Basic HTML Version
9/20
Dimensions
Section 10
Types of RCD
Protection
9
time
(s)
1000s
100s
10s
1s
100ms
100mA 300mA 600mA 1.5A 3A
500A
current (A)
Merlin Gerin
300mA time delayed rcd
Typical 100mA
instantaneous rcd
Typical 30mA
instantaneous rcd
Two families of RCD are offered in this catalogue
(1) Those without overcurrent protection (RCDs) e.g. RMGs
(2) Those with overcurrent protection (RCBOs) e.g. C60H rcbo’s and DPN
Miniature circuit breakers combined with a residual current device, this RCD/
MCB combination can be realised in the form of the C60H RCBO combined RCD/
MCB or by combining a vigi module with an MCB. All of these units can be used
to protect individual circuits or as devices mounted within individual enclosures.
Individual out going circuits within type A or type B MCB distribution boards can
be protected by installing RCD/MCB combinations on that particular outgoing way.
Groups of circuits can be protected within distribution boards by selection of either
split load or dual incomer units with selected circuits protected by RCD incoming
devices. Complete distribution systems can be given RCD protection by installing
an RMG RCD in a separate enclosure ahead of the distribution board or by using a
distribution board having an incoming RCD.
Unwanted tripping
The principal reasons for unwanted tripping of RCDs are lack of discrimination
between RCD’s (see following paragraph) and transient earth leakage currents,
which have various causes such as lightning strikes, switching surges (caused by
switching inductive loads) or switching capacitive loads (RF filter networks, mineral
insulated cables etc) All Merlin Gerin RCDs and RCD/MCBs incorporate a filtering
device which minimises their response to transients, virtually eliminating unwanted
tripping.
Discrimination of RCDs
Figure 3: characteristic of 231 16, 300mA time delayed RCD with 10, 30 and
100mA instantaneous devices superimposed to show discrimination. Wherever two
or more RCDs are installed in series with one another, measures must be taken
to ensure that they discriminate properly - in the event of an earth fault, only the
device next upstream should operate. RCDs do
not
discriminate on rated residual
operating current sensitivity alone. In other words, a 100mA device upstream of
a 30mA device will not offer inherent discrimination. To provide the necessary
discrimination, RCDs can be provided with an inbuilt time delay mechanism, usually
50ms. This inbuilt time delay is sufficient to allow the downstream device to open
the circuit before the upstream device starts to operate. Such an RCD must be
used as the incomer to a split load board incorporating two RCDs.
Zone Physiological Effects
1 Usually no reaction effects
2 Usually no harmful physiological effects
3 Usually no organic damage to be expected. Likelihood of muscular contraction and difficulty of breathing
reversible disturbances of formation and conduction of impulses in the heart and transient cardiac arrest without
ventricular fibrillation increases with current magnitude and time
4 In addition to the effects of zone 3 probability of ventricular fibrillation increased up to 5% (Curve C2) up to 50%
(Curve C3) and above 50% beyond Curve C3 Increasing with magnitude and time. pathyphysiological effects
such as cardiac arrest, breathing arrest and heavy burns may occur.
IEC publication 479 “effects of currents passing through the human body”
TIME/CURRENT ZONES OF EFFECTS OF A.C. CURRENT (15 - 100HZ) ON PERSONS
WITH STANDARD RCD CHARACTERISTICS SUPERIMPOSED
10
20
50
100
200
500
1000
2000
5000
10000
0.1 0.2 0.3 0.5 1 2 3 5 10 20 30 50 100 200 300 500 1000 2000 3000 5000 10000
time in milliseconds
current in milliamperes (R.M.S.)
typical current
limits due to body resistance
at 240V
a
b
c
1
c
2
c
3
IEC 479
10mA 30mA 100mA
1
2
3
4
(fig. 3)