When is a fuse required

Generally it is not possible to evaluate inrush currents with pre-arcing time-current characteristics, since the peak values of inrush currents change dramatically with time.

Evaluation process i Repeatedly measure the current waveform of the circuit from when the equipment is powered on inrush current to the steady-state current. If there is a component like a thermistor with a resistance that changes depending on the temperature, measure the current waveform at the minimum resistance.

Note that the instantaneous value of the current that flows through the circuit is represented by i m t. In actual practice, an even smaller sampling interval is used.

A larger value was selected to explain the process. As the necessary margin differs depending on the usage conditions, it is necessary to perform evaluations in the actual equipment the fuse will be used in. Measure the maximum possible abnormal current and select a fuse with a rated breaking current that can interrupt that abnormal current. Additionally, the minimum possible abnormal current should also be measured.

The judgement whether or not these two relationships are fulfilled, depending upon the protection conditions at what point and over what time the abnormal current is required to be interrupted, can be difficult in most cases.

Therefore it is both necessary and important to confirm whether the fuse can safely interrupt the abnormal current in the actual application. Before final fuse selection, always test the proposed fuse in your actual equipment to ensure that the fuse satisfies all your operational and safety requirements. Please contact your local SOC sales representative for help in selecting fuses. Maxi blade fuses also known as APX fuses tend to be used for higher current or 'power' applications and are available in higher ratings.

Circuit breakersprovide overload protection for the circuit in the same way that a fuse does by breaking the circuit continuity but, unlike a fuse, can be reset rather than having to be replaced.

These are useful in applications where, due to their nature, an over-current condition is expected to occur now and again under normal use and to have to frequently replace fuses would be inconvenient e.

They are also useful where you might need to reset the circuit quickly and don't want to spend time looking for a fuse. Most circuit breakers found in 12V or 24V systems operate thermally. The heat generated by the excess current during an overload causes the contacts inside the breaker to come apart and break the circuit. These are then reset either manually or automatically depending on the design of the circuit breaker. Circuit breakers that have to be manually reset provide an opportunity to check for any problems before using the circuit again and on some types you can manually trip the breaker, which is useful for isolating the circuit and for testing the breaker.

Whilst circuit breakers are very useful solution in certain applications it should be noted that a good quality fuse will ultimately be more reliable there are no moving parts so should be considered first, and where sensitive electronic equipment is involved a circuit breaker should never be used.

This is because the time taken for the circuit breaker to operate can be longer than it would take a traditional fuse to blow, potentially exposing the circuit to damaging overload currents for longer. Need some hints, tips or guidance on 12V electrics? Visit our Knowledge Centre for a wealth of useful information. Watch our YouTube "how to" guides here. We accept telephone orders Call Fusing Guide Overview Fuses are critical in any electrical system and are used to protect a circuit's cabling from excessive current that could lead to damage and, very often, an electrical fire.

Excessive current is most likely to be caused by three things: Incorrect wiring - wrong connections as a result of human error Damage to the circuit - e. Overloading the circuit - e. This keeps the excessive current from continuing down the circuit to more expensive equipment. However, they are not the only devices that you can use to protect your gear from overcurrent.

There are many other ways, like circuit breakers or protective relays, and here are 10 reasons why you might just want to consider fusing instead. Overcurrent protective devices that have tripped are often reset without first investigating the cause of the fault.

Electromechanical devices may not have the reserve capacity to open safely when a second or third fault occurs. When a fuse opens it is replaced with a new fuse, so the protection level is not degraded by previous faults. Fuses typically are the most cost-effective means of providing overcurrent protection. This is especially true where high fault currents exist or where small components such as Control Transformers or DC power supplies need protection.

Devices with low interrupting ratings are often rendered obsolete by service upgrades or increases in available fault current. Updated NEC and UL standards are causing the need for potentially expensive system upgrades to non-fused systems. Fuses can be easily coordinated to provide selectivity under both overload and short-circuit conditions.

Fuses do not require periodic recalibration as do some electromechanical overcurrent protective devices. As a fuse ages, the speed of response will not slow down or change.

A blown fuse can be easily replaced with the wrong size, or even jerry-rigged using a wire or small copper bar to replace the fuse creating a safety issue. Start-up tripping is an issue with fuses need to oversize the fuse for inrush current. Fuses can require larger wiring to compensate for inrush current. A circuit breaker can provide ground fault protection, a fuse cannot.

Single phasing on three-phase loads does not happen with a three-pole circuit breaker. All circuits trip at once. Using individual fuses for a three-phase power can result in single phasing and equipment damage. Fuses are simple and straight forward to use. Fuses trip faster than a circuit breaker.

Fuses are initially less expensive than circuit breakers. Fuses have a smaller footprint in a control cabinet. There are a variety of fuse types for different applications. Fuses can be more reliable over time, because they do not have moving parts. Fuses do not require regular maintenance. Molded Case Circuit Breakers and others, do. Because a fuse is replaced every time after an overcurrent trip, the same level of circuit protection performance is guaranteed.

A circuit breaker can wear out if it trips too many times. Our Products. Cam Switches. Control Stations.