SPECIFICATIONOn-line ground fault monitoring apparatus and methodThe present invention relates to a ground fault monitor of the type which continuously monitors the insulation to ground of an isolated AC installation, such as a motor or the like, wherein an aiarm is activated when the insulation falls below a preset value.
It is an object of the present invention to provide an on-line ground fault monitor which will provide continuous monitoring even when power is applied to the installation.
According to a first aspect, the present invention comprises a monitor adapted to be electrically connected to an isolated A.C. load and to generate an output signal which is a function of the resistance between the live side of said load and ground and which is independent of any A.C. power supply driving said load.
Preferably, the monitor includes a DC power supply connected to the neutral side of said load via a series resistance, said output signal being derived from the voltage developed across said series resistance.
Preferably also, the monitor further includes low-pass filter means in parallel with said series resistance and said output signal is a function of the filtered output of said filter means.
According to a second aspect of the invention, a method of monitoring ground faults in an AC load comprises filtering the voltage developed across a resistance connected between a DC power supply and the neutral side of an AC load using low-pass filter means whereby all AC components are removed from said voltage.
An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which shows a schematic circuit diagram of a monitor embodying the invention connected to an isolated A.C. power supply and load.
Referring now to the drawing, an A.C. load 10 is connected across an isolated A.C.
power supply such as the secondary winding 12 of an isolating power transformer, and is driven by the secondary voltage V 1 thereof.
In the drawing, a ground fault 14 is indicated between the live side of the load 10 and ground, having a resistance R1. An on line monitor 16 comprises a D.C. supply 18, developing a voltage V2, which is connected to the neutral side of the load 10 via a series resistance R2. The voltage developed acrossR2 is low pass-filtered by low-pass filter means 20 connected in parallel with R2 to provide an output signal (monitor viltage V3.
It can be shown that the voltage developed across R2 is a function of V1, V2, R1 and R2 which function includes a DC component.
V2 RR2 R1 R2
and an A.C. component:
vi R2 (R111R2) R2 The filter means 20 removes the A.C. component to provide the output signal:V2 R2V3=R1 +R2Thus a slowly varying D.C. signal is obtained which is dependent on R1 (the ground fault resistance) and independent of the A.C. power supply voltage V 1. Monitoring is therefore continuous, regardless of whether or not power is being applied to the load 10.
Furthermore, output signal V3 of the monitor 16 is not affected by a ground fault from the neutral side of the load 10 (or from any other point therein) since such a fault would affect only the A.C. component of the voltage across R2 and would therefore be removed by the filter means 20.
The output signal V3 may be applied to any one of a number of standard metering and trip devices to suit the particular application.
1. A ground fault monitor adapted to be electrically connected to an isolated AC load and to generate an output signal which is a function of the resistance between the live side of said load and grould, wherein a DC power supply is connected to the neutral side of said load via a series resistance, said output signal being derived from the voltage developed across said resistance such that the signal is substantially independent of any AC power supply driving said load.
2. A monitor is claimed in claim 1 further including low-pass filter means connected in parallel with said resistance and wherein said output signal is a function of the filtered output of said filter means.
3. A method of monitoring ground faults in an AC load comprises filtering the voltage developed across a resistance connected between a DC power supply and the neutral side of an AC load using low-pass filter means whereby substantially all AC components are removed from said voltage.
4. A ground fault monitor substantially as hereinbefore described with reference to the accompanying drawing.
5. A method of monitoring ground faults in an AC load substantially as hereinbefore described.
**WARNING** end of DESC field may overlap start of CLMS **.