Main switches and overloads

I often tell my clients that they have to upgrade their main switch. But why is that? The following article gives an overview of what a main switch does, and why it needs to be replaced.

Consumer Mains and Overloads

Main switches are often overlooked by electricians, but it has serious consequences if it is.

Main switch on an old switchboard
What is a main switch. This is what they could look like on an old switchboard.

In old houses, a main switch was simply that – a switch. It turned the whole switchboard on, or off. There was no current (load) limiting. You can think of load as the power you are using at any particular time.

As consumers have changed the way in which they use electricity, homes have increased the number of power points, lights, air conditioning etc., the load on the consumer mains (the supply from the street) has increased. The result may be that there is an increased likelihood that the consumer mains will experience an overload.

Where consumer mains are overloaded, the cable insulation will start to break down; a fault may occur, there will be an increasing likelihood of a fire occurring.

Why?

All houses have fault current limiters – service fuses. The service fuses only protect what is within the switchboard and the home from significant short circuits, they do NOT protect against overload. All cabling before the service fuses is considered un-protected..

If a fault occurs in the consumer mains which results in arcing, burning or a short circuit, the only protection that you have is the circuit protection in the nearest substation that supplies cables to your street.

This would be a very serious situation that would result in significant damage to your home. Your house will burn.

Switchboard fire
An overload could lead to a house fire!

What is overload current.

Most houses have either a 10mm2, or a 16mm2 consumer mains – some are only 6mm2 and have a rating of just 40A. They may be single phase (2 wires), or three phase (4 wires). Occasionally two phases may be present (3 wires)

In most instances a 10mm2 consumer mains has a capacity of 50A, while a 16mm2 consumer mains has a capacity of 63A – according to the Wiring Rules.

But…

The maximum rating of the cable may be different in different circumstances, but for this discussion, we will leave it at that.

Overload current is the current, above the rating of your cables (though we are talking about consumer mains in this instance), that occurs when too many appliances are connected and running. 

Why does all of this matter?

If you only have a main switch in your switchboard, like the one in the photo above, there is no overload protection.

Should too much current (load) be present on your circuits, your consumer mains will overheat and the insulation of your cables will break down. This will either cause a fire, arcing, or a short circuit – or all three.

This type of fire is catastrophic and causes substantial damage to a home.

The fire brigade has to disconnect the street power before your fire can be safely put out, and there can be delays in shutting off the street power – and your house will keep burning!

Am I just trying to make more money by over servicing?

Perhaps you think I am trying to get you to do more work than you need so that I can make more money?

Maybe you recently had some electrical work done and the other electrician didn’t say anything!

Well here’s some clauses from the Wiring Rules to clarify my statements.

Clause 2.5.1.2 of AS/NZS 3000:2018 (Clause 2.5.1.1 in AS/NZS 3000:2007), Overload protection of consumer mains shall be arranged in accordance with one of the following:

Note 5 (Note 6 in AS/NZS 3000:2007): Consumer mains supplying one or more circuits that are individually protected against overload should be provided with overload protection where the sum of the current ratings of the individual circuit-breakers so supplied exceeds the current-carrying capacity of the consumer mains.

So what does that mean in simple terms?

Let’s say I have a 10mm2 consumer mains – this generally gives us a 50A supply.

  • two 15A power circuits – total 30A
  • a 20A stove – 20A
  • a 20A water heater –20A
  • two 8A light circuits – total 16A

  • The total for the combined fuses is: 86A – far in excess of the 50A consumer mains capacity. The circuit doesn’t comply, and is overloaded.

What does this mean to the consumer?

If I am requested to add even one power point, I am expected to check the loading of the consumer mains and determine if the capacity can handle the increase in load –

Section 8 of the wiring rules lists mandatory checks and testing that must take place.

Clauses: 

8.2.2 (b) Consumer mains: (i) Current-carrying capacity

8.2.2 (c) Switchboard: (iii) Isolating devices e.g. main switches

If I have checked your main switch and found that it doesn’t have overload protection, I will report it to you- generally you will understand why it needs to be replaced.

Should you, or shouldn’t you fix it – the choice is always yours.

You are legally required to make repairs where they are shown to be required, but what if you don’t want to fix it?

Have a read over a previous article where I discuss this. To fix or not to fix, that is the question.

I am guided by the wiring rules

While I understand that changing over to a new main switch might be costly for you when you only wanted one extra power point, unfortunately I am LEGALLY required to do so!

If another electrician comes along and says that you don’t need to change your main switch, ask them to show you which clause in the Wiring Rules says so.

Get them to prove it to you as I have just done above.

Bye for now,

Greg.

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#MainSwitch #Overload #Compliance #Fire #HouseFire #ShortCircuit

electrician; electrical repairs; electrical installations; power points; lights; light fittings; light switches; electrical fault; electrical faults; electricity; electrical hazard; electrical safety; electrical contractor; Normanhurst,2076; Wahroonga,2076; Asquith,2077; Hornsby,2077; Hornsby Heights,2077; Waitara,2077; Mount Colah,2079; Mount Ku-ring-gai,2080; Berowra,2081; Berowra Heights,2082; Carlingford,2118; Beecroft,2119; Cheltenham,2119; Pennant Hills,2120; Thornleigh,2120; Westleigh,2120; Epping,2121; North Epping,2121; Cherrybrook,2126; Dural,2158; Middle Dural,2158; Arcadia,2159; Galston,2159; West Pennant Hills,2125;

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