What’s the best way to protect low-power circuits?

With the continued rise of 24 Vdc circuits, here’s how to keep cabinet components powered and protected.

By Control Design

As we’re building panels with more and more 24 Vdc circuits, what are the best options for protection? Fuses have always been a mainstay. What about electronic circuit protection modules? Electrostatic discharge devices? Miniature circuit breakers? Does each component warrant a different type of protection? How do we guard against overcurrent? Will a two-pole MCB suffice?

Also read: High-end protection for low power

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  • <p>I like the new electronic circuit protection modules. There are some out now that are the same size as a fuse holder but function like a push style circuit breaker where you push once to engage and a second time to disengage. If the circuit overloads then the button pops out and must be pushed back in to re-engage.</p>


  • <p>There's no one "best" option for circuit protection. Several factors need to be considered. First, what are the code requirements? Are there UL, CSA, or EN Safety Standards that must be met? What types of hazards are likely to be encountered? And what customer expectations must be met in terms of cost and features?</p> <p>Fuses are low cost and available with a wide variety of electrical characteristics ranging from ultra-fast types designed to protect semiconductor devices to time delay types designed to be work with inductive devices such as transformers and relays. Where fuses fall short is convenience and control. If a fuse opens in the field, an exact replacement may not be readily available which could lead to down time in a production setting, or the temptation to use whatever is available to get the equipment up and running. An end user’s actions in this situation are beyond the designer's control and an inappropriate substitution could lead to equipment damage or a safety hazard.</p> <p>Circuit breakers are another option, and a good choice for many applications. Thermal circuit breakers are rugged and cost competitive with fuses. In fact, many are designed to fit in cutouts originally designed for fuse holders. Where thermal circuit breakers come up short is reaction time and precision. Thermal circuit breakers are relatively slow acting and the trip point can vary considerably with temperature. These characteristics may render them inappropriate in applications where there are large variations in ambient temperature or fast reaction time is required. Magnetic circuit breakers overcome most of the limitations of thermal circuit breakers, but are considerably more expensive.</p> <p>PPTC devices, or resettable fuses are another viable option in many situations, but like thermal circuit breakers, their trip current and time are affected by changes in temperature.</p> <p>Each of the above options deal with over current protection. Many designs also require over voltage protection. Varistors, gas discharge tubes, Zener diodes, and TVS diodes can all be used to provide protection against transient voltages, and crowbar circuits can be designed to protect against over voltage conditions. Reverse polarity protection may also be appropriate for designs where the end user will be connecting the power source.</p> <p>Electronic circuit protection modules attempt to combine the best features of fuse and circuit breakers while adding a few new tricks including, in some cases, circuit isolation, remote monitoring and reset, and better tolerance to inrush current while still providing adequate overload protection. Electronic circuit protection modules provide a solution in a box which makes them a fantastic solution for industrial automation projects or low volume production, saving countless hours of engineering time; however, for volume production or price sensitive applications, electronic circuit protection modules may offer more features than are required or come at a cost too high to justify.</p> <p>Herm L. Harrison Vice President Foster Transformer Company 3820 Colerain Avenue Cincinnati, OH 45223 www.foster-transformer.com</p>


  • <p>While a specific power level was not mentioned in the question, we've had good results with resettable PTC fuses. We've used them to protect sensitive signal conditioning circuits powered from line voltages up to 250Vac.</p>


  • <p>There are many different protective devices available but which device to implement is determined by what task is required by such devices. For example, many codes and standards require that each branch be protected which includes protecting the load and the conductors. In many cases, a branch circuit rated fuse or circuit breaker can perform this task. As with any protective device, there are trip time characteristics associated with each unit for which a certain amount of current must flow within a specific timeframe in order to open the fuse or trip the circuit breaker. However, many power supplies available today current limit quickly preventing the necessary current needed to trip a protective device. Even robust high quality power supplies capable of delivering high peak current are still dependent on the length of wire, wire gauge and where the fault occurs to determine if the impedance is too high to allow sufficient current to flow. Electronic fuses are also available, but sometimes the trip characteristics can be too sensitive causing a nuisance trip on the startup of some applications.</p> <p>Depending on the severity of an overload on a power supply, the output voltage can drop significantly, causing the connected loads to shut down. If the protective devices do not open fast enough, even the controller can shut down causing a loss of control within the application, as lack of control voltage is not present. So although a fuse or circuit breaker is in the circuit and is providing the necessary branch circuit protection by code, if they do not open, this condition can render the power supply useless. For this reason, the market has seen an increase in the offering of DC Protection Modules.</p> <p>The PULS PISA 24VDC, Protection Module was designed to prevent such loss of control by a unique design concept. The PISA protection module monitors both the current and the voltage and shuts down the connected loads before the power supply voltage can be affected. This unique “Voltage Monitor” circuit ensures that the output voltage of the power supply never drops below 21VDC. This voltage level is typically acceptable for all 24VDC powered devices. The PISA amperage protection allows a larger power supply to be utilized and then distributes the current to four smaller rated channels providing the appropriate protection. Depending on the required codes and applicable standards, the PISA may not eliminate all fuses or circuit breakers but will provide additional protection that cannot be achieved by the other devices alone. PULS also offers a four channel NEC Class 2 protection module which can assist when a large non NEC Class 2 supply is used but NEC Class 2 circuits are required in the application. Technical data sheets, CAD drawings and other information can be found in the PISA product pages at the following link: <a href="http://www.pulspower.com/index.php?reqNav=serieInfo&amp;family=2&amp;serie=13">http://www.pulspower.com/index.php?reqNav=serieInfo&amp;family=2&amp;serie=13</a></p> <p>Ed Merkle - Engineering Manager - PULS, L.P. – www.pulspower.us </p>


  • <p>As we’re building panels with more and more 24V DC circuits, what are the best options for protection? Fuses have always been a mainstay. What about electronic circuit protection modules? Electrostatic discharge devices? Miniature circuit breakers? Does each component warrant a different type of protection? How do we guard against overcurrent? Will a two-pole MCB suffice?</p> <p>When Murrelektronik meets with integrators and OEM customers to consult on power management, electronic circuit protection is always an important topic. In the past, breakers and fuses were assumed to be adequate for protection on branch circuits. However, when you take the time to analyze the applications, you quickly find that in DC applications (specifically low voltage applications (&lt;48VDC)) fuses and breakers are marginal at best. There are two main conditions to protect from – short circuit and overload. Sizing fuses and breakers to satisfy both of the conditions and then provide a device that actually can protect the installation is difficult. Trip curves for the respective device will normally only satisfy one condition or the other. ECP devices like our MICO family of products can satisfy both conditions and add considerable monitoring and reset benefits. MICO monitors the current levels and measures the resistance along the branch circuit to ensure proper operation, reduced heat, and steady outputs. Adjustable current ranges for each branch, resettable circuits, no replacement parts needed, visual indicators and remote monitoring and reset capabilities are among the features that ECP devices and our products can provide. These features become more and more important when you, and your customers, consider the costs associated with replacing a fuse or resetting a breaker. The costs and time associated with a reset are minimized with ECP devices. You can find the problem, fix it and reset the circuit without ever opening a cabinet. </p> <p>Murrelektronik offers a wide variety of devices to satisfy different low conditions and applications including those with UL listings, NEC Class 2 circuit protection and more. Have a look at our offerings, review our white papers and more by visiting our web page www.murrinc.com #MICO</p> <p>Aaron Henry Marketing Manager Murrelektronik, Inc.</p>


  • <p>While fuses have their strengths and are a popular choice for 24VDC circuit protection, there are also reasons that a circuit breaker may be better suited for 24VDC applications. One major reason is because fuses are a “one-and-done” kind of deal. Circuit breakers allow you to reset and reuse the device over and over again, fuses do not. Circuit breakers also offer more functionality options. Many circuit breakers provide options for on/off switch capabilities, single and multi-pole configurations, auxiliary contacts, and more.</p> <p>And generally, circuit breakers are more robust in their design. They have more stable trip curves that enable them to be used for unique applications, like with identically current-rated wiring conductors. </p> <p>In terms of using miniature circuit breakers, the user would need to analyze his/her application’s specific requirements to protect in the right situations. Some applications require a faster or slower tripping time than others. Three of the most common circuit breaker categories are thermal, thermal-magnetic, and electronic. Each type of breaker reacts at various speeds due to the factors that trigger its tripping mechanisms. </p> <p>Thermal breakers are great for tripping when things get too hot. They are programmed to trip when they heat up beyond a certain temperature. This heat is generated when its connected equipment is drawing too much current over a long period of time. Thermal breakers are great for protecting things like motors, transformers, and electrical systems used in air, land, and sea vehicles. Thermal-magnetic breakers trip under both excess heat and magnetic effects. This means they trip when things get too hot, or when they draw too much current over a short period of time. The thermal part is the same as before. The magnetic parts refer to the way the breaker responds to high overload and short circuit currents. The breaker can also trip when there’s a rapid increase in current flowing through it. This is known as the magnetic tripping effect. Thermal-magnetic breakers are good for protecting in the event of overload, short circuits, and long cable paths. Typical applications include systems based on information and communication technology, and process and control applications.</p> <p>Electronic breakers are designed to trip at an exact point. They are highly sensitive in the event of overload, short circuit, and long cable paths. They are ideal for protecting extremely expensive, critical equipment. </p> <p>This leads to the next point: that because of their sensitivity, electronic breakers should be strategically placed. For example, if something has a huge start-up current, like a motor, the breaker could trip before you could even turn it on. You may be able to turn it on for a second, but it will shut off almost immediately after that. This is known as nuisance tripping. In this case, an electronic breaker would be too sensitive.</p> <p>Finally, electro-static discharge protection is not actually related to circuit breakers. To properly mitigate ESD transient anomalies, your design would require comprehensive shielding, grounding, and bonding techniques, along with the implementation of appropriate surge protective devices, which is a completely different topic in and of itself. </p>


  • <p>Thus far there have been several contributors on why they would choose a circuit breaker or a fuse. Others have commented on the location of the installation will determine what standards you will need to follow. In more general terms if you can assume that this protection will need to be in an industrial control panel, installed in the United States, a great reference is the UL 508A Standard for Industrial Control Panels, Sec In this part of the UL standard it describes how you would size your protection device. Once you have determined the protection required, cost, certification, size and performance, of the protection device, will most likely need to be considered. In most cases a fuse will be the obvious choice due to size and cost. If quick visual or electronic notification is required then there are good options with circuit breakers.</p> <p>Mark Lovell Proposals Manager Pepperl+Fuchs www.pepperl-fuchs.us/</p>


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