# 8 Key Points on How to Select a Fuse

A fuse is a component used to protect against short circuits. It is a safety feature thus needs to be selected correctly. Below are 8 key points on how to select a fuse.

# 1. How to Select a Fuse – Starts by Defining the Normal Operating Current

First on the line of the 8 key points on how to select a fuse is to define the normal operating current. The circuit designer should know the maximum current that will flow. From that maximum current, he/she can define the desired normal operating current of the fuse. There is a know rule of thumb to aid circuit designers. This is based on 80% utilization. For instance, if the circuit that want to protect has a full load current of 100A, the minimum fuse can use is 125A.

If the margin is less than 25%, the fuse may trip unintentionally. On the other hand, if the margin is more than 25%, this could be okay if the circuits or devices protected by the fuse will not break before the fuse blows up.

The fuse normal current is given in the datasheet. This current is specified under nominal ambient temperature which is 25’C usually.

# 2. Compute or Estimate the Breaking capacity or fault current rating

In selecting a fuse, it is a must to know the maximum short circuit current. This is also called the fault current. The fuse breaking capacity must be higher than this so that it can safely interrupt.

It is sometimes difficult to compute the fault current especially to non-engineers. However, you can pre-select a fuse and perform a fault test when there is an actual sample. Search for some fuses that have the same normal operating current. Select the fuse with the highest breaking capacity. This is a good start.

Breaking or fault rating is the fuse rating for short circuit condition. In some suppliers, this is called interrupting current rating. Breaking capacity is also provided in the datasheet.

# 3. Anticipate the Maximum Inrush Current

Inrush current is not normal current but is often a headache for circuit designers. Thus, designers must consider this on on how to select a fuse. Nobody wants a busted fuse every time a system starts up or turns on.

Therefore, designers must check the level of inrush current present in the circuit by computation or actual testing.

The same with the breaking capacity, it is no straight forward to compute the inrush current. To start, search for some fuses that have equal normal operating current and choose the one with the highest I2t rating or has the better time-current curve.

### 3.1. I2t or Melting

If the design is expecting to have high inrush currents, one useful fuse specification to refer to is the i2t.  I2t is related to the amount of thermal energy when the fuse element melts.

Fuse suppliers provided some reference waveforms to use in the computation of i2t. For instance, the very good reference file from Littelfuse.

Example 1:

The peak current is 100A. The duration is 10ms. The current waveshape falls to waveform D below (The illustration is courtesy of Little fuse. Little fuse is over the years is one of the best of the bests fuse suppliers.). Find the I2t.

I2t = (1/3) x Ip2 x t = (1/3) x (100 A)2 x 0.01 seconds = 33.33 A2 seconds

Example 2:

The melting I2t is 2245 A2 seconds. What is the allowable time for a 200A peak current? Assume the waveshape is same to D above.

I2t = (1/3) x Ip2 x t

t = I2t /((1/3) x Ip2) = 2245 A2 s / ((1/3) x 200 A)) = 33.67 seconds

### 3.2. Time-current curve

The time-current curve is also useful in determining the corresponding current when the fuse will open. This is useful during the design stage if there is already an initial value of the maximum inrush or transient current. Below is a sample time-current curve from Little Fuse.

### 3.3. Response or acting type

Another very helpful parameter so you can select the best fuse in terms of inrush current capability, or the speed of its protection is the response or acting type. For high inrush current applications, aim for a slow blow fuse. Some manufacturers called it time delay or time lagging fuse. From the term itself, it will delay a bit the tripping.

If you are anticipating no inrush current and want to protect a circuit almost instantly, you need a fast blow or fast acting fuse.

# 4. Know the Open Circuit Voltage

Does voltage matters on how to select a fuse? Fuse is a current-rated device. How come the voltage matters? This is the most asked question. Voltage rating is not understood by some as an important parameter of a fuse. It is very important. If the actual open circuit voltage is higher than the voltage rating of the fuse, it will explode by the time the fuse opens.

It is not difficult to size the fuse in terms of voltage. Whatever the voltage connected to the fuse; say for instance it is used in the 120Vac line, then the open circuit voltage of the fuse is expected to be 120Vac. Thus, the size of the fuse voltage is at least equal to that voltage. If cost will not matter, select a higher voltage rating for extra margin.

# 5. Define the Target Operating Temperature Range and De-rating

Every electronic or electrical component has a corresponding safe operating area. For the fuse, always consider the target operating range. For instance, the application is from -30’C to +85’C, ensuring that the fuse can function normally within this range.

### 5.1. Temperature Rerating or Derating

Datasheet highlights will say that the fuse is able to perform within the -30’C to +85’C operating temperature range. It is good. However, fuse current capability decreases with increasing temperature. Always refer to the current derating curve.

Below is an example rerating curve from one of the Little Fuse fuses. At 25’C, the current capacity is 100%. However, it is only at around 95% when at +85’C.

# 6. Know the Circuit Application

In general, the application will dictate what fuse to use. For example, in automotive, you should ensure the fuse to use is rated for automotive. If the application is aerospace, then the fuse to use must be compliant to it as well.

# 7. Don’t Forget the General Mechanical Considerations

Fuse physical size, form and how to mount or assemble are always relevant. If you plan to mount the fuse to a PCB, then select a PCB mountable fuse. On the other hand, if the purpose is to place the fuse in a din rail, then select one that can be placed in a din rail, so on and so forth.

# 8. Product Certificates

If you use the fuse in a product that will be marketed to the international consumer or end users, product certificates are necessary. For instance, if the product to sell in the US, it must be UL listed. Listed for Canada on the other hand if to sell in Canada. The European union also has their own international acceptance standards, so the fuse should comply with it is the target market is EU.

Environmental requirements also urge us to use environmentally friendly products like ROHS and REACH complaints.

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