INSPECTION
AND TESTING OF ELECTRICAL INSTALLATIONS
RESIDUAL CURRENT DEVICES
1. What is an RCD and what does it do?
An RCD
is defined, in BS 7671, as: ‘A mechanical switching device or association
of devices intended to cause the opening of the contacts when the
residual current attains a given value under specified conditions.
An RCD is a protective device used to automatically disconnect the electrical
supply when an imbalance is detected between live conductors. In the case of a
single-phase circuit, the device monitors the difference in currents between
the phase and neutral conductors. In a healthy circuit, where there is no earth
fault current or protective conductor current, the sum of the currents in the
phase and neutral conductors is zero. If a phase to earth fault develops, a
portion of the phase conductor current will not return through the neutral
conductor. The device monitors this difference, operates and disconnects the
circuit when the residual current reaches a preset limit, the residual
operating current (IΔn). RCDs are used to provide protection against the
specific dangers that may arise in electrical installations including protection
against indirect contact supplementary protection against direct contact
protection against fire and thermal effects An RCD on its own does not provide
protection against overcurrent's. Overcurrent protection is provided by a fuse
or a miniature circuit-breaker (MCB). However, combined RCD and MCBs are
available and are designated RCBOs.
2.
Types of RCDs
RCD is
the generic term for a device that operates when the residual current in the
circuit reaches a predetermined value. The list above indicates the different
types of RCD available, a description of each device and examples of how the
device is used.
2.1
Older installations with ELCBs
Historically,
two basic types of earth leakage circuit-breaker (ELCB) were recognized by the
Regulations; the familiar current-operated type and the earlier
voltage-operated type. The
voltage-operated
type ceased to be recognised by the Regulations in 1981 and today, only the
current-operated type is recognised. The voltageoperated device can be
distinguished by its two separate earthing terminals – one for the connection
of the earthing conductor of the installation
and the
other for a connection to a means of earthing. Such devices were often used on
installations forming part of a TT system where the means of earthing was an
earth electrode. The major drawback with the voltage operated earth leakage
circuit-breaker is that a parallel earth path can disable the device.
2.2
Recognised devices
RCDs are
manufactured to harmonised standards and can be identified by their BS EN
numbers. An RCD found in an older installation may not provide protection in
accordance with current standards. The following list identifies the applicable
current standards:
BS 4293: 1983 (1993)
Specification
for residual current operated circuit-breakers. (Replaced by BS EN 61008-1:
1995, BS EN 61008-2-1: 1995 and BS IEC 61008-2-2: 1990). This Standard remains
current.
BS 7071: 1992 (1998)
Specification
for portable residual current devices.
BS 7288: 1990 (1998)
Specification
for socket-outlets incorporating residual current devices. (SRCDs)
BS
EN 61008-1 : 1995 (2001)
Residual
current operated circuit-breakers without integral overcurrent protection for household
and similar uses (RCCBs)
BS
EN 61009-1 : 2004
Residual
current operated circuit-breakers with integral overcurrent protection for household
and similar uses (RCBOs)
2.3
Characteristics of RCDs
RCDs are
defined by a series of electrical characteristics; three main characteristics
are:
2. The rated residual operating current of the protective device in amperes.
3. Whether the device operates instantaneously or incorporates an intentional time delay to permit
discrimination.
Such devices are called ‘S’ or Selective.
Devices
are manufactured with different values of rated current and rated residual operating current, but we will just consider the rated residual operating current of the
protective device in amperes.
3.
Applications
The
correct device must be selected for the particular application. Choosing the
wrong device could have serious consequences and could result in electric shock
or fire. The list overleaf gives examples of particular applications of RCDs
and includes references to the relevant
Regulations
in BS 7671.
3.1
Unwanted tripping
Unwanted
tripping of RCDs can occur when a protective conductor current or leakage
current causes unnecessary operation of the RCD. An RCD must be so selected and
the electrical circuits so subdivided that any protective conductor current
that may be expected to occur during normal operation of the connected load(s)
will be unlikely to cause unnecessary
tripping
of the device (Regulation 531- 02-04 refers). Such tripping can occur on
heating elements, cooking appliances etc., which may have elements that absorb
a small amount of
moisture
through imperfect elementend seals when cold. When energised, this moisture
provides a conductive path for increased leakage and could operate the RCD. The
moisture dries out as the element heats up. Although not precluded in BS 7671,
it is not a requirement to use an RCD on such circuits if other satisfactory
means of protection are available. Providing an
RCD with
a higher rated residual operating current may solve the problem but the
requirements of the Regulations would still have to be met.
3.2
Discrimination
Where
two, or more, RCDs are connected in series, discrimination must be provided, if
necessary, to prevent danger (Regulation 531-02-09 refers). During a fault,
discrimination will be achieved when the device electrically nearest to the
fault operates and does not affect
other
upstream devices. Discrimination will be achieved when‘S’ (Selective) types are
used in
conjunction
with downstream general type RCDs. The ‘S’ type has a built-in time delay and
provides discrimination by simply ignoring the fault for a set period of time
allowing more sensitive downstream devices to operate and remove the fault. For
example, when two RCDs are connected in series, to provide discrimination, the
first RCD should be an ‘S’ type. RCDs with built in time delays should not be
used to provide personal protection.
4.
Labelling
Regulation
514-12-02, states that: “Where an installation incorporates a residual
current device a notice shall be fixed in a prominent position at or
near the origin of the installation. The notice shall be in
indelible characters not smaller than those here illustrated and shall
read as follows:”
5.
Testing
RCDs must be tested. The requirements are stated in the following Regulations:
a. The effectiveness of the RCD must be verified by a test simulating an appropriate fault condition and independent of any test facility, or test button, incorporated in the device (Regulation 713-13-01).
b. Where
an RCD of 30mA provides supplementary protection the operating time must not exceed
40 ms at a residual current of 5 IΔn. (Regulation 412-06-02 refers) Tests are
made on the load side of the RCD between the phase conductor of the protected
circuit and the
associated
cpc. Any load or appliances should be disconnected prior to testing. RCD test
instruments require a few milliamperes to operate; this is normally obtained
from the phase and
neutral
of the circuit under test. When testing a three-phase RCD protecting a
three-wire circuit, the instrument’s neutral is required to be connected to
earth. This means that the test current will be increased by the instrument
supply current and will cause some devices to operate
during
the 50% test, possibly indicating an incorrect operating time. Under this
circumstance it is
necessary
to check the operating parameters of the RCD with the manufacturer before
failing the RCD.
5.1
Range of tests
While
the following tests are not a specific requirement of BS 7671, it is recommended
that they are carried out.
5.2
Integral test device
An integral
test device is incorporated in each RCD. This device enables the mechanical
parts of the RCD to be verified by pressing the button marked ‘T’ or ‘Test’.
6.
Test Instrument
The test
instrument used to test RCDs should be capable of applying the full range of
test current to an in-service accuracy, as given in BS EN 61557-6. This
in-service reading accuracy will include the effects of voltage variations
around the nominal voltage of the tester. To check RCD operation and to
minimise danger during the test, the test current should be applied for no
longer than 2s. Instruments conforming to BS EN 61557-6 will fulfil the above
requirements.
Device Instrument test current setting Satisfactory result General purpose RCDs to BS 4293 and RCD protected socket-outlets to BS 7288
100% of operating current Device should operate in less than 200ms. Where the RCD incorporates an intentional time delay it should trip within a time range from 50% of the rated time delay plus 200ms’ to 100 % of the rated time delay plus 200ms
General
purpose RCCBs to BS EN 61008 or RCBOs to BS EN 61009
100% of operating current Device should operate in less than 300ms unless it is of
‘Type S’ (or selective) which incorporates an intentional time delay. In this case, it should trip within a time rangeb from 130ms to 500ms
Supplementary
protection against direct contact IΔn
≤ 30mA
Test
current at 5 IΔn the maximum test time must not be longer than 40ms, unless the
protective conductor potential does not exceed 50V. (The instrument supplier
will advise on
compliance).
This
installation, or part of it, is protected by a device which automatically
switches off the supply if an earth fault develops.
Test
quarterly by pressing the button marked ‘T’ or ‘Test’. The device should switch
off the supply and should then be switched on to restore the supply. If the
device does not switch off the supply when the button is pressed, seek expert
advice.
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