This piece was originally published in the May 2016 issue of ei, the magazine of the electroindustry.
Dave Loucks, PhD, Manager, Application & Advanced System Engineering, Eaton
The cost of microprocessors, temperature sensors, and electronic current sensors has decreased dramatically in the last 10 to 15 years. While much attention has been focused on using microprocessors to connect devices to a network, many of the use cases for residential device communication have focused more on lifestyle or convenience and perhaps less on human safety or property preservation. Having a washing machine email a user when a load is done only begins to hint at the safety potential that a smarter device could provide.
Consider the home electrical safety systems built around circuit breakers, ground-fault circuit interrupters (GFCIs), arc-fault circuit interrupters (AFCIs), and wiring devices that, today, largely do not examine the quality of the connections made to them. Here is what could happen if connections were able to self-check:
- Wiring devices could detect an improperly loose or otherwise damaged plug and alert or even trip the load off. By identifying this problem, overheating connections that degrade to the point of causing a fire could be prevented. An alerting device could provide the additional benefit of allowing the user to take action to mitigate the failure of the circuit.
- Internal behind-the-wall connections to the wiring device could be monitored for loose or loosening connections. Technology exists to detect problems even before arcing begins, providing even earlier detection of problems than that provided by standard AFCI systems. Detecting a problem before an arc occurs also has benefits for applications installed near combustible or explosive atmospheres.
Taking the Temperature of Safety
One method of self-checking a connection would be to take its temperature. Current-carrying connections can loosen the rough surface of the conductor. As parts loosen, relatively fewer peaks touch each other. Since connection resistance is proportional to the area over which the current passes, the resistance of the junction rises. Since energy loss through resistance is proportional to the resistance, the heating of that connection increases. See figure 1.
While this resistance does increase, however, it is still a small fraction of the total circuit resistance that is nearly imperceptible at the source. With such a small increase in series resistance, total load current does not decrease. This same magnitude of current flowing through a higher resistance creates more voltage drop and more heat; it is this heat that can be detected.
If allowed to persist, heating at the point of contact between conductors can rise above the boiling point of the metal and the metal will vaporize, further degrading the connection by reducing the surface area of contact further. The full load current then flows through this high temperature, and ionized gas forms an arc. Because arc temperatures far exceed the boiling point of any known material, destruction of the connectors accelerates as remaining contact material vaporizes and is lost.
One solution to this problem might be to detect the temperature rise within an outlet. UL places limits on these temperatures and they can be monitored. If these limits were approached or exceeded, that could indicate a developing failure mechanism within the wiring device.
The cost to add any technology must be scrutinized to be sure there are not any less costly ways of providing the same protection. But the cost to include this temperature protection would not need to be excessive. In fact, since the conductors used inside wiring devices are excellent thermal conductors as well as electrical conductors, careful placement of a temperature sensor might allow it to detect problems on either end of the conductor (plug or terminal). This would allow a single sensor (per conductor) to detect loosening at any point along the conductor. Protecting the hot and the neutral would require two sensors.
Another justification for this technology is evident in severe duty environments. Wiring devices can face accelerated degradation and shortened life if they are subject to severe service, e.g., environmental contaminants, repeated use, or even damaged plugs. Since remaining life is a function of the environment, it may not be easily determined based on numbers of uses. If, for example, a plug was repeatedly inserted and withdrawn from an outlet, the number of operations could be used as a proxy for remaining life. However, if a plug covered with abrasive debris were repeated inserted into an outlet, the wear on the surfaces would be accelerated. In this case, it would be more difficult to predict end of life based solely on the number plugging operations.
Using a temperature-protected wiring device could provide additional protection against accelerated degradation and prevent unsafe operation of a device beyond its end of life. The uncertainty related to holding force could be reduced if the outlet were self-protecting.
Going further, a control circuit within the outlet could be added to alert appropriate personnel by suitable means of the impending problem that would be leading to imminent tripping of the load. For medical or food storage applications, the benefits of this additional technology are clear.
These application ideas only scratch the surface of the potential safety and property preservation benefits that a more intelligent circuit-protective device could provide. Electrical distribution system components are now at the threshold of providing these and even more exciting safety features.
Dr. Loucks is a senior member of IEEE and registered professional engineer, a Certified Energy Manager®, who has been awarded eight patents, with five additional patents pending.