While fuses and circuit breakers offer tried-and-true electrical circuit protection for machines, new electronic circuit protectors provide far more advanced capabilities.
Although it may often not be top of mind, electrical circuit protection is critical to machine design. For automated machinery, the controllers and drives are typically powered by low voltage and low current, which must be carefully protected to preserve reliable operation.
Any disruption or failure leads to undesired behavior, so designers must carefully follow electrical codes and design practices for best results. Electrical conductors require protection from damaging overcurrent and short circuit conditions. This is most commonly done by incorporating overcurrent protection devices (OCPDs) such as traditional fuses and circuit breakers, according to the National Electrical Code (NEC). A newer and more advanced option is the electronic circuit breaker (ECB), which can be a good alternative to improve safety and uptime.
Control power basics
Industrial devices and input/output signals are regularly operated at 24 Vdc with relatively small currents. Most connections have a low inrush current unless they are associated with a solenoid or motor.
Because control equipment in North America is generally supplied by 120 Vac, control panels are typically designed with power supplies to convert this into 24 Vdc. These power supplies are downstream of feeder and branch circuit protection, so only supplementary protection is required.
Supplementary protection is designed to protect devices such as sensitive low-current PLC outputs, sensitive electronics, and power supplies. Protecting wires from overcurrent is usually not the goal of supplementary protection since wires are usually rated at a much higher current than the circuit will ever carry under non-fault conditions. The most common forms of control circuit protection are fuses and circuit breakers.
Fuses are available in many form factors and amperage ratings, but for control circuits in North America today it is quite common to see small 5 x 20-mm glass fuses installed into a fuse holder terminal block. Fuses are known for their fast response, effectiveness, and low cost. It’s straightforward to specify small glass fuses for control circuits, and they’re available in many different amperage ratings and trip classes.
So what happens when an overcurrent or short circuit occurs on a control circuit protected by a fuse? Well, the fuse blows, which means the conductive element inside of it vaporizes so the circuit is opened, and the downstream circuit is de-energized.
How do you know which fuse? What if you have dozens or hundreds of fuses in your control cabinet? An authorized technician can investigate the drawings and use a test meter. A more convenient option, although it adds cost, is to design panels with indicator fuse holders using LEDs to identify blown fuses. Another option is to wire the load side of each fuse to a PLC input for monitoring, but this is expensive in terms of design, wiring, PLC resources, and enclosure space.
Once a maintenance technician finds and removes a blown fuse, best practice is to inspect the removed fuse ratings, compare them with the design documents, and find a replacement. These are small components and it’s easy to lose them or put them in the wrong place, and in some cases a fuse is damaged to an extent that makes identification difficult. What happens when there is not an exact replacement in spares storage, or the technician installs a differently rated fuse?
Fuses can also be susceptible to premature failure in the form of nuisance tripping caused by vibration, and their use should therefore be avoided for these types of applications. Because fuses have been around so long, finding exact or functionally equal replacement fuses, holders, and other related accessories is almost always possible.
Circuit breaker protection
The other tried and true method of control circuit protection is with circuit breakers following clearly defined requirements in the NEC. There is a special category of “supplementary OCPDs” for providing a limited level of protection, usually within equipment. Normally these are not technically circuit breakers, but they look and behave like them, although they are only rated for specific applications downstream of properly protected feeder and branch circuits.
Supplementary OCPDs, or supplementary protectors, normally have a UL 1077 rating and are used on supplementary circuits only, such as control circuits. It’s acceptable to use a miniature circuit breaker on supplementary circuits, but circuit breakers are more expensive than supplementary protectors. Both circuit breakers and supplementary protectors are available in many different current ratings and trip curves—as well as in single, double and triple pole configurations.
The obvious advantage of circuit breakers and supplementary protectors is that if they trip (open the downstream circuit) they can be conveniently reset into service, without requiring replacement. Circuit breakers and supplementary protectors also fare better in high vibration environments than fuses, so they are better choices for machinery and equipment, where a lot of vibration is present.
However, these devices can fail on occasion and require replacement. Once again, the technician will need to review ratings, and then check in-house or distributor stock. Some circuit breakers and supplementary protectors use specifically sized accessories, such as bus bars, which can make future expansion a headache if compatible parts are not manufactured anymore. Circuit breakers and supplementary breakers are most commonly installed in control panels on DIN rail directly or clipped into a special terminal block base, but one disadvantage is that they typically take up more space than simple fuse holders.
ECBs: The modern OCPD option
After decades of using standard fuses and supplementary protectors, designers today have another option. Consider a device with the reusability of a circuit breaker or supplementary breaker. Add the ability for it to be remotely monitored, reset, turned on or off, and configured with an adjustable trip current. Package it for a more efficient use of space than a circuit breaker or supplementary protector. The resulting device is an electronic circuit breaker (ECB).
ECBs are a fast-acting supplementary circuit protection method, able to precisely, quickly, and repeatedly switch off in response to overcurrent and short circuit events, even with long cable runs and small conductor cross-sections. They are the logical choice for supplementary circuit protection and work especially well in conjunction with the switch-mode power supplies commonly used with control circuitry. Their flexibility can also reduce the need for spares inventory.
ECBs protect supplementary circuits by electronically monitoring load current. They can recognize overloads and short circuits rapidly, permitting brief current peaks but switching off during longer duration overloads. After a thermal waiting period, the output can be switched on locally like a circuit breaker, or remotely by a control signal.
ECBs come in many different configurations. Single-channel models are roughly the size of a standard IEC terminal block and come with either fixed or adjustable current ratings. For example, there may be a model with a current rating available in 1, 2, 4, 6 and 8 A — as well as adjustable range models that can be set anywhere from 1-8 A.
Multiple channel models are available, typically offering two to eight channels, and these provide even more flexibility and space savings. These types of ECBs take one power source and distribute it among the 2 to 8 channels. These channels can have adjustable current settings ranging from 0.5 to 10 A depending on the model, and each channel is protected individually. If one channel is tripped the other channels will maintain proper operation, and internal ECB circuitry is designed to prevent reverse feed into the other channels.
The NEC includes special provisions for low power ‘Class 2’ circuits, allowing designers to use non-UL-listed devices and more economical wiring methods. Of special note is the fact that ECBs are available in NEC Class 2 compliant models, with a fixed current setting of 3.8 A per channel and active current limiting. This allows an engineer or designer to use a standard (and larger) power supply not rated as an NEC Class 2, and to create Class 2 circuits.
ECBs normally have LED status lights indicating power on, power off, and tripped status for each channel. The ECB also can provide status signals to a PLC, using a communication protocol or pulse counting method, to provide feedback on if a channel is tripped. An ECB can accept a pulse sequence from a PLC to remotely reset, turn on, and turn off each channel.