FOR DETECTING AND
MONITORING CONDITIONS THAT COULD
CAUSE ELECTRICAL WIRING SYSTEM FIRES
U.S. Consumer Product Safety Commission
Underwriters Laboratories, Inc.
333 Pfingsten Road
Northbrook, IL 60062
1285 Walt Whitman Road
Melville, NY 11747
UL Project Number: NC233, 94ME78760
Contract Number: CPSC-C-94-1112
The CPSC Home Electrical System Fires Project
This project was
sponsored by the U.S. Consumer Product Safety Commission (CPSC)
as part of the Home Electrical System Fires Project, a priority
project for the CPSC in FY94 and FY95. The overall objective of
the Home Electrical System Fires Project is to reduce the rates
of death, injury and property loss from residential fires
associated with electrical wiring systems. The major elements of
the Home Electrical Systems Fires Project are to: 1) promote
electrical inspections of older dwellings to identify flagrant
hazards that need correction, 2) stimulate the repair and
correction of known hazards, and 3) demonstrate effective,
economical electrical products that can upgrade the safety of
electrical wiring systems in residences. The objective of these
activities is to permit the continued occupancy of these
dwellings without electrical symptoms that can cause fires.
The CPSC estimates that there were approximately 41,000 fires involving home electrical wiring systems in 1992. These fires resulted in 320 deaths, 1,600 injuries, and $511 million in property losses. CPSC studies have also shown that the frequency of wiring system fires is disproportionately high in homes more than 40 years old.
Identifying Technology for Reducing Residential Electrical Fires
The purpose of the project reported here was to conduct an in-depth study of technologies to detect and monitor precursory conditions that could lead to or directly cause fires in residential wiring systems in general, and how these technologies could be applied to older residential wiring systems in particular. The project included: 1) conducting a comprehensive review of published and unpublished literature on devices and systems that could decrease the likelihood of residential fires, 2) a survey of industry organizations and manufacturers for new products and systems that could decrease the likelihood of residential fires, and 3) the acquisition and analysis of promising devices and systems for ease of installation, reliability, cost and effectiveness in decreasing the likelihood of fires in residential wiring systems, particularly older residential wiring systems.
Literature Search to Identify Technologies
An extensive review of patents and of published and unpublished literature was conducted to acquire relevant information on technology for detecting or monitoring when conditions exist that could cause fires in residential electrical wiring systems. The review focused on innovative new technology or innovative uses for existing technology that is currently available or under development.
The search for published information included the use of a computer-aided information retrieval service through the New England Research Applications Center (NERAC), which has access to extensive database resources. Numerous documents were surveyed, and seven articles and sixteen patents describing relevant information and promising technology were acquired. The major types of new technologies, or new applications of existing technologies, that were identified fall into the following five general categories:
Arc-Fault Detection Technology - This technology
is intended to respond to arcing faults in the electrical
wiring system by looking for specific signature
characteristics of the current, voltage, or
electromagnetic fields associated with arcing faults.
2. Modified Trip Circuit-Breaker Technology - This technology is intended to modify the trip characteristics of conventional residential circuit breakers. Essentially, the technology causes a circuit breaker to trip magnetically (with or without an intentional delay) at levels of overcurrent that would normally result in the circuit breaker tripping thermally. The result is faster response time at levels of overcurrent that would typically correspond to the thermal trip region of a conventional circuit breaker.
3. Ground-Fault Interrupting Technology - This present technology is intended to respond to a ground-fault condition to reduce the risk of electric shock in the residential distribution system. The new application of this technology is to reduce the risk of fire in the residential distribution system.
4. Supplementary Protection Technology - This technology consists of the innovative application of conventional supplemental overcurrent and/or thermal protection technology incorporated at the outlet receptacle or attachment plug in order to respond to specific conditions of temperature or current.
5. Surge-Protection Technology - This existing technology is intended to limit the magnitude of transient overvoltages in the electrical distribution system. Consideration is given to the fact that repeated exposure to transient overvoltages can cause damage to electrical insulation.
Procurement of Products Exemplifying the Technologies Identified
An announcement letter seeking participation in this project was sent to approximately 800 individuals and manufacturers involved in product categories that were pre-identified as possibly having new or innovative electrical system protection technology. As a result, eleven products were procured in order to evaluate the technologies that they were intended to exemplify.
summarizes the individual and collective findings pertaining to
the technologies evaluated:
Arc-Fault Detection Technology - Arc-fault detection (AFD) technology, as exemplified by the involved products, demonstrated a capability of detecting and terminating certain types and levels of arcing fault current (but not all types and levels) that are beyond the present scope of conventional branch-circuit overcurrent protection technology. Consequently, it was found that arc-fault detection technology does indicate promise as a potential means for further reducing the incidence of fires in the electrical wiring system, that are either directly or indirectly caused by arcing faults.
There was a variety of performance of AFD products to the range of testing conducted under this project. Although each of the products both reacted to open the circuit and to apparently prevent ignition of the test indicator in a significant number of cases, none of the products responded in all of the cases tested. In order to fulfill the potential of AFD technology, further development is needed in order to detect and respond to a wider variety of arching fault conditions. Additional research is needed to obtain a more definitive understanding of the specific type or types of arcing conditions that can directly or indirectly result in fires in the residential wiring system so that AFD technology can be further developed to address those conditions.
Since AFD technology exemplified by the involved products is still developing, the evaluation of this technology with respect to unwanted tripping and operation inhibition was limited to a few tests. Some unwanted tripping and operation inhibition was observed. However, additional work is needed particularly with respect to unwanted tripping and operation inhibition at normal load current levels. For example, it appears that EMI filters may inhibit the operation of some AFDs.
Modified-Trip Circuit-Breaker Technology - Modified-trip circuit-breaker technology as exemplified by the involved products, indicated a capability to cause a conventional circuit breaker to trip magnetically at levels of overcurrent that typically correspond to the thermal trip region of conventional circuit breakers. However, since these approaches to protection are still based on the magnitude and duration of power frequency overcurrent, the challenge of balancing effectiveness, such as reduction in the probability of electrically caused ignition, especially ignition from arcing faults, and the desire to avoid unwanted tripping (and its consequences) over a wide range of inrush currents associated with the normal start up of loads, remains. In addition, faults involving currents below circuit breaker handle rating remain outside the scope of protection for products analyzed under this category.
Ground-Fault Interrupting Technology - Although ground fault protection technology, specifically that used in GFCIs (U.S. Ground-Fault Circuit Interrupters) and RCDs (European Residual Current Detectors), are used to provide protection to personnel from electric shock, the low trip current level (nominal 5mA for GFCIs and 30mA for RCDs), combined with the fast operating time (within a couple of cycles) makes ground fault- protection technology appear to be well suited to reduce the incidence of electrically caused fires. Since the devices only detect currents-to-ground, the concerns for unwanted tripping are limited to leakage current-to-ground and not load currents. The limited tests conducted under this project on ground-fault interrupting devices indicate that ground-fault detection technology has the capability to quickly detect and respond to the current arising from a ground fault (including an arcing fault) prior to igniting a fire indicator. A ground-fault interrupting device can only detect and respond to a fault that involves current-to-ground. It will not detect across-the-line or in-line series arcing faults unless such faults also involve current-to-ground. In those cases where the ground-fault current is a by-product of an across-the-line or in-line series arcing fault, operation of the ground-fault function may or may not preclude ignition.
Supplementary Protection Technology - The supplementary protection technologies exemplified by the involved products are intended to detect an overtemperature and/or overcurrent condition, or the effects of an overtemperature and/or overcurrent condition arising from the connection of a load(s) to a specific outlet receptacle. The technologies, as exemplified by the devices analyzed, demonstrated a capability of detecting and/or responding to certain types of abnormal conditions involving overtemperature and/or overcurrent and showing that supplementary protection technology can contribute to a reduction in the likelihood of an electrically caused fire. In contrast to the other technologies evaluated in this project, which were packaged or could be packaged so as to be permanently installed and continuously monitor conditions on a complete branch circuit or group of branch circuits, the zone of protection provided by supplemental protection technology is very limited and dedicated to specific applications.
When packaged as plug-in devices, reliance is placed on the user to plug the device into a receptacle and to keep it plugged into the receptacle. Also, the load current that can be obtained through these devices may be less than the normal load current that can be supplied by the outlet receptacle. On the other hand a protection-oriented homeowner can choose a supplementary protective device that matches the particular load without the risk of circuit interruption affecting other electrical loads.
Surge-Protection Technology - All of the previously indicated technologies are intended to mitigate the consequences of a failure or fault in the electrical distribution system.
Another approach is to reduce the likelihood of the fault occurring. Surge-protection technology, by limiting the exposure of the wiring system to transient overvoltages (e.g., from a lightning event or utility switching event), can reduce the likelihood of failure of electrical insulation from occurring. Repeated exposure to high voltages can degrade electrical insulation and ultimately result in an insulation fault. To the extent that a surge-protective device can limit the additional exposure of the electrical distribution system to transient overvoltages, it can help reduce the incidents of electrically caused fires. The overall significance of the reduction depends, however, on the significance of transient overvoltage caused insulation failure relative to other insulation failure modes.
It was determined that no single product or technology in the examined state of development would provide protection against all electrical ignition scenarios likely to be encountered in residential wiring systems. However, the present evaluation of the technologies that the involved products exemplified indicated that the potential exists to further combine certain technologies, once fully developed, into products that should significantly reduce the risk of fire beyond the scope of present conventional overcurrent protection technology.
From the technologies analyzed, arc-fault detection appeared to be very promising, especially when added to residential branch-circuit breakers and combined with other proven technologies, such as ground-fault protection. It is recommended that additional research be considered to better define the nature of residential electrical ignition sources, the levels of arc-fault protection needed, and standardized test methods to verify the effectiveness of practical products that would utilize this technology.
Copyright © 2002 Mike Holt Enterprises,Inc.