1. Field of the Invention
The present invention relates to transient voltage surge suppressors (TVSS) for protecting electrical equipment connected to alternating current (AC) electrical power lines, and in particular, to TVSS circuits for protecting electrical loads from sustained excessive AC voltages.
2. Description of the Related Art
Electrical power lines are often subject to surges and other transients of high current or voltage caused by various events, such as lightning switching on or off of significant electrical loads, or even occasional short circuits. Such surges or transients can cause permanent damage to electrical equipment connected to the power line, particularly equipment in which electronic devices are responsible for the consumption and use of the power. Transient voltage surge suppressors have long been used to detect and attempt to block such surges and transients before they reach the devices connected to the power line. The normal standards for determining the effectiveness of such TVSS devices focus upon the limiting of transients of short duraton, such as those resulting from lightning strikes. However, additional relatively common conditions exist which can produce sustained over-voltage conditions whereby the AC voltage presented via the power lines is significantly higher than that for which the connected electrical equipment is designed and capable of operating without damage. For example, poor voltage regulation by the electrical utility provider, improper wiring of the facility, use of standby generators, or incorrect or defective bonding of neutral and ground line connections can all lead to sustained AC over-voltage.
Under such sustained over-voltage conditions, conventional TVSS devices, which use varistors (e.g., metal oxide varistors or xe2x80x9cMOVxe2x80x9d""s) to limit transients of short duration, will either be ineffective (if their limiting, or xe2x80x9clet-through,xe2x80x9d voltage is more than the peak value of the power line voltage) or simply burn out, since MOV voltage limiters overheat in a very short time when absorbing the excessive power associated with the over-voltage. To address this limitation, one form of conventional TVSS device includes circuitry to detect such over-voltage conditions and cause a switch, such as a relay, to disconnect the load (as well as the MOV circuitry) from the power line when there is a sustained over-voltage condition between the power and neutral voltage lines.
Such devices have generally provided reasonable protection against sustained over-voltage conditions, but still exhibit damage in those applications where, although the line-neutral interterminal voltage was within acceptable limits, one or both of the line-ground or neutral-ground interterminal voltages exceeded such limits.
Accordingly, it would be desirable to have a TVSS device capable of protecting against sustained over-voltage conditions regardless of which of the incoming power lines introduces such condition.
An over-voltage protection circuit in accordance with the present invention monitors the line-neutral, line-ground and neutral-ground interterminal voltages for an over-voltage condition between any pair of terminals. If any one or more of such interterminal voltages exceeds a predetermined maximum voltage, even for a short time interval, the incoming power line connection is interrupted to protect the load circuitry (as well as any other additional circuitry, such as transient suppression circuitry using MOV devices) from exposure to such excessive voltage. This power interruption is maintained for so long as such over-voltage condition exists. Additionally, the incoming interterminal voltages can be monitored for under-voltage conditions, whereby the load can be protected from exposure to low voltage (e.g., xe2x80x9cbrownoutxe2x80x9d) conditions.
An over-voltage protection circuit for protecting against excessive voltages on two or more of a power line, neutral line and ground line in accordance with one embodiment of the present invention includes power, neutral, ground and load terminals, a voltage detection circuit and a switch circuit. The power, neutral, ground and load terminals provide for power, neutral, ground and load connections, respectively. The voltage detection circuit is coupled between the power, neutral and ground line terminals, and monitors interterminal voltages between such terminals. The switch circuit is operatively coupled to the voltage detection circuit and between the power line and load terminals, and provides an electrical current path between the power line and load terminals when the voltage detection circuit detects that all of the interterminal voltages are less than a predetermined maximum voltage, and interrupts the electrical current path when the voltage detection circuit detects that any one or more of the interterminal voltages is greater than the predetermined maximum voltage.
An over-voltage protection circuit for protecting against excessive voltages on two or more of a power line, neutral line and ground line in accordance with another embodiment of the present invention includes power, neutral, ground and load terminals, a voltage detection circuit and a switch circuit. The power, neutral, ground and load terminals provide for power, neutral, ground and load connections, respectively. The voltage detection circuit is coupled between the power, neutral and ground line terminals, and monitors interterminal voltages between such terminals. The switch circuit is operatively coupled to the voltage detection circuit and between the power line and load terminals, and: provides an electrical current path between the power line and load terminals when the voltage detection circuit detects that all of the interterminal voltages are less than a predetermined maximum voltage and the interterminal voltage between the power and neutral line terminals is greater than a predetermined minimum voltage; interrupts the electrical current path when the voltage detection circuit detects that any one or more of the interterminal voltages is greater than the predetermined maximum voltage; and interrupts the electrical current path when the voltage detection circuit detects that the interterminal voltage between the power and neutral line terminals is less than the predetermined minimum voltage.