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What Causes Electrical Faults: Understanding MCBs, GFIs and RCDs

My Breaker Box

My Breaker Box

Types of Circuit Breakers

An electrical panel or breaker box is fitted with several Miniature Circuit Breakers (MCBs) and a Ground Fault Circuit Interrupter (GFCI), also known as a Residual Current Device (RCD) in Europe. These safety devices help to protect you from being electrocuted in the event of a fault and also prevent overloading of wiring which can start a fire.

Typical MCBs

Typical MCBs

What is an MCB?

An MCB is an electro-mechanical device, and like a fuse, it acts as the "weak link in the chain". It will trip to protect cables from overload currents which can damage the cable or even cause a fire. An MCB has two mechanisms which act in different ways to give protection.

Firstly the thermal mechanism uses a heating coil to flex a bi-metalic strip strip which opens a pair of contacts when current becomes excessive (effectively a thermostat). This protects against moderate current overloads. The second element is an electromagnet which pulls open the contacts when current is excessive. This electromechanical element acts much faster than the thermal mechanism in the event of large current overloads.

What is a GFCI / RCD?

A GFCI will trip and shut off power if there is a flow of current from hot (live) to ground (earth). A GFCI measures the flow of current flowing out via the hot conductor to a circuit and returning via the neutral. Normally these currents are equal. If a fault occurs, some of the hot current doesn't return and takes a "detour" to ground.

The GFCI detects this and shuts off power in a split second. A typical fault that trips a GFCI is when the connector of the flex of a corded kettle is left in a pool of water on the sink. A GFCI can normally be identified because it is wider than an MCB in the breaker box and has a small test button on it. (which should be pressed regularly to check it works ok).

Faults That Can Trip an MCB

  • Connecting too many high powered appliances to an electrical circuit. In modern installations, there are usually lots of outlets and separate circuits for different sections of a home, e.g. upstairs and downstairs. In a kitchen, there are usually at least two circuits. So there is less chance of an overload as power demand is distributed between circuits. In an older installation, however, this may not be the case.
  • A fault in equipment causing a short circuit of current from hot to neutral. This could be due to insulation on a wire becoming compromised in some way. The exposed wire could then make contact with a neutral or ground conductor or terminal.
  • Breakdown of the windings in a transformer or motor. Wire in these devices is usually coated with a thin layer of polyurethane varnish or similar so that lots of turns of wire can be tightly packed together. Over time the varnish layer can be compromised. This can be due to heating of the windings as the appliance is loaded e.g. when a power tool is used for a long period of time and isn't allowed to cool properly. Overheating softens and cracks the insulation and in addition, abrasion due to particles of dust being sucked through a motor, wears away the insulation of the coils. This eventually causes arcing to occur between adjacent turns. Once this starts to happen, the winding can go into meltdown, but hopefully the breaker will trip and save the day before the device catches fire.
  • Power cord of an electric garden tool is cut through. The metal blades or teeth of the tool short hot and neutral together.
  • You drill through a wall and hit a power cable. The drill bit shorts out hot and neutral and causes an overload. Alternatively since the hot in the cable touches the drill bit, this could provide a path to ground through the body of the drill. Nowadays many power tools are "doubly insulated." This means that even though the outer casing may be metal, sufficient insulating barriers are built into the tool so that the external metal casing cannot become live due to an internal fault. These tools or appliances are only supplied with hot and neutral via the power cord, and not grounded. Most power tools have a plastic body though for absolute safety in damp environments.

How Does an Overload or Short Circuit Cause an MCB to Trip?

An electrical circuit can be represented by a diagram called a schematic or circuit diagram. In its simplest form, an electric circuit would have a voltage source connected to a load. The voltage source could be a battery or mains power supply. The load could be a bulb, motor or other electrical device or electronic component. In practice, electrical/electronic appliances may be much more complex and have several loads, connected to the same power supply. For instance in a washing machine, the electric element for the drum and also the motor would be powered from the mains supply.

Ohms law

Every load has a parameter associated with it called resistance, measured in ohms. When a voltage source is connected to a load, a current will flow through the load, measured in amps. The magnitude of the current equals the voltage of the source divided by the resistance of the load in ohms. For a more detailed explanation of this, see my article "How to Understand Electricity: Volts, Amps, Watts and Electrical Appliances".


If the number of loads connected in parallel with a voltage source is too great, or the resistance of a load is too small, this results in current being greater than the rating of the MCB, eventually causing it to trip.

Short circuits

A short circuit is a type of overload when a much lower resistance path than that of a normal load is created in parallel with the voltage source. Typically this could occur if wires touch inadvertently, are connected incorrectly or a fault occurs in a piece of equipment. Since Ohm's law states that current = voltage / resistance, current will be very high if resistance is low (e.g. the resistance of a short section of conductor such as a wire).

A schematic of of a simple circuit. The voltage source V causes current I to flow around in a loop through the resistance or load R.

A schematic of of a simple circuit. The voltage source V causes current I to flow around in a loop through the resistance or load R.

Ohm's Law

Ohm's law.

Ohm's law.

A short circuit has a low resistance and causes a large current Is to flow.

A short circuit has a low resistance and causes a large current Is to flow.

Faults That Can Trip a GFI (RCD)

  • Someone touches a live conductor. A current in excess of 30 ma flowing to ground through their body should trip the breaker.
  • The flex of a kettle is left in a pool of water on a sink
  • The cable of a garden power tool is cut through, If the appliance is grounded, the teeth or blade of the tool would cause a short circuit from hot to ground.
  • Exposed wiring or terminals inside an appliance touch the metal body of the appliance.
  • The outer sheath of the element in a kettle, hot water tank or washing machine becomes cracked. This allows water to seep into the powder which insulates the heating wire from the sheath, causing an electrical leak to ground.
An RCD(GFCI) with test button

An RCD(GFCI) with test button

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2012 Eugene Brennan


Eugene Brennan (author) from Ireland on February 07, 2017:

Hi Dee Dee. Usually the gas or oil feed will shutoff if a sensor doesn't detect a flame, however for absolute safety, it's probably best to turn everything off (fuel and electricity) until you can get an electrician to check out your stove.

Dee Dee on February 07, 2017:

My stoves burner control indicator is staying on, the burners are cool but I want to know if this will cause a fire.

Eugene Brennan (author) from Ireland on May 08, 2015:

Hi Peter, I think I heard Dr Karl going on about this on BBC or ABC's Triple J channel. Supposedly switches are turned on or off sideways in Japan so that falling objects during an earthquake can't turn the switch on!

Peter from Australia on May 08, 2015:

It gets confusing here in Australia because the 'normal' switch to turn a light/power point to the 'ON' position is pushed downwards.

However to reset the 'Circuit Breaker' one must push it upwards.

I had to reset mine today due to a faulty water pump :)

Fortunately it is easy to locate as you only have to look for the CB that is in a different position to all of the others :)

Eugene Brennan (author) from Ireland on June 19, 2012:

There appears to be a difference between the European and American system for resetting breakers. I asked someone about this before publishing the hub but I was misinformed so thanks for the info and pulling me up on the error!

Sam from Tennessee on June 19, 2012:

Unless things have changed drastically. The circuit breakers are re-set by turning them to the off position and then turning them on again...