Patent Application
20070262779
This invention relates to a method and apparatus for detecting and alarming unbalanced power feed conditions in both AC and DC power distribution applications.
A fault condition exists in a power distribution system when unequal currents appear in a pair of power feeder wires that supply power to a system or subsystem. For example, if a fault is present in a system or subsystem that causes part of the current to leak to earth ground, one of the two wires supplying power to the system will carry less current than the other since a part of the current that it would normally carry is being leaked to the earth ground. It has been recognized that the presence of unequal currents in power feeder wires is a cause of system inefficiencies, presents reliability issues, and a primary safety issue to the human interface. It is highly desirable to correct the fault condition as quickly as possible.
The present arrangements for detecting unequal current flow in a power feeder are not economical and not very sensitive. Typically, if a system is suspected of having unequal power feed currents in the two directions, a clamp-on current meter may be placed successively on the two power feeder wires and if the readings are different this is an indication of an unbalanced power feed current. Monitoring the current in the two directions individually on a steady basis can also provide the information necessary to determine that the currents are unbalanced. Ground Fault Interruption Circuitry (GFCI) may also be employed but these devices remove power when a fault condition is detected.
A problem of the prior art therefore is that there is no quick, economical, and sensitive arrangement for detecting unbalanced currents in a pair of power feeders during installation or while the circuit is operating.
The above problem is overcome and an advance is made over the teachings of the prior art in accordance with this invention wherein a clamp-on device is installed around a power feeder pair normally carrying equal and opposite currents. The clamp-on device contains a Hall effect device. When the probe is clamped around two conductors with different polarities, the resulting reading will be the difference between the two currents. If the currents are the same, the reading will be zero. When a reading other than zero is obtained, the reading is proportional to the amount of leakage current that is occurring. The reading is then converted into an alarm signal for providing alarm information with respect to the pair of power feeder wires being monitored. Advantageously, this arrangement provides a continuous, inexpensive, and sensitive method and apparatus for detecting unbalanced currents in a feeder pair during and after installation of the system.
In accordance with one aspect of Applicant's invention, monitoring devices may be installed on various feeders of a system. Each monitoring device can be connected to send an alarm signal to a centralized alarm control system wherein an alarm controller controls audible alarms and/or visual alarms for use by the craft attending a system. Advantageously, this arrangement allows the craft to rapidly identify, locate, and isolate the faulty area.
Hall effect devices can be used for both AC and DC applications. They can be programmed for sensitivity ranging from milliamps to several hundred amperes. The Hall effect devices are commonly available and inexpensive.
In contrast, wires 11 and 12 carry equal and opposite electric currents so that the net magnetic field 13 surrounding these two wires is zero. Consequently, sensor 14 detects no net magnetic field and does not send an alarm signal 15 to the associated alarm unit.
The above description is of one preferred embodiment of Applicant's invention. Other embodiments will be apparent to those of ordinary skill in the art. The invention is limited only by the attached claims.
