The selection of slow break (delay type) fuses is mainly to deal with situations where there is a transient surge current in the circuit that is significantly higher than the normal operating current. These surge currents are usually normal phenomena during equipment startup or operation, rather than fault currents. If a fast break fuse is used, it may melt under these normal surge impacts, causing the device to fail to start properly or frequently malfunction.
Here are some specific situations and application scenarios that require the use of slow break fuses:
Equipment containing electric motors or compressors:
Reason: Electric motors (especially AC induction motors) and compressors require a very large current (usually 5-8 times or even higher than the rated current) at the moment of starting (when the rotor is stationary) to overcome static inertia and establish a magnetic field/pressure. Although the starting current (locked rotor current) is large, its duration is very short (usually several hundred milliseconds to a few seconds).
Applications: refrigerators, air conditioners, washing machines, electric fans, power tools, water pumps, blowers, etc. A slow break fuse allows this startup surge to pass through without melting, but still provides protection in the event of sustained overload or short circuit.
Circuits containing large capacity capacitors:
Reason: Switching power supplies, frequency converters, high-power LED drivers, audio amplifiers, and other devices typically have large capacity filtering capacitors at the input end. When the device is first powered on (especially during cold start), these capacitors will exhibit a nearly short-circuit state at the initial charging moment, generating a very large surge charging current (usually referred to as "Inrush Current").
Application: Almost all modern switching power supply devices, computer power supplies, industrial power supplies, power factor correction circuits. Slow breaking fuses can withstand this brief charging surge.
Transformer excitation inrush current:
Reason: When a transformer (especially a large power transformer) is closed and energized at a specific phase angle (such as voltage zero crossing), the iron core may be in a deep saturation state, resulting in a very large and relatively long (up to several cycles) surge current called excitation inrush current. It may reach more than 10 times the rated current.
Application: Power supply for various equipment using transformers, primary side protection for power systems. Slow breaking fuses can avoid excitation surges.
Incandescent or halogen lamp load:
Reason: The cold resistance of incandescent lamp filaments is much lower than the hot resistance during normal operation. At the moment of turning on the light, the current flowing through will be much higher than the rated current of the lamp during normal operation (up to 10-15 times), until the filament heats up and reaches a stable state.
Application: incandescent lamp, halogen lamp lighting circuit (especially multi lamp parallel circuit). Slow breaking fuses can prevent them from melting every time the lights are turned on.
Some types of power converters/controllers:
Reason: Some power topologies may require a brief overload state during startup to establish a working point or complete a soft start process.
Applications: DC-DC converters, inverters, etc.
In situations where frequent misoperations need to be avoided:
Reason: Even without the typical surges mentioned above, if the circuit occasionally generates short-term small overshoot or noise interference under normal operating conditions, the quick break fuse may be too sensitive and cause unnecessary melting.
Application: Industrial equipment, communication equipment, etc. that require high reliability.
Key points for selecting slow break fuses:
Understanding surge characteristics: It is necessary to know the peak value, duration, and waveform (whether single or repetitive) of surge current in the circuit.
Matching the melting characteristic curve: Slow breaking fuses have a specific time current characteristic curve. To ensure that the characteristic curve of the selected fuse can cover the peak and duration of surge current (the surge point is located below the curve), while also providing effective protection against sustained overload and short circuit (the sustained overload point is located above the curve).
Rated current selection: The rated current of a slow break fuse usually needs to be selected based on the steady-state operating current of the device (which may be closer to the operating current than the rated current selected for a fast break fuse in the same application), because it can withstand start-up surges. The rated current cannot be simply selected based on the surge current value.
Breaking ability: Like fast breaking fuses, slow breaking fuses must also have sufficient breaking ability to safely cut off the maximum possible fault current (short-circuit current).
Summary:
When there is a brief, high amplitude surge current caused by normal operation in the circuit, in order to protect the equipment from continuous overload and short circuit damage, and to enable the equipment to start and operate normally, slow break (delay type) fuses need to be selected. It tolerates these necessary surges and avoids unnecessary false fuses through its longer melting time characteristic. When selecting, it is necessary to carefully analyze the surge parameters and refer to the time current characteristic curve of the fuse.