A tandem isolator is a pair of isolators of the same rating mounted and operated together so that two circuits, or two breaks of one circuit, open and close with a single mechanism. It is common in 33 kV to 400 kV substations where space and cost matter.
What Is a Tandem Isolator?
In a tandem arrangement, two isolator poles sit side by side and are linked by an interconnecting rod or common operating shaft. One drive operates both, so the isolators open and close together. The term is sometimes also used for isolators connected in series on the same line to increase the isolating gap.
How a Tandem Isolator Works
- The operator turns the handle or starts the motor drive.
- The torque passes through the common shaft or pipe to both isolator poles.
- The blades rotate and open the contacts at the same time.
- Earth switches, if fitted, are interlocked so they cannot close while the isolator is closed.
Always open the circuit breaker first. Isolators are off-load devices and must not break current.
Benefits of a Tandem Isolator
- Single operation: One drive controls both poles, saving operator time and reducing mistakes.
- Space saving: Compact layout reduces the switchyard footprint.
- Lower cost: One mechanism and one control cabinet instead of two.
- Better isolation gap: Two breaks in series improve the safety margin on high voltage lines.
- Easy interlocking: Common mechanism makes breaker and earth switch interlocks simpler.
Where Tandem Isolators Are Used
- Double circuit lines where two feeders are switched together
- Bus section and bus coupler arrangements
- Transformer and capacitor bank bays
- Industrial and railway traction substations
Selection Checklist
- Rated voltage, current and short-circuit withstand
- Single break, double break or centre rotating type
- Manual or motorised operation
- Creepage distance for local pollution levels
- Insulator type: porcelain or polymer
Maintenance Tips
- Clean and lubricate the shaft bearings every year
- Check contact alignment and pressure
- Inspect the insulators for cracks or tracking
- Test the interlocks before every shutdown
Isolator Basics Before We Go Further
An isolator, also called a disconnector, is a mechanical switch that gives a visible, safe gap in a circuit so that work can be done on equipment that has been de-energised. It is designed to open and close when the circuit carries negligible current. It cannot interrupt load or fault current, which is the job of the circuit breaker. Because the isolator is the point of protection for maintenance crews, its operation, locking and earthing are tightly controlled in every substation.
Tandem Arrangement Explained in More Detail
The word tandem simply means one behind another or side by side and working together. In substation practice it is used for a few related setups, and it helps to know which one a drawing is showing.
- Two poles on one drive: two isolator poles, often for two circuits or two phases of a bus section, are connected by a coupling rod and operated by one handle or motor.
- Two breaks in series: two isolating gaps in the same phase give a greater clearance, useful for higher voltage lines and for improved safety on long outages.
- Isolator with earth switch: an isolator and an earthing switch mounted together and mechanically interlocked so that one cannot close when the other is closed.
In every version, the aim is the same: operate related switching devices together, reduce the number of separate manual actions and make wrong sequences impossible.
Main Parts of a Tandem Isolator
- Base frame: hot dip galvanised steel channel that carries the whole assembly and is bolted to the support structure.
- Post insulators: porcelain or polymer columns that carry the live parts and insulate them from the frame.
- Current carrying parts: the blade, fixed contacts and terminal pads, usually made of electrolytic copper or aluminium alloy, with silver plated contact surfaces on better designs.
- Operating mechanism: a handle, gearbox or motor drive with a mechanical linkage that turns the insulators or moves the blade.
- Interlock: mechanical and electrical interlocks that link the isolator with the circuit breaker and the earth switch.
- Auxiliary contacts: position indication for the control room.
Step by Step Operating Sequence
- Confirm that the circuit breaker on the bay is open and that its position is verified on the panel.
- Release the interlock key or the electrical permission signal.
- Operate the isolator drive. Both poles should rotate fully and evenly.
- Check the visible gap on each pole or break from the ground.
- Close the earth switch if the work needs earthing, and lock it in position.
- Fit the safety tags and complete the permit to work documents.
The reverse order is used when returning the equipment to service: remove the earth, close the isolator, then close the circuit breaker.
Why Substations Choose Tandem Arrangements
Operational safety
When two switching devices are operated from one drive, the chance of closing one while the other remains open is removed. Linked earth and main blades make it mechanically impossible to earth a live line.
Savings in cost and space
A shared drive, a single control cabinet and a common frame reduce material, cabling and civil work. In an older yard with limited space, a compact tandem layout can fit a bay that would not fit with separate units.
Lower maintenance workload
One mechanism to lubricate, adjust and test means fewer moving parts to keep healthy. Gear boxes, bearings and linkage are inspected once instead of twice.
Limitations and Points to Watch
- A fault in the common mechanism affects both poles, so spare parts and quick repair matter.
- Alignment is critical. If the linkage drifts, one pole may not fully open or close.
- Replacement of one pole may need shutdown of both circuits if they share a frame.
- Ratings must suit the higher of the two circuits when they share one structure.
Ratings and Specification Points
| Parameter | What to specify |
|---|---|
| Rated voltage | Highest system voltage, for example 36 kV, 72.5 kV, 145 kV or 245 kV |
| Rated current | Continuous current such as 630 A, 1250 A, 1600 A or 2000 A |
| Short-time withstand | Fault current and duration, for example 31.5 kA for 1 second |
| Insulation level | Power-frequency and lightning impulse withstand |
| Creepage distance | Based on the pollution level at the site |
| Mechanism | Manual, motor operated or both, with local and remote control |
| Mounting | Upright, underhung or vertical break, as the layout needs |
Maintenance Checklist
- Clean insulators and check for cracks every year, more often in polluted areas.
- Check contact surfaces for pitting, burning and loss of silver plating, and restore them as needed.
- Measure contact resistance and compare it with the commissioning value.
- Lubricate bearings and gears with the grease the manufacturer recommends.
- Test the drive motor, limit switches and interlocks.
- Check earthing connections and flexible braids for fatigue.
- Operate the isolator several times before a planned shutdown so any sticking shows early.
Selecting a Manufacturer
Choose a manufacturer that can show type test reports for short-time current, temperature rise and mechanical endurance, and one who can supply spares for the full life of the equipment. Ask for drawings that clearly show the interlock arrangement. SPKN India manufactures isolators in Faridabad for substations and industrial plants, and also supplies spares, so that an old installation can be maintained for many years.
To compare arrangements, read single break vs double break isolators and the guide to how isolators work in substations.
Typical Faults and How to Avoid Them
Poles out of step
If one pole closes before the other, the contacts see unequal pressure and may overheat. Re-set the linkage lengths and check that both blades reach the end stop at the same time.
Stiff or jammed mechanism
Dust, corrosion and old grease are the usual causes. Clean the bearings and fit weather covers on the gearbox. In coastal sites, use stainless steel fasteners and sealed bearings.
Overheating at contacts
Loose bolts, oxidised surfaces or worn silver plating raise contact resistance. A thermal camera scan during peak load will show hot spots before they cause an outage.
Interlock failure
A bent interlock rod or a faulty auxiliary switch can allow a wrong operation. Test the interlock with the breaker closed and the isolator closed, and confirm that the operation is blocked.
Case Example: A Compact Bay Upgrade
Consider an industrial 33 kV switchyard with two incoming feeders and limited space. The plant wants to isolate both feeders for yearly maintenance without a long shutdown. A tandem isolator with a common drive, an integral earth switch and a mechanical interlock with the incoming breaker allows the crew to isolate and earth both feeders in a single, controlled sequence. The same area would otherwise need separate isolators, separate earth switches and two sets of interlock cables. This is a typical illustration of the benefits, and the exact layout depends on your single line diagram.
Looking for Isolators for Your Substation?
SPKN India manufactures 11 kV to 400 kV isolators, including single break, double break and tandem arrangements, from Faridabad. Send us your single line diagram and we will quote.
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