Patent Application
20130328657
1. Field Of The Invention
The present invention relates generally to electrical distribution equipment and control apparatus therefore, such as circuit breakers. The invention relates more particularly to an indicator of thermal activity within the circuit breaker and especially to an inexpensive thermal indicator within high commercial volume breakers commonly used in the residential market and known to those in the art as miniature circuit breakers.
2. Discussion of the Known Art
Within the general context of the art and the present disclosure it will be understood that a common miniature circuit breaker will have a dielectric, i.e. nonconducting, case enclosing a current path comprising a line terminal connected to the central electrical distribution bus for line power, a braided wire connection leading from the line terminal to a thermo-magnetic trip unit comprising a magnetic yoke and thermally sensitive bimetal which in ordinary operation pass current through to a load terminal and then on to the branch circuit being protected by the breaker. In the case of overcurrent conditions the thermo-magnetic trip unit will react to trip the breaker causing a movable electrical contact to separate from a fixed contact and thereby open the circuit to protect the load.
In the present art, when it comes to the operational state of an inexpensive thermo-mechanical breaker such as an miniature circuit breaker, there is no easy way to readily determine either the present current draw or in what position the thermal element, i.e. bimetal, might be within the breaker (while still conducting) and therefore how close the breaker may be to tripping.
In the known art one may have to purchase and install a secondary measurement system e.g., such as a POWERLOGIC brand Branch Circuit Power Meter available from Schneider Electric USA, Inc., to know how much electricity is being used by a circuit or remove the deadfront from the circuit breaker panel to access a load wire for current usage measurement. This requires proper personal protective equipment (PPE) and labor time even if all that is needed is a simple estimate, e.g. that load use is at greater than 50% of total breaker capacity, to answer a diagnostic question.
Further in terms of breaker operation, current draw measurement does not always provide a complete understanding of how close a breaker is to tripping, as current draw is a secondary parameter of latch engagement. If the breaker has a loose connection, a normally low current load may generate excessive heat making a circuit breaker trip thermally. In applications such as a data center which require constant access to power, such nuisance tripping is a big issue.
Also, miniature circuit breaker's have a limited life and can only handle a relatively small number of high current interruptions before losing continuity. in the current path due to contact erosion or fouling. This extreme current level elevates the bimetal temperature significantly above the normal thermal tripping temperature and may change the shape or operating characteristics, or both, of the bimetal or other internal parts and hence the operation of the breaker. There is little way of telling that such high current interruptions have taken place other than a gross physical inspection of the outside of the breaker.
Due to the above limitations a means to readily ascertain thermal activity within the breaker would be welcome in the art to indicate either or both of present operating conditions and indications of the remaining useful life of the breaker. To that end, the present invention in its various aspects and embodiments teaches and provides a thermal indicator for the breaker by which both of these conditions can be made visible to the user without exposure to live circuit components and at a very reasonable product cost.
Miniature circuit breakers typically have a bimetal element in the current path that moves with current draw by a load on a circuit. The bimetal then releases a latch if the current magnitude and time (I2*t) exceeds the trip setting of manufacturer. Change in temperature drives the bimetal movement and has a strong correlation to the trip point of the breaker. A thermal gauge is a simple method to visually indicate the amount of latch engagement and how close a circuit breaker is to the trip point, or possibly how much current is being drawn by a load.
In the current path of the circuit breaker a thermal transducer is located for forming a directly observable indication of circuit breaker thermal operating conditions visible from outside of the circuit breaker. The thermal transducer may provide a measurement, visual display, and if desired, a record of the extreme temperature reading. The thermal transducer operates without additional power, i.e. it is not parasitic. The electrical power normally passing through the circuit breaker generates heat which is registered as a color change of a chromic element or a mechanical movement against a scale. The temperature indication can be a direct measurement of, or may be correlated to, the relative position of the mechanical latch members of the trip unit. The thermal transducer preferably but not necessarily has a physical connection to the current path of the circuit breaker to receive heat transfer through conduction. Typically the transducer is sitting on top of a bimetal of the miniature circuit breaker trip unit where heat generation is near its maximum and the physical location is adjacent the front wall or face of the circuit breaker case. An opening within the case of the circuit breaker through which the thermal transducer may be observed visually is then easily provided.
Thus several advantages may be provided by various embodiments of the present invention including providing indications useful to electricians for system maintenance. For example, a non-reversible thermal transducer that captures maximum thermal load can show if a circuit breaker has experienced extreme elevated condition causing significant breaker life consumption and allow a user to plan a product replacement at an opportune time versus peak usage of a circuit. The thermal gauge need not require additional space or have any power requirements, and the costs are minimal due to small size and low part count. In some aspects of the invention the only change to a present circuit breaker case would be an additional viewing window formed therein.
In some aspects the invention may provide a thermo-mechanical transducer utilizing movement of the bimetal of the circuit breaker trip unit, or may be constructed of a bimetallic material in itself. Any desired scale may then be provided in conjunction with an indicator on the bimetal. The user merely has to look at a needle position on the gauge with a desired scale. The indicator may either passively move with the bimetal, or can move due to its own bimetallic properties based on the temperature of the surrounding breaker parts in an active manner. The scale may be adjustable to calibrate the thermal indication. A secondary bimetal is contemplated if the indicator on the gauge needs more movement.
In some aspects of the invention an embodiment may be formed with a thermochromic transducer. This transducer may, like a mechanical transducer, be attached to the current path and is preferably located at or near the top of the bimetal of the miniature circuit breaker trip unit. Although thermochromism is often described as a phenomenon in which certain dyes made from liquid crystals change color reversibly when their temperature is changed, it will be appreciated that some aspects of the present invention contemplate that the thermochromic transducer can include an thermosensitive material with irreversible color change used to indicate and record a temperature extreme which has occurred within the breaker. indicia, including but not limited to alphabetic, numeric, or symbolic indicators; may be included integrally or in conjunction with the thermosensitive color material of the indicator.
Some aspects of the invention may be particularly suited or adaptable for miniature circuit breakers such as QO or HOMELINE breakers sold under the SQUARE D brand by Schneider Electric USA, Inc. Some aspects of the invention may be expanded to encompass indicators for larger breakers utilizing the principles expressed herein.
The foregoing and other advantages of the present disclosure will become apparent upon reading the following detailed description and upon reference to the drawings of which:
Referring to
As seen in
Referring to
Also in
As seen in
As seen as in graph of
Having thus described a thermal load visual gauge for a circuit breaker; it will be appreciated that many variations thereon will occur to the artisan upon an understanding of the present invention, which is therefore to be limited only by the appended claims.
