Understanding Fuses and Circuits

by DoItYourself Staff

Fuses Safety Tips

Always use a fuse puller when changing a cartridge fuse.

Before changing or adding a circuit breaker, switch off the main breaker that lets power into the rest of the box.

Never put fingers into or touch the empty spaces in a circuit breaker box.

Never remove the knockout covering an empty space in a breaker box until you’re ready to use it.

Plug Fuse

Also known as Edison base fuse, plug-in fuse or glass fuse.

A safety device that breaks an electrical circuit when it is overloaded.

The most commonly used fuse. Available in 5- to 30-amp sizes. It has a threaded base like a light bulb and a small window to view a metal linkage. Electricity flows through the linkage. When the circuit is overloaded, the linkage melts and turns black, cutting off the flow of electricity through it. The fuse then must be replaced

Type S Fuse

Also known as Fustats™ or Nontamperable

Similar in design and use to a plug fuse. However, it prevents anyone from replacing a lower-rated fuse with a higher one.

Has two parts: the fuse and the adapter. The adapter has a different diameter for each fuse ampere rating. Once the adapter of a particular size fuse has been inserted into the fuse socket, it cannot be removed. Only fuses with the same rating can be used in that socket.

Cartridge Fuse

Cartridges fuses for circuits above 60 amps are also known as knife-blade cartridges. They look like rifle cartridges with metal caps and blades sticking out of each end.

Cartridges fuses for circuits 60 amps or less are also known as ferrule contact or round cartridge fuses. They look like rifle cartridges with plain, capped ends.

Used in high-current applications, such as in the main service box and in clamp- or bar-type fuse boxes that serve electric ranges, water heaters, clothes dryers and air conditioners.

Unlike plug fuses, you cannot tell if the fuse is blown by merely looking at it.

Use a special pair of pliers known as fuse pullers to remove these fuses.

Time Delay Fuse

Similar in appearance to a plug fuse.

Provides a minimum time delay for small household motors that cause an electrical surge when started. That surge would cause a regular fuse to blow needlessly.

Circuit Breaker

Two styles are available: Push button (less common) and toggle.

Breakers can be single, double or thin.

Contains a bi-metal strip that breaks the circuit when a current exceeds a predetermined rating.

A breaker indicates a broken circuit when the switch is in the mid-point, or tripped, position.

Reset the circuit breaker after correcting the overload problem. Switch it to the off position, then to the on position.

Another type of circuit breaker is a screw-in type that looks similar to a fuse, but has a button on the top. The button pops out when the circuit is broken.

Arc Fault Circuit Interrupter

An arc fault occurs when the insulation surrounding wire and cable is damaged or deteriorates. The arc fault can flare at temperatures in excess of 10,000ºF and ignite surrounding combustible material.

Standard circuit breakers do not respond quickly enough to arc faults. An arc fault circuit interrupter (AFCI) recognizes an arcing fault and acts instantly