KYORITSU 6050 - Manual (Page 22)

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6.1 Principles of Measurement
The RCD tester is connected between phase and protective connectors
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 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 current.
There are two classifications for RCDs; the first one 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.
● RCD type G. In this case G stands for general type (without trip- out
time delay)and is for general use and applications.
● RCD type S where S stands for selective type (with trip-out time
delay).This type of RCD is specifically designed for installations
where the selectivity characteristic is required. In order to assure
successful protection on an electrical installation using RCD's they
should be checked to test trip- out time t⊿.
● Trip- out time t⊿ is the time needed by the RCD to trip at a rated
residual operating current of I⊿n. The standard values of tripping
time are defined by IEC 61009 (EN61009) and IEC 61008 (EN
61008) and are listed in the table below for I⊿n and 5I⊿n.
6. RCD TESTS