Transcript
ALL CABLES ARE NOT THE SAME...
Nail Penetration Test Cables of this type must pass a nail penetration test in order to meet the requirements of current cable specification BS8436:2011. The purpose of the nail penetration test is to confirm that the metallic screen will carry a fault current and enable electrical protection devices to operate in case the cable is penetrated by a nail or similar fixing. This vital safety test assesses the performance of the cable with a specified test current, applied for 1 second. This test current is derived from the conductor size and its maximum current rating should be taken into account when selecting a suitable protective device. The device should be capable of operating at this specified current in a time not greater than 0.1s. Cables manufactured to BS8436:2011 are primarily for installation within walls, partitions and building voids. They must have a suitable metallic screen to carry the entire fault current without failing, for example melting under the heating produced by the current. The test ensures suitable circuit protection can be provided by the cable; but most importantly the design ensures a Type B circuit breaker can operate swiftly and fully. To perform the test, the cable is first energised in the rig. The 1.6 mm diameter nail is placed onto the cable sheath and retained firmly by the chuck. With equipment isolated and the sample interlocked safely in the chamber, the steel nail is driven through the sheath into a live conductor by a 900 gram weight. This weight falls from a height of 400mm and guided so that it produces the statistically consistent effect. The test must be repeated five times and six straight passes must be achieved in order to comply with the specification in the British Standard. A pass for the selected conductor-current pairing is given if the sample of cable can maintain the fault current for 1 second or longer and the voltage measured on the nail is less than 25V. However, the sample fails for the selected current if the fault current flows for less than 1 second or the value of voltage measured on the nails is 25 volts or more. The implication of such a failure is melting of the metallic screen, rendering the nail live and exposing the operator or user to the full fault current carried within the nail. The market should be tested regularly with a number of samples collected from a range of manufacturers. In many instances, samples fail this simple test.
