Solid-state circuit breaker with solid-state interlock mechanism

Abstract

The system includes a first circuit breaker (10B-10N) including a first solid-state switch (100), a first mechanical contact (500A), and a current sensor configured to sense a current flowing through the first circuit breaker; and a second circuit breaker (10A) electrically coupled to the first circuit breaker (10B-10N) and configured to interrupt a current flowing through the first circuit breaker, wherein the first circuit breaker (10B-10N) issues a request (SSI REQUEST) to the second circuit breaker (10A) upon detecting a failure mode of the first solid-state switch (100). The second circuit breaker (10A) is configured to transmit a request (SIGNAL), the second circuit breaker (10A) is configured to interrupt the current flowing through the first circuit breakers (10B-10N) in response to receiving the request, and the first circuit breakers (10B-10N) are further configured to open the first mechanical contacts (500A) when the current flowing through the first circuit breakers (10B-10N) drops to a predetermined level.

Description

[Technical field]

[0001] The disclosed concepts relate generally to circuit breakers and, more particularly, to circuit breakers having a solid state interlock mechanism. [Background technology]

[0002] Circuit interrupters, such as, but not limited to, circuit breakers, are typically used to protect electrical circuits from damage due to an overload condition, an overcurrent condition such as a short circuit, or another fault condition such as an arcing fault or a ground fault. Solid-state circuit breakers use solid-state components, for example, using semiconductor devices to switch on and off the current flowing from a power source to a load. Solid-state circuit breakers may offer advantages over traditional mechanical circuit breakers, such as faster interruption, smaller size, or better reliability.

[0003] UL 489I is an outline of the requirements to be investigated for solid molded case circuit breakers. It should be used in conjunction with UL 489, the standard for the safety of molded case circuit breakers, molded case switches and circuit breaker enclosures. The scope of UL 489I covers solid molded case circuit breakers rated at 1000Vac and 1500Vdc that use semiconductors to switch and have an integral air gap to provide isolation. The semiconductor devices used are known to have leakage currents, so an air gap is still required to provide galvanic isolation to the load. Mechanical contacts are commonly used to create this air gap.

[0004] One advantage that solid-state circuit breakers have over conventional circuit breakers is the size of this gap. Solid-state breakers can use a much smaller gap and do not require the use of an arc chute because the energy is dissipated within the semiconductor device itself. The reliability of the breaker is based on the ability of the semiconductor to interrupt the current before the gap is created. Summary of the Invention [Problem to be solved by the invention]

[0005] However, one of the failure modes of solid-state components, e.g., semiconductor devices, is a short circuit. When such a short circuit fault occurs, the solid-state circuit breaker fails to open. Such a fault that trips the circuit breaker can have adverse effects on the solid-state circuit breaker, the circuit interruption system that includes the failing solid-state circuit, and the load.

[0006] There is room for improvement in solid-state circuit breakers and systems that include solid-state circuit breakers.

[0007] These and other needs are met by embodiments of the disclosed concepts that require a solid-state circuit breaker in a failure mode to interrupt current flow to an upstream circuit breaker so that the mechanical contacts can open.

[0008] According to one aspect of the disclosed concept, a system includes a first circuit breaker comprising a first solid-state switch, a first mechanical contact, and a current sensor configured to sense current flowing through the first circuit breaker; and a second circuit breaker electrically coupled to the first circuit breaker and configured to interrupt current flowing through the first circuit breaker, wherein the first circuit breaker is configured to send a request to the second circuit breaker upon detecting a fault mode, the second circuit breaker is configured to interrupt current flowing through the first circuit breaker in response to receiving the request, and the first circuit breaker is further configured to open the first mechanical contact when current flowing through the first circuit breaker drops to a predetermined level.

[0009] In accordance with another aspect of the disclosed concepts, a solid-state circuit breaker comprises a solid-state switch configured to open to interrupt current flowing through the solid-state circuit breaker, mechanical contacts configured to open to provide galvanic isolation, a current sensor configured to sense current flowing through the solid-state circuit breaker, and an electronic trip unit configured to control the opening or closing of the solid-state switch to control the opening of the mechanical contacts, wherein the solid-state circuit breaker is configured to detect a failure mode of the solid-state switch and in response transmit a request to an upstream circuit breaker to interrupt current flowing through the solid-state circuit breaker.

[0010] According to another aspect of the disclosed concepts, a method includes detecting a failure mode by a first circuit breaker, the failure mode including a failure of a first solid-state switch of the first circuit breaker to open; sending a request by the first circuit breaker to a second circuit breaker; receiving the request by the second circuit breaker; interrupting, by the second circuit breaker, a current through the first circuit breaker in response to receiving the request; detecting that the current through the first circuit breaker has dropped to a predetermined level; and opening a mechanical contact by the first circuit breaker in response to detecting that the current through the first circuit breaker has dropped to the predetermined level. [Brief description of the drawings]

[0011] A complete understanding of the concepts of the present disclosure can be gained from the following detailed description when read in conjunction with the accompanying drawings. [Figure 1] FIG. 1 is a diagram of a solid-state circuit breaker in accordance with an exemplary embodiment of the disclosed concepts. [Diagram 2] FIG. 2 is another diagram of a system according to an exemplary embodiment of the disclosed concepts. [Diagram 3] FIG. 3 illustrates the waveform of the current flowing through a circuit breaker in accordance with an exemplary embodiment of the disclosed concepts. [Figure 4] FIG. 4 is a flow chart of a method of solid state interlock according to an exemplary embodiment of the disclosed concepts. [Diagram 5] FIG. 5 is another flow chart of a method of solid interlock according to an exemplary embodiment of the disclosed concepts. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Directional phrases used herein, such as left, right, front, back, top, bottom, and variations thereof, refer to the orientation of the elements shown in the drawings and do not limit the scope of the claims, unless expressly stated therein.

[0013] As used herein, a statement that two or more parts are "coupled" together is intended to mean that the parts are joined together either directly or through one or more intermediate parts.

[0014] 1 is a diagram of a solid-state circuit breaker 10 in accordance with an exemplary embodiment of the concepts of the present disclosure. The solid-state circuit breaker 10 may be part of a larger system that includes additional circuit breakers, solid-state and / or mechanical circuit breakers, such as the system shown in FIG.

[0015] The solid-state circuit breaker 10 is configured to be electrically connected between a power source and a load 12 via the LINE and LOAD conductors 2, 4. An upstream circuit breaker may be disposed between the power source and the solid-state circuit breaker 10. The solid-state circuit breaker 10 is configured to trip or switch open to interrupt current flow to the load 12 in the event of a fault condition (such as, but not limited to, an overcurrent condition) to protect the load 12, the circuit associated with the load 12, and the components within the solid-state circuit breaker 10.

[0016] The solid-state circuit breaker 10 includes a solid-state switch 100, an electronic trip unit 200, an operating mechanism 300, a sensor 400, and mechanical contacts 500. The solid-state switch 100 includes solid-state switching elements (such as, but not limited to, metal-oxide-semiconductor-field-effect-transistors (MOSFETs) and insulated-gate bipolar transistors (IGBTs)) that are configured to turn on and off (i.e., open and close) to allow or interrupt current flow to the load 12. The solid-state switch 100 is electrically coupled to the load 12 and the electronic trip unit 200.

[0017] The electronic trip unit 200 is configured to control the solid-state switch 100 to open and close, and is also configured to control the operating mechanism 300 to trip open the mechanical contacts 500 based on a signal from the sensor 400. The electronic trip unit 200 may include a processing unit that may include a processor and a memory. The processor may be, for example, but not limited to, a microprocessor, a microcontroller, or other suitable processing device or circuit. The memory may be any one or more of various types of internal and / or external storage media, such as RAM, ROM, EPROM, EEPROM, FLASH, etc., a storage register, i.e., a machine-readable medium, for storing data, such as an internal storage area of ​​a computer, and may be a volatile or non-volatile memory. The memory may store a firmware solution for solid state interlocking (SSI), in which a first solid-state circuit breaker, e.g., a downstream circuit breaker, fails to open the solid-state switch, such as due to a short circuit of the solid-state switch. SSI is implemented by a first solid-state circuit breaker and a second circuit breaker, e.g., an upstream circuit breaker relative to the first solid-state circuit breaker. The second circuit breaker may or may not be a solid-state circuit breaker. The SSI-enabled downstream circuit and the SSI-enabled upstream circuit breaker may together monitor and respond to the interrupting capabilities of their power semiconductors. SSI is further discussed with reference to FIG.

[0018] The electronic trip unit 200 is configured to determine a fault condition (such as, but not limited to, an overcurrent fault) and control the solid-state switch 100 to open in response to the fault condition. Opening the solid-state switch 100 interrupts current flow to the load 12.

[0019] The operating mechanism 300 is configured to open the mechanical contacts 500 in response to a signal from the electronic trip unit 200. Opening the mechanical contacts 500 provides galvanic isolation between the power source and the load 12. For example, but not limited to, the operating mechanism 300 is configured to open the mechanical contacts 500, for example, but not limited to, by moving a movable arm to separate the mechanical contacts 500. The electronic trip unit 200 is configured to control the operating mechanism 300 to open the mechanical contacts 500 only after the solid-state switch 100 is opened to interrupt the current flowing through the solid-state circuit breaker 10. In some exemplary embodiments, the mechanical contacts 500 are not designed to interrupt the rated current flowing through the solid-state circuit breaker 10, and the solid-state circuit breaker 10 may lack components such as arc chutes to address the effects of the mechanical contacts 500 interrupting the rated current. Thus, the mechanical contacts 500 should only be opened when the current flowing through the solid-state circuit breaker 10 has dropped to a level that is safe for the mechanical contacts 500 to open.

[0020] The sensor 400 may be a current sensor (e.g., but not limited to, a current transformer, a Hall effect sensor, etc.) configured to sense the current flowing through the solid-state circuit breaker 10. The output of the sensor 400 may be provided to the electronic trip unit 200.

[0021] The electronic trip unit 200 is configured to detect a failure mode of the solid-state circuit breaker 10. The failure mode is a failure of the solid-state switch 100 to not open. In response to detecting the failure mode, the electronic trip unit 200 is configured to output a request to the upstream circuit breaker. In response to the request, the upstream circuit breaker is configured to interrupt the current flowing to the solid-state circuit breaker 10, resulting in a drop in the current flowing through the solid-state circuit breaker 10. The electronic trip unit 200 is configured to monitor the current flowing through the solid-state circuit breaker 10 during the drop and determine when the current drops to a predetermined level. The predetermined level is a level that is safe for opening the mechanical contacts 500. An exemplary predetermined level is 5A. However, the predetermined level may be modified without departing from the scope of the disclosed concepts. When the current reaches the predetermined level, the electronic trip unit 200 controls the operating mechanism 300 to open the mechanical contacts 500. After opening the mechanical contacts 500, the electronic trip unit 200 sends a clear request to the upstream circuit breaker. The clear request indicates that the upstream circuit breaker may stop interrupting current flowing through the solid-state circuit breaker 10. The drop in current flowing through the solid-state circuit breaker 10 allows the mechanical contacts 500 to safely open. The clear request allows the upstream circuit breaker to quickly restore current, thereby minimizing interruption to other loads downstream of the upstream circuit breaker. In some exemplary embodiments, the duration of the interruption may be on the order of microseconds and will have little effect on other loads downstream of the upstream circuit breaker.

[0022] 2 is a diagram of a system 1 according to an exemplary embodiment of the concepts of the present disclosure. System 1 may include one or more solid-state circuit breakers 10. In FIG. 2, system 1 includes multiple circuit breakers, e.g., upstream circuit breaker 10A and downstream solid-state circuit breakers 10B-N, where N is an integer. Upstream circuit breaker 10A may or may not be a solid-state circuit breaker.

[0023] The circuit breakers 10A-N are electrically coupled to each other and communicate with each other via any suitable communication protocol. For example, the circuit breakers 10A-N may be configured to communicate via wired or wireless communication. In wired communication, the circuit breakers 10A-N may communicate via power line communication or via control lines. It will be understood that any suitable form of communication may be used between the circuit breakers 10A-N without departing from the scope of the disclosed concepts. The circuit breakers 10A-N may communicate with each other to request or perform a solid-state interlock (SSI), for example, when a failure mode, e.g., a solid-state switch fail-to-open failure, is detected and one or more of the solid-state circuit breakers fail to open. For example, if the solid-state switch 100 of the downstream solid-state circuit breaker 10B is shorted 100A, the solid-state switch 100 of the downstream solid-state circuit breaker 10B may not open. When the solid-state switch 100 fails to open and current continues to flow through the downstream solid-state circuit breaker 10B, it is not safe to open the mechanical contacts 500A. In response to the failure mode, the downstream circuit breaker 10B sends a request (eg, an SSI request) to the upstream circuit breaker 10A.

[0024] SSI is both a hardware and firmware solution to one of the failure modes associated with the power semiconductor devices in a solid-state circuit breaker. SSI provides a firmware solution in that it provides a communication scheme and allows an upstream circuit breaker to recognize and respond to a failure mode, such as when the solid-state circuit breaker cannot open during a short circuit event. For example, when an SSI-enabled solid-state circuit breaker senses a short circuit fault in one or more of the semiconductor devices, it sends an SSI request to the nearest upstream circuit breaker, which in response interrupts current flowing to the downstream circuit breaker. The SSI request may indicate that there has been a failure in which the solid-state switch of the downstream circuit breaker 10B does not open, and may request the upstream circuit breaker 10A to interrupt current flowing to the downstream circuit breaker 10B. SSI may be performed via interruption logic embedded in the circuit breaker.

[0025] The downstream circuit breaker 10B is configured to monitor the current drop due to interruption and then open its mechanical contacts 500A when the current reaches a predetermined level. The predetermined level may be a current level at which the mechanical contacts of the requesting circuit breaker can safely open at all stages. The upstream circuit breaker 10A may create an interrupting current flowing through the downstream circuit breaker 10B by temporarily turning off its semiconductor devices, e.g., SiC MOSFETs, IGBTs, etc. The duration of the current interruption by the upstream circuit breaker may be adjusted to have minimal impact on other loads downstream of the upstream circuit breaker 10A. The duration of the current zero may be in microseconds (μs). In examples where the upstream circuit breaker is a non-solid state circuit breaker (non-SSCB), the non-SSCB upstream circuit breaker may interrupt the current by opening its mechanical contacts. Thus, the non-SSCB may also require an additional action to reclose itself. However, in some exemplary embodiments, the non-SSCB may be capable of reclosing its mechanical contacts.

[0026] When the current is interrupted, the downstream solid-state circuit breaker 10B detects a drop in current through it that reaches a predetermined level via a current sensor (e.g., current sensor 400 described with reference to FIG. 1) and opens its mechanical contacts 500A. Upon opening the mechanical contacts 500A, the downstream circuit breaker 10B may send a clear request to the upstream circuit breaker 10A. The clear request may indicate that the failure mode has now been repaired, e.g., the mechanical contacts of the downstream circuit breaker 10B are now open, and it is clear that the upstream circuit breaker 10A will terminate the SSI mechanism and resume normal operation.

[0027] Upon receiving the clear request, the upstream circuit breaker 10A may terminate the SSI, for example, by closing its mechanical contacts or solid-state switch. The upstream circuit breaker 10A then resumes its normal operation.

[0028] 3 includes waveforms of current flowing through downstream and upstream circuit breakers in accordance with an exemplary embodiment of the disclosed concepts: A is the waveform of current flowing through the downstream circuit breaker, e.g., an SSI-enabled downstream solid-state circuit breaker; B is the waveform of current flowing through the upstream circuit breaker, e.g., an SSI-enabled upstream circuit breaker;

[0029] At time t1, both the downstream circuit breaker and the upstream circuit breaker are closed.

[0030] Following time t1, but before time t2, a failure mode of the downstream circuit breaker occurs (i.e., failure of the solid-state switch to open). The downstream circuit breaker detects this failure mode.

[0031] At time t2, the downstream circuit breaker sends a request to the upstream circuit breaker. In response to the request, the upstream circuit breaker interrupts the current flowing to the downstream circuit breaker to reduce the current flowing through it. The downstream circuit breaker then waits for the current flowing through it to reduce to a predetermined level at which it is safe to open its mechanical contacts.

[0032] At time t3, the current flowing through the downstream circuit breaker reaches a predetermined level. In one example, the predetermined level may be 5 A or less for a 100 A solid-state circuit breaker. However, it will be understood that other predetermined levels may be used without departing from the scope of the concepts of the present disclosure. At time t3, the downstream may begin the process of opening its mechanical contacts. Because this process takes a certain amount of time, the mechanical contacts may not actually open until t4. Once the mechanical contacts are open, the downstream circuit breaker sends a clear request to the upstream circuit breaker indicating that the upstream circuit breaker may stop interrupting current flowing to the downstream circuit breaker.

[0033] At time t5, the upstream circuit breaker may receive a clear request from the downstream circuit breaker and stop interrupting current to the downstream circuit breaker, for example, by closing its solid-state switch or mechanical contacts.

[0034] During time t5, when the upstream circuit breaker stops interrupting current, by time t6, the current flowing through the upstream circuit breaker rises to its normal level, which is reached at time t6.

[0035] 4 is a flow diagram of a method 400 according to an exemplary embodiment of the disclosed concepts, which may be performed by the circuit breaker and the upstream and downstream circuit breakers of the respective processing units as described with reference to FIGS.

[0036] At 410, a first circuit breaker detects the failure mode (ie, a failure of the solid-state switch to open). The first circuit breaker may be a downstream circuit breaker, as described with reference to FIG.

[0037] The request may indicate that there has been a failure of the solid-state switch of the first circuit breaker to open and may request the second circuit breaker to interrupt current through the first circuit breaker. At 420, the first circuit breaker sends a request to the second circuit breaker. The second circuit breaker may be an upstream circuit breaker, as described with reference to FIG. 2. There may be one or more upstream circuit breakers within a circuit breaker, and the downstream circuit breaker sends the SSI request signal to the closest upstream circuit breaker. The upstream circuit breaker may be either a solid-state circuit breaker or a non-solid-state circuit breaker.

[0038] At 430, the second circuit breaker receives the request from the first circuit breaker.

[0039] At 440, the second circuit breaker interrupts the current flowing to the first circuit breaker. The second circuit breaker may interrupt the current by temporarily turning off its semiconductor device, e.g., SiC MOSFET, IGBT, etc., via an interruption logic embedded in the memory of the upstream circuit breaker. The duration of the interruption by the second circuit breaker may be adjusted to have minimal impact on the load of the downstream breaker. The duration is in microseconds (μs).

[0040] At 450, the first circuit breaker opens its mechanical contacts when the current flowing through it drops to a predetermined level. The second circuit breaker waits for a clear request signal from the first circuit breaker. The clear request signal may indicate that the mechanical contacts of the first circuit breaker have been opened, and therefore it is clear for the second circuit breaker to stop interrupting and resume its normal operation.

[0041] At 460, the first circuit breaker sends a clear request to the second circuit breaker.

[0042] At 470, the second circuit breaker stops interrupting current to the first circuit breaker based at least in part on the received clear request. The second circuit then resumes normal operation.

[0043] 5 is another flow chart of a method 500 in accordance with an exemplary embodiment of the disclosed concepts. The method may be performed by respective processing units coupled to the upstream and downstream circuit breakers of a circuit breaker and respective memories therein, as described with reference to FIGS.

[0044] At 510, both the downstream circuit breaker and the upstream circuit breaker are closed and normal operation is performed.

[0045] At 512, a fault event occurs in the downstream circuit breaker. The fault event may include an overcurrent event.

[0046] At 514, the downstream circuit breaker detects that a failure mode has occurred, i.e., the downstream circuit breaker detects that its solid-state switch has failed to open.

[0047] At 516, the downstream circuit breaker sends a request signal to the upstream circuit breaker.

[0048] At 518, the upstream circuit breaker receives the request from the downstream circuit breaker.

[0049] At 520, the upstream circuit breaker opens its solid-state switch to interrupt current flow to the downstream circuit breaker.

[0050] At 522, the downstream circuit breaker detects the level of current flowing therethrough, and when it detects that the level of current has dropped to a predetermined level, the method proceeds to 526. The downstream circuit breaker may detect zero current via a current sensor (e.g., current sensor 400 discussed with reference to FIG. 1 ). The predetermined level is a level that is safe for the downstream circuit breaker to open its mechanical contacts.

[0051] At 524, the downstream circuit breaker opens its mechanical contacts.

[0052] At 526, the downstream circuit breaker sends a clear request to the upstream circuit breaker.

[0053] At 528, the upstream circuit breaker receives a clear request from the downstream circuit breaker.

[0054] At 530, the upstream circuit breaker stops interrupting current flowing to the downstream circuit breaker, for example, by opening its solid-state switch.

[0055] At 532, the upstream circuit breaker resumes normal operation.

[0056] Although specific embodiments of the concepts of the present disclosure have been described in detail, those skilled in the art will appreciate that various modifications and alternatives to those details may be developed in light of the overall teachings of the present disclosure. Accordingly, the particular arrangements disclosed are meant to be merely illustrative and not limiting as to the scope of the concepts of the present disclosure, which is given the full scope of the appended claims and any and all equivalents thereof.

Claims

1. 1. A system comprising: a first circuit breaker comprising a first solid-state switch, a first mechanical contact, and a current sensor configured to sense a current flowing through the first circuit breaker; a second circuit breaker electrically coupled to the first circuit breaker and configured to interrupt current flow through the first circuit breaker; the first circuit breaker is configured to send a request to the second circuit breaker upon detecting a failure mode; the second circuit breaker is configured to interrupt current through the first circuit breaker in response to receiving the request, the first circuit breaker being further configured to open the first mechanical contact when current through the first circuit breaker drops to a predetermined level.

2. 2. The system of claim 1, wherein the second circuit breaker includes a second solid-state switch, and wherein the second circuit breaker is configured, in response to receiving the request from the first circuit breaker, to open the second solid-state switch to interrupt current flow through the first circuit breaker.

3. 3. The system of claim 2, wherein the first circuit breaker is further configured to send a clear request to the second circuit breaker when the first mechanical contacts are opened, and the second circuit breaker is configured to close the second solid-state switch to stop interrupting current flow through the first circuit breaker in response to receiving the clear request from the first circuit breaker.

4. 2. The system of claim 1, wherein the second circuit breaker includes a second mechanical contact, the second circuit breaker is configured to open the second mechanical contact to interrupt current through the first circuit breaker in response to receiving the request from the first circuit breaker, the first circuit breaker is further configured to send a clear request to the second circuit breaker when the first mechanical contact is open, and the second circuit breaker is configured to close the second mechanical contact to stop interrupting current through the first circuit breaker in response to receiving the clear request from the first circuit breaker.

5. The system of claim 1 , wherein the predetermined current level comprises a current value that allows the first circuit breaker to safely open the first mechanical contacts.

6. 2. The system of claim 1, wherein the second circuit breaker is configured to stop interrupting current through the first circuit breaker for a predetermined time after interrupting current through the first circuit breaker.

7. 2. The system of claim 1, wherein the second circuit breaker is disposed upstream of the first circuit breaker, the system further comprising a third circuit breaker disposed downstream of the second circuit breaker.

8. 2. The system of claim 1, wherein the failure mode comprises a failure of the first solid-state switch to open.

9. 1. A solid-state circuit breaker comprising: a solid-state switch configured to open to interrupt current flow through the solid-state circuit breaker; a mechanical contact configured to open to provide galvanic isolation; a current sensor configured to sense a current flowing through the solid-state circuit breaker; an electronic trip unit configured to control the opening or closing of the solid-state switch and to control the opening of the mechanical contacts; The solid-state circuit breaker is configured to detect a failure mode of the solid-state switch and, in response, send a request to an upstream circuit breaker to interrupt current flow through the solid-state circuit breaker.

10. 10. The solid-state circuit breaker of claim 9, wherein the electronic trip unit is configured to detect when the current flowing through the solid-state circuit breaker drops to a predetermined level and to control the mechanical contacts to open when the current flowing through the solid-state circuit breaker drops to the predetermined level.

11. 11. The solid-state circuit breaker of claim 10, wherein the electronic trip unit is configured to send a clearing request to the upstream circuit breaker after controlling the mechanical contacts to open, and the predetermined level comprises a current value that allows the solid-state circuit breaker to safely open the mechanical contacts.

12. 1. A method comprising: detecting a failure mode by a first circuit breaker, the failure mode comprising a failure of a first solid-state switch of the first circuit breaker to open; sending a request by the first circuit breaker to a second circuit breaker; receiving the request by the second circuit breaker; interrupting, by the second circuit breaker, current through the first circuit breaker in response to receiving the request; and detecting when a current flowing through the first circuit breaker drops to a predetermined level; and opening mechanical contacts by the first circuit breaker in response to detecting that the current flowing through the first circuit breaker has dropped to the predetermined level.

13. The method further comprising: sending a clear request by the first circuit breaker to the second circuit breaker when the mechanical contacts open; 13. The method of claim 12, further comprising: in response to receiving the clear request, ceasing interruption of current flow through the first circuit breaker by the second circuit breaker.

14. 14. The method of claim 13, wherein interrupting the current through the first circuit breaker comprises opening a second solid-state switch of the second circuit breaker, and wherein stopping interrupting the current through the first circuit breaker comprises closing the second solid-state switch of the second circuit breaker.

15. The method further comprising:

13. The method of claim 12, further comprising: after interrupting the current through the first circuit breaker, stopping interrupting the current through the first circuit breaker for a predetermined time.

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