Electronic circuit breakers protect 24 V dc circuits in industrial control panels
For industrial control panels, electronic breakers offer distinct advantages over mechanical breakers. An industrial control panel consists of an array of circuits. There are feeder, branch, control, or class II circuits that are needed to establish the specified function(s). All except Class II circuits need to be protected by a fuse or circuit breaker. For feeder and branch circuits the use of fuses or molded circuit breakers is common, as it also is for 24 V dc control circuits. In view of the progress of electronic development of programmable logic controllers (PLCs), sensors, and the like, the electronic components become more sensitive to voltage dips and interruptions.
The standard way of fusing 24 V dc circuits is with the use of standard circuit breakers. If more than one circuit is needed, the output of the used power supply is to be split into the needed numbers of circuits by using fuses or breakers. It is a basic requirement that fuses or breakers used in control circuits have to comply with the applicable standard for supplementary overcurrent protectors. These well-known standards like UL 1077 or UL 248-14 are not applicable for electronic breakers. In view of that, UL established a new standard UL 2367 — Solid State Overcurrent Protectors. This type of protectors is intended to be used on the load side of an isolating transformer, power supply, or battery to provide a means of supplementary protection.
Standard fuses or breakers have some issues that occur when a fault or short comes up within one of the spread circuits fed by the same power supply: a voltage drop on all connected loads with an amplitude based on the shorted path’s impedance. A mechanically operated breaker or melting fuse will need some time to operate before shutting down the affected circuit. During this period the voltage can drop until 0 Vdc for more than 10 ms and may lead PLCs into an unsafe or uncertain condition. When using an electronic fuse (solid state overcurrent protector), overcurrents are detected much more accurately and shut down much faster than by a fuse or breaker. This reduces the risk of misoperations to near zero.
Active current limiting
To avoid these problems, advanced circuit breakers can provide a solution. Variants provide different features.
One design follows a thermo-magnetic curve and behaves like a standard molded circuit breaker, but much faster and more accurately based on electronic detection. High inrush currents (>50.000µF) can be handled without problems.
Another series of breakers have an active current limiting implemented, which avoids any drop on the output circuits other than those affected. Based on that 100% individual channel design and sequential switch-on functionality, high inrush currents are manageable without any problems or shut downs. An advanced design can avoid shutdown of outputs during one single-channel failure (other than the affected channel), such as with some with electronic breakers available.
Finally, the electronic measurement of each channel’s current allows designers to reduce the installed power of the power supply used, because there is no need for backup power for letting circuit breakers trip in case of a short.
Benefits like two-wire communication, remote reset, or signaling contacts also are available. Some reduced space designs are especially advantageous for 4- or 8-channel units. For mechanical breakers, 8 breakers can require approximately 5.7 in.; some electronic breakers needs as little as 1.7 in.
Only two more wires are needed for remote operation. NEC Class II electronic breakers can be used to establish a NEC Class II circuit being supplied by the same power supply as with other circuits.
– Christoph Wesner, Dipl.-Ing. (FH), Block USA; edited by Mark T. Hoske, content manager, Control Engineering, email@example.com.
Note: This file, originally posted on March 16 as part of the Control Engineering March 2015 print and digital edition, was amended on April 15, in response to March 26 feedback from a reader.
Reader feedback, reply
A reader asked if electronic breakers can be used to supply NEC Class 2 circuits and asked if doing so complies with 725.121(A) in the 2014 National Electrical Code.
Christoph Wesner, p.p. Dipl.-Ing. (FH), head of Acc. Testlab / Standards & Approvals, Block Transformatoren-Elektronik GmbH, said, "NEC2014 Article 725.121 (A) defines, that power sources for Class 2 or Class 3 shall comply with:
- A listed Class 2 or Class 3 transformer, or
- A listed Class 2 or Class 3 power supply, or
- Other listed equipment marked to identify the Class 2 or Class 3 power supply.
"Per exception No 2 to (3) a current limiting impedance (per example 2) is also applicable as long as the limits per Chapter 9 Table 11(A) and (B) are fulfilled.
"Our Class 2 eBreakers are UL recognized per UL2367 as ‘Solid-state Overcurrent Protector’ in File E356250 and UL508 listed (E219022). In addition to UL2367 the units PM-9824-076-x and PM-9724-152-x were evaluated to comply with Limited Power Source requirements per NFPA 70 (NEC) and additionally for Class 2 Outputs (refer CoA #13)."
This article has additional information and a link to a related product, below.