Patent Application
20120050933
This disclosure relates to protection of branch circuits, and more particularly to overcurrent protection of a branch circuit with an in-line solid state switch.
Modern building codes, such as the National Electric Code 2008, require arc fault circuit interrupter (“AFCI”) devices to be used on numerous branch circuits in a home. AFCI devices are operable to de-energize a branch circuit in response to detected current signatures indicative of arcing and sparking. However, the response time of AFCI devices is slow enough that a downstream solid-state control device may be damaged before the AFCI device can de-energize its associated branch circuit.
A method of protecting against overcurrent conditions controls a solid state switch to turn OFF to prevent a flow of current along a first current path in response to an overcurrent condition exceeding a predefined current threshold and a predefined energy threshold. Current is selectively passed along a second current path that is parallel to the first current path in response to the solid state switch turning OFF. An overcurrent detection device that is upstream of the first current path and the second current path is used to detect at least partially via the second current path whether the overcurrent condition corresponds to a predefined fault condition downstream of the current paths.
These and other features of the present disclosure can be best understood from the following specification and drawings, the following of which is a brief description.
A current and energy sensing module 20 is operable to determine if a flow of current through the current path 12 exceeds a predefined current threshold and a predefined energy threshold (which would indicate an overcurrent condition). If the flow of current through the current path 12 exceeds the predefined current and energy thresholds (e.g. the current exceeds the predefined current threshold for an amount of time such that the flow of current exceeds the predefined energy threshold) then the current and energy sensing module 20 commands switch control 22 to turn the switch 16 OFF such that the flow of current along path 12 is interrupted. By turning OFF the switch 16, the switch 16 is protected from potential damage from the overcurrent condition.
The second current path 14 includes a transient voltage suppression (“TVS”) device 24. In one example the TVS device is a zener diode or a metal oxide varistor (“MOV”). Of course, other TVS devices 24 could be used. Under normal non-fault conditions the TVS is not conducting such that no current passes along the second current path 14, and current is only able to pass through the current path 12 (depending on the state of the switch 16). The branch circuit also includes an overcurrent detection device 26 and an inductor 28 that are both upstream of the current paths 12, 14. In one example the overcurrent detection device 26 includes an arc fault circuit interrupter (“AFCI”) device operable to detect if the overcurrent condition corresponds to an arc fault condition. In one example the overcurrent detection device 26 includes a ground fault circuit interrupter (“GFCI”) device operable to detect if the overcurrent condition corresponds to a ground fault condition. In one example the overcurrent detection device 26 includes a standard circuit breaker operable to detect if the overcurrent condition corresponds to a current exceeding a certain maximum allowable current threshold. Of course, other overcurrent protection devices could also be used.
In the event of an overcurrent condition downstream of the current paths 12, 14 (e.g. at node 30), the switch 16 turns OFF to stop a flow of current through the first current path 12. As will be discussed below, this abrupt change in current flow causes the inductor 28 to discharge as a voltage spike. If this voltage spike is of a sufficient magnitude, the TVS device 24 begins conducting, and the overcurrent detection device 26 may detect at least partially via the second current path 14 whether the overcurrent condition corresponds to a predefined fault condition. Upon detection of a predefined fault condition (e.g. arc fault condition), the overcurrent detection device 26 (e.g. AFCI device) turns OFF to prevent current from flowing through either of the current paths 12, 14. This process will now be described in greater detail.
As described above, if the current and energy sensing module 20 detects a current spike that exceeds the predefined current and energy thresholds, the switch 16 is commanded OFF. In one example the current threshold may be within the range of 50-75 Amps. In one example the energy threshold may be in the range of 100-300 mJ. Of course, other current and energy thresholds could be used. The time from detection of the overcurrent condition to turn OFF of the switch 16 may be on the order of several microseconds such that the switch 16 turns OFF quickly enough to protect itself. However, the overcurrent detection device 26 may need a greater amount of time (e.g. several milliseconds) to detect its predefined fault condition. Therefore, the quick turnoff time of the switch 16 prevents the overcurrent detection device 26 from detecting the fault condition through the first current path 12.
Once the switch 16 turns OFF to interrupt the flow of current along current path 12, the inductor 28 discharges its stored energy as current. Because of the abrupt change in current (ΔI) within the time period following the turn OFF of the switch 16 (Δt), a voltage across the TVS device 24 is produced as a voltage spike. The magnitude of the voltage generated by the inductor 28 may be expressed using equation #1 below, and the magnitude of the energy stored in the inductor 28 may be expressed using equation #2 below.
where
The energy stored in the inductor 28 is discharged through the TVS device 24 if the generated voltage V is greater than the breakdown voltage of the TVS. The inductance of the inductor 28 and the breakdown voltage of the TVS device 24 may be selected such that this discharge is sufficient to cause the TVS device 24 to start conducting along the second current path 14. In one example the TVS device 24 is selected to have a conduction threshold that is 10-20 volts lower than a rating of the solid state switch 16. However, these are only example values and it is understood that other conduction thresholds could be used.
When the TVS device 24 is conducting, the inductor 28 discharges through the TVS device 24. Because the discharged current resembles the downstream fault condition that caused the switch 16 to open, by analyzing the discharged current, the overcurrent detection device 26 is able to determine if the overcurrent condition corresponds to a predefined fault condition (e.g. at node 30). As described above, the predefined fault condition may include an arc fault condition or a ground fault condition, for example. If the overcurrent condition that caused the switch 16 to turn OFF corresponds to an arc fault condition, the overcurrent detection device 26 is able to trip to protect the circuit 10 despite the possible premature stop of current flow along the first current path 12. In one example the overcurrent detection device 26 analyzes the downstream overcurrent condition exclusively through the second current path 14. However, it is possible that the overcurrent detection device 26 could analyze the downstream overcurrent condition partially through the first current path 12 (before the switch 16 turns OFF) and partially through the second current path 14.
In one example, the switch 16 will automatically turn ON again after a delay (e.g. at a zero-crossing voltage of a subsequent half AC cycle), at which time the circuit 10 is ready to react to the next potential downstream overcurrent condition. Thus, the circuit 10 enables effective protection of the switch 16 and effective compliance with electrical codes requiring AFCI usage such that the switch 16 is able to open fast enough to protect itself and the current path 14 is able to provide a current representing a downstream arc and the arc pattern to enable the overcurrent detection device 26 to trip.
A current and energy sensing module 50 is operable to determine if a flow of current through the current path 42 exceeds a predefined current threshold and a predefined energy threshold. If the flow of current through the current path 42 exceeds the predefined current and energy thresholds (e.g. the current exceeds the predefined current threshold for an amount of time such that the flow of current exceeds the energy threshold) then the current and energy sensing module 50 commands switch control 52 to turn the switch 46 OFF such that the flow of current along path 42 is interrupted and current flows through the second current path 44. As with the circuit 10, by turning OFF the switch 46, the switch 46 is protected from potential damage from the overcurrent condition.
The second current path 44 includes an electro-mechanical switch 54. Under normal non-fault operating conditions, the switch 54 is OFF such that current is only able to pass through the current path 42 (depending on the state of the switch 46). In response to the solid state switch 46 turning OFF, the electromechanical switch 54 turns ON to permit a flow of current along the current path 44. The flow of current along current path 44 enables the overcurrent detection device 56 to trip and protect the circuit 40 and load 48 if the overcurrent condition corresponds to a predefined fault condition (e.g. at node 60). As described above in connection with the branch circuit 10, the overcurrent detection device 56 may include at least one of an arc fault circuit interrupter, a ground fault circuit interrupter, or a standard circuit breaker, and the predefined fault condition may includes at least one of an arc fault condition, a ground fault condition, or an overcurrent condition of a predefined magnitude.
In one example the overcurrent detection device 56 analyzes the downstream overcurrent condition exclusively through the second current path 44. However, it is possible that the overcurrent detection device 56 could analyze the downstream overcurrent condition partially through the first current path 42 (before the switch 46 turns OFF) and partially through the second current path 44.
Thus, like the circuit 10, the circuit 40 enables effective protection of its switch 46 and effective compliance with electrical codes requiring AFCI usage such that the switch 46 is able to open fast enough to protect itself and the current path 44 is able to provide access to the downstream overcurrent condition to enable the overcurrent detection device 56 to detect downstream fault conditions.
Without the use of current paths 14, 44 the overcurrent detection devices 26, 56 may not be able to trip because the solid state switches 16, 46 may open prematurely before the overcurrent detection devices 26, 56 were able to detect predefined fault conditions. This could result in the circuit 10, 40 potentially failing building inspections due to the lack of arc fault detection compatibility through the overcurrent detection devices 26, 56. By selectively passing current along the second current paths 14, 44 that are parallel to the first current paths 12, 42, the overcurrent detection devices 26, 56 are able to detect arc fault conditions downstream of the current paths 12-14 despite the overcurrent detection devices 26, 56 being located upstream of the current paths 12-14, 42-44. Thus, operation of the overcurrent detection devices 26, 56 is not interrupted by operation of solid state switches 16, 46.
The controller 74 may include one of the branch circuits 10, 40 with overcurrent protection such that overcurrent protection (e.g. AFCI protection) could be included in the wireless switching application 70. In one example the controller 74 incorporates the branch circuit 10 with arc fault protection such that the controller 74 includes the second current path 14 and a plurality of the first current paths 12, with each of the plurality of first current paths corresponding to a channel of the multi-channel controller 74. In this example the controller 74 could include only a single inductor 28 and a single TVS device 24 for the compatibility of fault condition detection through the overcurrent detection device 26.
In one example, the controller 74 incorporates the branch circuit 40 such that the controller 74 includes the current paths 42, 44 in each of its channels and such that each channel includes both the switch 46 and the switch 54.
Although embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope and content of this disclosure.
This application claims priority to U.S. Provisional Application No. 61/378,408, which was filed Aug. 31, 2010.
| Number | Date | Country | |
|---|---|---|---|
| 61378408 | Aug 2010 | US |
