—
6
—
EMC:
EN
55022
:
1998
+A
1
+A
2
EN
61000
-
4
-
2
:
1995
+A
1
+A
2
EN
61000
-
4
-
3
:
1996
+A
1
+A
2
Note:
Measurement Category lll is for measurements
performed in the building installation. Examples are
measurements on distribution boards, circuit breakers, wiring
including cables, bus-bars, junction boxes, socket outlets and
permanently connected equipment in the fixed installation.
4
Principles of measurement
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. If the fault current to earth is not a
pure sine wave but has a DC component, it is possible that a
normal breaker will not trip at its rated tripping current. It is
only those breakers that are also sensitive to DC fault current
that will trip with this type of fault current. 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.
The ISO-TECH IRT-
2310
is capable of testing these breakers
by introducing a DC fault current (Ref. IEC
1008
) with a
fluctuating waveform as shown in Figure
1
above.
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
Time - t
Fig.
1
