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10. RCD TESTS
10.1 Principles of RCD Measurement
The RCD tester is connected between phase and protective conductor on the load side of
the RCD after disconnecting the load.
A precisely measured current for a carefully timed period is drawn from the phase and
returns via the earth, thus tripping the device. The instrument measures and displays the
exact time taken for the circuit to be opened.
An RCD is a switching device designed for breaking currents when the residual current
attains a specific value. It works on the basis of the current difference between phase
currents flowing to different loads and returning current flowing through the neutral
conductor (for a single-phase installation). In the case where the current difference is higher
than the RCD tripping current, the device will trip and disconnect the supply from the load.
There are two parameters for RCDs; the first due to the shape of the residual current wave
form (types AC and A) and the second due to the tripping time (types G and S).
● RCD type AC will trip when presented with residual sinusoidal alternating currents
whether applied suddenly or slowly rising. This type is the most frequently used on
electrical installations.
● RCD type A will trip when presented with residual sinusoidal alternating currents
(similar to type AC) and residual pulsating direct currents (DC) whether suddenly applied
or slowly rising. This type of RCD is not commonly used at present, however, it is
increasing in popularity and is required by the local regulations in some countries.
Making measurement with setting uses pulsating direct currents for test.
● RCD type G. In this case G stands for general type (without tripping time delay) and is for
general use and applications.
● RCD type S where S stands for selective type (with tripping time delay).This type of RCD
is specifically designed for installations where the time delay characteristic is required.
Given that when the protective device is an RCD, Ia is typically 5 times the rated residual
operating current I
n, then the RCD must be tested recommending the tripping time,
measured by RCD testers or Multifunction testers, shall be lower than the maximum
disconnecting times required in IEC 60364-41 (see also LOOP/PSC/PFC section) that are:
200 ms for final circuits not exceeding 32A
TT system
(at 230V / 400V AC)
1000 ms for distribution circuits and circuits over 32A
400 ms for final circuits not exceeding 32A
TN system
(at 230V / 400V AC)
5 s for distribution circuits and circuits over 32A
S
