1. Field of the Invention
This invention pertains generally to short tracing devices for wiring, and more particularly to systems and methods for isolating vehicle short circuits.
2. Description of Related Art
A short in the electrical system of a vehicle is dangerous and can render the vehicle inoperable. An electrical short is created in a circuit whenever an unwanted current path develops from any portion of the circuit to other paths in the circuit or to elements which may be coupled to a potential in relation with the circuit, such as to a chassis ground.
It is often necessary to quickly locate short circuits in order to facilitate a rapid repair. For example, in the case of a short circuit in the electrical wiring of a vehicle, trailer, or tractor-trailer rig, the safety of the operator and other parties can be compromised as key vehicle systems may cease to operate properly and sparks or heat can be generated at the site of the short. Shorts which arise in wiring for vehicles, trucks, trailers, tractor-trailer rigs and so forth often result from shorts between one or more wires and the frame of the vehicle, and is often difficult to trace. It should be appreciated, however, that a short with continuous conductance is much easier to diagnose than a short which appears only intermittently, because unless the circuit is in a shorted condition one cannot perform electrical tests to locate the short. Complicating the tracing process is the fact that the vehicle circuit being tested typically comprises a single wire that is usually contained somewhere within a somewhat stiff bundle of similar wires which is often protected by a sleeve or overwrap of some form.
Electrical short tracing devices that are commercially available in the automotive repair industry typically utilize an intermittent pulsing mode, such as generated by a self-resetting circuit breaker. The tester with the self-resetting circuit breaker is placed into the shorted circuit through whatever connecting leads are being used, and power is applied to the shorted circuit through the resettable circuit breaker. The attachment of the short tracing device is typically established at a point in the circuit at which the original protection device for the circuit (i.e. fuse, fusible link, or a circuit breaker), has failed or become disabled as a result of excessive current flow due to the shorted condition. However, it is generally possible for the connection to be made anywhere along the length of the circuit, such as at the load or intermediary locations along the circuit.
A common method of testing for shorts is by sending the intermittent pulses down the shorted circuit while performing a tracing process, such as using handheld current sensors to detect the magnetic field which results from current flow along the circuit to the short. The pulse emitting device is electrically connected on a portion of the shorted circuit and power is applied. In response to an excessive current flow condition (exceeding the fixed circuit breaker limit) the circuit breaker device breaks the circuit, and then subsequently resets. The making and breaking of the circuit results in the generation of intermittent high current pulses that an operator can attempt to trace on the circuit toward finding the short.
In another form of automotive circuit testing device, a radio-frequency (RF) signal is coupled to the circuit under test and radiated along the wire to which it attaches. However, this form of RF testing does not work well for wires within wire bundles as found within automotive circuits, as the signal is shielded by the surrounding wires.
A number of drawbacks exist with the current short tracing devices and methods which have not been fully appreciated in the art, some of which are now described.
It should be noted that in the brief moment after power is applied very large electrical currents may flow through the circuit, prior to the resettable circuit breaker of the prior art tester being triggered into an open state. Test devices utilizing resettable breakers often utilize a ten ampere resetting breaker to provide sufficient signal strength for magnetic circuit tracing. With a ten amp trip point it will be appreciated that the actual current through the circuit could reach a peak that is about an order of magnitude above the threshold. For instance with a ten amp resetting breaker the peak current can easily reach one hundred amps for a short duration prior to tripping of the circuit breaker into an open state. It will be noted that the current peaks through the breaker typically vary with cycling and temperature. The current peaks through a resettable breaker, which initially exceed one hundred amperes (i.e. when cold), can upon being heated by repeated actuations drop significantly, such as to fifty amperes.
It should be appreciated that circuit breakers always exhibit a lag time between the conditions meeting the trip threshold and the circuit breaker entering an open-circuit state. Typically most high current circuit breakers are of a mechanical construction having elements which must heat up and overcome mechanical inertia between the time the current threshold is exceeded and the breaker trips into an open state. Electronic breakers also typically have a significant time lag, however, it is one that is intentionally built-in to the devices for limiting false triggering in response to noise pulses.
The current through the circuit-breaker during testing is generally concentrated through the path created by the shorted condition, yet some portions of the current may flow through other current paths within the circuit, such as to a load. Passing high current pulses through the shorted circuit can lead to a number of problems, such as damage to sensitive wiring or circuitry, and the inadvertent conversion of a static short to an intermittent short, which further complicates locating and repairing of a wiring short.
As a consequence of the large current flow passing through the shorted path, a significant magnetic field is generated around the associated conductor(s). The magnetic field obviously only arises along the portion of the conductor through which the current is flowing, which is typically between the point at which power is applied through the short tracing device (i.e. at a fuse box, at a load, at a connector, or other point in the circuit) and the point at which the circuit is shorted. It should be appreciated, however, that some portion of the total current may still flow to a load, or loads, that has not been disconnected. In many cases the current flow to the short, which generates the associated magnetic field, is traced using a handheld current detector, for example a magnetic needle-pointer type current detector (i.e. often a compass or compass-like device), Hall-effect detector (magnetic field strength sensing), or similar.
In using the self-resetting circuit breaker approach, the test device intermittently reapplies power to the circuit, resulting in repetitive current pulses being generated through the circuit. The duty cycle of this pulsed output signal is typically very low, in particular for circuits having a high conductance short, because the time over which current is passed through the shorted circuit is much less than the time required for the circuit breaker to break the circuit and then reset back to a closed circuit condition. With the typically slow-reacting resettable breakers, the fact that the trace signal is only available intermittently complicates locating the short. Even if faster-reacting resettable breakers are utilized, the average flux density of the magnetic field about the circuit path is still significantly impacted due to the low duty cycle output making circuit tracing more difficult or even infeasible in some instances.
In utilizing intermittent test pulses as described, the amplitude of the current pulses through the circuit-breaker often can exceed the maximum allowable current for the circuit, such as determined by the conventional fuse, or circuit breaker element, found in the vehicle. This is especially true for modern vehicles which often utilize a number of sensors or other low power electronic devices.
The effect of a huge current pulse, such as generated by the circuit breaker approach, can make the short circuit untraceable. In one situation sufficient heat can be generated at the short to separate the conductors to temporarily eliminate the short circuit condition. Although temporarily eliminating the short may at first glance sound like a benefit, it can be more accurately considered as masking a potential short circuit, or creating an intermittent short circuit condition. It is generally well appreciated that intermittent problems are far more difficult to diagnose than those which are constant and which more readily avail themselves of fault isolation.
High current pulses also result in high heat build-up on the wiring of the shorted circuit which often exacerbates the condition of the wiring near the short and perhaps elsewhere along the path of the short, increasing the severity of future shorts. The heat created by the large pulses of “testing current” may far exceed the maximum allowable current levels for that circuit, typically an indicator for which is the recommended amperage value for the fuse. The current passing through the short can cause other fuses or circuit breakers in the system to open up, and can damage other wiring besides the section damaged from the original short circuit.
An additional effect that is created by large current pulses which can produce temporary elimination of a short, converting it to an intermittent short, is a physical repulsion response of conductors subject to large current pulses. This mechanical repulsion reaction results from the substantial magnetic field created by the large current pulses and can be observed to cause wires to jump or move in response to the current pulse. In response to this force, the conductor(s) of the shorted circuit may pull apart enough to temporarily eliminate the short, typically resulting in an intermittent short or a short which will manifest itself at a later time, for example at night when the driver may be an inconvenient distance from repair facilities or populated areas.
Another drawback to the use of current pulsing, such as utilizing the circuit breaker approach, is that the signal duty cycle is dramatically reduced. In many cases the time for a resettable breaker to open and then reset back to a closed condition can exceed ten seconds, although it varies depending on construction and manufacturer. With the active time for the current pulse being less a few hundred milliseconds, the duty cycle of the testing pulses can be very low. Consequently, personnel attempting to test the circuit are provided with only short intermittent pulses with which to test the circuit, making isolation that much more difficult and time consuming.
Additionally, the intermittent pulses produce an uncertainty condition as a detector is moved from one point along the circuitry to the next because the person performing the testing may not be able to discern the pulse occurrence when it does not produce a response on the detector. When a pulse is not detected at any given point after a seemingly sufficient delay, it becomes necessary for test personnel to backtrack to a point at which the signal can again be detected, which considerably impacts the fault tracing process.
Another drawback which results from using an intermittently traceable signal, is that no indication can be provided as to when a short condition disappears, such as in response to a service technician handling the wiring along the circuit where a short has arisen. This is a critical drawback, because it is often necessary to physically handle circuitry in the process of tracing an electrical short. When the shorted condition results from contact between two conductors (i.e. between adjacent wiring, or between one or more wires and adjacent structures) which are not firmly held in contact with one another, physical handling of the circuit under test can result in the loss of contact at the point at which the circuit is shorted. In the absence of a continuous indication of the short, it can become impossible to locate where the short was occurring, and to determine what action resulted in the elimination of the shorted condition. The inability to correlate action, such as movement of specific wiring, with a responsive change in the short condition makes short locating far less definitive, and more prone to guesswork. Consequently, as a result of intermittent responses it is unlikely that whatever action produced the response can be directly associated with a narrowly restricted area of the vehicle circuitry.
Accordingly there is a need for a short circuit testing and isolation device that is particularly well-suited for troubleshooting electrical circuit wiring in a variety of vehicles and other systems employing similar current-carrying circuitry. The present invention satisfies those needs and others, and may be implemented at low cost.