Technical Details – HT Isolator
1. Overview
An HT Oil-Immersed Isolator Switch Fuse Unit (SFU) is a combined switching and protection device used in medium-voltage (MV) systems (typically 6.6 kV to 33 kV). It consists of:
- An isolator switch for safe disconnection,
- Current-limiting HRC fuses for fault protection, and
- An oil-immersed enclosure for insulation and arc-quenching.
These are widely used in 11 kV/22 kV/33 kV substations, distribution panels, and transformer protection.
2. Typical Applications
- Protection of distribution transformers
- Incoming/outgoing feeders in substations
- Ring main units (RMU)
- Industrial and utility MV networks
- Rural electrification and pole-mounted switchgear
3. Key Technical Specifications
|
Parameter
|
Typical Range/Value
|
|---|---|
|
System Voltage
|
6.6 kV, 11 kV, 22 kV, 33 kV
|
|
Rated Current
|
100 A to 630 A (depending on model)
|
|
Short Circuit Breaking Capacity
|
16 kA to 25 kA for 3 seconds
|
|
Insulation Medium
|
Mineral insulating oil
|
|
Switching Mechanism
|
Manual / Motor-operated
|
|
Fuse Type
|
HRC (High Rupturing Capacity) cartridge fuses
|
|
Poles
|
3-pole (usually)
|
|
Installation
|
Outdoor/Indoor; Pole-mounted or floor-mounted
|
|
Standard Ratings
|
12 kV/400 A/50 Hz, 24 kV/630 A/50 Hz etc.
|
4. Construction Features
- Isolator switch: Provides visible isolation for safety during maintenance.
- Fuse holder: Supports high-rupturing capacity fuses (usually DIN type).
- Oil tank: Filled with transformer-grade insulating oil for arc suppression and cooling.
- Arc-quenching chamber: Reduces arc duration during switching.
- Bushings: Porcelain/epoxy insulators for incoming and outgoing terminations.
- Earthing provision: Ensures safety during operations and maintenance.
5. Protection Features
- Fuses provide fast-acting protection against overloads and short circuits.
- Current limitation minimizes damage to downstream equipment.
- Manual trip/operation allows safe isolation.
- Optionally, blown fuse indication may be available.
6. Standard Conformity
- IEC 62271-105 – High-voltage alternating current switch-fuse combinations
- IEC 60265 / IS 9920 – High-voltage switches
- IS 9385 – Fuses for voltages exceeding 1000 V AC
- IS 3427 – Metal-enclosed switchgear
- IS 3637 – Switches and isolators
7. Advantages
- Combines switching and protection in a single compact unit
- Arc suppression via oil ensures safety and longevity
- Suitable for outdoor and harsh environments
- Low maintenance compared to vacuum/air-insulated units
- Cost-effective for distribution-level protection
8. Limitations
- Oil maintenance required (filtering, testing, or replacement)
- Environmental concerns with oil leakage
- Slower switching speed than vacuum circuit breakers (VCBs)
- Not ideal for frequent operations (limited mechanical endurance)
9. Optional Accessories
- Fuse blown indicator
- Earth switch
- Pad-lockable operating handle
- Oil level indicator
- Temperature gauge
CONNECT WITH VOLTAMP®