EMC looks at the testing of residual current operated devices: why is it necessary and how should it be carried out in practice?
Residual current operated devices (rcds) are often fitted in electrical installations to provide additional protection against fire and electrical shocks.

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.

Meeting standards
A 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 16th Edition, BS 7671: 2001.

The Wiring Regulations address the requirements for test equipment used in installation testing. The Regulations consist of the general requirements for test equipment and specific requirements for combined measuring equipment. They also cover the specific requirements for measuring/testing:

  • insulation resistance;
  • loop impedance;
  • continuity of protective conductors;
  • resistance to earth;
  • rcd performance in TT and TN systems;
  • phase sequence.

Verifying protection
The verification of the effectiveness of the measures for protection against indirect contact by automatic disconnection of supply depends on the type of system. In summary, it is as follows:

  • for TN systems – measurement of the fault loop impedance and verification of the characteristics of the associated protective device, ie visual inspection of the nominal current setting for circuit-breakers, the current ratings for fuses and testing rcds;
  • 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, ie rcds by visual inspection and by test.

In all these procedures, verifying the characteristics and operation of protective devices such as circuit-breakers, fuses and rcds is critical.

Why do we test rcds?
Most rcds have an integral test button, but even a successfully completed test using this facility does not necessarily confirm that the device 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 be carried out to determine the actual tripping current.

The 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. When connecting at distribution boards, connections are made to the line, neutral and earth conductors at convenient points on the load side of the rcd. The test is performed with a live circuit, with the load 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 recorded. The measured tripping time can be compared with the maximum time permitted in the IEE Guidance note 3.

How to test rcds
Installation testers that are specifically designed to carry out the tests specified in BS 7671, in the safest and most efficient way are now freely available on the market.

Examples of such products are those in the Fluke/Robin 1650 Series. Of these products, the 1652 and 1653 testers induce a calibrated fault current in the circuit, causing the rcd to trip. The instrument measures and displays the time taken for the trip to occur.

The testers also perform a pre-test to determine if the actual test will cause a fault voltage exceeding the 50 or 25 V limit. To avoid having an inaccurate trip time for S type (time delay) rcds, a 30-second delay is activated between the pre-test and the actual test.

To simplify and speed up testing, the Fluke/Robin 1652 and 1653 models have an auto-mode for measuring rcd tripping time: six tests (x 0·5, 1 and 5 at 0° and 180°) are automatically carried out in sequence. This eliminates the need for the test engineer to keep returning to the installation tester after resetting a tripped rcd, saving a considerable amount of time on site. The 1653 also has an internal memory for storing results for later recall or importing into a report made using FlukeView Forms software.

In addition to measuring trip time, the 1652 and 1653 models can measure rcd tripping current by gradually increasing an applied current until the rcd trips. This is commonly referred to as a ramp rcd test. Once again, the rcd trip current rating, rcd type, and test current phase must be selected using soft keys before commencing the test.

Finally, after the electrical tests have been completed, the test buttons on the rcd should be operated to confirm that all mechanical parts are functioning correctly.

The 1650 Series multifunction testers measure up to 500 Vac and simultaneously display the line voltage level (primary display) and frequency (secondary display). They are easy to set up for making measurements, with a clearly marked rotary control for setting the range and a straightforward user interface with simple menus for setting test conditions.

The testers are lightweight and ergonomically curved for comfortable operation in the field. The 1650 series testers use a patented advanced loop measurement technology to prevent electronic and passive type rcds from tripping during earth-loop testing, while giving highly consistent and repeatable results.

The 1652 performs all the basic installation tests and has rcd test functions; the 1653 also offers low voltage insulation resistance, earth resistance measurements and a phase sequence indication for three-phase systems.

In addition the 1653 features an internal memory for storing up to 500 measurements and a pc interface for extra convenience in documentation and reporting. This simplifies generating reports in combination with the optional FlukeView Forms software to comply with legal requirements for documented results.