Technology Tip: A Primer on Fuses
By Peter Ruplinger
Reprinted from Utah Molders Quarterly Newsletter May 2008
Caution: Always check with a qualified electrician, engineer, or machinery manufacturer before servicing electrical devices or wiring.
Fuses vs Circuit Breakers: A question we frequently hear is, “What is better, a fuse or a circuit breaker?” It depends on the application. Fuses are always cheaper and they generally give better protection. That is to say, they blow quicker and with greater accuracy and dependability. The obvious advantage to circuit breakers is that all you have to do is flip a switch and you’re up and going again.
Contrary to popular belief, circuit breakers do not last forever. After several trips, they become less reliable and eventually fail. In critical applications such as aircraft, fuses are often preferred, due to their reliability and light weight. In planes a spare fuse is often mounted directly beside the one in use. Machinery manufacturers like fuses because they are cheap and take up less space. One problem, however, is that users may install the wrong fuse and then blame subsequent problems on the manufacturer. I recently had to replace two vacuum loaders because the night crew had replaced 10-amp fuses with 15-amp fuses. One fuse holder was entirely melted from over heating. I suspect when they ran out of 15-amp fuses, they stuck a bolt in the fuse holder. What creativity!
Back to basics: If you are new to electricity, please remember that fuses protect from over amperage, not voltage. Amperage is the amount, or flow of electricity. Voltage is the force. A good analogy is water. Pressure is analogous to voltage. Volume is similar to amperage. For example a garden hose with a nozzle may be high pressure (voltage), but low volume (amperage). A fire hydrant may be huge volume (amperage), but may only push water out a few feet due to its low pressure (voltage). When you multiply amps by volts you get watts.
Every electrical circuit whether a motor or a computer, is designed to handle a specific amperage. The wires are only so big. If you try and run too many amps through the wires, they get hot and burn up. The purpose of a fuse or circuit breaker is to blow or trip before damage is done to the device or wiring.
More than amps If fuses are rated for amps, why are fuses also labeled with a voltage rating? This is because not all fuses will work properly with the same voltage. For example a small ¼” by 1.25” glass fuse rated at 20 amps may work well in a 24-volt control circuit, but if the same fuse was used in a 250-volt application it could shatter and blow glass fragments into the user’s eyes. A ceramic rather than glass fuse is recommended for the higher voltage in most applications. Before installing a fuse, look at the fuse to see how many volts it is rated for. Check with a qualified electrician.
Current Interrupting Rating is another critical rating. A fuse may be rated to blow at 50 amps, but it may actually see several thousand amps at the instant it blows. Consequently every fuse is also rated as to how many amps it can safely interrupt. The fuse, in the photo below, is rated to blow at 200 amps, but it is rated to safely interrupt a surge up to 100,000 amps. How could a fuse possibly see so many amps if a fuse ahead of it, perhaps on the power pole is only rated at 400 amps?
Induction is the reason. Electrical circuits with coils of wire, such as motors, ballasts or transformers store huge amounts of electricity. This is called “induction”. Circuits with purely resistive loads, such as electrical heaters, do not. When a fuse protecting high inductive loads blows, it may experience thousands of amps. Many fuses commonly used to protect a single motor are only rated for 10,000 amps. In some applications, especially with just one motor, this is adequate. It is important, however to consult a qualified electrician, or engineer to verify that the fuse has a proper interrupting rating. A fuse or circuit breaker, which does not have proper interrupting rating may violently explode. In the process it can destroy the fused disconnect, even the wall, shower the area with molten metal, and injure or kill personnel.
In some applications a fuse holder may be designed to only accept fuses that have a higher current interrupting rating. Such fuse holders are called “rejection fuse holders”. Notice that the fuse in the previous photo has a large slot in the blade. This slot is designed so that it can be used in a rejection type holder or standard fuse holder. The rejection type holder is equipped with a pin that fits into the slot on the fuse blade. The pin will block a lower rated fuse from being inserted.
How fast? The forth rating for fuses is how fast will they blow. There are very fast, fast, and slow blowing fuses. Very fast fuses will blow in as little as one two-hundredth of a second. They are designed to protect electronic circuits such as rectifiers and solid state electronics. Slow blowing fuses are designed for motors. They will blow fast as well, but only if they see an amperage much higher than what they are rated for. This is because a motor typically draws three to five times its rated amperage during its first few seconds of operation. For example, a three phase, two horsepower, 230-volt motor may be rated at 6.2 amps, but during its first three seconds of operation it may draw up to twenty-five amps. Such a motor would typically be fused with a slow-blow, sometimes called “dual-element” or “time delay” fuse, rated at 7.5 amps. This fuse is designed with heavy lugs on its fusing link. The link will only get hot enough to blow if it sees an inrushing current sustained for more than the normal start up time of a few seconds. If, however, it sees a current significantly higher than the expected inrush current, for example seventy amps, it will blow immediately. So the fuse gives dual protection. It protects from a direct short with fast response, and it also protects a motor which is slowly overheating due to an abnormally heavy load or operation at less than the appropriate voltage.
In Summary, fuses and circuit breakers have four ratings:
- Amperage at which they blow or trip
- Safe operating voltage
- Interrupting rating
- Time to blow or trip
Maintenance: It is very unusual that a fuse would blow without a reason. It is important to look for the cause before replacing the fuse. Common reasons for blowing are overloading, a direct short circuit, the environment is too hot, the voltage may be too low, or one of the three phases has been lost. If the fuse is a small fractional amperage fuse protecting an electronic circuit, there may not be an adequate ground to dissipate spikes from the spike protection device. The system ground wire should be the same sizeor larger than the current carrying conductors. It should go all the way back to the distribution panel. Conduit is not adequate for grounding spike protectors.
Fuse clips should be replaced when necessary. They tend to lose spring tension over the years. Terminals on the fuse blocks as well as circuit breakers should be checked for tightness. They tend to come loose with time. Loose terminals and fuse clips can result in heating, loss of voltage, and drawing too many amps.
A fuse should be replaced if it is corroded or several years old. Notice the corrosion on the 200 amp fuse in the previous photo. It was removed from an exterior fused disconnect. It is eighteen years old.
Supply voltage should be checked routinely, especially during hot weather. Low voltage will result in drawing more amps. A 10% drop in line voltage may result in a significant increase in amperage as well as a dramatic increase in operating temperature.
When devices are properly fused, the fuses will usually blow if one of the three phases is lost. Protective devices can be installed on motors and distribution panels to shut off current and alarm if a phase is lost or reversed.
Naturally, only appropriately trained and authorized maintenance personnel with appropriate safety and lock-out devices should attempt maintenance on electrical devices
