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Home >Better protection for 24VDC control circuits

Better protection for 24VDC control circuits

26 August 2014

When it comes to protecting 24VDC control circuits, traditional circuit breakers and fuses are not without their drawbacks. Modern circuit protection and monitoring devices based on solid-state electronics offer an alternative.

Traditional circuit breakers and fuses have some distinct drawbacks, such as slow reaction time, limited sensitivity, and offer no help in troubleshooting. It can be time-consuming and costly to open a control cabinet - or worse, shut down a whole production line - while technicians strive to troubleshoot a circuit.

An alternative are circuit protection and monitoring devices based on solid-state electronics. These can do things that traditional breakers and fuses were never designed to do. For example, monitor and analyse circuits: modern solid-state devices are designed to analyse any fault, or possible fault, conditions and react quickly.

When these new devices sense an overload, they can isolate that circuit to prevent any damage, then monitor it to see if the overload subsides. This enables these devices to accommodate a machine start-up without any nuisance tripping. Some can even be programmed for a cascading start, with a certain time delay for each branch that eliminates start-up peaks.

Problem tracing

Some newer solid-state devices can monitor a circuit, and send a relay contact signal to another device, such as a stack light, HMI or PLC, to alert an operator of a fault. LEDs on the monitoring device (or other contacts) can show the affected circuit, and help a technician trace the problem and fix the cable. Once the repair is complete, a quick and easy remote reset can be done to clear the fault - without ever needing to open the control cabinet.

While 8A of current takes 9s to trip a 2A breaker, the same current will trip the electronic device in only 10ms - 900 times faster. This protects all cables, conductors, and devices from exposure to unnecessary current, hundreds or thousands of times faster than a traditional breaker.

The newer devices install just like a breaker, but with less wiring. Some of the newer devices provide current-selection switches for each branch, which makes any future adjustments quick and easy. UL and CSA have categories for testing and use of electronic circuit protectors, removing any issues about where to use them safely.

Murrelektronik’s MICO family of electronic circuit-protection devices features current selection, overload monitoring, remote reset, fault notification and remote on/off for energy savings. While fuses may blow, solid-state devices don’t, so they never need to be replaced.

MICO+, the newest member of the MICO family, additionally helps to improve the energy balance. Often current is flowing, even though no productive activity is being carried out. During pause times,  60% of the energy is often consumed as stand-by power: pumps are running, displays lighten, loads are heating.

With MICO+, machine parts not currently needed are switched off. A signal from the control is enough to put the module into a non-operating state. MICO+ reactivates the machine part quickly when it is needed again.

MICO's 90% visual warning is a perfect tool for setting up machines and systems. If channels are near the limit, the LED starts blinking. Often the total current needed increases during operation. This happens because valves and motors wear out, for example. To solve this problem, the MICO+ 90% warning is additionally available as a digital signal, which sets off the alarm – and counter-measures can be taken immediately.

Symptoms of 24V DC control circuit problems

  • Sensor and valve cables get very hot, melt or catch fire without the breaker reacting
  • Have to shut down the machine to reset a breaker, repair sensor cables or replace a fuse
  • Have to open the control cabinet to see which breaker or fuse failed, and keep the cabinet open to troubleshoot a circuit
  • The power supply shuts down during a short circuit or overload
  • Need to oversize power supply to prevent it drawing down before our breakers trip
  • Need to oversize our breaker or fuse to avoid them blowing during machine start-up
  • Have to keep many different breakers and fuses in stock

Key Points

  • Unlike traditional circuit breaker, devices based on solid-state electronics can monitor and analyse circuits
  • When the new devices sense an overload, they can isolate the circuit to prevent damage, then monitor to see if the overload subsides
  • Some solid-state devices can monitor a circuit, and send a relay contact signal to another device