In a switch-fuse combination, the transfer current is the symmetrical three-phase value at which the fuse and the switch exchange the clearing duty. Above it, only the fuses clear. IEC 62271-105 requires it to be lower than the primary current of a solid secondary fault on the transformer.
Short answer
In a medium-voltage switch-fuse combination, the fuse clears the high currents and the switch clears the low ones. The transfer current is the symmetrical three-phase current value at which both exchange that duty (IEC 62271-105, edition 2.0, 2012, clause 3.7.109). Above that value, the current is interrupted by the fuses alone.
Clause 4.104 defines the rated transfer current as the maximum RMS value of transfer current that the switch of the combination can interrupt. It is a value declared by the manufacturer after testing.
What the combination is
IEC 62271-105 defines the switch-fuse combination as the assembly of a three-pole switch with three fuses fitted with strikers. The operation of any striker makes all three poles of the switch open automatically (clause 3.4.101). The fuses are added to extend the short-circuit breaking capacity of the combination beyond that of the switch alone.
The standard applies to combinations above 1 kV up to 52 kV, in three-phase 50 Hz or 60 Hz systems. The manufacturer defines a reference list of fuses: any combination with one of those fuses is demonstrated to comply. The short-circuit making capacity of the combination is 2.5 times (50 Hz) or 2.6 times (60 Hz) the rated short-circuit breaking current (clause 4.103).
The switch of the combination must have been tested separately according to IEC 62271-103, except for the short-time current and short-circuit making requirements, because the fuses influence those parameters (clause 6.1).
What happens near the transfer point
Clause 8.102.4 describes the sequence in a three-phase fault near that point:
- The fuse that melts fastest clears the first phase.
- Its striker starts tripping the switch.
- The other two phases see a reduced current, 87 %. They are interrupted by the switch or by the remaining fuses.
The transfer point is the one at which the switch opens and the fuse elements melt at the same time. It depends on two things: the opening time of the switch initiated by the fuse and the time-current curve of the fuse.
- Three-phase fault. Current close to the transfer current
- The fastest fuse melts. It clears the first phase
- Its striker starts the trip. The switch begins to open
- The other two phases see 87 %. Reduced current
- The switch or the remaining fuses clear them. Depending on tolerances
Why it must be lower than the secondary fault
A solid fault at the transformer secondary terminals produces a very high current on the primary and very high transient recovery voltage values. The switch of the combination is neither designed nor tested for that condition. That is why the standard asks for the fuses to be selected so that they clear that fault on their own, without passing breaking duty to the switch (clause 8.101.3).
In practice, the transfer current of the combination must be lower than the primary current of that fault:
Isc = 100 · IT / Z- IT is the rated transformer current.
- Z is its short-circuit impedance, in percent.
Once that condition is met, the transfer currents correspond to faults with arc or line impedance, which reduce the current and the transient voltage and increase the power factor.
Example with a 1,250 kVA transformer
A 1,250 kVA transformer at 13.2 kV, with short-circuit impedance of 6 %. The primary rated current is 1,250 / (√3 × 13.2) = 54.7 A. The primary current of a solid secondary fault is 100 × 54.7 / 6 = 911 A. With those data, the transfer current of the combination must be lower than 911 A.
This is an arithmetic example of the rule in the standard. For your transformer, use the nameplate data and the curves of your fuse.
- Is the transfer current of the combination lower than 100 · IT / Z?
- Yes: The fuses clear the solid secondary fault on their own
- No: Does the design of the low-voltage connections prevent a solid fault at the secondary terminals?
- Yes: The standard allows that fault to be disregarded when selecting the fuses
- No: The standard indicates that a switch according to IEC 62271-103 should not be applied
The exception in clause 8.101.3 applies when the system supplier considers that the design of the low-voltage connections between the transformer and the switchgear, for example inside prefabricated substations, prevents a solid short circuit at the secondary terminals.
Transfer and take-over
These are two similar but not identical concepts:
| Concept | When it applies | What it is |
|---|---|---|
| Transfer current | The switch opens through the fuse strikers | Symmetrical three-phase current at which fuses and switch exchange the clearing duty |
| Take-over current | Combinations with an overcurrent release or relay trip | Current at which the fuse and switch curves cross, with its release |
The take-over depends on the opening time of the switch initiated by the release and on the fuse curve. Above that value, the fuses take over the clearing function. The standard requires the take-over current to be lower than the maximum take-over value of the combination (clause 8.102.5).
How it is verified
The type test is called TDItransfer (clause 6.101.3.3). It demonstrates the coordination between switch and fuses in the zone where breaking duty passes from the fuses to the switch. Three breaks are made in a three-phase circuit, with the fuses of two poles replaced by solid links, and the fused pole changes at each break. In combinations with a relay trip, the test can be omitted if the take-over current is equal to or greater than the transfer current.
What not to do
- Do not select the fuses only by the rated transformer current: also check the transfer current.
- Do not use a combination whose transfer current exceeds the secondary fault primary current, unless the design prevents that fault.
- Do not confuse transfer with take-over: they apply to different tripping configurations.
- Do not mix fuses of different types or currents in the same combination.
Related topics
The choice between fuse and switch to protect a transformer is covered in protecting the transformer with a fuse or a circuit breaker. The fuse is described in medium-voltage fuse. The switch of the combination is the one in the guide on the medium-voltage switch-disconnector. The settings of the low-side protection are explained in industrial transformer protection: settings.
The striker that trips the switch after a fuse melts is explained in fuse striker and switch tripping, and the equipment that brings fuses and switch together in a single block, in fused circuit-switcher.
The map of standards for primary equipment testing places this part among the others of IEC 62271.
When to call us
At Tiza Group we verify with you the coordination between the fuses and the switch protecting your transformers. See our protection coordination service. Write to us and we will send you a quote.
Related guides
- Fuse striker and switch tripping
- Medium-voltage fused circuit-switcher (IEC 62271-107)
- Fuse or circuit breaker to protect an MV transformer
- Medium-voltage fuse: current-limiting, expulsion, coordination
- Medium-voltage switch-disconnector and RMU and their tests
References
- IEC 62271-105, High-voltage switchgear and controlgear - Part 105: Alternating current switch-fuse combinations for rated voltages above 1 kV up to and including 52 kV, 2012 edition: clauses 3.4.101, 3.7.109 to 3.7.112, 4.101 to 4.105, 6.101.3.3, 8.101.3 (Figure 8) and 8.102.4.
Frequently asked questions
What happens when the fault exceeds the transfer current?
The three-phase current is interrupted by the fuses alone. Just below the transfer current, the phase that clears first is broken by its fuse, and the other two are broken by the switch or by the fuses, depending on the tolerances of the fuse curve and of the switch opening time.
How is the transfer current related to the fault on the transformer secondary?
The transfer current of the combination must be lower than the primary current produced by a solid short circuit on the transformer secondary, equal to 100 times the rated transformer current divided by its short-circuit impedance in percent. That way this fault, which brings very high transient recovery voltages, is cleared by the fuses alone.
Is the transfer current the same as the take-over current?
No. The transfer current applies when the switch opens through the fuse strikers. The take-over current applies to combinations with an overcurrent release or relay trip: it is the current at which the fuse and switch curves cross, with its relay. In both cases, above that value the fuses clear.