An Isolator is a mechanical switching device which, in the open position, provides an isolating distance between two contacts. It can open/close a circuit when negligible current is broken/made, and can carry currents under normal conditions plus, for a specified time, abnormal (short-circuit) currents. Conventional isolators are operated manually; when operated by a motor instead, it is called a Motorised Isolator. Single- and double-pole motorised isolators are used in 25kV AC OHE, Traction Substations (TSS), and Switching Stations (SSP) for maintenance and segregation. They are mounted on a mast/structure, operated via a DC motor fed through a multi-core insulated copper cable, and can be operated from a REMOTE LOCATION AT MORE THAN 500 METRES — with open/closed status available at the station master's chamber/cabin or another desired location.
- Advantages of Motorised Isolator: manpower saving; increased power block time due to quick isolator operation; quick segregation of a faulty section, saving time.
Key Terminology
- Earthing Switch: a mechanical switching device for earthing part of a circuit, capable of withstanding short-circuit-type abnormal currents for a specified time, but NOT required to carry current under normal conditions.
- Pole: the portion of an isolator/earthing switch associated exclusively with one electrically-separated conducting path of the main circuit, excluding parts that mount/operate all poles together.
- Normal Current: the current the main circuit is capable of carrying CONTINUOUSLY under specified use conditions.
- Short-Time Withstand Current: the current the isolator/earthing switch can carry in the CLOSED position for a short specified period under prescribed conditions.
- Isolating Distance: the clearance between open contacts (or conducting parts connected to them) in a pole, meeting safety requirements — distinct from ordinary 'clearance,' the general shortest-path distance between any two conducting parts.
- Short Time Rating: specifies the load at which the motor, starting at ambient temperature, may be operated for the period/conditions specified on the rating plate.
- Over Load: any load condition in excess of rated load, usually expressed as a percentage excess torque for the motor.
RDSO Specification (TI/SPC/PSI/ISOLTR/1060) — Three Distinct Types
Motor-operated, horizontally mounted on pole, vertical break type, suitable for outdoor use. One phase, one or two poles. Isolating distance (in air): 500mm minimum. Rated frequency: 50Hz ±3%.
| Parameter | Type I | Type II | Type III |
|---|---|---|---|
| Rated current | 1250 A (used in OHE at SP, SSP) | 1600 A (used at feeding post & traction substation) | 3150 A (used in OHE during DC-AC transition & 30MVA traction substation, Mumbai suburban) |
| Operating mechanism | 24V DC motor + battery charger; 24V 40Ah battery capable of 10 CO operations fully charged; 240V AC supply, 170-270V variation | 110V DC motor, no charger | 110V DC motor, no charger |
| Rated short-time withstand current (main & earthing switch) | 20 kA (rms) for 3s | 20 kA (rms) for 3s | 50 kA (rms) for 3s |
| Rated peak withstand current (main & earthing switch) | 50 kA | 50 kA | 125 kA |
| Rated Impulse Withstand Voltage (1.2/50µs, peak) | Value |
|---|---|
| To earth and between poles | 250 kV |
| Across the isolating distance | 290 kV |
| Wet power frequency 1-min withstand — to earth | 95 kV (rms) |
| Wet power frequency 1-min withstand — across isolating distance | 110 kV (rms) |
| Part | Max Temperature | Max Temperature Rise |
|---|---|---|
| Copper contacts in air | 90°C | 75°C |
| Bare copper or tinned aluminium alloy terminals (external conductor connection) | — | Limited so elasticity of material is not impaired; pure copper limit = 75°C |
Environmental & Climatic Conditions
| Parameter | Value |
|---|---|
| Maximum ambient air temperature | 55°C |
| Minimum temperature of air in shade | -5°C |
| Maximum relative humidity | 100% |
| Annual rainfall | 1750mm to 6250mm |
| Number of thunderstorm days/annum | 85 (max) |
| Number of dust storm days/annum | 35 (max) |
| Number of rainy days/annum | 120 (max) |
| Maximum basic wind pressure | 200 kgf/m² |
| Altitude | Not exceeding 1000m |
| Pollution level | Very heavy, per IEC 815-1986 & IS 13134-1992 |
Intended for use as outdoor line-sectioning switches, including in normally polluted atmospheres (dust, smoke, chemical-plant effluents) and coastal areas.
Design & Constructional Features
- Supply voltage: motor suitable for 24V DC (Type I) or 110V DC (Types II/III); battery-terminal voltage may vary 85-110% of normal. The isolator must be able to operate MANUALLY if the motor circuit fails. Once a closing/opening operation is initiated, it must complete AUTOMATICALLY (snap action) even if the DC supply fails mid-operation.
- Motor Drive Unit: installed in a weather/corrosion-proof, adequately ventilated cubicle of sheet steel NOT LESS THAN 2mm thick with stiffeners, sloping roof, and a metal-clad space heater (thermostat-controlled) to prevent condensation. Cubicle top height: ~1.5m from rail level. Manual operating-handle hinge height: ~1.1m from rail level.
- Control Circuit: operated from RCC by TPC, and also from the isolator cubicle itself, via a local/remote spring-loaded switch — with tele-signal/tele-command terminations for RCC operation, plus motor protection relays.
- Operating Mechanism: closes when the handle is moved UPWARDS. Single-pole isolators permit coupling for gang operation from one handle — but NOT MORE THAN TWO single-pole isolators may be coupled. 'OFF'/'ON' plates indicate status. Hinge pin height: 1.1m from ground.
- Pole/Blade: all poles operate simultaneously; movable joints use maintenance-free brass bushes/bearings. Blades are electrolytic high-conductivity copper tubes per IS:191 (Part V)-1980, sized to withstand thermal/bending stress especially under short-circuit conditions.
- Fixed Contact: extruded/rolled/drawn copper strips, spring-loaded, non-ferrous or stainless steel components only. Springs are phosphor bronze per IS:7608-1975, insulated with nylon strip to prevent current passing through the springs themselves.
- Bolts/Nuts/Lock Nuts: stainless steel, 12mm dia (larger sizes may be hot-dip galvanized steel), per RDSO spec ETI/OHE/18.
- Earthing: frame earthing terminal with clamping screw diameter NOT LESS THAN 12mm, marked with the 'Earth' symbol.
- Operating Pipe: no joints permitted; minimum FOUR guides; supplied to the exact length for the particular mast/structure, from approved manufacturers only.
- Malleable Cast Iron & Ferrous Parts: black-heart malleable cast iron grade BM-340 per IS:2108-1977; all ferrous parts hot-dip galvanized per ETI/OHE/13 (4/84).
- Steel Structure: withstands 200 kgf/m² wind pressure acting on 150% of projected area, in addition to all other loads — no shock permitted during operation, and no backlash tendency to re-open after closing due to jerk.
- Locking: isolator interlocks with associated CB/Interrupter to prevent mal-operation (including manual). A bolt-type integral lock/interlock permits locking in BOTH open and closed positions — the key cannot be withdrawn except when locked in one of these two positions. Padlocking is additionally permitted. Internal mechanism: stainless steel/non-ferrous; outer housing: chrome-plated steel, theft-resistant construction. Two keys per lock, with key/lock number inscribed.
- Earth Contact Assembly: the design must allow a LATER-DATE addition of an earth contact pair (to earth the fixed contact when open) WITHOUT modifying the frame — two holes of 15.5±0.5mm dia are pre-provided on the mounting channel for this purpose.
- Arcing Horn: single-pole isolators only, bright steel Gr. 20-C-8 per IS:1570 (Pt. II)-1979. Arcing horns make contact BEFORE main contacts close, and break contact AFTER main contacts break — protecting the main contacts from the arc.
Real Implementation 1: DMRC, New Delhi
A 25kV single-pole motorised isolator with a Monitoring Box is provided at DMRC New Delhi, operating at 110V DC via a three-position switch (Remote/Local/Manual). Its accessories: 110V DC supply for the motor, 240V AC supply, 110V DC control supply, terminal block for DC & AC, and the monitoring box. The 110V DC motor supply is taken from terminal block-2 (points 51-52), fed through a 2-pole MCB, then through an overload relay (OLR) and open/close contactor contacts (OC & CC) to the motor. The 240V AC supply (from terminal block-2, points 55-56) feeds the cubicle heater, thermostat, and power socket via a single-pole-with-neutral MCB. The 110V DC control supply energises the open/close contactor solenoid coils via terminal block-1 (points 1-2).
Real Implementation 2: South Central Railway (Tirupati)
A genuinely distinctive cross-department engineering solution: at Tirupati, the motorised isolator is driven by a 143mm-throw ELECTRIC POINT MACHINE — borrowed directly from the Signalling department's point-machine technology, complete with internal locking. The 25kV/240V AC supply comes from an auxiliary transformer at the cabin/ASM room; an AC-DC converter steps this down to 110V DC, which reaches the point machine via a junction box over multi-core copper cable. A spring-type rotary switch in the junction box issues the open/close command, with OPEN/CLOSE status also indicated there.
Mechanically: 110V DC power reaches the motor via a rotary switch; the motor drives the point mechanism through a friction clutch, reduction gears, and throw bar. The motor's rotary motion converts to rectilinear (straight-line) movement of a toothed driving bar via a transmission assembly. A detector slide, attached to the isolator's lever and operating rod, moves OUTSIDE to open the isolator (with open indication via contactor to the junction box) and INSIDE to close it (close indication likewise via contactor). If the motor or power supply fails, a CRANK HANDLE can be inserted into the point machine for fully manual operation.
| DC Motor — Parameter | Value |
|---|---|
| Type | DC split-field series motor |
| Rated voltage | 110V DC (capable of ±25% of rated voltage) |
| RPM | 1700 ±15% |
| Rated current / max current | 5.3A / 8.5A |
| Operating time | 4 to 5 seconds |
| Power | 440 Watts |
| Gear oil | SAE 30 |
| AC-DC Converter — Parameter | Value |
|---|---|
| Type | Single-phase, air-cooled, for indoor installation |
| Input voltage | 240V AC |
| Output voltage | 110V DC |
| Current | 10A |
| Rated frequency | 50 Hz |
| Power | 1.1 kW |
| Type of cooling | AN (natural air) |