The fuse selection procedure is very similar to contactor selection and complementary parameters help to select EV fuses. Fuse datasheets include breaking current-time characteristics. Some of the fuse manufacturers also provide thermal and mechanical characteristics.
Thermal and Mechanical Specifications
Fuses are simple busbars with melting elements inside. Therefore, the initial step is always to define the mechanical and thermal boundaries of the fuse. This is because mechanical overload can create micro-cracks on melting elements. Thus, it can cause pre-mature tripping. Also, thermal conditions directly affect fuse characteristics. Fuse melting curves change according to the ambient temperature.
System Specifications
As we use fuses and contactors together, let’s take the same driving profile as contactor selection.
- 500A for 10s,
- 350 A for 100s,
- 200 A for continuous operation.
Can continuous operation current value define the fuse rating? Let’s look at a breaking current-time chart [1].
If we locate the current profile, the 200A fuse breaking curve is above the load current curve. However, is it really that simple? Can we select a 200A fuse?
Fuse Selection based on Rated Current: Not that Straightforward
Unlike contactor carry current-time chart on the datasheet, fuse datasheets usually specify breaking current-time characteristics. Therefore, we need to think about not only the continuous current but also short and long-term pulse currents.
Fuses are not complex elements but the manufacturers make optimizations such as coating and melting element refining. Therefore, application-specific fuse behaviors can differ. EV fuse manufacturers suggest not to exceed 50% of the fuse melting currents in a normal operation. Therefore, 50% of the fuse melting curve needs to be considered to compare with the current profile. Let’s bring these together.
Violating aging limits can cause internal chemical reactions as follows [2]:
If we look at the double logarithmic chart below, we can see that our driving current is just between 300A and 400A fuse according to manufacturer data [1].
Therefore, we can start by taking into account the continuous current by doubling it. However, we should also consider the 50% of the fuse curve. If our pulse current was not 500A but 600A, even we could not use a 400A fuse in that case. Thus, analyzing the fuse breaking curve and considering its 50% to avoid aging is a safe start for selecting fuses.
Breaking Energy: I2t
As we specified, fuses react according to thermal energy consumed by the melting elements. This thermal energy is calculated by:
- E [J] = I2 · R · t
where
- I [A] : Current flow through the fuse
- R [ohm] : Fuse internal resistance
- t [s] : Duration of current
Since the fuse’s internal resistance is almost constant, fuse-breaking energy is shown as I2t in the datasheets.
At fuse-breaking current, the melting element increases in temperature, changing its phase from solid to liquid. During this change, bottlenecks break the circuit and electrical arcs happen. The arc extinguishing parts and starting to isolate the two edges of the melting element.
In this scenario, if we measure current and voltage over the fuse terminals, we can expect a current rise until melting. Then, gaps occur inside the fuse, and the arc starts. During arcing, we can expect current flow decreases due to increased resistance caused by gaps. Therefore, voltage increases as a result of this arcing. However, arc extinguishing starts to decrease voltage, and consequently, the circuit between fuse terminals becomes non-conductive. The following chart shows how a fuse current and voltage waveforms are during breaking [3]:
So, we can say that the total clearing time is the sum of melting and arcing durations.
We use the breaking energy calculation to check if the neighboring components can withstand until the fuse breaks the current. For instance, if the I2Rt energy limit of a contactor is less than the selected fuse, we should change the selected contactor. Thus, the weakest component in terms of breaking energy must be the fuse.
Minimum Breaking Current
Let’s assume that we have a failure current that is not sufficient to trip the fuse immediately. Our battery system design must be designed to handle such cases as well. Therefore, we choose contactors to switch-off failure currents up to a certain level. However, what if the contactor failed? The minimum breaking current of the fuse is important at this point. Fuse manufacturers also specify the minimum breaking current values in the datasheets as follows [1]:
The table defines the minimum and maximum fuse breaking times with respect to current ratings. As an example, if the failure current is 300% of 400A, the fuse breaks the circuit within 30 seconds. Therefore, we know that even if the contactor is not capable to switch off 1200A, the fuse will react within 30 seconds at a maximum.
Short Circuit Current Breaking
Contactor selection example has shown us we can assume 2867 A for a short circuit current. This value is particularly important for the fuse selection. The following chart shows how long it takes the fuse to break the current. A 400A fuse can break a 2867A short circuit current in 10ms to 20ms. This duration differs according to temperature, altitude, current pulses, neighboring components, and manufacturer design [1]. We should always consider tolerances by the nature of manufacturing processes.
5 Steps for EV Fuse Selection
- Select a fuse rated double as continous current (e.g. initially take 400A fuse for 200A continuous current) and draw the load profile next to 50% of the fuse breaking current-time chart to check if pulse currents can be carried by the fuse without aging.
- Calculate the components and the fuse breaking energy (I2Rt) to ensure that the weakest component is the fuse.
- Identify the minimum breaking current for the failure cases when the contactor cannot switch-off.
- Analyze the short circuit clearing time and check if the contactor can withstand until the fuse breaks the circuit.
- Check the contactor-fuse coordination for normal operation, overloads and failure currents.
References
- “PEC Fuse Catalog“, Pasific Engineering Corporation, 2019, accessed 13 Jan. 2022.
- “Guide to Fuse Selection“, Schurter, accessed 13 Jan. 2022.
- C. P. Yang, R. Ball, A. McGordon and G. Dhadyalla, “Simulation methodologies to support novel fuse design for energy storage systems using COMSOL,” IET Hybrid and Electric Vehicles Conference 2013 (HEVC 2013), 2013, pp. 1-5, doi: 10.1049/cp.2013.1888.
Time constant and circuit inductance are also factors that are often overlooked but could give you sleepless nights if you don’t make the right verifications on time