Day 1: The Simplest Distribution Circuit (SLD)

Today you will read your first Single Line Diagram (SLD) — the most fundamental drawing type in switchboard drafting.

Don't worry about symbols, standards, or component ratings yet. Just look at the drawing and think about how electricity moves through it.

The Drawing

This is the simplest possible distribution circuit: one supply, one main switch, one circuit breaker, one busbar, and one load.

   Incoming Supply
        |
        |
   +-----------+
   | Main SW   |
   | 100A      |
   +-----------+
        |
        |
   +-----------+
   |   MCCB    |
   |   80A     |
   +-----------+
        |
        |
   =================
        BUSBAR
   =================
        |
        |
   +-----------+
   |   MCB     |
   |   20A     |
   +-----------+
        |
        |
     LOAD 1
    Lighting

5 Questions

Try to answer each question before expanding its answer. It's completely fine to guess — the point is to engage with the drawing, not to be correct.

Q1

Where does the power enter the switchboard?

Click to see the answer

From the utility grid at the top of the drawing — three phase supply.

But as a draftsperson, watch the expression on the drawing. A real drawing would not say:

Grid 380V 400A

It would say:

Incoming Supply
400V 3 Phase + Neutral + Earth
From Utility Transformer

The New Zealand low-voltage system is normally:

400V 3 Phase
230V Phase-Neutral

Note: China commonly says 380V, but NZ/AU engineering documents write 400V three phase.

Q2

What is the direction of current flow?

For example:

Supply → ? → ? → Load

Click to see the answer

For this drawing:

Utility Supply
      ↓
 Main Switch
      ↓
    MCCB
      ↓
   Busbar
      ↓
    MCB
      ↓
    Load

There is no transformer in this drawing — and that's worth pausing on. Many installations look like:

Transformer
     ↓
Main Switchboard
     ↓
Sub Switchboard
     ↓
   Loads

But a small commercial board may simply be:

Grid
  ↓
Switchboard
  ↓
Loads

So whenever you open a drawing, first ask: which level of the distribution hierarchy is this board?

Q3

What is the difference between the Main Switch and the MCCB?

(It's okay if you don't know — take a guess.)

Click to see the answer

Main Switch — switches the entire board on and off. Its job is isolation:

ON  → the whole switchboard is live
OFF → the whole switchboard is dead

MCCB (Moulded Case Circuit Breaker) — its job is protection against overload and short circuit:

Abnormal current → MCCB trips → equipment protected

A simple way to remember:

Device	Role
Main Switch	Switches the whole system on/off (isolation)
MCCB	Protects the main circuit
MCB	Protects a small branch circuit

Q4

Why do we need a Busbar?

Click to see the answer

Imagine there were no busbar. Every circuit would need its own incoming cable:

Incoming Cable → MCB1 → Load 1
Incoming Cable → MCB2 → Load 2
Incoming Cable → MCB3 → Load 3

Messy. The busbar is one large electrical bar that distributes the supply to multiple circuits:

     Incoming
        ↓
     Busbar
   |    |    |
 MCB1 MCB2 MCB3
   |    |    |
  L1   L2   L3

A switchboard is fundamentally a power distribution system — and the busbar is its core.

Q5

If we add a second load, for example:

Motor

where should it be connected?

Click to see the answer

Correct direction: hang a new protective device off the busbar, then the motor. But a motor is usually not just:

MCB → Motor

because a motor needs both control and protection. The typical motor circuit is:

       BUSBAR
          |
          |
        MCCB
          |
      Contactor
          |
    Overload Relay
          |
        MOTOR

You'll meet the contactor and overload relay properly in a later day — for now, just remember that motors get their own control-and-protection chain, not a bare MCB.

Today's Learning Goal

Today is not about learning:

• MCCB parameters
• IEC standards
• Symbol conventions

Today is about building one core concept:

A switchboard is fundamentally a device that takes one large power supply and safely distributes it to multiple loads.

If you can look at the drawing above and see that idea — a supply at the top, protection in the middle, loads at the bottom — Day 1 is done.

Summary

Every switchboard, no matter how large, follows the same chain:

Incoming Power
      ↓
  Isolation
      ↓
  Protection
      ↓
 Distribution
      ↓
  Protection
      ↓
     Load

Map it back to today's drawing: the Main Switch is the isolation, the MCCB is the upstream protection, the Busbar is the distribution, the MCB is the per-circuit protection, and Load 1 is the load. Hold onto this chain — every drawing in the next four days is just this pattern repeated and expanded.