Radar and Bath Test

Press, Volume XCIX, Issue 29239, 24 June 1960, Page 10

Radar and Bath Test

On thin - walled insulated wires, where the radical thickness of insulation may be as low as O.OOfiin, the specified maximum variation from the thickness is 0.001 inch. In Christchurch an electronic concentricity gauge is built into the extruder line to ensure that the conductor is correctly centred in the insulation. This equipment employs as indicator cathode-ray tubes with a display unit very much like a radar screen.

The position of the conductor in relation to the overall diameter of the insulated wire is shown by a bright point of light. The instrument is accurate to plus or minus 0.0003 inch.

In addition to all the automatic control equipment, routine checks are taken of every drum of insulated wire or sheathed cable continuously throughout the period of production. Samples carrying identifying code numbers are passed to the laboratory for complete physical examination.

First determinations are made by fiducial micrometers to ensure compliance with the required specification. Measurements are made of overall dimensions, on individual insulated conductors, and the copper conductors themselves. Determination of eccentricity Is made on a binocular microscope which has fitted a microfjmeter scale where maximum and minimum thickness of a sheath or insulation are measured and recorded.

Insalation resistance is measured on a new type of instrument, also electronic. This instrament reads directly high resistance between 300,000 ohms and 20 million million ohms, i.e. 20 million megohms.

For this test the cable under inspection is suspended in a water tank with the ends of the insulation trimmed back and held above the water level. The same water tank is also used for high voltage testing, where pressures op to 20 kV are available from a special testing transformer.

This pressure test is frequently used to determine breakdown voltage of cables insulated with new types of plastic compound, or cables of new design.