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TECHN I CAL DATA
RCD s
214
SPECIFICATION
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●
BSEN61008
(
Voltage independent)
●
Range of current ratings 16–100A
●
Range of sensitivities
10–300
mA
●
Operation General
Type A or AC
Selective Type S (Time Delay)
●
Pole configurations
DP and TP&N
●
Voltage ratings DP
230
V
TP&N 240V/415V
●
Frequency ratings
50/60
Hz
OPERATION
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The RCD employs the current balance principle which involves the supply
conductors to the load (phase and neutral) being wound onto a common
transformer core to form the primary windings. The secondary winding of the
current transformer is then connected to the electro-magnetic relay. Under
healthy circuit conditions, the current in the phase conductor is equal to the
current in the neutral and the vector sum of the current is zero. In the event
of an earth fault, an amount of current will flow to earth, creating an out of
balance situation in the transformer assembly. This out of balance is detected
by the secondary winding of the transformer and at a pre-determined level of
out of balance will activate the trip mechanism.
Single phase and neutral or three phase and neutral units (suitable for 3 or 4
wire systems) are available, the latter being suitable for balanced or
unbalanced 3 phase loads.
The RCD trip mechanism will operate at a residual current of between
50–100%
of its rated tripping current (sensitivity).
TEST BUTTON
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A test button is provided on all RCDs to enable the operation of the device to
be checked.
It is recommended that an RCD is tested at least quarterly. (See BS7671)
Regulation 514-12-02.
RCDs
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APPLICATIONS
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Residual Current Devices (RCDs) may be required for one of two main
reasons:
(
a) to ensure the compliance of an installation with BS7671.
An RCD may be installed to meet the requirements where a high earth fault
loop impedance disqualifies the use of overcurrent protection devices as a
means of providing protection against indirect contact. To comply with
Regulation 413-02-16 the earth fault loop impedance in ohms multiplied
by the rated tripping current of the RCD in amperes must not exceed 50.
With the RCD having a sensitivity of 30mA, the maximum permissible
earth fault loop impedance is calculated as follows:
Z
S
(
max)=50/0.03=1666 Ohms
Rated tripping current
Max permissible
of RCD
earth fault loop impedance
30
mA
1666
Ohms
100
mA
500
Ohms
300
mA
166
Ohms
(
b) to provide a higher degree of protection than that given by direct
earthing, against fire or shock risks caused by earth leakage currents.
Overcurrent protection devices cannot detect earth fault currents below their
operating current. If they are the only means of earth fault protection, it is
possible for sufficient earth fault current to flow undetected to constitute a
fire risk.
By using an RCD, the flow of the sustained earth fault current, above the
tripping current of the RCD, is prevented. The shock risk associated with these
earth fault currents is also greatly reduced.
For personnel protection, a high sensitivity RCD with a maximum tripping
current of 30mA should be used. This is particularly important with portable
appliances where there is a danger of losing earth continuity due to damage
or fatigue.
Residual current devices are completely selective in their operation. They are
unaffected by parallel earth paths and are thus ideally suitable for the
protection of installations in modern high density dwellings or office blocks.
They are virtually tamperproof and provide a predetermined level of
protection. Even if earthing conditions deteriorate substantially, they will
continue to provide a higher degree of protection than would have been
given by direct earthing.
SENSITIVITIES
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The choice of RCD depends upon the application of the degree of protection
required.
300
mA
provide the means to achieve compliance with the Wiring Regulations
in conditions of poor earth loop impedance and also give a good level of fire
risk protection.
100
mA
provide the means to achieve compliance with the Wiring
Regulations, a high level of fire risk protection and a degree of indirect shock
risk protection.
30
mA
for use where a higher degree of protection is required, with portable
equipment or equipment used in hazardous conditions. BS7671 regulation
471-16-01
indicates that where a socket outlet may reasonably be expected
to supply equipment to be used outside the zone, protection shall be afforded
by a residual current device having a rated residual operating current not
exceeding 30mA.
If using RCDs in series, discrimination can only be achieved by using Type S
devices in series with Types A or AC. See chart below.
TRANSIENT EARTH LEAKAGE CURRENTS
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All Crabtree residual current devices incorporate a high degree of immunity
to tripping when subjected to transient earth leakage currents.
Such transients can occur when there is a significant level of capacitance to
earth as can result from cable capacitance (particularly MICC) or RF filter
networks. Crabtree RCDs are therefore less susceptible to nuisance tripping
due to transient earth leakage currents.
Test button
Test resistor
Incoming
supply
Load
circuits
Trip
relay
L1
N
L1
N
L2
L3
L2
L3
Test button
Test resistor
Incoming
supply
Load
circuits
Trip
relay
L
N
L
N
RCD circuit diagram (double pole)
Note
Exposed installation
metal work must be
earthed.
Note
Exposed installation
metal work must be
earthed.
RCD circuit diagram (triple pole and neutral)
BSEN61008-1:1995
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Rated Residual Current (In)
Tripping
Type of RCD RCD to Trip between
times
50%-100%
I
n
1
x
I
n
2
x
I
n
5
x
I
n
500
Amps
Scope
Standard
Any Value, eg 10, 30, 100mA 300ms 150ms 40ms
40
ms
Maximum-Trip
A & AC
Time Delay
Greater than 30mA, eg 100mA
500
ms 200ms 150ms
150
ms Maximum-Trip
(
S)
130
ms 60ms 50ms
40
ms Minimum-Non Trip