Automatic Transfer Switch - Bridged Contactor Diagram

This diagram shows a simple automatic transfer switch using two bridged contactors (one Normally Open, one Normally Closed) to automatically switch single-phase loads between grid power and an emergency power source.

Main Circuit Diagram

flowchart TB
    subgraph SOURCES[" "]
        direction LR
        subgraph GRID["⚡ GRID POWER (Mains)"]
            GL["L - Live"]
            GN["N - Neutral"]
        end
        subgraph EMERG["🔋 EMERGENCY POWER"]
            EL["L - Live"]
            EN["N - Neutral"]
        end
    end

    subgraph CONTACTORS[" "]
        direction LR
        subgraph K1BOX["K1 - NO Contactor"]
            K1["[ / / ]
Normally Open
Closes when energised"]
        end
        subgraph K2BOX["K2 - NC Contactor"]
            K2["[ — ]
Normally Closed
Opens when energised"]
        end
    end

    subgraph LOADBOX["🏠 LOAD (Protected Circuits)"]
        LOAD["Single Phase Circuits
L + N"]
    end

    GL --> K1
    GN --> K1
    EL --> K2
    EN --> K2
    K1 --> LOAD
    K2 --> LOAD

    style GRID fill:#e3f2fd,stroke:#1976d2,stroke-width:2px
    style EMERG fill:#fff3e0,stroke:#f57c00,stroke-width:2px
    style K1BOX fill:#c8e6c9,stroke:#388e3c,stroke-width:2px
    style K2BOX fill:#ffcdd2,stroke:#d32f2f,stroke-width:2px
    style LOADBOX fill:#f3e5f5,stroke:#7b1fa2,stroke-width:2px

K1 and K2 must be mechanically interlocked to prevent simultaneous closure

Contactor Coil Control Circuit

flowchart LR
    subgraph SUPPLY["Grid Supply"]
        L["Grid L"]
        N["Grid N"]
    end

    subgraph COILS["Contactor Coils (Both fed from Grid)"]
        C1["K1 Coil
(NO Contactor)"]
        C2["K2 Coil
(NC Contactor)"]
    end

    L -->|"Live"| C1
    L -->|"Live"| C2
    C1 -->|"Return"| N
    C2 -->|"Return"| N

    style SUPPLY fill:#e3f2fd,stroke:#1976d2,stroke-width:2px
    style COILS fill:#fff9c4,stroke:#fbc02d,stroke-width:2px

When grid fails, both coils de-energise simultaneously

State Diagram

stateDiagram-v2
    [*] --> GridPower: System Start

    GridPower: Grid Power Active
    GridPower: K1 (NO) = CLOSED
    GridPower: K2 (NC) = OPEN
    GridPower: Load fed by GRID

    EmergencyPower: Emergency Power Active
    EmergencyPower: K1 (NO) = OPEN
    EmergencyPower: K2 (NC) = CLOSED
    EmergencyPower: Load fed by BACKUP

    GridPower --> EmergencyPower: Grid Fails\n(Coils de-energise)
    EmergencyPower --> GridPower: Grid Restored\n(Coils energise)

Operation Summary

Grid Status	K1 (NO Contactor)	K2 (NC Contactor)	Load Fed By
ENERGISED	CLOSED ✓	OPEN	GRID POWER
FAILED	OPEN	CLOSED ✓	EMERGENCY POWER

Grid Status

K1 (NO Contactor)

K2 (NC Contactor)

Load Fed By

ENERGISED

CLOSED ✓

OPEN

GRID POWER

FAILED

OPEN

CLOSED ✓

EMERGENCY POWER

How It Works

Normal Operation (Grid Present): Both coils are energised. K1 (NO) closes to connect grid power to load. K2 (NC) opens to disconnect emergency power.

Grid Failure: Both coils de-energise. K1 (NO) opens to disconnect grid. K2 (NC) closes to connect emergency power to load.

Grid Restored: Both coils re-energise. K2 (NC) opens first, then K1 (NO) closes, returning load to grid power.

Key Features

Purely electromechanical operation - no control logic required

Automatic and instantaneous transfer on grid failure

Automatic return to grid when power is restored

Simple and reliable design with minimal components

⚠ SAFETY WARNINGS

MECHANICAL INTERLOCK REQUIRED: Install a mechanical interlock between K1 and K2 to prevent both contactors closing simultaneously. This is critical to prevent backfeeding the grid.
QUALIFIED INSTALLATION: This circuit must be installed by a qualified electrician in compliance with local electrical codes and regulations.
CONTACTOR RATING: Use contactors rated for your maximum load current and voltage (typically AC-3 or AC-1 rating as appropriate).
TIME DELAY: Consider adding a time delay relay to prevent rapid switching during grid voltage fluctuations or brownouts.
ISOLATION: Ensure the emergency power source is properly isolated from the grid when not in use.
TESTING: Test the transfer switch regularly to ensure proper operation.

Component List

Item	Description	Qty
K1	Contactor with Normally Open (NO) main contacts, coil voltage to match grid	1
K2	Contactor with Normally Closed (NC) main contacts, coil voltage to match grid	1
-	Mechanical interlock kit (for K1/K2 pair)	1
-	DIN rail / mounting hardware	As required
-	Appropriately rated wiring	As required

Item

Description

Qty

Contactor with Normally Open (NO) main contacts, coil voltage to match grid

Contactor with Normally Closed (NC) main contacts, coil voltage to match grid

Mechanical interlock kit (for K1/K2 pair)

DIN rail / mounting hardware

As required

Appropriately rated wiring

As required

AUTOMATIC TRANSFER SWITCH (ATS)
Bridged Contactor Configuration

Overview

Main Circuit Diagram

Contactor Coil Control Circuit

State Diagram

Operation Summary

How It Works

Key Features

⚠ SAFETY WARNINGS

Component List

AUTOMATIC TRANSFER SWITCH (ATS)Bridged Contactor Configuration

Overview

Main Circuit Diagram

Contactor Coil Control Circuit

State Diagram

Operation Summary

How It Works

Key Features

⚠ SAFETY WARNINGS

Component List

AUTOMATIC TRANSFER SWITCH (ATS)
Bridged Contactor Configuration