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Fuses

The fuse is the most widely used electrical protective device.  Fuses are made in an endless variety of configurations to fit any need.  Once installed, they require no periodic maintenance or testing.

The basic fuse consists of a “fusible” link or links inside a tube and connected to terminals on each end.  Electrical resistance of the link is so low that it acts as a conductor.  However, when a destructive current occurs, the link quickly melts and opens the circuit to protect conductors and other components of the circuit.

Seldom are we involved in the initial selection of fuses.  An electrical engineer that designed the circuits for a system did the original selections.  The average person or service technician’s encounter with fuses is when they are required to replace a “blown” fuse.  Usually, replacing a blown fuse with exactly the same rated fuse is the best policy to follow.  Investigating why the fuse blew is the most important part of any fuse replacement.

It is possible that unauthorized personnel have replaced fuses with inappropriately rated fuses, which could lead to dangerous situations.  One should be familiar with the various ratings found on fuses to ensure proper replacements are being used.

The voltage rating of a fuse must be equal to or greater than the circuit voltage.  A fuse rated 600 volts can be used in a 240-volt circuit, but a fuse rated 250 volts should never be used in a 480-volt circuit.

The voltage rating of a fuse determines the ability of the fuse to suppress the internal arcing that occurs when the fuse “blows,” that is, when the fuse link melts.  When the link melts, an arc is produced.  If a fuse with a voltage rating lower than the circuit voltage is used, arc suppression will be impaired, and the fuse will not clear the fault quickly enough to prevent conductor or component damage.

Every fuse has a specific ampere rating.  When originally selecting the amp rating of a fuse, the electrical engineer considered the type of load or loads on the circuit and code requirements.  The amp rating of a fuse must not exceed the carrying capacity of a circuit.

For instance, if the conductors, or wires, of a circuit are rated to carry 20 amps, a 20-amp fuse should be the largest fuse used.  (There are some exceptions involving motor circuits, but we are discussing fuses, not circuit design.)

Another rating found on fuses is the “interrupting” rating often noted “IR.”  A fuse must be able to withstand the destructive energy of a short circuit.  If a fault current were to exceed a level beyond a fuse’s capability, the fuse may actually rupture or explode!  This is extremely dangerous, and certainly would cause serious damage.  A fuse needs to retain its physical integrity when reacting to fault currents.

Short circuit currents get very high very fast.  Just because a circuit may have a 30-amp rated fuse, it doesn’t mean that a short circuit current of 30 amps will be present during a fault.  Short circuit currents depend on available current, and short circuit currents of up to 75,000 amps or more are common.  A fuse must be able to withstand this massive surge of current without rupturing.

Modern fuses have an IR rating of either 200,000 or 300,000 amps.  As mentioned, short circuit currents can reach massive levels of 30,000 or more amps in just a half cycle (.008 seconds at 60 HZ) at the start of a short circuit.  Instant heat is produced in the circuit by this huge amount of energy.  Severe damage to wires and other components of a circuit will occur, even explosions.  Fuses should cut off a short circuit in less than one-half cycle to prevent the total available current from reaching destructive values.

Figure 1 shows a non-current limiting fuse that would let an immense amount of destructive short circuit energy build up before opening the circuit.

Figure 1.

Figure 2 is a depiction of a modern “Current – Limiting” fuse cutting off a short circuit before it can build to its peak value.

Figure 2.

If a fuse is current limiting, it will be noted on the body of the fuse.  Most fuses are current limiting today.

Non-time delay fuses have a very high-speed response to over currents.  They provide excellent short circuit protection.  However, temporary surge currents or harmless overloads cause these fuses to “blow.”  Applied to a wrong type of circuit, they can cause nuisance shutdowns and often result in replacement using oversized fuses — not a good idea.  Non-time delay fuses are for circuits not subject to temporary overloads, but where high-speed response to short circuits is required.

Circuits with inductive loads, such as motors, transformers, and solenoids, are subject to temporary overloads.  These overload currents are usually two to six times normal current, and only last for a relatively short time.  Time delay fuses should be applied to these circuits.  Oversizing non-time delay fuses is not necessary.

Time delay fuses are dual element fuses.  Two distinct elements are series connected in the body of the fuse.  One element provides short circuit protection as in a non-time delay fuse, while the overload element provides protection against low level over currents.  Time-delay fuses will hold an overload five times greater than the fuse’s amp rating for up to 10 seconds before opening.

All this information is printed on the body of a cartridge fuse: voltage rating, amperage, IR rating, current limiting or not, and time-delay or non-time-delay.  The ratings are shown for both AC and DC circuits.

So, the next time you replace a “blown” fuse; make sure you are using the correct fuse for the application.  Most of what you want to know is right there on the fuse.

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