-
..........................................................................Collection
Technique
Cahier technique n 167
Energy-based discrimination forlow-voltage protective
devices
M. SerpinetR. Morel
n Merlin Gerin n Modicon n Square D n Telemecanique
-
Cahiers Techniques are a collection of documents intended for
engineersand technicians people in the industry who are looking for
information ingreater depth in order to complement that given in
display productcatalogues.
These Cahiers Techniques go beyond this stage and constitute
praticaltraining tools.They contain data allowing to design and
implement electrical equipement,industrial electronics and
electrical transmission and distribution.Each Cahier Technique
provides an in-depth study of a precise subject inthe fields of
electrical networks, protection devices, monitoring and controland
industrial automation systems.
The latest publications can be downloaded on Internet from
theSchneider server.code: http://www.schneiderelectric.comsection:
mastering electrical power
Please contact your Schneider representative if you want either
a CahierTechnique or the list of available titles.
The Cahiers Techniques collection is part of the Groupe
SchneidersCollection Technique.
ForewordThe author disclaims all responsibility further to
incorrect use of informationor diagrams reproduced in this
document, and cannot be held responsiblefor any errors or
oversights, or for the consequences of using informationand
diagrams contained in this document.
Reproduction of all or part of a Cahier Technique is authorised
with theprior consent of the Scientific and Technical Division. The
statementExtracted from Schneider Cahier Technique no..... (please
specify) iscompulsory.
-
Robert MOREL
Graduated with an engineering degree from ENSMM in Besanon
andjoined Merlin Gerin in 1971. Specialised in designing low
voltageswitchgear and participated in designing the Sellim
system.In 1980, took over development of Compact circuit-breakers
andInterpact switches.In 1985, became manager of the Low-Voltage
Current Interruptiondesign office in the Low-voltage Power
Components division.
n 167Energy-based discriminationfor low-voltage
protectivedevices
ECT167 first issued, march 1998
Marc SERPINET
Joined Merlin Gerin in 1972 and worked until 1975 in the
low-voltageequipment design offices, in charge of designing
electrical cabinetsfor various installation layouts. Since 1975, he
has managed researchand development testing for low-voltage
circuit-breakers. Graduatedin 1981 from the ENSIEG engineering
school in Grenoble.In 1991, after managing a Compact
circuit-breaker project from thepreliminary studies on through to
production, he was appointed headof the electromechanical design
office in charge of anticipatingfuture developments.
-
Cahier Technique Schneider n 167/ p.2
Lexicon
EbEnergy let through by the protective deviceduring breaking.
This energy is characterised by
i dt tb b2 2
IibLimited short-circuit current actually flowingthrough the
circuit-breaker (the break current isless than Ip).
IpProspective short-circuit current that woulddevelop in the
absence of protective devices(rms value).
IrCorresponds to the overload protection setting.
tbThe actual breaking time (arc extinction).
UTElectronic processing unit.
ActuatorDevice capable of producing a mechanicalaction.
Circuit-breaker ratingCorresponds to the models of the range(ex.
160 A, 250 A, 630 A, 800 A, etc.).
Current-limiting circuit-breakerCircuit-breaker which, when
interrupting a short-circuit current, limits the current to a
valueconsiderably less than the prospective current (Ip).
High-set instantaneous release (HIN)Instantaneous release used
to limit thermalstress during a short-circuit.
Instantaneous release (INS)Release without an intentional time
delaysystem. It trips at a low multiple of In to
ensureshort-circuit protection.
Long-time release (LT)Release with an intentional time delay
system(several seconds) for overload protection.
Partial discriminationDiscrimination is said to be partial when
it isensured only up to a certain level of theprospective current
(Ip).
Selective circuit-breakerCircuit-breaker with an intentional
time delaysystem (time discrimination).
Short-time release (ST)Release with an intentional time delay
systemranging from ten to several hundredmilliseconds. If the time
delay is reduced as Ipincreases, the system is referred to
asdependent short-time (DST).
Total discriminationDiscrimination is said to be total when it
is ensuredfor all values of the prospective fault current.
Trip-unit ratingCorresponds to the maximum current setting ofthe
trip unit.
-
Cahier Technique Schneider n 167 / p.3
Energy-based discrimination forlow-voltage protective
devices
The purpose of this Cahier Technique publication is to present
the newenergy-based discrimination technique that ensures tripping
discriminationbetween protective devices during a short-circuit.
Both simpler and moreeffective than standard discrimination
techniques, it has been implementedon the Compact NS range of
circuit-breakers used in low-voltage powerdistribution networks.
Discrimination is ensured for all prospective faultcurrents on the
condition that upstream and downstream circuit-breakershave
different current ratings (ratio u 2.5) with a trip-unit rating
ratio u 1.6.Following a brief review of standard discrimination
techniques, the authorsexamine the behaviour of circuit-breakers
and various trip units from theenergy standpoint.They then
demonstrate that total discrimination is possible up to
thecircuit-breaker breaking capacity, over several levels, without
using timediscrimination techniques.
Contents1. Discrimination in low-voltage 1.1 Definition p. 4
1.2 Enhanced safety and availability p. 51.3 Discrimination
zones p. 5
2. Discrimination techniques for short-circuits 2.1 Current
discrimination p. 72.2 Time discrimination p. 72.3 SELLIM
discrimination p. 82.4 Zone selective interlocking p. 92.5
Combining the different types of discrimination p. 9
3. Energy-based discrimination 3.1 Choice of operating curves p.
103.2 Characterisation of a Compact NS circuit-breaker p. 113.3
Characterisation of the trip units p. 13
4. Advantages and implementation of 4.1 Current-limiting
circuit-breaker fitted with a pressure trip system p. 164.2
Discrimination with Compact NS circuit-breakers p. 184.3
Combination with traditional protective devices p. 19
5. Conclusion p. 216. Appendix - indications concerning breaking
with current limiting p. 22
energy-based discrimination
protective devices
-
Cahier Technique Schneider n 167 / p.4
1 Discrimination in low-voltage protective devices
1.1 DefinitionIn an electrical installation, loads are
connectedto sources via a succession of protection,isolation and
control devices. This CahierTechnique publication deals essentially
with theprotection function using circuit-breakers.In a radial
feeder layout (see fig. 1 ), the purposeof discrimination is to
disconnect only the faultyload or feeder from the network and no
others,thus ensuring maximum continuity of service.If
discrimination studies are not or are incorrectlycarried out, an
electrical fault may cause severalprotective devices to trip, thus
provoking aninterruption in the supply of power to a large partof
the network. That constitutes an abnormal lossin the availability
of electrical power for thoseparts of the network where no fault
occurred.
Fig. 1: several circuit-breakers are concerned by the fault
If.
Several types of overcurrents may beencountered in an
installation:c overloads,c short-circuits,c inrush currents,as well
as:c earth faults,c transient currents due to voltage dips
ormomentary loss of supply.To ensure maximum continuity of service,
theremust be coordination between protectivedevices.Note that
voltage dips may provoke unnecessaryopening of circuit-breakers by
actuatingundervoltage releases.
ACB1
CB3
CB4
BCB2
If
If passes through CB1, CB2, CB3, CB4.
-
Cahier Technique Schneider n 167 / p.5
Fig. 2: circuit-breaker behaviour during a fault.
1.2 Enhanced safety and availabilityA specific type of
protective device exists foreach type of fault (overloads,
short-circuits, earthfaults, undervoltages, etc.). However, a fault
maysimultaneously bring several types of protectivedevices into
play, either directly or indirectly.
Examplesc A high short-circuit current creates anundervoltage
and may trip the undervoltageprotective device.c An insulation
fault may be interpreted as azero-phase sequence fault by an
earth-leakageprotective device and as an overcurrent by
theshort-circuit protective device (applicable for TNand IT
earthing systems).c A high short-circuit current may trip the
earth-leakage protective device (in TT earthingsystems) due to
local saturation of thesummation toroid which creates a false
zero-phase sequence current.For a given network, discrimination
studies andthe evaluation of the protection system in generalare
based on the protective devicecharacteristics published by the
manufacturers.
Studies begin with an analysis of requirementsconcerning
protective devices needed foreach type of fault. The next step is
an evaluationof coordination possibilities between theprotective
devices concerned by a given fault.The result is improved
continuity of service whilestill guaranteeing protection of life
and property.The following chapter will deal exclusively
withdiscrimination in the event of overcurrents(overloads and
short-circuits).In this context, the existence of
discriminationbetween circuit-breakers is determined quitesimply by
whether several circuit-breakers openor not (see fig. 2 ).Total
discriminationDiscrimination is said to be total if and only
if,among the circuit-breakers potentially concernedby a fault, only
the most downstream circuit-breaker trips and remains open, for all
faultcurrent values.
Partial discriminationDiscrimination is said to be partial if
the abovecondition is no longer valid for fault currentsexceeding a
certain level.
CB2
CB1
CB2
CB1
a) CB1 and CB2 open. discrimination is not ensured, i.e. power
is notavailable for the feeders where no fault occurred.
b) CB1 opens CB2, remains closed. discrimination is ensured,
i.e. power is available for thefeeders where no fault occurred
(continuity of service).
1.3 Discrimination zonesTwo types of overcurrent faults may
beencountered in an electrical distribution network:c overloads,c
short-circuits.Overcurrents ranging from 1.1 to 10 times the
ser-vice current are generally considered as overloads.
Overcurrents with higher values are short-circuitsthat must be
cleared as rapidly as possible byinstantaneous (INS) or short-time
(ST) releaseson the circuit-breaker.Discrimination studies are
different for each typeof fault.
-
Cahier Technique Schneider n 167 / p.6
Overload zoneThis zone starts at the ILT operating threshold
ofthe long-time (LT) release. The tripping (or time-current) curve
tb = f (Ip) is generally of theinverse-time type to remain below
thepermissible thermal stress curve of the cables.Using the most
common method, the curves ofthe LT releases concerned by the fault
are plottedin a system of log-log coordinates (see fig. 3 ).For a
given overcurrent value, discrimination isensured during an
overload if the non-trippingtime of the upstream circuit-breaker
CB1 isgreater than the maximum breaking time(including the arcing
time) of circuit-breaker CB2Practically speaking, this condition is
met if theratio ILT1/ILT2 is greater than 1.6.
Short-circuit zoneDiscrimination is analysed by comparing
thecurves of the upstream and downstream circuit-breakers.The
techniques that make discriminationpossible between two
circuit-breakers during ashort-circuit are based on combinations
ofcircuit-breakers and/or releases of different typesor with
different settings designed to ensure thatthe tripping curves never
cross.A number of such techniques exist and arepresented in the
next chapter.
Overload discri-mination zone
ILT2 ILT1 I ins2
Ip
tb
Overloads Short-circuits
CB2 CB1
Fig. 3: overload discrimination.
-
Cahier Technique Schneider n 167 / p.7
2 Discrimination techniques for short-circuits
Several techniques can be used to ensurediscrimination between
two circuit-breakersduring a short-circuit:c current
discrimination,c time discrimination,
2.1 Current discriminationThis type of discrimination is the
result of thedifference between the thresholds of theinstantaneous
or ST releases of the successivecircuit-breakers.Applied primarily
in final distribution systems, it isimplemented using rapid
circuit-breakers notincluding an intentional tripping time-delay
system.It protects against short-circuits and generallyresults in
only partial discrimination.This form of discrimination is all the
moreeffective when the fault currents are different,depending on
where they occur in the network,due to the non-negligible
resistance ofconductors with small cross-sectional areas(see fig. 4
).The discrimination zone increases with thedifference between the
thresholds of theinstantaneous releases on circuit-breakers CB1and
CB2 and with the distance of the fault fromCB2 (low Isc < Iins
of CB1).The minimum ratio between Iins1 and Iins2 mustbe 1.5 to
take into account threshold accuracies.
2.2 Time discriminationTo ensure total discrimination, the
time-currentcurves of the two circuit-breakers must nevercross,
whatever the value of the prospectiveshort-circuit current. For
high fault currents, totaldiscrimination is guaranteed if the
horizontalsections of the curves to the right of Iins1 are notone
on top of another.Several solutions may be implemented toachieve
total discrimination:c the most common involves installing
selectivecircuit-breakers including an intentional time-delay
system,c the second applies only to the last distributionstage and
involves using current-limiting circuit-breakers.
Fig. 4: current discrimination.
Short-circuitdiscrimination limit
Iins2
Ip
tb
Iins1
Short-circuitdiscrimination zone
CB2 CB1
Use of selective circuit-breakersThe term selective means that:c
the circuit-breaker trip unit has a fixed oradjustable time-delay
system;c the installation and the circuit-breaker canwithstand the
fault current for the duration of theintentional time delay
(sufficient thermal andelectrodynamic withstand capacities).A
selective circuit-breaker is generallypreceded in the network by
another selectivecircuit-breaker that has a longer intentional
timedelay.Use of this type of circuit-breaker, correspondingto time
discrimination solutions, results in totalbreaking times greater
than 20 ms (one period)
c SELLIM discrimination,c zone selective interlocking,c
energy-based discrimination (see chapters 3and 4).
-
Cahier Technique Schneider n 167 / p.8
in the event of a fault. This figure may run up to afew hundred
milliseconds (see fig. 5 ).When the installation (and perhaps even
thecircuit-breaker) cannot withstand a high short-circuit current
(Isc) for the entire time delay,circuit-breaker CB1 must be
equipped with ahigh-set instantaneous release (HIN).In this case,
the discrimination zone is limited tothe high-set threshold of the
upstream circuit-breaker (see fig. 5 ).Use of current-limiting
circuit-breakers andpseudo-time discriminationThese
circuit-breakers have two maincharacteristics:
Fig. 5: time discrimination.
IDIN1
CB2
CB2 : rapidCB1 : selective with1-2-3 ST settings
tbCB1
Iins1
Installation and/or circuit-breaker thermal withstandcapacity
limit
Ip1
32
Note: use of a high-sed instantaneous releasedetermines the
discrimination limit.
Ip
CB2tb CB1 CB2 : rapid current limitingCB1 : rapid
Fig. 6: pseudo-time discrimination.
Note: use of dependent ST releases (dotted line) onCB1 improves
discrimination.
c they severely limit short-circuit currents due tofast opening
times and high arcing voltages,c the higher the prospective
short-circuit current,the faster they act.Use of a current-limiting
circuit-breakerdownstream thus makes it possible to
ensurepseudo-time discrimination between twoprotection levels. This
solution, due to thecurrent limiting effect and rapid clearing of
thefault, limits thermal and electrodynamic stressesin the
installation (see fig. 6 ).
2.3 SELLIM discrimination
Fig. 7: SELLIM discrimination.(CB1 - Compact C250 L SBCB2 -
Compact C125 N).
CB1
CB2
A
B
2.5 ms
i3
u3
i2
u2
i1
u1
26 k
Fault at B
i3
u3
i2
u2
i1
u1
3.5 ms12 ms
Fault at A
34 k
-
Cahier Technique Schneider n 167 / p.9
2.5 Zone selective interlocking
The SELLIM system offers a number ofadvantages: discrimination,
cascading, reducedstresses in the installation.Upstream from a
rapid circuit-breaker CB2, thesystem requires an ultra
current-limiting circuit-breaker CB1 fitted with a special release
thatdoes not trip during the first half-wave of the faultcurrent
(see fig. 7 ).A major fault at B is detected by both
circuit-breakers.CB2, equipped with an instantaneous release,opens
as soon as the fault current exceeds its
This technique requires data transmissionbetween the trip units
of the circuit-breakers atthe various levels in a radial feeder
network.The operating principle is simple (see fig. 8 ):c each trip
unit that detects a current greaterthan its tripping threshold
sends a logic waitorder to the next trip unit upstream,c the trip
unit of the circuit-breaker located justupstream of the
short-circuit does not receive await order and reacts
immediately.With this system, fault clearing times remain lowat all
levels in the network.Zone selective interlocking is a technique
usedwith high-amp selective LV circuit-breakers,though its main
application remains HV industrialnetworks. For further information,
refer to CahierTechnique Publication Number 2, entitledProtection
of electrical distribution networks bythe logic selectivity
system.
Fig. 8: zone selective interlocking.
Logicrelay
Logicrelay
Logic waitorder
CB1
CB2
2.6 Combining the different types of discriminationThe different
types of discrimination presentedabove are generally combined to
ensure thehighest degree of availability of electrical power.See
figure 9 for an example.Discrimination studies are still carried
out using thetables supplied by manufacturers. The tables indi-cate
the discrimination limits for each combinationof circuit-breakers
and for the various trip units.
The costs of non-discrimination and of thevarious devices
selected are taken into account.The energy-based discrimination
techniquepresented in the next chapter constitutes a trueinnovation
that will considerably simplify LVdiscrimination studies and make
possible totaldiscrimination over several levels at
minimumcost.
Fig. 9: example of uses for different types of
discrimination.
Circuitsconcerned Zone selective
interlocking
Powerdistribution
Finaldistribution
Type of discrimination Type ofcircuit-breakerTime Pseudo-time
SELLIM
SelectivelogicSelectiveRapid/currentlimiting SELLIMRapid
Head ofLV network
Rapid/currentlimiting
trip threshold and clears the fault in less than ahalf-period.
CB1 detects only a single currentwave and does not trip. The fault
currentnonetheless causes contact repulsion, thuslimiting the
current and the resulting stresses.This limiting of the fault
current means thatdownstream circuit-breakers may have
breakingcapacities less than the prospective fault current.A fault
at A causes repulsion of the contacts of thecurrent-limiting
circuit-breaker, thus limiting thestresses produced by the fault
current. CB1 opensafter the second half-wave of limited
current.
-
Cahier Technique Schneider n 167 / p.10
3 Energy-based discrimination
Energy-based discrimination is an improved andgeneralised
version of the pseudo-timetechnique described in the preceding
chapter.Discrimination is total if, for all values of Ip, theenergy
that the downstream circuit-breaker letsthrough is less than that
required to actuate thetrip unit of the upstream
circuit-breaker.The actual implementation of the energy-based
dis-crimination principle is covered by a Merlin Gerinpatent and
has been incorporated in the design ofthe new Compact NS range of
circuit-breakers.These rapid and highly current-limiting
circuit-breakers meet the rapidly evolving criteria of themarket
concerning:
c increases in installed power, which lead tohigher
short-circuit currents and correspondinglyhigher breaking
capacities;c the need to limit stresses in the installation aswell
as the level and duration of fault currents.When reasoning in terms
of energy and in orderto understand energy-based discrimination,
thechoice of the means of presenting the operatingcurves is
essential and the subject of the nextsection.Following that
discussion is an analysis of thebehaviour in terms of energy for
current-limitingcircuit-breakers and the various trip units.
3.1 Choice of operating curvesThe tb = f (Ip) curves commonly
used fordiscrimination studies are of no use with current-limiting
circuit-breakers when currents exceed25 In (breaking times are less
than 10 ms at afrequency of 50 Hz).Discrimination studies may no
longer be carriedout on the basis of periodic phenomena, butrather
require analysis of transient phenomena.An understanding of
energy-baseddiscrimination requires that the followingelements be
characterised:c the current wave that the circuit-breaker
letsthrough during breaking, which is characterisedby its Joule
integral i dt2 (often expressed asI2 t ), and corresponds to the
breaking energy Eb,c the sensitivity of the releases to the
energycorresponding to the current pulse.Thus, quite logically, the
above characteristicsare represented using I2 t = f (Ip)
curvesinstead of tb = f (Ip) curves (see fig. 10 ).It should be
noted that standard IEC 947-2specifies characterisation of
circuit-breakersusing such curves.For practical reasons the I2 t =
f (Ip) curve ispresented in a system of log-log coordinates.For
discrimination studies, the limits of thebreaking I2 t value (Eb
for circuit-breakers) arebetween 104 and 107 A2 s for
prospectivecurrents ranging from 1 to 100 kA. Three powersof ten
are therefore used for Eb and two for thecurrent.Assuming that the
half-wave of the interruptedcurrent is equivalent to half of a
sine-wave withthe same initial slope as the prospective current,the
breaking energy Eb may be expressed as afunction of Ip using the
following expressions
Fig. 10: tb = f (Ip) and I2 t = f (Ip) curves for a
circuit-breaker equipped with an electronic trip unit.
10 In 15 In 30 In
10 In
I2 t
INS
ST1ST2
t(s)
Ip(A)
LT
(A2 s)
Ip(A)
-
Cahier Technique Schneider n 167 / p.11
(see the appendix on breaking with currentlimiting):v for t u 10
ms(2) Eb = Ip2 tv for t < 10 ms(3) Eb = 4 f2 Ip2 tvb3or
(4)
bf p
3
4 2 IOn the basis of these equations, the Ip2 t / Ipsystem can
be improved, thus providing furtherinformation on the virtual
breaking time (tvb) andthe limited peak current value (b).Time
lines (see fig. 11 )A series of lines representing constant
breakingtimes can be included in the log-logrepresentation for a
given frequency.For example, when f = 50 Hz, the line for:c t = 20
ms corresponds to the most commonbreaking time when Ip is greater
than the
Fig. 11: graph representing energies.1
40 m
s20
ms
10 m
s
7 m
s
5 m
s
3 10 50 100
107
106
105
104
= 40 kA
= 20 kA
= 10 kA
Ip (kA) = 5 kA
I2 t
2.5 ms
(A2 s)
5 30
instantaneous thresholds and less than thecontact repulsion
threshold:(2) Eb = Ip2 x 2 x 10-2.c t = 10 ms is the breaking time
at the current-limiting threshold:(2) Eb = Ip2 x 10-2.c t = 9 to 4
ms which indicate circuit-breakerbehaviour when current
limiting:(3) Eb = Ip2 tvb3 x 104.Peak-current linesSimilarly, on
the basis of equation (4)Eb =
bf p
3
4 2 Ia series of lines corresponding to constant,limited peak
currents can be included in therepresentation (see fig. 11 ).It
should be noted that this method ofrepresentation makes it possible
to characterisecircuit-breakers and trip units at 50 Hz for
three-pole, two-pole and single-pole faults.
3.2 Characterisation of a Compact NS circuit-breakerDisplay of
the breaking I2 t valueThe I2 t values that a circuit-breaker
letsthrough are determined by standardised typetests or by computer
models run for a givenvoltage and frequency.
The curves presented here correspond to three-phase faults at
400V/50Hz.The same curves may be generated for othervoltages and
other frequencies. The indicatedvalues are the maximum values
obtained
-
Cahier Technique Schneider n 167 / p.12
irrespective of the moment at which the faultoccurs (upper
limits) (see fig. 12 ).Curve analysisA great deal of information is
available from thegraph in figure 12 which corresponds to a250 A
Compact NS circuit-breaker, equippedwith a dependent ST (DST)
electro-mechanicalrelease with a 10 In threshold.The information
characterises the differentphases in the breaking behaviour of the
current-limiting circuit-breaker depending on the value ofthe
prospective short-circuit current Ip.c Point A: when the fault
current reaches the tripthreshold of the release, the breaking time
istypically 50 ms for an INS or DST release.c Point B: when the
fault current is greater thanthe trip threshold of the release, the
breakingtime drops and stablises at 20 ms beginningat 16 In.c Point
C: when the fault current reaches thecontact repulsion level,
current limiting starts dueto the insertion of an arc voltage in
the circuit.Current limiting results in the return to
in-phaseconditions for the voltage and the current andconsequently
a drop in fault clearing timesfrom 20 ms to 10 ms as Ip
increases.
Fig. 12: breaking curve for a current-limiting
circuit-breaker.
c Point D: when the fault current reachesapproximately 1.7 times
the contact repulsionlevel, the energy is sufficient to totally
open thecontacts. At that point, the breaking time istypically 10
ms.This reflex-type breaking is autonomous and atrip unit is
required only to confirm the trippedstatus of the circuit-breaker
and avoid untimelyreclosing of the contacts.c Zone E: when the
fault currents runs beyond2 times the contact-repulsion level,
currentlimiting is increasingly effective and results
inincreasingly short breaking times.c Point F: the end of the curve
represents thebreaking capacity limit of the circuit-breaker.The
curve provides a great deal of information:c tripping threshold (I
threshold, point A);c breaking I2 t value as a function of
theprospective current;c contact-repulsion level (Ir, point C);c
breaking capacity (point F);c breaking time (tvb) as a function of
the prospec-tive current;c limited peak current (b) as a function
of theprospective current;c current value above which tvb < 10
ms(beginning of current limiting).
1
40 m
s20
ms
10 m
s
7 m
s
5 m
s
3 10 50 100
107
106
105
104
= 40 kA
= 20 kA
= 10 kA
Ip (kA) = 5 kA
2 t
2.5 ms
(A2 s)
5 30(10 In)
DC
(B)
(E)A F
-
Cahier Technique Schneider n 167 / p.13
3.3 Characterisation of the trip unitsTrip units are
characterised by their responsetime to a given current (full-wave,
half-wave,etc.).By modifying the duration and the peak value ofthe
current, which corresponds to the variouscurrents limited by a
circuit-breaker, a number oftests can be run to obtain a series of
pointswhich may be plotted on the previouslydescribed graph, thus
producing the curvecharacterising a trip unit.
Magnetic trip unitsc Instantaneous releaseGenerally made up of a
magnetic U and a blade,it ensures short-circuit protection. The
responsetime is under 50 ms at its operating threshold(between 5
and 10 times the rated current), thendrops rapidly to below 10 ms
when the currentincreases (see fig. 13 ).c High-set releaseAs
indicated in the time discrimination section,the role of high-set
releases in timediscrimination systems is to limit thermalstresses
(see fig. 5 ) in the installation and thecircuit-breaker.
The high-set release is an instantaneous unitwith a threshold of
15 to 50 In.The release may be either electro-mechanical
orelectronic.c Constant time-delay releaseThis is an instantaneous
release fitted with aclock-type time delay system intended to
maketripping selective with respect to the
downstreamcircuit-breaker.The time delay may range from 10 to 500
msand is generally set using notched dials.Figure 13 shows the
curve (20 ms setting) for ashort-time delay.If the thermal stess
(I2 t) resulting from a longtime delay must be limited, the
high-set releaseenters into play (see fig. 13 ).c Dependent time
delay release (function of Ip,dependent short-time - DST).The time
delay results from the inertia of a massand is therefore inversely
proportional to Ip(see fig. 13 ).Electronic trip unitsThe
instantaneous thresholds in electronic tripunits are sensitive to
the rms value or the peak
Fig. 13: curves for various magnetic releases.
1
40 m
s20
ms
10 m
s
7 m
s
5 m
s
3 10 50 100
107
106
105
104
= 40 kA
= 20 kA
= 10 kA
Ip (kA) = 5 kA
2 t
2.5 ms
(A2 s)
5 30(10 In)
20 m
s fixe
dTi
me
delay
(ST)
Depende
nt
time dela
y (DST)
Instantaneous (INS)
Hig
h se
t
-
Cahier Technique Schneider n 167 / p.14
current value. For high fault currents, their I2 tcharacteristic
is theoretically a straight line(b = constant).In fact, the above
is true for current pulsedurations greater than the response time
of theactuating elements of the trip unit (generally4 ms). Below
this value, the inertia of themechanical elements of the trip unit
produces,for high Ip values, a characteristic similar tothat of an
instantaneous electro-mechanicalrelease.The trip unit must
therefore be characterised byits Eb = f (Ip) curve by carrying out
testsidentical to those for magnetic trip units.These trip units
may be of either theinstantaneous or time delay type.It is possible
to combine several types ofelectronic trip units, for example:c 10
to 15 In - ST (40 ms),c 15 to 30 In - ST (10 ms),c > 30 In -
INS.Figure 14 is an illustration of this example. Thecurves for
this combination should be compared
Fig. 14: examples of combinations of electronic trip-unit
curves.
1
40 m
s20
ms
10 m
s
7 m
s
5 m
s
3 10 50 100
107
106
105
104
= 40 kA
= 20 kA
= 10 kA
Ip (kA) = 5 kA
I2 t
2.5 ms
(A2 s)
5 30
(INS)
40 m
s tim
e de
lay (S
T)
10 m
s tim
e de
lay
(10 In)
with those in figure 10 for the breakingenergies of the
circuit-breaker.
Trip units with arc detectionGenerally combined with electronic
trip units,arc detectors may be used to provide protectionfor:c a
cubicle: if an arc occurs in a cubicle, thedetector orders opening
of the incoming circuit-breaker,c a selective circuit-breaker:
positioned in thebreaking unit, the detector provokes via
theelectronic trip unit the instantaneous tripping ofthe
circuit-breaker.The circuit-breaker is thus self-protected and
cantherefore be used up to the limit of itselectrodynamic withstand
capacity.Pressure trip systemThe pressure that develops in the
breaking unitof a circuit-breaker is a result of the energyproduced
by the arc.Above a certain fault current level, this pressuremay be
used for detection and tripping.This is possible by directing the
expanding gases
-
Cahier Technique Schneider n 167 / p.15
in the unit toward a piston that trips the circuit-breaker (see
fig. 15 ).Pressure trip systems may be used to:c ensure
self-protection of a selective circuit-breaker (similar to the arc
detector),c improve breaking and operating reliability of arapid
current-limiting circuit-breaker.If each circuit-breaker is fitted
with a correctlydesigned pressure trip system, discrimination
is
P1 P2 P3 Breaking units
Flap valves
PistonFault on phase 1pressure P1 pressure P2 and P3
Fig. 15: operation of the pressure trip system.
ensured between circuit-breakers with differentratings for all
overcurrents greater than 20 In.It is this energy-based trip system
(constant I2 tvalue) that makes possible the
energy-baseddiscrimination technique employed in theCompact NS
current-limiting circuit-breakers.
-
Cahier Technique Schneider n 167 / p.16
4 Advantages and implementationof energy-based
discrimination
Note that the circuit-breaker trip-unit system,whether
electromechanical, electronic or acombination of the two, must
offer the followingfeatures:c minimum stresses in the installation
(limited and I2 t values),
c tripping dependability (safety),c minimum disturbance for
correctly functioningcircuits (voltage dips),c ease of
discrimination studies.
4.1 Current-limiting circuit-breaker fitted with a pressure trip
systemThe above requirements may best be met with apressure trip
system, combined with either anelectromechanical or electronic trip
unit.Figure 16 indicates the energy sensitivity of thiscombination.
The higher the prospective short-
Fig. 16: trip-unit combination curves (electromagnetic and
pressure or electronic and pressure).
circuit current, the shorter the response time, whichleads to a
virtually constant tripping time at I2 t.The energy let through by
the current-limitingcircuit-breaker during a break follows the
samecurve, but with a slight shift.
1
40 m
s20
ms
10 m
s
7 m
s
5 m
s
3 10 50 100
107
106
105
104
= 40 kA
= 20 kA
= 10 kA
Ip (kA) = 5 kA
I2 t
2.5 ms
(A2 s)
5 30
Pressure trip system
ST 40
ms
ST 10
ms
-
Cahier Technique Schneider n 167 / p.17
Stresses in the installationStresses are limited compared to
those observedin current-limiting circuit-breakers of the
previousgeneration.On the basis of the example in figure 16 ,
thefigures for a Compact NS 250 A and an Ip of40 kA are:c 4 ms for
the breaking time;c 20 kA for the peak current;c 8 x 105 A2 s for
the I2 t.
Tripping dependabilityThe pressure trip system is a part of the
openingmechanism for short-circuits and thereforedepends on the
current rating of the circuit-breaker.The adjustable DST release,
whetherelectromechanical (see fig. 13 ) or electronic(see fig. 14
), is physically independent of thepressure trip system. Physical
independenceenhances operating dependability.
Voltage dipsVoltage dips in an installation can tripundervoltage
releases in circuit-breakers andcontactors.Unnecessary opening,
following a voltage dipcaused by a short-circuit, results in
reducedcontinuity of service.Consequently, discrimination studies
must alsotake into account the reactions of undervoltagereleases
and contactors during voltage dips.A voltage dip in a network lasts
until the arcvoltage that opposes the source voltage
enablesinterruption of the current. It follows that thevoltage dip
depends on the type of circuit-breaker and/or trip unit used:c with
non-limiting circuit-breakers, the voltagedip is more pronounced
and can last from 10 to15 ms (see fig.17 ),c with current-limiting
circuit-breakers, the rapiddevelopment of a high arc voltage
reduces thevoltage dip both in duration and in amplitude(see fig.17
).The voltage dip lasts approximately 5 ms andamounts to 50 % of
the rated voltage for currentsclose to the level required for
contact repulsion.The voltage dip amounts to 30 % of the
ratedvoltage for higher currents, but the duration isreduced to 3
to 4 ms. The higher the Isc, theshorter the voltage dip.Any
undervoltage releases equipping the circuit-breakers are not
affected by such voltage dips.
DiscriminationThe severely limited energy let through by
thecircuit-breaker is insufficient to trip the trip unit on
Fig. 17: the voltage dip on the network depends onthe type of
circuit-breaker.
a) non-limiting circuit-breaker
b) highly limiting circuit-breaker
Ur
t (ms)10 20
Ua
Ur
i
Ua
i
i
Uaic
Ur 5 10 20
t (ms)
the upstream circuit-breaker which remainsclosed.
-
Cahier Technique Schneider n 167 / p.18
4.2 Discrimination with Compact NS circuit-breakersUsing Compact
NS circuit-breakers,discrimination is total up to 150 kA.To ensure
total discrimination, the energy that acircuit-breaker lets through
must be less thanthat required to trip the upstream
circuit-breaker.
General ruleDiscrimination is total and without anyrestrictions
if:ccccc the ratio between the ratings of thesuccessive
circuit-breakers is equal to orgreater than 2.5,ccccc the ratio
between the trip unit ratings isgreater than 1.6.
Fig. 18: total discrimination between 100 A, 160 A and 250 A
Compact NS circuit-breakers.
1
40 m
s20
ms
10 m
s
7 m
s
5 m
s
3 10 50 100
107
106
105
104
= 40 kA
= 20 kA
= 10 kA
Ip (kA) = 5 kA
I2 t
2,5 ms
(A2 s)
5 30
Breaking
Non-tripping
Breaking
Non-tripping
Breaking
ST 1
60ST
200
ST 2
50
100 A
100 A
Mag
netic
100
A
ST 4
00ST
500
630 AST 6
30
630 A
250 A
250 A
Non-tripping2.5 ms
NoteST 160, ST 200 and ST 250: electronic trip unitsfor 250 A
circuit-breakers.ST 400, ST 500 and ST 630: electronic trip
unitsfor 630 A circuit-breakers.
Using the energy-based discrimination techniqueand depending on
the ratios between theupstream and downstream circuit-breaker
ratingsand the trip unit ratings, the Compact NS range(100, 160,
250, 400 and 630 A) offers eitherpartial or total discrimination up
to the breakingcapacity.
Total discriminationFigure 18 provides an example of
totaldiscrimination up to 100 kA over three levels with100 A, 250 A
and 630 A circuit-breakers fittedwith various trip units.
-
Cahier Technique Schneider n 167 / p.19
Fig. 19: partial discrimination between two Compact NS
circuit-breakers, 160 and 250 A.
1
40 m
s20
ms
10 m
s
7 m
s
5 m
s3 10 50 100
107
106
105
104
= 40 kA
= 20 kA
= 10 kA
Ip (kA) = 5 kA
I2 t
2.5 ms
(A2 s)
5 30
Breaking 160 ANon-tripping 250 A
Discrimination limit
160
A( 8
In)
250
A(10
In)
Partial discriminationIf the general rule presented above is
notrespected, discrimination is only partial.Figure 19 indicates
that between a 160 A circuit-breaker and a 250 A circuit-breaker
fitted with a250 A trip unit, discrimination is ensured up to
aprospective short-circuit current of 4 800 A. Thislevel is higher
than that observed, under the sameconditions, with standard Compact
circuit-breakers.Cascading with the Compact NSCascading, covered by
standard NF C 15-100,enables the upstream circuit-breaker to help
the
downstream device to break high short-circuitcurrents.Note that
this is detrimental to discrimination(except with the SELLIM
system).For the Compact NS, cascading in no way modifiesthe total
and partial discrimination characteristicsmentioned above.A Compact
NS circuit-breaker can however alwaysassist a downstream
circuit-breaker of a differenttype and with insufficient breaking
capacity.
4.3 Combination with traditional protective devicesStandard
circuit-breakersIn an existing installation, the highly
limitingCompact NS circuit-breakers may be used forextensions or to
replace existing circuit-breakers
without reducing the previous discrimination limit. Onthe
contrary, if the new circuit-breaker is installed:c downstream, its
current-limiting capacity canonly improve the discrimination level,
possibly to
-
Cahier Technique Schneider n 167 / p.20
Fig. 20: replacement of a Compact C250 N, H or L by a Compact NS
250 provides improved discrimination. In thisexample,
discrimination becomes total.
1
40 m
s20
ms
10 m
s
7 m
s
5 m
s
3 10 50 100
107
106
105
104
= 40 kA
= 20 kA
= 10 kA
Ip (kA) = 5 kA
I2 t
2.5 ms
(A2 s)
5 30
H
N
C 250 L
NS 250
C 250
Non-tripping
Mag
netic
630
A
the point of making discrimination total(see fig. 20 ),c
upstream, the discrimination level is at leastequal to the previous
level and the high current-limiting capacity of the Compact NS can
be usedto reinforce cascading.
FusesThe I2 t = f (Ip) curves (supplied bymanufacturers)
concern:c the energy required to blow the fuse(prearcing),
c the energy that flows through the fuse duringthe break.To
ensure discrimination between an upstreamcircuit-breaker and a
fuse, the circuit-breaker tripunit must not react to the sum of
these twoenergies.
-
Cahier Technique Schneider n 167 / p.21
5 Conclusion
Using a few simple rules, highly limiting circuit-breakers that
operate faster for higherprospective short-circuit currents can
beimplemented to provide total discrimination overseveral network
levels. They may alsoimplement time-discrimination techniques.This
is a major technical innovation that can beused to:c considerably
simplify discrimination studies,c minimize electrodynamic forces,
thermalstresses and voltage dips resulting from short-circuits.This
new discrimination technique, referred to asenergy discrimination
and based on total controlover the energy let through by the
circuit-breakers during breaking and on the sensitivityof the trip
units to the same energy, is animportant contribution to improving
theavailability of electrical power.
-
Cahier Technique Schneider n 167 / p.22
6 Appendix - indications concerning breakingwith current
limiting
Figure 21 shows the currents and voltages for ahalf-period
current-limiting phenomenon.The short-circuit current (ib) obeys
the followingrelationship:
Ur Ua = r i + L didt
L didt
c at the beginning of the short-circuit, Ua is zero,ib and ip
are equal and have identical slopes,c when Ua is equal to the
network voltage Ur, ibattains its maximum value (b) because
itsderivative is equal to zero,c when Ua is greater than Ur, ib
declines to zeroat tb.The interrupted current wave is equivalent to
asinusoidal half wave with a period equal to twicethe virtual
breaking time (tvb).With the above information, it is easy
todetermine the energy dissipated in theimpedances of the concerned
circuit.Expressed in other terms, the formula for thisenergy,
called the breaking energy, is:
Eb i dtbtvb
= 20where ib is a sinusoidal function:
Eb tb vb ( ).=
12
12
It is useful to express Eb as a function of Ip andthe duration
(tvb) of the break:c tvb u 10 msFor such a duration, the fault
current is low, thecircuit-breaker contacts do not repel each
otherand there is therefore no arcing voltage:
i i andb p b p ;= = 2 I
and formula 1 may be expressed as:
Eb p t ( )= I 2 2
c tvb 10 msThe circuit-breaker limits the fault current.ib and
ip have the same initial slope, therefore:
didt
p b = = I 2
where = pi tvb
t pvb b piI 2 = hence:
b vbt f p = 2 2Ior
t
f pvbb
=
2 2I
If we express equation (1) as:
b
vb
Ebt
2 2
=
we obtain:
2 2 22
Ebt
t f pvb
vb= ( )Ihence:
Eb f p tvb ( )= 4 32 2 3IAgain on the basis of (1), but with b
in mind:
t Eb
f pvb bb
= =
22 22
I
we obtain:
Eb
f pb
( )= 3
4 24
I
Formulas (3) and (4) can be used to plot the timeand peak
current curves.
-
Cahier Technique Schneider n 167 / p.23
Fig. 21: breaking with current limitation.
Ua
Ur
ip
t
b
ir
0 tr ta tb
ib
^t tvb T/2
di/dto
Ua: arcing votlageUr: network voltageip: prospective currentib:
break current (limited)b: maximum break currentir: contact
repulsion current
: time corresponding to bta: time at which the arc appearstb:
breaking timetr: time at which contact repulsion occurstvb: virtual
breking time: angular frequency of the interrupted wave
t
-
Cahier Technique Schneider n 167 / p.24
-
Schneider Direction Scientifique et Technique,Service
Communication TechniqueF-38050 Grenoble cedex 9Fax. (33) 04 76 57
98 60
Real.: Sodipe - ValenceEdition: SEST Grenoble03.98 - 1500 -
Printing: ClercPrinted in France
19
98 S
chne
ider
03-9863645