The FZN(R)N25-12DG indoor high-voltage vacuum load break switch is a three-phase 50 Hz, 12 kV switching unit that combines the load break switch, the isolating switch and the earthing switch on one frame. It is built to GB 3804 for AC load break switches and GB 16926 for switch-fuse combination units, and serves as the incoming, sectionalizing and transformer-protection unit of 12 kV metal-enclosed and air-insulated switchgear.
Two rated currents are offered, 630 A and 1250 A, both with 25 kA short-time withstand current and a spring energy-storage mechanism. In the FZR N25-12DG combination unit the circuit is completed with back-up fuses that clear short-circuit current, so the switch itself only makes and breaks load current.
Model Designation
Two designation formats are used, one for the load break switch and one for the combination unit:
Load break switch — FZN 25-12DG/T 630-25 or 1250-25
- F: load break switch
- Z: vacuum
- N: indoor type
- 25: design sequence number
- 12: rated voltage, kV
- D: with earthing switch
- G: with isolating switch
- T: spring energy-storage mechanism
- 630 / 1250: rated current, A
- 25: rated short-time withstand current, kA
Combination unit — FZR N25-12DG/T 200-31.5
- R: combined with fuse
- 200: rated current of the fuse, A
- 31.5: rated short-circuit breaking current, kA
The D and G suffixes are ordered separately when only one of the two switches is required, and a 125 A fuse is available where the feeder demands a lower fuse rating.
Construction and Design
The three poles are carried on a common metal frame, with the vacuum interrupters and the isolating and earthing switches supported on insulator bases and joined by flexible connections. Controls, position indication and the mechanism are grouped on the front operating panel, so all three switch positions can be operated and verified from the cubicle front. In the combination unit the fuse is held between upper and lower clips and acts on the pole through a trip plate.
Operating Function and Application
In service the unit makes and breaks rated load current and closed-loop current up to 1250 A, and switches no-load transformers up to 2000 kVA; the isolating switch provides the maintenance break and the earthing switch bonds the de-energised side. Short-circuit current is cleared by the back-up fuses of the combination unit, rated 31.5 kA or 50 kA depending on the fuse fitted, with a rated transfer current of 3150 A. Typical duties are incoming and sectionalizing feeders and transformer protection in 12 kV indoor switchgear.
Technical Parameters
Service Conditions
- Ambient temperature: +40℃ upper, -10℃ lower
- Altitude: Not more than 1000 m
- Relative humidity: Daily average not more than 95%, monthly average not more than 90%
- Vapour pressure: Daily average ≤ 2.2 × 10⁻³ MPa, monthly average ≤ 1.8 × 10⁻³ MPa
- Seismic intensity: Not more than 8 degrees
- Site condition: Indoor installation with no dripping, no fire or explosion hazard, no chemical corrosive gas and no violent vibration
Technical Data
| No. | Item | Unit | FZN25-12DG/T 630-25 (load break switch) | FZN25-12DG/T 1250-25 (load break switch) | FZR N25-12DG/T 200-31.5 (switch-fuse combination) |
|---|---|---|---|---|---|
| 1 | Rated voltage | kV | 12 | 12 | 12 |
| 2 | Rated frequency | Hz | 50 | 50 | 50 |
| 3 | Rated current | A | 630 | 1250 | 200 (fuse) |
| 4 | Rated short-time withstand current | kA | 25 | 25 | 25 |
| 5 | Rated peak withstand current | kA | 63 | 63 | 63 |
| 6 | Rated short-circuit making current | kA | 63 | 63 | — |
| 7 | Rated active load breaking current | A | 1250 | 1250 | 1250 |
| 8 | Rated closed-loop breaking current | A | 1250 | 1250 | 1250 |
| 9 | No-load transformer breaking capacity | kVA | 2000 | 2000 | 2000 |
| 10 | Earth fault current | A | 20 | 20 | 20 |
| 11 | Line and cable charging current under earth fault | A | 20 | 20 | 20 |
| 12 | Rated short-circuit breaking current | kA | — | — | 31.5 or 50 (by fuse) |
| 13 | Rated transfer current | A | 3150 | 3150 | 3150 |
| 14 | Inherent opening time | ms | 45 | 45 | 45 |
| 15 | 1 min power-frequency withstand voltage (phase and earth, vacuum gap / isolating gap) | kV | 42/48 | 42/48 | 42/48 |
| 16 | Lightning impulse withstand voltage (phase and earth, vacuum gap / isolating gap) | kV | 75/85 | 75/85 | 75/85 |
| 17 | Mechanical life | operations | > 10000 | > 10000 | > 10000 |
Standards
- GB 3804 — AC high-voltage load break switches, 3.6 kV to 40.5 kV
- GB 16926 — High-voltage load break switch-fuse combination units
- IEC 60265 and IEC 60420 — Indoor switch-disconnectors and switch-fuse combinations
Installation & Wiring
Installation and Commissioning
- Panel mounting: the three-pole unit is bolted into the cubicle frame across an overall width of 886 mm, with a 650 mm mounting envelope, 600 mm between fixing centres, 210 mm phase spacing and an overall height of 997 mm; check busbar and cable clearance against the approved outline drawing before the frame is torqued.
- Mechanism checks: charge the spring, confirm the closed, open and earthed positions on all three poles, verify the interlocking between load break switch, isolating switch and earthing switch, and check the position indication at the operating panel.
- Fuse loading (combination unit): fit the specified fuse between the upper and lower clips, confirm the trip plate engagement, and verify that the fuse rating matches the tabulated 31.5 kA or 50 kA breaking capability.
- Before energization: carry out the 1 min power-frequency withstand test at 42 kV on the main circuit and 48 kV across the isolating gap, and bond the frame to the earth network of the switchgear.
Component Reference
- 1, 9: upper and lower outgoing lines
- 2: disconnector, the isolating switch
- 3: insulating seat
- 4: soft flexible connection
- 5, 7: upper and lower fuse clamps
- 6: circuit breaker, the fuse link, combination unit only
- 8: release plate
- 10: metal frame
- 11: vacuum arc extinction chamber
- 12: spring operating mechanism
- 13: operation panel
- 14: earthing switch
Applications
- Incoming and outgoing feeders in 12 kV metal-enclosed switchgear
- Transformer protection and control units with fuse combination
- Bus sectionalizing and ring-network distribution in indoor substations
- Cable branch units where a maintained earth is required for safety
Selection & FAQs
Selection Guide
- Fix the rated current. Choose 630 A or 1250 A; both carry 25 kA short-time withstand and a 63 kA peak, so the choice follows the feeder load rather than the fault duty.
- Decide which switches the cubicle needs. The D and G suffixes add the earthing switch and the isolating switch; order them together where the unit is the visible break point of the panel.
- Choose the plain switch or the combination unit. Transformer feeders normally take the FZR N25-12DG unit, where the fuse clears the fault; the plain switch relies on upstream protection.
- Coordinate the fuse with the transfer current. The combination unit is rated 200 A with 31.5 kA or 50 kA breaking capability and a 3150 A rated transfer current; a 125 A fuse is available for smaller transformers.
- Match the panel to the frame. The 886 mm width, 600 mm fixing centres and 997 mm height must fit the cubicle bay, and the front operating panel must stay accessible.
FAQs
Q1: What is the FZN25-12DG used for?
It is an indoor three-phase 12 kV, 50 Hz vacuum load break switch for switchgear cubicles, used to make and break load current, to isolate the circuit with a defined break and to earth the de-energised side for maintenance. With back-up fuses fitted it also protects transformers and feeders against short-circuit faults.
Q2: How do I read the model designation FZN 25-12DG/T 630-25?
F = load break switch, Z = vacuum, N = indoor type, 25 = design sequence number, 12 = rated voltage in kV, D = with earthing switch, G = with isolating switch, T = spring energy-storage mechanism, 630 = rated current in A and 25 = rated short-time withstand current in kA. In the combination unit the letter R after FZ denotes the added fuse, followed by the fuse rated current and the short-circuit breaking current.
Q3: Why are the load break switch, isolating switch and earthing switch built into one frame?
Each has a different duty: the vacuum interrupter breaks load current, the isolating switch provides the insulation of a maintained open gap, and the earthing switch bonds the dead side to the cubicle earth. Mounting all three on one frame with mechanical interlocking keeps the operating sequence correct and saves panel space.
Q4: Can the FZN25-12DG interrupt short-circuit current?
The load break switch itself does not; it withstands 25 kA for one second and makes 63 kA peak but breaks only load, closed-loop, cable charging and no-load transformer currents. In the FZR N25-12DG combination unit the back-up fuse interrupts the fault at 31.5 kA or 50 kA, and the 3150 A rated transfer current marks the point above which the fuse and the switch must clear the fault together.
Q5: How does the FZN25-12DG compare with the VZF-12R?
Both are indoor 12 kV vacuum switching units for metal-enclosed cubicles, and both are offered with back-up fuses. The VZF-12R is an embedded-pole, centre-mounted unit rated 630 A with 20 kA short-time withstand and a 125 A fuse option, while the FZN25-12DG frame carries the load, isolating and earthing switches openly on one assembly and is offered at 630 A and 1250 A with 25 kA withstand and a 200 A fuse. Select the VZF-12R for compact embedded panels and the FZN25-12DG where higher continuous current and a visible three-switch arrangement are required.
Q6: How does the FZN25-12DG compare with the SF6 FLN36-12 and the outdoor FZW32-12?
All three are load break switches rather than fault-interrupting devices. The FZN25-12DG interrupts in a vacuum chamber on an indoor open frame and is built for cubicles up to 1250 A; the FLN36-12 uses a sealed low-pressure SF6 three-position interrupter for 630 A indoor duty; and the FZW32-12 is the pole-mounted outdoor equivalent at 630 A with a visible isolating gap for overhead lines. Choose by installation site, insulation medium and whether the switch is panel-mounted or pole-mounted.