Proper spark testing at critical points in the insulated wire manufacturing process can identify manufacturing and material defects, greatly reducing scrap and enhancing profitability. Marianne Savage, President of Clinton Instrument Co., USA, has recently highlighted seven steps which can insure that operators will find the defects in the product:
1- Use the correct spark tester and electrode: Clinton offers several types of spark testers and a wide range of electrodes, each designed for a particular product type. For instance, round wire tested to a UL specification must use a bead chain electrode; brushes are UL-approved for flat or irregularly shaped products only. UL specifications also state that faults must be able to be detected every 24" at any wire line speed. Because of this, a wire line running faster than 3,000 FPM requires a spark tester with fast fault resolution capabilities and a longer bead chain electrode.
2- Ground the product: DC testing requires grounding the conductor or the shield for a safe and effective spark test. While high-frequency testing theoretically works under optimum conditions without grounding the conductor, most test standards, including UL, mandate product grounding.
3- Make sure the test product is dry: water on the wire can cause false counting, cause printers and other devices to malfunction, and even damage your spark tester. High-quality air wipes and other wiping devices will limit moisture related problems.
4- Test at the right voltage for the product: generally, published standards specify test voltages. If a high-frequency spark tester is used, but the correct test voltage is unknown, a simple experiment can be used to determine an effective potential. First establish a lower limit where ionisation, or corona, begins to form in the electrode around the wire. Next, find the upper limit (where the insulation fails). A satisfactory test voltage will be between these limits. This procedure is described in detail in the Clinton paper, "Selection of 3kHz spark testing potentials for insulated wire".
5- Maintain the electrode regularly: replace beads or brushes when they become dirty or worn. Wire bouncing around between bent, filthy brushes or through a hole in the bead chain electrode is not being tested properly, since a much greater test potential is needed to bridge a large air gap. Dirty electrode mounting plates with evidence of tracking should also be replaced.
6- Stabilise the wire as it runs through the electrode: Wildly vibrating wire can cause bead chains in the electrodes to be thrown out of contact with the wire, causing air gaps that lead to a poor test. In some cases, smaller wires running at very high speeds are prone to lateral vibrations that may cause false counting. Wire guides positioned before and after the electrode can reduce or eliminate problematic vibration. A diagram showing proper placement of these guides is available from the American company.
7- Calibrate the spark tester on a regular basis: UL specifications require regular voltage calibration. It is important that spark testers, particularly the high-frequency type be calibrated with equipment that will provide accurate results. However, voltage calibration only checks the accuracy of the applied test voltage. It is equally important to check the function of the detection circuitry. Some standards, including NEMA, BSI and UTE, require precise tests for measuring fault detection sensitivity, operating load and output current. Clinton supplies equipment that performs all of these tests.
