For more than 20 years, companies have been developing and marketing products that are connected between the AC supply system and electronic equipment, in an attempt to protect the connected electronic equipment from a variety of power-line disturbances. Currently, many designs incorporate hybrid technology that combines both filter circuits, such as passive LC filter networks, and suppressor technologies, such as metal oxide varistors (MOVs), and gas discharge tubes (GDTs), in an attempt to protect against a broader range of disturbances than either technology on its own.
In order to divert common mode disturbances away from connected equipment, all protective products, regardless of technology, (i.e. filter, suppressor, hybrid, transformer based, etc.), must be connected to a properly wired supply system with continuity of the safety ground conductor. Loss of ground continuity results in loss of common mode protection for connected equipment. Without ground continuity common mode disturbances, such as ground-referenced transients, can find alternate paths through connected equipment to ground via network cabling and other properly grounded equipment. In the process, disruption and possible damage can occur to both the improperly and properly grounded equipment. In addition to causing potential power quality problems, loss of ground continuity increases the risk of electrical shock for operators of electronic equipment.
In the prior art, diagnostic devices such as power-line monitors exist that can be used to check wiring as well as the power quality conditions of supply system outlets and, if available, can be an important aspect of an overall power quality program. Power quality, however, is a dynamic condition that needs to be addressed for the life of the protected equipment. Facility renovations, for instance, can result in wiring faults such as loss of ground continuity for an outlet that was previously checked and found to be correctly wired. In addition, movement of equipment from one outlet to another outlet can result in potentially disruptive/damaging wiring faults.
In the applicant's experience, most conventional power protection designs pass power to the connected equipment and allow for continued equipment operation after loss of continuity of the safety ground conductor. Some available designs contain diagnostic indicators that turn on or off in response to various wiring fault conditions such as loss of ground continuity. The problem with this approach is that the operator must recognize and understand these diagnostic indicators before a potential problem can be rectified. Because most power protection products are installed behind the equipment to be protected, it is unlikely that the operator will ever notice the diagnostic indicators and a potentially problematic wiring fault.
Virtually every power protection design of which the applicant is aware uses several MOVs to suppress various modes of transient over-voltages. While MOVs can be very effective at suppressing transients, they are vulnerable to failure as a result of extended over-voltage conditions that cause one or more MOVs to conduct and continue to conduct for the duration of the over-voltage condition. Continued conduction causes these suppressor components to heat up and eventually fail. Failures of MOVs can be both a shock and fire hazard, and growing concern about this problem has resulted in significant revisions to applicable CSA and UL safety standards. In particular, so-called abnormal over-voltage tests have been added which simulate these extended over-voltage conditions. Compliance with the revised standards requires that the design respond to the abnormal over-voltage tests in a way that does not present a shock or fire hazard; that is components cannot fail short across supply system conductor pairs and failure of components cannot cause ignition of surrounding flammables.
To comply with revised CSA/UL safety standards, most power protection designs have been modified with the addition of one or more thermal cut-offs (TCOs). These TCOs are usually connected in series with one or more MOVs across various supply system conductor pairs, (that is shunt connected). During over-voltage conditions one or more MOVs will go into conduction and heat up. One or more TCOs, preferably sandwiched between the MOVs, will in turn heat up and will trip when its trip temperature is reached. Tripping of a TCO will, therefore, disconnect one or more MOVs from across various supply system conductor pairs and, designed correctly, can prevent potentially dangerous MOV failures.
While the above approach usually satisfies revised CSA and UL standards, it can result in continued operation of connected equipment after loss of some or all of the transient protection. Operation of one or more TCOs, in this type of design, will disconnect one or more suppressor components from across the line while allowing power to pass to the connected equipment. This allows for continued equipment operation after loss of this mode of transient protection. While some designs contain diagnostic lights that indicate loss of protection, similar to the problems associated with wiring fault indicators, these indicators may not be noticed on a product that is typically installed behind the connected equipment.
An alternative approach that has been used is to electrically connect the TCO in series with the line conductor on the supply side of the MOV(s) to be protected. With this configuration, tripping of the TCO will not only disconnect one or more MOVs from across various supply system conductor pairs, but will also disconnect power to the connected equipment. This will immediately notify the operator of a failure of the protection product. One of the difficulties with this approach stems from the fact that series connected TCOs must carry connected equipment load currents. The steady-state current carried by the TCO causes self-heating of the component that requires selection of a TCO with a much higher trip temperature in order to avoid nuisance tripping. This increases the difficulty of coordination of the TCO with the MOVs that it is protecting.
While the series connected TCO configuration can be used to break continuity of the line conductor, CSA and UL do not allow for a similar TCO configuration that breaks neutral continuity. The reason for this is that breaking of neutral continuity will shut off the connected equipment but will still pass line to ground voltages to the protection product outputs which could be a potential shock hazard for anyone servicing the equipment. CSA and UL, therefore, do not allow for breakage of neutral continuity unless it occurs simultaneously with breakage of line continuity, (such as a double pole circuit breaker).
As a result, designs that contain a “line-connected” TCO will also require at least one “shunt-connected” TCO in order to protect all modes of suppressor components. In such designs, tripping of shunt connected TCOs will disconnect one or more modes of transient protection while still allowing power to pass to connected equipment, leaving it vulnerable to subsequent transient and extended over-voltage conditions.
In addition to the problems described above, TCO-only designs provide a one-time solution to extended over-voltage conditions. Every time a TCO functions, the power-protection product must be returned to the manufacturer for repair or replacement. For a product that is designed to improve equipment uptime, this repair/replacement related downtime is a significant nuisance.
In the prior art, U.S. Pat. No. 6,229,682 purports to teach a device that contains circuitry that “senses the incoming voltage and electrically disconnects its output to the office equipment if a voltage surge above an established level is sensed.” In the applicant's view, the problem with the design disclosed is that it only responds to line voltage levels above 165 VAC, a level that far exceeds the 130 VAC rating of the MOVs that are used in the device. In addition, the design contains no transient protection on the supply side of the sensing circuitry and relays. This may leave the supply side circuitry vulnerable to transient related damage when the line-connected relay disconnects power to the device output. Finally, the design contains no backup to the sensing/relay circuitry. Failure of the sensing/relay circuitry may result in a potentially dangerous failure of one or more device MOVs.