A better alternative to troublesome 24V circuit breakers

Control engineers and end users expect circuit breakers and fuses to react faster than the power supply in the event of a cable being severed or damaged. However, traditional circuit breakers suffer from slow reaction times, limited sensitivity and are of little help in troubleshooting. Taken together, these drawbacks can result in the following:

• Sensor and valve cables overheating, melting or catching fire without the circuit breaker reacting
• It being necessary to shut down a machine in order to reset breakers, repair cables or replace fuses
• Control cabinets being opened in order to identify the breaker or fuse, then the cabinet being kept open while troubleshooting circuits
• Power supplies shutting down during short circuits or overloads
• Power supplies being oversized to prevent them from drawing down before breakers trip
• Breakers or fuses being oversized to prevent them blowing during machine start-ups
• Many different breakers and fuses being held in stock

The downtime associated with such problems can be extremely costly, which is why 'electronic fuses' can pay for themselves in a single incident. Compared with their traditional counterparts, solid-state circuit protection devices react faster, are more precise in terms of the switching point, and are less sensitive to mechanical shock, vibration, humidity and magnetic fields. In addition, they consume less power, generate less electrical noise, are smaller and lighter, and have a longer life expectancy.

More functionality than a fuse or breaker

Solid-state devices also create new possibilities for control system design. For example, some devices can be programmed for a cascading start, with time delays for each branch to eliminate start-up peaks. Another way in which solid-state devices can avoid nuisance trips at start-up is by reacting intelligently to an overload; the overloaded circuit can be isolated, then monitored to see if the overload subsides.

Given the high cost of downtime, a further attractive feature of 'electronic fuses' is integral diagnostics in the form of LEDs that indicate the branch requiring attention, and some have a status output that can be monitored remotely; many of these smart devices can also be reset remotely.

As well as end users who stand to benefit form these features, machine builders and system integrators that are contracted to maintain the equipment they have supplied will appreciate the advantages. And although the purchase cost per device will be higher than for traditional fuses or circuit breakers, the installation is faster and easier, requiring less wiring.

Murrelektronik offers an extensive range of solid-state circuit protection. The company's MICO devices feature current selection, overload monitoring, remote reset, fault notification and remote on/off (which can save energy). Four MICO models are offered:

• MICO Basic with preset nominal currents and a 90 per cent visual warning
• MICO Classic with adjustable current ranges,individual channel switch on/off plus a 90 per cent visual warning
• MICO+ with remote group channels switch off plus a digital and visual signal
• MICO Fuse with sockets for glass tube fuses and status indication at 24V DC

Email [email protected] to request a copy of the report Give circuit breakers a break: How to protect your 24V DC circuits better, or follow the link to download a brochure for the range of MICO solid-state circuit protection devices (2.6MB PDF).