We have witnessed an increase in reports from several well-informed installers relating to Type B devices that do not function correctly in the presence of DC components. For example, one design was marked as bidirectional but failed smooth DC tripping test.
Although the basic requirements of BS 7671 do not require verification that the Type B RCD protection feature works, it would seem prudent to spend a few minutes checking that modern complex domestic systems costing possibly £10,000 or more are protected by a functioning RCD. A simple test with the correct instrument can be performed to verify that the RCD trips when subject to smooth DC.
RCD Tests and Thresholds
The existing BS 7671 1x RCD verification test required, for example for 30mA RCDs, relates to an AC sinusoidal residual current irrespective of the type of RCD.
- All RCCBs subjected to 50Hz residual current are designed to trip between more than 50% and less than 100% IΔn, which equates to 15 to 30mA. The maximum switch-off time at 1x is 0.3s.
- Type A, Type F and Type B pulsed residual current at 50Hz use a tripping current threshold of more than 35% and less than 140% IΔn.
Type A must still trip with the pulsed waveform superimposed on 6mA smooth DC. Type F must still trip with the pulsed waveform superimposed on 10mA smooth DC. Type B subjected to smooth DC residual current must trip between more than 50% and less than 200% IΔn. For 30mA, that equates to 15 to 60mA DC on both cycles 0° and 180°.
The standards define the tripping thresholds and times based on the residual current components. Although BS 7671 only requires verification of the AC trip threshold irrespective of the type, other tests are not precluded.
Why DC Tripping Characteristics Matter
RCD Types A and F use a magnetic core to detect imbalance between live and neutral conductors. Smooth DC residual current above the design current can saturate this core, a phenomenon known as DC blinding.
When the core is saturated, the RCD becomes insensitive to further imbalance. Trip thresholds increase unpredictably and the device may fail to trip entirely. This is a major safety concern because a circuit could carry a dangerous fault current without any protective disconnection.
Type B RCDs are designed to detect smooth DC and trip when it exceeds the default value. This maintains sensitivity to both AC and pulsating DC components while also responding to pure DC leakage.
Test Instruments
For test instruments equipped with a Type B RCD setting, a multiplier of 2 × IΔn is applied in accordance with BS EN 62423. We have witnessed complaints relating to a number of Type B devices that do not function correctly in the presence of DC components, particularly those marked as bidirectional. A simple test can be performed to verify DC tripping on DC currents.
Testing Procedure for Type B RCDs
The testing procedure for a Type B RCD is relatively straightforward. Connect the live, neutral and earth to probes on the appropriate terminals on the load side and carry out the tests. Once complete, carry out the same test on the supply side to check whether the device is truly bidirectional.
Carry out the standard AC test first, following all testing safety procedures.
DC tripping test applicable to any 2-pole 30mA Type B where the outgoing circuit should be disconnected:
- Turn the rotary switch to the IΔn position.
- Press F1 to select the test current phase, 0° or 180°.
- Press F3 to select the RCD test-current waveform: smooth DC current to test a Type B RCD.
- Press F4 to select the RCD current rating, such as 10, 30, 100, 300 or 500mA.
- Press and release test, then wait for completion.
Repeat the same test on the opposite side of the device. For a true bidirectional RCD, it should still trip to the relevant parameters.
Conclusion
So, does a Type B RCCB trip on DC? It should do, but some do not. If it is detecting DC it should trip between more than 50% and less than 200% IΔn, which equates to 15 to 60mA on both cycles 0° and 180° for a 30mA device. If it is truly bidirectional it will trip on DC irrespective of which side the supply is connected.
The testing procedure for a Type B RCD including the DC test takes only a few minutes in addition to the standard AC test. If it is a complex installation requiring bidirectional RCD functionality, you can verify the operation by repeating the test on the reverse side.
Spend a few additional minutes checking that your modern complex domestic system, and the people using it, are adequately protected by the RCD you have specified and installed.
Written by Martin Plumbridge – TMIET, Technical Manager, Doepke.