2.4) checks the zone below the melting curve of the fuses. Three breaks are made with a current equal to one third of the rated take-over current, with the fuses replaced by solid links.
The standard adds a case: if the transformer type and the medium-voltage connections give a very low capacitance, the transient voltage of the take-over test may not cover a solid short circuit on the secondary. In that case, the behavior must be specified according to special test T30 of Annex M of IEC 62271-100:2008.
Versus the switch-fuse combination and the circuit breaker
Clause 8.109 places the fused circuit-switcher at an intermediate point between the switch-fuse combination (IEC 62271-105) and the circuit breaker (IEC 62271-100).
| Characteristic | Switch-fuse combination | Fused circuit-switcher |
|---|---|---|
| Range of correct operation | Between melting and transfer, depending on the transient voltage; above transfer, always | Complete |
| Protection setting | By the choice of fuse | By an external device |
| Strikers | Basic | Optional |
| Transfer current | Relevant and limits the applications | Not relevant |
| Take-over | Relevant if it has a release | Basic |
| Characteristic | Fused circuit-switcher | Circuit breaker |
|---|---|---|
| Range of correct operation | Complete | Complete |
| Fault current limitation | Yes | No |
| Reclosing | No, if a fuse has operated | Yes, at any current |
Operation
Clause 8.108 requires the three fuses to be of the same type and the same current, and to be inserted with the strikers, if any, in the correct orientation. After a three-phase fault, only two fuses or only two strikers may have operated, which is not abnormal. All three fuses must be replaced if those of one or two poles operated, unless it is known for certain that no overcurrent passed through those that did not melt. If tripping occurred without melting any fuse, the minimum precaution is to measure the cold resistance of the fuses before re-energizing.
What not to do
- Do not use a fused circuit-switcher for lines, cables, motors or capacitor banks: the standard excludes them.
- Do not install fuses whose maximum breaking current is lower than the Isc of the circuit-switcher.
- Do not consider the zone between Ith and I3 protected without an external trip.
- Do not confuse this equipment with the switch-fuse combination of IEC 62271-105: they have their own name, tests and limits.
Related topics
The fuse is described in medium-voltage fuse. If you must choose between a fuse and a circuit breaker for a transformer, see protecting the transformer with a fuse or a circuit breaker. The medium-voltage circuit breaker is explained in medium-voltage vacuum circuit breaker. The switching equipment of the switchgear is in medium-voltage switchgear.
The comparison with the switch-fuse combination is expanded in transfer current of the switch-fuse combination, and striker tripping in fuse striker and switch tripping.
To see how it fits with the rest of the IEC 62271 series, see the standards for primary equipment testing.
When to call us
At Tiza Group we help you define the protection of your medium-voltage transformers: coordination between fuses, external tripping and circuit-switcher. See our protection coordination service. Write to us and we will send you a quote.
Related guides
- Transfer current in switch-fuse combinations
- Fuse striker and switch tripping
- Medium-voltage fuse: current-limiting, expulsion, coordination
- Fuse or circuit breaker to protect an MV transformer
- Medium-voltage vacuum circuit breaker: function and tests
References
- IEC 62271-107, High-voltage switchgear and controlgear - Part 107: Alternating current fused circuit-switchers for rated voltages above 1 kV up to and including 52 kV, 2012 edition: clauses 3.4.101 to 3.4.103, 4.4.101, 4.101 to 4.104, 6.101.2.4 and 8.101 to 8.109 (Tables 3 and 4).
Frequently asked questions
What is the maximum thermal current Ith of a fused circuit-switcher?
It is the maximum value of the thermal current of the assembly, assigned by the manufacturer and verified by the temperature-rise test. The actual thermal current depends on the fuses installed and need not be taken from the R10 series. It may have to be reduced if the ambient temperature in service exceeds the normal one.
What protects a fused circuit-switcher between the thermal current and the fuse minimum breaking current?
Only an external trip, such as an overcurrent or over-temperature relay. With currents between the thermal current and I3 (the minimum breaking current of the fuse, per IEC 60282-1), the fuse does not protect by itself. A striker actuated by overcurrent or temperature can give the trip order.
How does it differ from a switch-fuse combination?
The fused circuit-switcher operates correctly over the whole range, is not limited by the transfer current, and its tripping comes from an external device. In the switch-fuse combination, protection is set by the choice of fuse, the strikers are basic, and the transfer current limits the applications.