Reliable Fuse and Circuit Breaker Tripping with the RACPRO1 Series

A 2.5mm² cable with a total length of 50m (25m there and back) has a total resistance of 174mOhm. The circuit breaker has an internal resistance of 56mOhm. Application #2 has a typical load of 5A.

The resulting supply voltage for Application #2 is:
Vsupply – IR = 24V – (5A x 230mOhm) = 22.85V.

This supply voltage falls below the nominal 24V supply level.

The RACPRO1 series includes a front panel adjustment trimmer that allows fine-tuning of the output voltage to offset voltage drops in long cables (Figure 2). In this example, the output can be trimmed up to 24.6V, balancing the voltage drop.

• Application #1 receives 24.6V (600mV above nominal).
• Application #2 receives 23.4V (600mV below nominal).

Both supply voltages remain within an acceptable ±2.5% range.

A thermomagnetic DC circuit breaker operates with an overload tripping curve that combines two distinct characteristics:

• The thermal trip responds to sustained overloads, disconnecting the load if the current remains excessive.
• The magnet trip reacts much faster, but it requires a higher overcurrent surge to activate.

Both tripping mechanisms have built-in tolerances, as shown below (Figure 3):



At nominal load current (5A) the circuit breaker remains unaffected. Even with high start-up currents reaching 10A, it will not trip as long as the overcurrent situation lasts less than 5s (thermal characteristic). Under a short circuit fault condition, when the power supply delivers its maximum output current (20A), the thermal trip eventually activates, but this takes between 1.5s and 10s (dotted line). This delay may be too long to protect the load from damage or overheating.

Fortunately, the RACPRO1 series can supply a short-term fuse trip current of 250% for up to 20ms. For the 480W version, this amounts to 250% x 20A = 50A. At this level, the circuit breaker enters its fast reacting magnetic trip mode, allowing it to trip within 2ms and 10ms (dotted yellow line).

Instead of a circuit breaker, a DC fuse can be used. This is a more cost-effective option, and DIN-Rail-mounted fuse holders with built-in LED indicators are readily available (Figure 4). However, a higher-rated fuse is required to prevent nuisance tripping from high inrush currents and to account for long-term performance reduction due to aging or high operating temperatures. For a 5A load, a 7.5A fuse is the recommended choice.



At a 5A load current, the 7.5A fuse remains intact during normal operation. Even with high start-up currents of up to 10A, the fuse will not trip if the overcurrent event lasts less than 5s (dotted line). In the event of a short circuit fault, and at the power supply’s maximum output current (20A) the fuse will blow, but its reaction time is 200ms (dotted red line). Although relatively fast, this delay may still be too long for certain sensitive applications.

Fortunately, the RACPRO1 series provides a short-term fuse tripping current of 250% for 20ms. For the 480W version, this equates to 250% x 20A = 50A, ensuring the DC fuse trips in under 4ms. (dotted yellow line).

Even with a short circuit-protected power supply, there are applications where installing DC circuit breakers or fuses on the output is beneficial for extra protection. However, these components, as do any long supply cables, include a voltage drop. To compensate for these drops, the front panel output voltage trim can be used.

When using an external circuit breaker or fuse, it is important that the power supply can deliver a short-term overcurrent without going into immediate short circuit protection, ensuring that the overcurrent protection devices are properly activated. The RACPRO1 series features a 250%/20ms easy fuse tripping current.