November 2009Maintenance, Test & Measurement, Calibration
Web exclusive – Application note courtesy Fluke and Comtest
Testing RCDs with the Fluke 1650B Series.
Residual current operated devices (RCDs) are often fitted in electrical installations to provide additional protection against fire and electrical shocks. Verifying the correct and safe operation of RCDs involves a number of dedicated tests, all of which can be carried out using the new Fluke 1650B Series Multifunction Installation Testers.
Why use RCDs?
An RCD detects fault currents flowing to earth that are too small to trip over-current protection devices (such as fuses), but are still sufficient to cause a dangerous electric shock (see also Figures 1 and 2 online at http://instrumentation.co.za/+C13214) or an electrical fire. Verifying their operation is critical for safety, and is covered in IEC 60364 (and its various national equivalent standards). This standard specifies the requirements for fixed electrical installations in buildings.
Why test RCDs?
Most RCDs have an integral test button, but even a successfully completed test using this facility does not necessarily confirm that the RCD is working correctly. Additional tests to measure tripping time are necessary to verify that the RCD will perform correctly under fault conditions, while extra tests may also be carried out to determine the actual tripping current. In standard regulations, testing RCDs falls under ‘Verifying protection by automatic supply disconnection’. Depending on the type of system, that is TN, TT or IT, various test procedures are used. These include measurement of the fault loop impedance, measurement of the earth-electrode resistance for exposed-conductive-parts of the installation, and measurement or calculation of the first fault current. In all these procedures, verifying the characteristics and operation of protective devices such as circuit breakers, fuses and RCDs is critical.
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