ABB S200-C series, taken from the ETAP program shown in Figure 1, is a type C automatic fuse breaking curve with a nominal current of 3 Amperes. The horizontal axis shows the amount of current passing through the automatic fuse, and the vertical axis shows the tripping time of the fuse.

Thermal Protection

It is the sloping region at the top of the curve. When the current slightly exceeds the nominal value (3A), the device heats up and the bimetal plate bends, breaking the circuit. As the current increases, the breaking time becomes shorter, showing an inverse time characteristic. For example, when 6A current passes through the fuse, when the fuse is in the thermal protection zone, it breaks in 7.43 seconds at the earliest and in 343.5 seconds at the latest as seen Figure 1. Ambient temperature, cold/hot temperature of the device and manufacturing tolerances create tolerance for the same tripping current. The main reason for a wide time range (tolerance) is the thermal tripping zone. The current ambient temperature of the automatic circuit breaker and whether the device is currently "cold" or "hot" because current has passed through it before determines where the tripping time will occur between these two values.



Figure 1. C-Type Automatic Fuse Trip Curve (3 Amps)



The IEC standard specifies a thermal trip initiation range between 1.13 x In and 1.45 x In, i.e. 1.13 x 3 =3.39A and 4.35A. These values ​​may be different in manufacturer catalogues. Figure 2 indicates the manufacturer's range 3.25A-4.73A.

Şekil 2. Thermal trip initiation 

Magnetic Protection 

It is the steep region at the bottom of the chart. At very high currents, the device turns on instantly, thanks to an electromagnetic coil, without waiting for heating.  Type C automatic fuse usually magnetically trips between 5 and 10 times the rated current. In this case, the zone will be between 15A-30A.

The 15A current value is where the lower limit curve (earliest tripping time) suddenly steepens downwards, as seen in Figure 1. When the current reaches 15 Amperes, the electromagnetic coil inside the switch says "I can step in". At the full 15A level, there is a tolerance uncertainty as to whether the fuse will break thermally (slowly with heating) or magnetically (fast with coil). 15A, 23.5 seconds is the upper limit for the possibility of tripping the zone at the latest. This point is still in the sloping thermal trip zone. Due to production tolerances, the magnetic coil inside the automatic fuse may not be activated immediately when it sees 15A. According to the standards, it has the right to wait up to 30A (5 - 10 x In). If the coil is not activated, the automatic fuse or thermal magnetic switch just waits to trip by heating up. In this case, in the worst and slowest scenario, it may take a maximum of 23.5 seconds for the switch to break the circuit when 15A current is flowing. 

If the automatic fuse is already in a hot environment or if current has passed through it before, the thermal mechanism (bimetal) can break the circuit in 1.41 seconds at the earliest, even if the magnetic coil is never triggered. This is the fastest heating response that the switch can achieve mechanically, even if the magnetic protection is not activated.

If the magnetic coil does its job at 5 times the current (15A), the thermal waiting time is completely eliminated and the switch immediately breaks the circuit in a short time of 0.0187 seconds. At 15A, the device can turn on very quickly or wait up to 1.41 seconds.

When it reaches 30A, the possibility of thermal tripping disappears. At 30A, the tripping time is 7.2 seconds. At the earliest, it can break in less than a second. When 30A current passes through a C type 3 Ampere switch, the magnetic coil inside is expected to react instantly and break the circuit in a very short time such as 0.01 to 0.1 seconds (close to the lower limit). Considering the production tolerance and the ambient temperature where the fuse operates, the manufacturer states that the fuse will break in a maximum of 7.2 seconds when the current reaches 30A.

Maximum Breaking Capacity

In Figure 1, the maximum breaking capacity is selected as 10kA. This value is the short circuit breaking capacity of the automatic fuse (Ultimate Breaking - Icu). The maximum limit that the thermal magnetic switch can break safely without breaking or burning is 10 kA. Above this value, the switch cannot be physically turned on and a fire may occur.

Figure 3 is the visual representation, the product is ABB B16 model. It is physically similar to the C type ABB switch.



Figure 3. Image Source: Author VoltCrafter, used under the [Creative Commons Attribution-Share Alike 4.0] license via Wikimedia Commons.