Residual Current Devices
Selection and Nuisance tripping
Why’s and wherefore’s
Gary Busbridge
February 2014
The Residual Current Device
●The RCD is a device that disconnects a circuit
whenever it detects that the electric current is
not balanced between the energised conductor
and the return neutral conductor.
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Designed to Trip
●Should the primary means of protection, the
insulation, fail, the RCD is designed to trip
before…
a set time and at a current so low as to
prevent permanent injury or death from an
electric shock.
●Tripping may be as low as half the related
sensitivity but must trip at its rated sensitivity.
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An RCD or a Safety Switch?
●Introduced into the market as an Earth Leakage
Circuit Breaker.
●Heavily promoted to the installers and
standards groups as a supplementary safety
protection
●Entrepreneurial retailer in SA dubbed the unit
as a Safety Switch so that he could penetrate
the consumer market
●Standards and industry speak now refers to the
units as RCD’s
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“Normal” Residual Current
●A sustained residual current which is above the
predetermined level.
●Using a real-life scenario, the leakage on a
circuit made up of a fridge, freezer and electric
kettle
●Each of these may have its own sustained
residual current and when added together could
be above the threshold of the RCD and..
as such will trip the breaker, ceasing the
flow of that residual current.
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The Residual Current Device
A path to Earth
thru tapware
Human body becomes
path to Earth
Appliance becomes faulty
RCD on socket
or switchboard
●30mA and 300ms is the cutoff before heart
fibrillation
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The Residual Current Device
A path to Earth
thru tapware
Human body becomes
path to Earth
Appliance becomes faulty
RCD on socket
or switchboard
●10mA and 40ms is the cutoff before muscular
contraction
Schneider Electric - Division - Name – Date
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RCD’s & Protection
●The use of RCDs with a residual current not
exceeding 30 mA, is recognized as providing
additional protection.
●The use of RCDs is intended only to augment
other measures of basic protection.
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RCD’s & Protection
●RCDs do not provide protection against:
Faults between live conductors
Voltages imported into the installation
earthing system through the supply neutral.
●The use of such devices is not recognized as a
sole means of protection and does not obviate
the need to apply the protective measures
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Disturbances : nuisance tripping
earth-leakage protection can operate
incorrectly and cause:
• Power outages,
• production downtimes or data losses
• Lighting outages
• End-user dissatisfaction
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“Abnormal” Residual Current
●Any residual current that flows after an initial
surge or impulse and then automatically ceases
is a non-sustained residual current.
●This is not associated with insulation
breakdown and therefore not sustained, and…
is largely due to lightning surges, reactive
load switching and momentary flows to earth.
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General principle of nuisance tripping
●Although not dangerous, a high
frequency current is seen as a fault
by the residual current circuitbreaker
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Causes of the phenomenon
●It seems that electro-magnetic compatibility
can be partly attributed to this recent
phenomenon as..
manufacturers are including mains input
filters in their electronic equipment and..
High frequency ballasts into fluorescent
lighting.
●Manufacturers of these devices are
capacitively coupling current to earth to
reduce radio frequency signals emitted into
the atmosphere.
Schneider Electric - Division - Name – Date
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Causes of the phenomenon
●The switch-on phase in appliances provide a
brief period where the capacitors are passing
more leakage to earth than usual or..
in the case of lighting using electronic
starters, there is an ignition current of hundreds
of amperes with a duration of a few
milliseconds..
which result in large capacitive current flow
to earth and a lot of harmonic noise for a similar
period.
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Causes of the phenomenon
●This current is not returning to earth through
the RCD via the neutral but still causes the RCD
to see a difference in the current out and
current in.
●Often the difference exceeds the allowed
threshold and hence it trips the breaker.
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What does the Installer need to
understand?
●The installer must understand the practical
requirements of the circuits being installed and
consider the effects of this residual
current in relation to circuit protection.
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The phenomenon and the effect on
computers
●Any computer, whether new or old, may have a
residual current of between 1 and 2mA.
●It would seem that the number of computers
that may be connected to an RCD may be
between 7 and 10.
●Public Schools Departments in Australia have
limited the number of computers on one RCD to
seven.
Schneider Electric - Division - Name – Date
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Microcomputers, servers, data
centers
●Each PC amplifies the sensitivity of the RCD;
the more PCs there are, the greater the risk of
tripping during high-frequency interference
1.5 mA
30 mA
13 mA
Schneider Electric - Division - Name – Date
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The phenomenon and effect on
lighting
●Between 0.3mA and 1mA can be expected on
any electronic ballast for fluorescent lighting
●It would seem that the maximum number of
fluorescent lamps with ballasts protected by
one RCD should be limited to 15 and 20.
●Consideration should be given to battens with
more than one fluorescent lamp fitted, as each
tube carries its own ballast.
Schneider Electric - Division - Name – Date
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Very low voltage
fluorescent lighting
●When banks of lighting units are turned off,
the high-frequency oscillations will be seen
as a fault
200 pF/m
100 kHz - 10 MHz
Transformer
Lighting unit
1
nF
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Fluorescent lighting with electronic
ballast
●Upwards of 20 lighting units, there is a risk of
tripping due to the stray capacitances
10 -100
kHz
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The Phenomenon and effect on other
devices
●Similar principles should apply to other
devices but the non-sustained residual current
may vary..
and will need to be checked before
determining the maximum number of devices
to be protected by any one RCD.
●Be aware that even though the residual
current on any device may not be sufficient to
trip the RCD it may have an overall effect on
the background leakage.
Schneider Electric - Division - Name – Date
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EarthEarth-leakage protection
downstream a UPS
●Slow and damped oscillations may cause
nuisance tripping
N
U
n
100 V
U
n
-100 V
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Activating a capacitor bank
●Activating the capacitor bank causes an
overvoltage with high-frequency oscillations,
which can cause nuisance tripping
I∆
I
∆
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U
U
U
1000
V
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Variable speed drives with frequency
converters
●Square voltage modulated between 2 and 15
kHz according to the frequency of the motor:
This presents no danger, but the personnel
protection devices may trip
2-20 kHz
EMC
M
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Lightning
●The lightning pulses flow to ground, and the
RCD will trip
10 kA
50 µs
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Quantified Effect
1mA
2mA
0.5mA
1mA
0.5mA
0.75mA
0.5mA
1mA
0.5mA
1.5mA
Around 1mA
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What does the Installer need to do?
●Be aware of the type of load being put on the
circuit.
●Divide the circuits to prevent a surplus of load
on any one circuit.
●Limit the number of ballasts in the lighting
circuit.
●Use a selective type RCD (Class A Type Si) on
those circuits that may be affected. (These types
of RCD’s are specifically designed to be less
sensitive to the effect of high frequency
disturbances whilst at the same time ensuring
personal safety.)
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Si type RCDs
1 - Current
transformer (toroid)
2 Signal processing
devices
3 Electromech.
actuator
SN
SI
Transient
filter
- waveform
types :
8/20 µs
- 0,5µs
100 kHz
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Pulsed
current
detector
HF filter
Energy
accumulator
Trip
signal
check
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Common Myths
●Testing instruments are required to check the
activation of an RCD.
WRONG! Field testing has verified that the Test button operation
is sufficient
●RCD’s cannot be used to protect Cookers
because of nuisance tripping
WRONG! There is now sufficient evidence from Cooker
manufacturers that this is not a problem
●RCD’s protect against active to neutral faults
WRONG! Common misconception amongst home-owners
Schneider Electric - Division - Name – Date
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Common Myths
●The new rules for 3 circuits per RCD are
sufficient
RIGHT & WRONG! It’s safe enough to install in this manner but
how do you fault find without having 1 circuit per RCD
●RCD’s have been added to lighting circuits
and if more than one then shared across
RCD’s for extra safety
RIGHT & WRONG! It’s helps keep the ceiling cavity safe but it
also ensures that if one RCD trips then there is still a chance that
the light circuit on the other is still working. Elderly concern!
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In Summary
●Being more aware of the practical
requirements of the circuit being designed and
installed..
●May well be the nuisance value in the future..
and perhaps go a long way in negating the
nuisance trip problems that installers and
designers face today.
Schneider Electric - Division - Name – Date
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