Electrical transmission or interconnection systems – Switching systems – Switch actuation
Reexamination Certificate
1998-12-31
2001-10-02
Jackson, Stephen W. (Department: 2836)
Electrical transmission or interconnection systems
Switching systems
Switch actuation
C307S125000, C307S113000
Reexamination Certificate
active
06297569
ABSTRACT:
BACKGROUND OF THE INVENTION
Electrically powered systems rely on a variety of switches for controlling their operation. These switches may be solid state devices such as SCRs or triacs, or may be electromechanical relays. (The term “switch” will be used hereafter to refer to any device having a pair of power terminals whose conductive state is controlled by an electrical signal on a control terminal of the device.) In either case it is important for safe operation of some systems that electrical power applied to them can be reliably controlled. In particular, many systems require near absolute certainty that power can be removed from them when desired. Combustion systems having fuel valves for controlling flow of pressurized fuel are one classic example of this situation.
Where solid state devices are involved, their failure mode is typically an open circuit which of course removes power from the system. A relay on the other hand, is notorious for failing with its switch contacts closed, so that removing power from the actuator coil does not remove power from the powered system. This condition of the contacts is referred to as welding. It is even possible that solid state switches can fail in a conducting mode, the solid state equivalent of relay contact welding. Because of their low switch resistance and the preferences of preexisting safety codes, relays are still usually used to switch power to the fuel valves in burner systems, so safe operation requires that relay contact welding not result in continued power flow to the controlled component of the system.
One expedient for increasing the reliability of disconnecting power from load for such switching systems is to use redundant switches, with two pairs of switch contacts in series. Thus if one pair of contacts weld, the other pair continues to safely provide switching for the powered system. One problem that arises with this arrangement however, is that once one contact pair welds, redundancy has been lost but the system continues to operate normally. The system is thus at risk of failure through welding of both contact pairs. Depending on how the switch control operates and the individual switch characteristics, it is possible that both switches will weld in near succession, say if both contact pairs have experienced approximately the same number of load switching operations. For systems where switch failure by welding creates an unsafe condition, the possibility of this type of failure should be avoided.
BRIEF DESCRIPTION OF THE INVENTION
We disclose below a power connection system which substantially reduces the likelihood of this contact weld failure mode of redundant switch pairs. This system relies on the realization that the activities which usually cause a contact pair to fail are closing and opening of the contacts. That is, simply carrying current does not usually cause contact pair deterioration or welding. Accordingly, if in a redundant switching system, one switch is dedicated to handling power switching, then that switch is one which is far and away the most likely to fail.
Such a power connection system is intended to supply power from a source to a load during a demand signal interval in which a demand signal exists. The connection system has a first electrically controlled switch having first and second power terminals, and a first control terminal for receiving a first connect signal. The first switch establishes electrical contact between the first and second power terminals responsive to the first connect signal. A second electrically controlled switch also has first and second power terminals, and a second control terminal for receiving a second connect signal. The second switch is responsive to the second connect to establish electrical contact between the switch's first and second power terminals. The first switch's first power terminal is for connection to the power source. The first switch's second power terminal is connected to the second switch's first power terminal. The second switch's second power terminal is for connection to the load. It is convenient to consider the first switch as the upstream switch, as it is to be connected to the power source. The second switch can be designated the downstream switch as it is connected directly to the load.
The power connection system includes a switch operating system having a first voltage sensor having a sensor terminal connected to the first switch's second power terminal, and providing at a signal terminal a power signal having a first value responsive to power voltage at the sensor terminal and having a second value otherwise. There is a switch status detector receiving the demand signal and the power signal which provides a status signal having a first value except when the demand signal and the second value of the power signal simultaneously exist, which causes this detector to provide a status signal having a second value.
A switch controller receives the status signal and responsive to the second value thereof, provides the connect signal to the second switch's control terminal and after a preselected interval, provides the connect signal to the first switch's control terminal. Thus according to this description, the first (upstream) switch always closes after the second (downstream) switch closes. Experience teaches that the switch which actually makes and breaks the connection is much more likely to fail, and such a failure is often a welded mode failure. Accordingly, the first switch is much more likely than the second switch to fail in a welded mode. With this arrangement, failure of the switch more likely to fail can be detected without having the other switch closed and therefore liable itself to welding, a safer arrangement. That is, if the first switch performs the power switching function, and experiences a welded mode failure, this is detectable regardless of the status of the second switch.
A complementary operation is used to open the switches. To open the switches, a first disconnect element in the switch controller ends the first connect signal responsive to the end of the demand signal. A second delay element receives the first connect signal and provides a delayed first connect signal. A second disconnect element ends the second connect signal responsive to the end of the delayed first connect signal.
There are a number of switch status configurations which indicate switch or other failures, and a preferred embodiment of the invention detects many of these and provides an error signal or locks out the switch operating system.
REFERENCES:
patent: 4298334 (1981-11-01), Clark et al.
patent: 4850852 (1989-07-01), Ballard
patent: 5041775 (1991-08-01), Erdman
patent: 5076780 (1991-12-01), Erdman
patent: 5277575 (1994-01-01), Newberry
Bartels James I.
Juntunen Robert D.
Patton Paul B.
Solosky Richard M.
Honeywell International , Inc.
Jackson Stephen W.
Polk Sharon
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