Question

Explain with a neat sketch the pantograph type of isolator. State the sequence of operation of isolator, CB, and earthing switch while opening and closing.

Answer 1

### Pantograph Type Isolator

A pantograph type isolator is commonly used in electrical substations and overhead transmission lines to isolate a section of the circuit for maintenance purposes. It is designed to ensure that the line is disconnected from the power source and provides a physical separation of the circuit.

#### Neat Sketch

Imagine a simple diagram with the following components:

1. **Fixed Contact**: A stationary terminal connected to the busbar or power line.
2. **Moving Contact**: A pivoting arm that connects to the fixed contact when closed.
3. **Support Insulators**: Insulators that support the moving arm and keep it isolated from the grounded structure.
4. **Operating Mechanism**: A handle or motor mechanism to operate the isolator.
5. **Grounding Switch**: A switch that connects the circuit to the ground for safety.

Here's a simple description of how to visualize the sketch:
- Draw a vertical line representing the busbar.
- Connect a horizontal line representing the overhead line.
- Show the fixed contact connected to the busbar.
- Illustrate the moving contact on a pivot that swings away from the fixed contact to open the isolator.
- Place insulators below the moving arm to support it.

### Sequence of Operation

The operation of the isolator, circuit breaker (CB), and earthing switch involves a systematic sequence to ensure safety and effectiveness. Here's the sequence when opening and closing the isolator:

#### Opening Sequence

1. **Circuit Breaker Operation**:
   - The circuit breaker is opened first. This interrupts the current flowing through the line, ensuring that there is no electrical load when the isolator is operated. The CB is designed to handle fault currents and should be opened under normal conditions first.

2. **Isolator Operation**:
   - Once the circuit breaker is open, the isolator can be operated. The moving contact is swung away from the fixed contact, creating a physical disconnection in the circuit. This operation is performed with the mechanism (manual or motorized).

3. **Earthing Switch Operation**:
   - After the isolator is open, the earthing switch is closed. This connects the line to the ground, ensuring that any residual charge is safely dissipated. The earthing switch provides a safety ground to prevent electrical shock or accidents during maintenance.

#### Closing Sequence

1. **Earthing Switch Operation**:
   - The sequence begins with opening the earthing switch. This disconnects the ground, ensuring that the line is safe to re-energize. The earthing switch must be opened first to avoid any back-feed of current.

2. **Isolator Operation**:
   - After the earthing switch is open, the isolator is closed. The moving contact swings back to connect with the fixed contact, thus re-establishing the circuit.

3. **Circuit Breaker Operation**:
   - Finally, the circuit breaker is closed to restore power to the line. The CB checks for any faults before re-energizing the circuit to ensure that it is safe to do so.

### Summary

The pantograph type isolator plays a crucial role in safely isolating circuits in power systems. Its operation, along with that of the circuit breaker and earthing switch, ensures a safe working environment during maintenance. Following the correct sequence of operations is vital to prevent electrical hazards.

Answer 2

### Pantograph Type Isolator

#### **Description**

A Pantograph-type isolator is a switch used in electrical systems to isolate a section of the circuit for maintenance. It is called a "pantograph" because its mechanism resembles the pantograph used in drawing, which can trace a pattern while maintaining a consistent scale.

**Features of Pantograph Isolator:**
- **Mechanism:** It uses a pantograph linkage system to move the contacts. This design helps maintain a consistent gap between the contacts and allows for reliable isolation.
- **Construction:** It typically includes an arm with multiple pivot points and a central linkage that moves the contacts apart or together. The contacts themselves are often mounted on insulators.

**Sketch:**

```plaintext
      [--]                        [--]
       |                            |
       |                            |
  +----+----+                +----+----+
  |         |                |         |
  |         |                |         |
  |  Fixed  |                |  Moving |
  |  Contact|                |  Contact|
  |         |                |         |
  +----+----+                +----+----+
       |                            |
       |                            |
      [--]                        [--]
      Isolator                    Pantograph Mechanism
```

In this simplified sketch:
- The **Fixed Contacts** are stationary.
- The **Moving Contacts** are connected to a pantograph mechanism that moves them apart or together.
- When the isolator is in the open position, the contacts are separated, isolating the section of the circuit.

### Sequence of Operation

#### **Opening**

1. **Isolator Operation:**
   - The isolator is operated first. The pantograph mechanism moves the moving contacts away from the fixed contacts, breaking the circuit.
   - The isolator mechanism ensures that the contacts are separated completely before the circuit breaker is operated.

2. **Circuit Breaker (CB) Operation:**
   - In some systems, the circuit breaker is operated only after the isolator is fully open to ensure there is no load current passing through the circuit breaker. This is because the circuit breaker’s main function is to interrupt the current under fault conditions, and it should not have to deal with current flow when being operated for maintenance.

3. **Earthing Switch:**
   - After the isolator is fully open and the circuit breaker is opened (if necessary), the earthing switch is operated. This switch connects the isolated circuit to the ground, ensuring that it is safely grounded before maintenance is performed.

#### **Closing**

1. **Earthing Switch:**
   - The earthing switch should be the first to be operated if it was used during opening. This removes the grounding and prepares the system for re-energization.

2. **Circuit Breaker (CB):**
   - Once the earthing switch is in its normal position (i.e., not grounded), the circuit breaker can be closed. This re-establishes the circuit and prepares it for normal operation.

3. **Isolator Operation:**
   - Finally, the isolator is closed. The moving contacts are brought back into contact with the fixed contacts, re-energizing the circuit.

**Sequence Summary:**

- **Opening:**
  1. Operate Isolator
  2. Operate Circuit Breaker (if applicable)
  3. Operate Earthing Switch

- **Closing:**
  1. Operate Earthing Switch
  2. Operate Circuit Breaker
  3. Operate Isolator

This sequence ensures safety by isolating the circuit before grounding it and by re-establishing the circuit only after ensuring that it is safe to do so.