1. Field of the Invention
This invention relates to measuring and recording the current drawn by an electrical load, and more particularly, to using the measured and recorded data for direct evaluation of the current usage versus capacity function for given loads and circuit elements in order to analyze the capabilities of current-limiting devices within such circuits.
2. Description of Related Art
Installations of electrical equipment in residential, commercial, and industrial settings include fuses or circuit breakers as protective current-interrupting safety devices. These devices function to protect both the equipment and the circuit elements against overcurrent conditions, and are nominally rated in amperes. The numerical ampere rating of a fuse or circuit breaker is the normal current it will carry for an indefinite time period without opening the circuit. Although this nominal designation properly identifies the circuit size, it somewhat oversimplifies the actual current-carrying characteristics of the circuit.
Fuses and circuit breakers operate on a time-current curve and will melt or trip, respectively, on the basis of both magnitude and duration of current flow. When subjected to currents above their ampere rating they will open the circuit after a predetermined amount of time, the time being inversely related to the current--the greater the current, the shorter the time. A fuse can carry its rated current indefinitely without melting. But the same fuse might carry an additional twenty percent of its rated current for several minutes before melting. It may also carry twice its rated current for a few seconds, and it may sustain a yet higher multiple, perhaps ten times its rated current for only a fraction of a second. The "average melt curve" of a fuse is a graph that plots this current magnitude versus time-interval characteristic.
Manufacturers design fuses to have particular average melt curve shapes depending on the intended application, and they publish these curves for their users. An example of the application of a specialty fuse is the type used to protect a motor circuit, which must allow for a large motor-starting current. In this case a dual-element fuse is available that will carry on the order of ten times its rated current for a fraction of a second without melting. Similar considerations apply to circuit breakers. The manufacturers of circuit breakers publish "trip curves" that describe the time-current characteristics of their products.
In an effort to anticipate or analyze overload conditions, electricians and engineers measure current flow in distribution components and relate it to the capacity of the associated circuit. By definition, current-limiting safety devices represent the most current-inhibiting elements in an electrical distribution system, and current-flow information used in direct comparison to the current-allowance characteristics of these devices effectively quantifies the function of usage versus capacity for the entire circuit under inspection. Many devices are presently available to measure current flow in electrical distribution systems; these include handheld ammeters, chart recorders, and other power monitoring products. Some of these devices provide both continuous display of a short-term current measurement as well as retention of the maximum value observed during a survey period.
The desirable characteristics of fuse and circuit breaker operation stem from the fact that these and similar devices operate on the fundamentals of heating theory, and heat transfer calculations involve time averaging as well as magnitude information. The measure of the heating effect of an alternating current is its RMS (root-mean-square) value. By definition this is the value of any periodic current equal to the value of the direct current which, flowing through a resistance, produces the same heating effect in the resistance as the periodic current does. It is the evaluation of the RMS function on a given current waveform that determines its heating effect on current-limiting safety devices.
Devices that measure the RMS value of current for the purpose of electrical distribution system analysis are readily available. These devices evaluate the mean in the root-mean-square (RMS) function over a predetermined, fixed-time interval, and present the result in real time to the user. More advanced chart recording and storage versions of these devices plot or save the data as they are being generated, typically in an effort to locate the peak value of RMS current during a survey period. In the case of the latter, more advanced device, the output is the peak of the root-mean-square calculation carried out continuously over a defined mean interval--say one-eighth of a second. In other words, the device shows the maximum amount of current-flow for any one-eighth second during the circuit survey period.
What all of these existing tools fail to produce is maximum current-flow information in a unit form that can be used in direct comparison with the manufacturer published "average melt" characteristics of standard current interruption devices (typically fuses and circuit breakers). Average melt curves plot peak-current magnitude versus time-interval of application, and represent a significantly more complex interpretation of the current versus time information than can be obtained from existing measurement tools. In relation to the average melt fuse curve, a measurement of instaneous direct current using an ammeter, for example, or a peak value using an RMS type of measurement represents but a single point of relevant data. In some cases this single point is sufficient to estimate the entire load current curve, given that the load conforms to assumed boundaries of normality. Unfortunately, for varying reasons, this type of estimation is quite disadvantageous because it can significantly misrepresent the actual peak-current magnitude versus time-interval characteristics of a circuit.