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All about Household Fuses

The invention of the modern electric fuse is generally attributed to Thomas Edison. Beginning around 1880 his team of inventors were the source of many of today’s standard electrical products. As his electrical ideas came to fruition many products we now take for granted had to be perfected. The device we simply call a “fuse” is one of these inventions.

Most people think the purpose of the fuse is to protect the electrical device being used -- this is incorrect. Actually the primary purpose of the fuse is to protect the wiring that runs through the walls of your home. Without the fuse the wiring could overheat, causing the wood in the walls to become warm, and eventually cause a fire. The fuse is the first line of defense in the protection of your home and family.

Basically a fuse is a safety device that stops the flow of electricity if an electrical limit is surpassed. In this way the amount of electricity is never allowed to exceed this preset limit. At the heart of its simplicity is the fact that once it blows it must be manually replaced. This forces the homeowner to question why the fuse failed. The answer is usually because of a failure of something within that particular circuit.

Most household fuses are either " P " type or " D " type. This is easily confirmed by the capital letter stamped onto the top of the fuse. The P means PLUG and the D means DELAY. Fuses come in many different sizes and shapes but the most common type used in homes is still the “plug” fuse. It has a cylindrical glass body, with a threaded metal base that allows it to be screwed into a matching fuse holder in your appliance or fuse box. At its tip is a metal contact through which the electricity flows once contact with the fuse box is made. Inside every fuse is a soft metal link (usually lead) through which the electricity flows. The link is calibrated to disintegrate if the fuses maximum rating is exceeded. It can disintegrate either from too much electricity passing through it, or because it became overheated.

Fuses are rated in amperes, which is a measurement of electrical flow. The higher the amperage rating the more electricity can flow through it before the fuse blows. For 90% of your household electrical circuits the maximum allowable fuse is 15 Amps. The exceptions to this rule are your dryer, range, and hot water tank. They are higher amperage because they are specially wired to safely surpass the standard 15 Amp limit.

The first rule to remember about fuses is to never replace any fuse with one of a higher rating. Secondly, never replace a fuse with any other material or device. We have all heard horror stories about people replacing fuses with coins, pieces of metal, or even metal bottle caps. Anyone who does this is placing both themselves and their family at risk. So when replacing a defective fuse only use the exact match to the type being removed. If unsure of fuse amperage remove it and look for a number at the tip of the fuse, on top of the glass section, or printed on the paper label inside the fuse. If all else fails make note of the color of the paper label inside the fuse. Each different size of fuse has a specially colored label representing different amperage ratings. The label inside a 15 Amp fuse is always blue, 20 Amp is brownish orange, 25 Amp is red, and a 30 Amp is green.

While most people recognize the simple plug fuse the delay type is generally misunderstood. While physically identical to the plug type the delay fuse is used for special high amperage circumstances. For instance, electrical devices that use large motors may require high starting amperage. Examples might be a power saw, furnace motor, or sump pump. The delay fuse solves this problem by allowing high amperage for the first few seconds that the device starts. One of the most common uses of a delay fuse is when using a window air conditioner. It may require 40 or 50 Amps of electricity during the first few seconds it starts, before dropping down to its normal operating amperage of 5 to 10 Amps. If a 15 Amp plug fuse were used it would blow the fuse every time the air conditioner started. But by substituting a 15 Amp delay fuse it will allow a momentary high amperage while still protecting the household wiring. The disadvantage of delay fuses is that they are much more expensive. Usually two or three times the cost of a plug fuse. Therefore, only purchase a delay fuse when absolutely necessary, or when required for special appliances such as a window air conditioner.

When replacing a fuse remember that it may be hot to the touch -- therefore use caution. To remove it grasp the glass top of the fuse and unscrew it by turning counter-clockwise. It should turn easily. If the fuse will not turn don't force it or try to remove by using pliers. If it can't be removed contact an electrician -- it may have melted inside the fuse holder and only a professional will be able to correct this problem. Replace the blown fuse by screwing it back into the fuse holder and turning it clockwise. Turn until you feel the fuse stop. Again don't force it any further or apply unnecessary torque. The circuit the fuse protect should now have electricity flowing throughout. A plug fuse is very durable and should only have to be replace very rarely. If you repeatedly replace a fuse you should contact an electrician for advise. You may have wiring problems that will need to be corrected.

By Donald Grummett

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