Residual Current Operated Devices (RCDs) are fitted in electrical installations to provide additional protection. 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 or an electrical fire. Verifying their operation is critical for safety:
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Growing concern for public safety and the increasing complexity of today's fixed electrical installations in domestic, commercial and industrial premises places extra responsibility on electrical test engineers who are charged with verifying conformity to stringent international standards. It is therefore important to have suitable test tools for carrying out the stringent tests imposed by the IEE Wiring Regulations 17th Edition, BS 7671.
Verification of the effectiveness of the measures for protection against indirect contact by automatic disconnection of supply depends on the type of system:
- For TN systems: measurement of the fault loop impedance, and verification of the characteristics of the associated protective device.
- For TT systems: measurement of the earth electrode resistance for exposed-conductive-parts of the installation, and verification of the characteristics of the associated protective device.
In all these procedures, verifying the characteristics and operation of protective devices such as circuit breakers, fuses and RCDs is critical.
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. Basic testing of RCDs involves determining the tripping time (in milliseconds) by inducing a fault current in the circuit. This test can be performed at distribution panels with test leads or at socket outlets. The test is performed on a live circuit with all known loads disconnected.
Because some RCDs are more sensitive in one half cycle of the mains supply waveform than the other, the test must be carried out for both zero and 180 degree phase settings, and the longest time should be recorded. The measured tripping time can be compared with the maximum time permitted in the IEE guidance note 3.
The instruments:
The latest Fluke 1651B, 1652B and 1653B multifunction installation testers are specifically designed to carry out the tests specified in BS 7671 17th Edition Wiring Regulations as well as IEC 60364 and HD 384, in the safest and most efficient way. They induce a fault current in the circuit, causing the RCD to trip. The instrument measures and displays the time taken for the RCD to trip. The testers also perform a pre-test to determine if the actual test will cause a fault voltage exceeding the 50V or 25V limit, and offer a variable RCD current mode for customised settings. To avoid having an inaccurate trip time for S type RCDs, a 30s delay is activated between the pre-test and the actual test.
To simplify testing, the 1652B and 1653B have an auto mode for measuring RCD tripping time to eliminate the need for the engineer to keep returning to the installation tester after resetting a tripped RCD. They can measure RCD tripping current by gradually increasing an applied current until the RCD trips, commonly referred to as a ramp RCD test. The 1653B also has an internal memory for storing results for later recall or importing into a report made using FlukeView Forms software.
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