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It is the smaller size which can be achieved
which enables the electronic designs to be incorporated in
the above units.
RCDs detect fault currents leaking to
earth either through metalwork or unfortunately in some cases
through a person's or animal's body.
Because of the way in which they work
they can detect very small currents which cannot be achieved
by any other device and provide the only practical method
of protecting against the effects of these small currents.
If a person comes into contact with metal which
is carrying electric current and at the same time is earthed
either through the other hand or the feet, a current will
flow through the body dependant on the resistance of the body
and the voltage of the supply. At 240 volts this current could
vary between 240 thousandths of an amp. shortened to 240 milliamps
and 80 milliamps.
For more details on the effect of ELECTRIC
SHOCK click here
Firstly to have a low
enough voltage that a dangerous current could not flow though
the body. This would be 50 volts or below. Although this is
used in special cases it is not practical for normal domestic
or commercial premises.
Secondly to prevent people coming
into contact with dangerous voltages. This is the object of
barriers and enclosures. As we know from experience these
are not foolproof.
The third and only practical way
on mains voltage is the RCD which will detect that
a fault current is leaking to earth and then cut off the supply
rapidly enough to prevent dangerous effects.
For details of
how RCD's/RCCB's operate click here
You will recall earlier that
the level of current which flows through the body in the event
of a shock situation is between 80 mA and 240 mA. It is essential
that the detection of the RCD is below the minimum anticipated
current through the body and in fact the recommended tripping
current for shock protection is a maximum of 30 mA and
this is the current recommended in the Wiring Regulations.
A tripping level of 100mA will
a give degree of shock protection if it is not possible to
use a 30 mA device.
A 300mA device should never be
recommended for shock protection and is only intended for
equipment and fire protection.
A 10 mA RCD should be used for
110 volt supplies because levels of less than 30mA could flow
through a body in the event of a fault. A 10 mA device is
sometimes recommended when the likelihood of shock is increased
such as when people are working with live equipment
However at 10mA the likelihood of unwanted
operation could increase at 240 volts so the 30 mA RCD should
be used if possible.
The British Standard requirements for
RCDs states that the RCD should operate between 50% and 100%
of its rated tripping current. That is 15 mA and 30 mA for
the 30 mA RCD. Most 30 mA RCDs operate at levels between 18
mA and 23 mA.
The Wiring Regulations recognize two ways
to receive an electric shock. Firstly, indirect contact where
a shock is received from metalwork made live by a fault and
direct contact where a shock is received from metal intended
to carry current such as a cable or electrical element.
In the case where a RCD is intended to
provide indirect contact protection only, it is required to
trip in less than 200 milliseconds at rated tripping current.
In addition, for direct contact protection the RCD must trip
in less than 40 milliseconds when 5 times rate tripping current
is applied.
The only suitable instrument for testing
a RCD is a RCD tester. A loop impedance tester cannot
give an indication of the correct operation of a RCD and must
never be used.
One RCD which does not comply with the
requirements just discussed is the time delay RCCB. This device
is time delayed or slugged so that it does not commence to
operate for a predetermined time for instance 50 milliseconds
which will allow downstream instantaneous RCDs to clear the
supply without affecting the TD RCCB.
The time delay RCD is ideal in situations
where the fastest tripping times are not required such as
fire protection but must never be recommended for shock protection.
The time delay RCCB must always be of lower sensitivity than
the instantaneous RCD it is feeding in order to guarantee
discrimination.
There are a number of queries
which arise on RCDs.
Firstly the use of RCDs on reduced voltage
such as 110 volts for a 240 volt RCD.
In the case of electronic RCDs they can
only be used at their stated voltage because of the requirements
of the electronic circuit.
As far as protection in the event of an
earth fault is concerned on electromagnetic RCCBs there is
no problem since the operation is dependant on residual current
only. However the test circuit has to generate sufficient
current to trip the RCCB and this is dependant on voltage.
Generally a 240 volt RCD will operate
on the test button down to 110 volts. However according to
BS4293 the test button is required to operate at 85% plus
and minus 5% of the rated voltage. That is 192 volts for a
240 volt supply and 88 volts for a 110 volt supply. This latter
operation cannot be guaranteed so although a 240 volt RCD
will operate at 110 volts you cannot claim compliance with
IEC1008 at that voltage.
RCDs are designed to operate at specific
frequencies; in the case of the standard devices 50/60 Hz.
They can be used at higher or lower frequencies but their
tripping characteristics will change and they therefore cannot
be recommended.
RCCBs to the latest standards are resistant
to transient surges up to 250 amps over a period of 30 microseconds,
and are marked accordingly.This makes them a lot more resistant
to spikes created by lawnmowers and mixers etc.
RCDs above 30 amps rating are tested
for an endurance of 2000 operations which is a mixture of
manual switching, test button and out of balance.
RCDs have a fault breaking capacity of
3000 amps on their own., 10000 amps if used with a M10 MCB
and 10000 amps if backed up with a BS88 fuse of 100 amps maximum
rating.
This then is the basic technical information
on RCDs.We will now look at applications and the types of
problems which can occur once the RCDs are installed.
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