Circuit Breakers Tripping: What Causes Electrical Faults?
Protecting Your Home
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). 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.
Types of Circuit Breakers
There are two types of breakers mounted in your electrical panel; MCBs (Miniature Circuit Breakers) and GFCIs (Ground Fault Circuit Interrupters). A GFCI is more commonly referred to as an RCD (Residual Current Device) in Europe.
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.
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.
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