A wide variety of modern commercial and consumer electronics employ microchip technology which is particularly sensitive to power line transients or surges, RFI (radio frequency interference), EMI (electromagnetic interference) and ESD (electrostatic discharge). Such devices as computers, stereos, televisions, radios and the like can all be affected if not seriously damaged by unfiltered, unprotected or generally "dirty" AC power.
Of these four general types of power line disturbances, power surges or transients are by far the most damaging. These surges are found on power lines from a wide variety of sources, such as lightning, power company switching, collapsing electric fields when electric motors are shut down, surges when electric motors are started, sparking between power lines, arcing between opening and closing relay contacts, and the like. These voltage surges of 400 V or more can find their way into sensitive microchip technology electronics and damage or destroy circuitry. Thus, at a minimum, surge protection is extremely important for commercial or consumer electronics, and especially important for personal computing systems which can suffer data dropout and circuit failures in the presence of power line surges.
A wide variety of prior art surge or transient protection and surge or transient suppression devices are available for removing or dampening offending power line surges and transients in order to protect the consumer electronics connected thereto. May of these prior art transient or surge suppression devices are typically tested to specifications produced by, for example, the Institute of Electrical and Electronic Engineers (IEEE), Underwriters Laboratories (UL1449 and UL1283), the Federal Communications Commission (FCC) or the Consultant Committee, International Telegraph and Telephone (CCITT). The most common among these standards is the IEEE Standard 587 (ANSI C62.41-1981). The portion of this technical specification which applies to indoor applications describes transient conditions occurring in low voltage (less than 600 V) AC power circuits in which the transients exceed twice the peak operating voltage with durations ranging from a fraction of a microsecond to a millisecond and originating primarily from system switching and lightening effects. This standard has been promulgated to evaluate the survival capability of equipment connected to power circuits as described in UL1449. In testing against this standard, many of the prior art surge protection devices combine transient suppression components with additional components to perform RFI and EMI filtering which may also be required to comply with FCC standards.
To survive the impulse characteristics of IEEE 587-1981 for indoor applications and to meet the standards of UL1449 (and become listed by UL), prior art surge protection devices are typically designed to shunt the offending surge current and thus limit the over-voltage portion of the transient to the neutral or return line, thus creating a momentary low-impedance path between the power line and the return path. The clamping device must be designed to operate above a certain voltage threshold and must be capable of withstanding the current which is momentarily shunted to ground. Typical components used in prior art devices to clamp or limit the overvoltage portions of transients have been gas-filled spark gap surge arrestors, MOV's (metal oxide varistors) available from Sanken Electric, GE and other vendors, and silicon transient voltage suppressors such as the TVS 1.5KE series devices available from World Products Inc., the TransZorb.RTM. transient voltage suppressor devices available from General Semiconductor Industries, Inc., and other vendors. These aforementioned shunt or clamping electronic devices are designed to withstand a plurality of overvoltage transients and safely shunt them to ground, thus protecting the device to which they are attached.
A problem with the prior art surge protection devices is when the shunt or clamp electronic components fail during repeated overvoltage transients. All of the aforementioned transient suppression devices can fail "open" (although they will momentarily fail "closed"), which in effect removes them from the circuit and leaves the power line and the devices connected thereto unprotected from subsequent power line surges. To remedy this situation, prior art power line surge protection devices have been designed such that a monitor light or LED indicator is used to indicate that the surge suppression devices are still operating properly. Should the power line surge protection devices fail, the indicator light will be extinguished, indicating to the user that the power line protection has been lost. A further problem with the prior art surge protection devices and their associated indicator lights is that the indicators are not always readily accessible or visible by the user. The surge protection devices are often located near a wall outlet or on the floor under a desk. The location of these suppressor devices inhibits the user or owner in identifying a failed condition on the surge or transient suppression device. Thus, protection may be lost and go unnoticed for quite some time, leaving the electronic equipment which is attached to the failed surge protector open to damage by power line surges.
To alleviate this problem, some prior art surge protection devices interrupt the power upon the failure of the transient suppression components. An example of a power line transient surge suppressor which interrupts power in this fashion is U.S. Pat. No. 4,587,588. The device described in this patent senses the failure of a transient suppression component and removes power from the load by using a thermal cutout which actuates within a few minutes of the failure of one of the surge suppression components of the device. However, the delay between the failure of a surge suppression device and the opening of the thermal cutout leaves the load vulnerable to a second and subsequent transient which may then damage the equipment connected to the transient suppression device.
There therefore is a need in the prior art for a transient or surge protection device which disconnects power from the load immediately upon the failure of any of the transient suppression components of the device.