Patent GB-1173288-A (GB)
Title: Method and Apparatus for Testing the Total Insulation Resistance of an Electrical Supply
IPC: G01R27/18
Filed: 1966-11-23
Abstract: 1,173,288. Protective arrangements. COMPAGNIE DES COMPTEURS. 23 Nov., 1966 [13 Dec., 1965], No. 52373/66. Heading H2K. [Also in Division G1] The resistive component of the effective insulation impedance between the lines of a supply circuit and earth is tested by applying a low frequency test voltage to the circuit to cause current I, to leak to earth through the insulation, applying a voltage of the same frequency, phase and amplitude across a reference to provide a reference current I Ro,  deriving a current representing the total leakage current through the insulation and producing a further current, in response to this representative current and the reference current, whose phase is that of the vector difference #I r -#I RO  between the leakage current and the reference current. A current whose polarity indicates the phase difference between the further current and reference current relative to 90 degrees is derived and used to indicate whether the leakage resistance is greater or less than the reference resistance. If the phase difference is less than 90 degrees, the leakage resistance is less than the reference, and this condition is used to operate an alarm. The method is applicable both to D.C. supply circuits (M) or balanced or unbalanced threephase A.C. circuits (P or Q), whenever it is necessary to test for the resistive component of the insulation impedance in the presence of the capacitive component. An apparatus 30 for carrying out the test incorporates a low frequency signal generator 14 which applies a test voltage to a supply circuit (M, P or Q) under test via an inductance 11 and capacitors, e.g. 1, 2, tuned together to block the supply frequency from the test apparatus. The apparatus 30 incorporates a tuned circuit 9, 12 tuned to filter any supply frequency signal passing back through the inductance 11. The test current I passing to the supply circuit and circuit 9, 12 is detected by the transformer 15, and a current corresponding to the actual leakage current is established in the primary 17a of the transformer 17 by vectorially subtracting from I a current I q  derived from a circuit 21, 23 connected across the supply 14 and exactly simulating the filter circuit 9, 12. A reference current is established through primary windings 17b, 18a of the transformers 17, 18 by applying the supply voltage at 29 across a reference resistor R o  and a circuit comprising the transformer secondary 24b and resistance 16 in parallel, in series with the windings 17b, 18a. The parallel circuit 24b, 16 produces a voltage drop equivalent to that in the blocking circuits between the point 29 and the supply circuit under test. The output from the winding 17c of transformer 17 represents #I r -#I RO , while that from winding 18b of transformer 18 represents I R0 . These outputs are applied across opposite diagonals of a diode bridge phase demodulator, while the centre taps of the windings are connected together through the energizing winding 27a of polarity sensitive alarm relay 27. The polarity of the current in the winding 27a changes direction as the vector angle between the outputs from the two transformers passes through 90 degrees in such a way that the alarm operates when the angle is less than 90 degrees to indicate that the insulation resistance is less than the reference.