Rcd Testing - Iso-Tech IMT-3300 Manuel D'instructions

4.4 RCD testing

An RCD is a switching device designed to break a circuit
when a residual or unbalanced current in the circuit attains a
specific value. It works by monitoring the difference in currents
flowing through the phase and neutral conductors, which are
unbalanced when a fault condition occurs (for a single-phase
installation). When the difference exceeds the RCD tripping
current, the RCD will trip and disconnect the supply from the
circuit.
The RCD tester is connected between the phase and
protective conductors on the load side of the RCD. A
predetermined current is drawn from the phase conductor
and returns via the earth, causing the RCD device to trip. The
instrument measures and displays the exact time taken for the
circuit to be opened under this simulated fault condition.
There are two parameters which designate RCD types; the first
relates to the shape of the residual current waveform (types
AC and A) and the second relates to the tripping time (types
G and S). A typical RCD is designated "Type AC-G". These
designations are explained below:
Type AC: An RCD of the AC type 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.
Type A: An RCD type A will trip when presented with residual
sinusoidal alternating currents (similar to the AC type) and
residual pulsating direct currents (DC) whether suddenly
applied or slowly rising. This type of RCD is not in common use
at present, but is increasing in popularity and is required by the
local regulations in some countries.
Type G: In this case G stands for general type without a trip-out
time delay and is for general use and applications.
Type S: An RCD designated type S means a selective type with
a trip-out time delay. This type of RCD is specifically designed
for installations where the selectivity characteristic is required.
In order to ensure the protection of an electrical installation
protected by RCD's, they should be checked to confirm the
trip-out time tΔ is correct.
The trip-out time tΔis the time required by the RCD to trip at
a rated residual operating current of IΔn. 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 IΔn
and 5IΔn.
Type of RCD
300ms
General(G)
max allowed value
500ms
max allowed value
Selective(S)
130ms
min allowed value
*Maximum 5IΔn tripping time is limited to 50 ms as required
by BS7671 when 'OL' is displayed.
Typical examples of instrument connection:
Practical example of 3-phase + neutral RCD test in a TT system.
— 10 —
IΔn 5 IΔn
40ms
max allowed value
*150ms
max allowed value
*50ms
min allowed value
RCD
Fig 8