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Manufacturing Faults

At a Geelong company, we recently encountered a number of leads with a manufacturing fault. The Earth resistance of the leads was greater than the 1.0 Ohm permitted by AS/NZS 3760. An old computer lead had failed testing and the OH&S manager said they had a box full of spare leads, one of which could be used as a replacement. The spares were as a result of common practice when installing a new computer: the computer is supplied with new leads but the old ones already in place are used and the new ones stored as spares.

We unwrapped a new lead and after passing the visual test it was connected to the PAT (Portable Appliance Tester) for the electrical tests. The new lead failed for the same reason as the old lead; the Earth resistance was greater than the 1.0 Ohm permitted by AS/NZS 3760. We destroyed the lead and opened another brand new lead. This also failed the Earth resistance test. It wasn’t until the third lead was unwrapped that we found one that would pass.

The Earth resistance needs to be below 1.0 Ohm in order to carry electricity away from the housing of a faulty appliance. In this case, the leads had excessively high Earth resistance that would not have protected a user had the computer developed a fault that caused the metal computer housing to become live. The most likely cause is faulty soldering in either the plug or socket.

Billions of electrical appliances and accessories are manufactured each year. It would be unreasonable to assume that manufacturing errors are never made.