The FZN21A-12D indoor high-voltage vacuum load break switch and the FZRN21A-12D switch-fuse combination unit serve three-phase AC 50 Hz distribution systems of 10 kV, where 12 kV is the rated voltage of the switchgear. The switch breaks rated and overload current at 630 A, while the combination unit clears short-circuit current at 31.5 kA and keeps plant from running on two phases.
Compact and light, the units are built into cubicles, ring main units and package substations for urban network work, industry, and high-rise and public buildings, as a ring supply unit or a terminal unit.
Model Designation
Two designations are used, one for the switch and one for the combination unit:
Load break switch — FZN21A-12D/T 630-20
- F: Load break switch
- Z: Vacuum
- N: Indoor
- 21A: Design sequence number
- 12: Rated voltage, kV (applied in three-phase AC 50 Hz systems of 10 kV)
- D: With earthing switch
- T: With spring operating mechanism
- 630: Rated current, A
- 20: Rated short-time withstand current, kA
Switch-fuse combination unit — FZRN21A-12D/T 125-31.5
- R: With fuse
- 125: Rated current, A, of the combination unit and of the fuse it carries
- 31.5: Rated short-circuit breaking current, kA
The remaining letters carry the same meaning as in the switch designation.
Construction and Operation
The three poles stand on a common steel frame carried on the lower support frame and insulators, which holds the live parts clear of the frame and keeps the unit compact. The operating mechanism turns the main shaft and the drive linkage, and the insulating pull rods carry that movement to the contacts of the vacuum chamber.
The isolating switch gives the visible isolation gap and the earthing blade, working against the static earthing contact head, bonds the de-energised side before maintenance, with mechanical interlocks between switch, earthing switch and fuse compartment meeting the five-prevention requirements.
Mechanism and Control
The closing energy comes from a charged closing spring, so contact speed is set by the mechanism rather than by the operator and the switch can be worked as often as the network requires. The release device acts on the signal from the fuse, which is how the combination unit reports the loss of a phase.
With the motor-charged mechanism the spring is restored from the control supply, giving remote operation and remote signalling and tripping on a relay command, while manual operation stays available if that supply is lost.
Technical Parameters
Service Conditions
- Ambient temperature: +40℃ upper, -25℃ lower
- Altitude: Not more than 1000 m
- Relative humidity: Daily average not more than 95%, monthly average not more than 90%
- Seismic intensity: Not more than 8 degrees
- Site condition: No fire, explosion hazard, chemical corrosion or violent vibration
- Pollution level: II
Technical Data
| No. | Item | Unit | Value |
|---|---|---|---|
| 1 | Rated voltage | kV | 12 |
| 2 | Rated frequency | Hz | 50 |
| 3 | Rated current, load break switch FZN21A-12D/T 630-20 | A | 630 |
| 4 | Rated current, combination unit FZRN21A-12D/T 125-31.5 | A | 125 |
| 5 | Rated short-time withstand current, load break switch | kA | 20 |
| 6 | Rated short-circuit breaking current, combination unit | kA | 31.5 |
Installation & Wiring
Installation and Commissioning
The unit is bolted to the cubicle frame on its lower support frame, the closed, open and earthed positions and the interlocks are checked on all three poles, the specified fuse of the 125 A class is fitted with its striker engaging the release device, and the main circuit resistance and contact travel are measured before energisation.
Outline Dimensions
- Front view: 765 mm overall width with 560 mm marked inside it, 210 mm between adjacent poles marked twice, and 580 mm on the vertical.
- Side view: 530 mm across the top and 880 mm vertical.
- Panel details: the outline drawing carries the complete dimension set; request the drawing for panel cut-outs and busbar drilling.
Component Reference
The item numbers below follow the outline and installation drawing of the series:
- Frame
- Isolating switch
- Fuse
- Insulating pull rod
- Upper support frame
- Arc chute
- Earthing contact head (static)
- Insulator
- Earthing blade
- Lower support frame
- Closing spring
- Insulating pull rod
- Main shaft
- Release mechanism
- Auxiliary shaft
- Drive linkage
- Operating mechanism
Applications
- Urban network construction and renewal projects
- Industrial and mining enterprises
- High-rise buildings and public buildings
- Ring supply and terminal feeders in cubicles, ring main units and package substations
Selection & FAQs
Selection Guide
- Choose the switch or the combination unit. The FZN21A-12D/T 630-20 carries 630 A of load and overload current and relies on upstream protection for faults; a transformer or cable feeder that must also clear short-circuit current takes the FZRN21A-12D/T 125-31.5 unit, where the fuse breaks the fault at 31.5 kA and the release device reports the loss of a phase.
- Confirm the current class of the fuse. The combination unit is drawn at 125 A, so check the full-load current and the inrush of the transformer or cable box against that rating before ordering.
- Check the short-time duty. 20 kA as the rated short-time withstand current of the switch sets the busbar and the upstream protection coordination; verify it against the network before ordering.
- Decide the control arrangement. The T suffix carries the spring operating mechanism, and the motor-charged version restores that spring from the control supply, which is what allows remote operation and remote signalling and tripping on a relay command.
- Match the insulation to the cubicle. The unit is for indoor use at pollution level II, not more than 1000 m above sea level, between -25 and +40 degrees, in a location free from fire, explosion hazard, chemical corrosion and violent vibration.
FAQs
Q1: What is the FZN21A-12D used for?
It is an indoor three-phase vacuum load break switch for 50 Hz distribution systems of 10 kV, used to make and break rated and overload current, to isolate the circuit through a visible gap and to earth the de-energised side for maintenance. The FZRN21A-12D combination unit adds fuses so the same frame also clears short-circuit current and protects a transformer or cable feeder. Both are built into cubicles, ring main units and package substations, as a ring supply unit or a terminal unit.
Q2: How do I read FZRN21A-12D/T 125-31.5?
F is the load break switch, Z the vacuum medium, R indicates that fuses are combined, N that the unit is for indoor use, 21A the design sequence number, 12 the rated voltage in kV, D that an earthing switch is fitted, and T that a spring operating mechanism is fitted. The trailing numbers give the current: 125 A is the rated current of the combination unit and of the fuse it carries, and 31.5 is the rated short-circuit breaking current in kA. Read as FZN21A-12D/T 630-20, without the R, the same designation describes the switch alone at 630 A with a rated short-time withstand current of 20 kA.
Q3: Why does the page mention 12 kV and 10 kV?
They are two different quantities. 12 kV is the rated voltage printed on the designation of the switchgear, which is the voltage class the insulation and the vacuum chamber are designed for, while 10 kV names the system the unit is applied in, the three-phase AC 50 Hz distribution network. Equipment of a higher voltage class is normally selected for a network so that the insulation and the switching duties carry margin.
Q4: How does it compare with the FZN16A-12D?
Both are indoor 12 kV vacuum units with a 630 A switch and a 125 A combination unit, so either can be used on a comparable feeder. The FZN16A-12D is the earlier frame and its data sheet publishes the wider family of breaking currents, from cable charging to capacitive current. This FZN21A-12D is built around a motor-charged spring mechanism with remote operation and remote signalling, an isolating switch with a visible gap and an earthing switch with making capacity, which is why it is specified for urban network work and for buildings where the switch is operated often and watched from a control point.
Q5: Which member of the range suits a 40.5 kV or a larger current application?
For the higher voltage the FZN21-40.5 is the indoor unit of this family for 40.5 kV and the 35 kV side of box-type substations, on a 1250 A frame, and its combination unit clears 20 kA. The FZN25-12DG stays at 12 kV and covers the larger current frames. None of these can be substituted across the voltage class: the insulation levels, the fuse class and the clearances are designed for one class only.
Q6: Can the unit be operated remotely, and how is it maintained?
Yes. The spring mechanism is charged by an electric motor from the control supply, so the switch can be closed and tripped from a remote control point and can respond to a protective relay, while manual operation stays available if that supply is lost. Maintenance covers the mechanism and the insulation: keep the main shaft, the auxiliary shaft and the drive linkage clean and lubricated, check the interlocks and the position indication, and replace the vacuum chamber when the contact wear reaches its permissible value. Where a different interrupting medium is required, the FLN36-12 seals a low-pressure SF6 three-position interrupter, the FKN12A-12D blows air over the arc, and for pole-mounted outdoor duty the FZW32-12 is the vacuum equivalent.