Industry-Building
Electrical distribution
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Design of a protection system
The example
installation
In this document,
we design the
protection
system for a
low-voltage
installation.
This consists
in precisely
determining
the necessary
protective
devices while
ensuring
maximum
discrimination at
all points in the
installation.
The low-voltage installation used for this
example is supplied by two incoming feeders.
The design objectives include coordination of the
protective devices in the LV section with those
located upstream of each MV/LV transformer.
As indicated in the diagram below, the
installation is made up of two 20 kV
medium-voltage feeders protected by fuses,
20 kV
Main LV
switchboard
80 A
80 A
20 kV/415 V
1600 kVA
In 2253 A
Isc 36 kA
20 kV/415 V
1600 kVA
In 2253 A
Isc 36 kA
B
A
Masterpact
NW25H1
Masterpact
NW25H1
C
Isc 72 kA
D
Masterpact
NT08L1
700 A
cable
Masterpact
NW25H1
Isc 36 kA
Isc 72 kA
E
Masterpact
NW10H2
750 A
cable
Isc 50 kA
F
Compact
NSX250H
185 A
Installation diagram
DBTP107S0L1/EN
each supplying a MV/LV transformer
(20 kV / 415 V, 1600 kVA), followed downstream
by a low-voltage incoming circuit breaker (circuit
breakers (A) and (B) respectively).
A coupling circuit breaker (C) is used to
interconnect or isolate the two parts of the
installation, in view of optimising power
availability if one of the incoming feeders fails.
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Isc 50 kA
G
Compact
NSX250H
185 A
Sizing of the protective
devices
Discrimination is determined by
selecting and setting the most
downstream circuit breakers such
that the forces exerted on the
installation are limited in the event
of a fault, then moving upstream
to ensure the best possible
discrimination between each pair of
upstream and downstream circuit
breakers.
Ratings of fuses on the MV feeders
The rated current on the MV feeders must be
calculated:
In = 1 600 000 / (20 000 x √3) = 46 A.
Fuses with an 80 A rating should be selected,
in accordance with the manufacturer selection
tables.
The inrush current and the overload current have
to be taken into consideration.
Ratings of circuit breakers (A) and (B) on
the LV feeders
The rated current on these LV incomers must be
calculated:
1600 kVA at 410 V corresponds to a rated
current of
1 600 000 / (410 x √3) = 2253 A.
Circuit breakers with a 2500 A rating are suitable.
Breaking capacity of the various devices
b Short-circuit currents at various points in the
installation:
Each transformer has a short-circuit current Isc
equal to 36 kA (due to the short-circuit power
and voltage of the transformer).
When the coupling circuit breaker is closed,
the short-circuit power downstream of circuit
breakers (A) and (B) is 2 x 36 = 72 kArms (if the
busbar impedances are neglected). Taking into
account the cable impedances, the short-circuit
current flowing through circuit breakers (F) and
(G) is only approximately 50 kA.
b Breaking capacity of the devices:
On the basis of the short-circuit currents at the
various points in the installation, it is possible
to determine the required breaking capacity for
each device.
Circuit breakers (D) and (E) must have a
breaking capacity greater than 72 kA. Circuit
breakers (A), (B) and (C) must have a breaking
capacity greater than 36 kA. Circuit breakers
(F) and (G) must have a breaking capacity of at
least 50 kA.
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Industry-Building
Alternative with zone selective
interlocking
This alternative requires that circuit breakers (A),
(B), (C), (D) et (E) be equipped with control units
offering the zone selective interlocking function
(e.g. Micrologic 5.0 A).
Description:
Each control unit is equipped with two input
terminals, connected to the downstream devices,
and two output terminals, connected to the
upstream devices.
When a control unit detects a fault greater than
its short-time threshold, it shorts its two output
terminals. If an upstream control unit detects the
shorted input terminals, it activates the shorttime delay. Otherwise, its trips immediately.
Implementation
b The inputs on the first circuit breakers (D) and
(E) are short-circuited on a permanent basis
to ensure that their short-time delay is always
activated. This guarantees discrimination with
the downstream circuit breakers (Compact NS).
b Next the various devices are wired as shown
in the diagram opposite.
The direct wiring between (E) and (B) and
between (D) and (A) ensure discrimination
between these devices when the coupling
(C) is open. Diodes are used to maintain
the independence of the two halves of the
installation in this case by preventing (D) from
acting on (B) and (E) from acting on (A).
out
out
A
B
ST delay 2
IN
ST delay 2
IN
out
C
out
out
ST delay 1
IN
ST delay 1
IN
E
Operation
b In the event of a fault downstream of circuit
breaker (G):
v if (C) is closed,(G) trips instantaneously and
(E) is delayed 100 milliseconds, i.e. it does not
trip. (E) sends a signal to (C) and (B), which
are delayed 200 milliseconds and do not trip. In
turn, (C) sends a signal to (A) and (B), which are
delayed 200 milliseconds and do not trip. As a
result, only circuit breaker (G) trips,
Alternative with two feeders
with higher ratings
Consider the same installation as presented
above, but in which the rating of the transformers
is increased to 2500 kVA and the current of the
circuit downstream of (E) is increased to 2200 A.
Rating of the fuses on the MV feeders.
The MV current is 72 A. Fuses rated 125 A
are recommended, in compliance with the
manufacturer selection tables.
ST delay 2
IN
D
v if (C) is open, it will not send a signal to (A),
however (A) will not be subject to the shortcircuit.
b In the event of a fault between (G) and (E):
v if (C) is closed, circuit breaker (E) is delayed
100 milliseconds and it sends a signal to (C) and
(B), which are delayed 200 milliseconds and do
not trip. In turn, (C) sends a signal to (A) and (B),
which are delayed 200 milliseconds and do not
trip,
v if (C) is open, it will not send a signal to (A),
however (A) will not be subject to the shortcircuit.
b In the event of a fault between (E) and (C):
v if (C) is closed, it trips instantaneously and
sends a signal to (A) and (B), which are delayed
200 milliseconds. In this way, the fault is no
longer supplied by the two feeders in parallel.
(A) remains closed and the left side of the
installation remains in service. After 200 ms, (B)
will trip to interrupt the supply of current by the
transformer on the right.
v if (C) is open, it will not send a signal to (B)
and (B) will trip instantaneously.
b In the event of a fault between (C) and (B),
circuit breaker (B) trips instantaneously.
It is clear that zone selective interlocking
significantly limits the forces exerted on the
installation and all the more so the higher the
fault occurs in the system. Without this function,
a fault occurring just downstream of (A) or
(B) would produce tripping in over 300 ms,
compared to just a few tens of milliseconds with
the function.
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Breaking capacity of the various circuit breakers:
b Circuit breaker (E)
Given that the Isc is greater than 100 kA, a
Masterpact NW25H2 (breaking capacity =
100 kA) cannot be used. A current-limiting circuit
breaker cannot be used either, because the rated
current does not exceed 2000 A (NW20L1).
b Circuit breakers (A), (B) and (C)
For the In = 3520 A rating, Masterpact NW40H1
circuit breakers (In = 4000 A, breaking capacity =
65 kA, Icw = 65 kA/1 s) are suitable.
Device selection to ensure discrimination
No fundamental modifications are required. The
time delay on circuit breaker (E) must be set to
0.1s. The time delay on circuit breaker (C) must
be set to 0.2s. On circuit breakers (A) and (B),
the time delay must be set to 0.3s.
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As standards, specifications and designs change from time to time, please ask for confirmation
of the information given in this publication.
This document has been printed on ecological paper.
Design: Schneider Electric
Photos: Schneider Electric
Printed:
DBTP107SOL1/EN
04-2008
© 2008 - Schneider Electric - All rights reserved
Schneider Electric Industries SAS
Discrimination between circuit breakers
(E) and (C)
Discrimination is not indispensable between these
two circuit breakers if the two incoming feeders are
operational (in this case, opening of the coupling
circuit breaker does not interrupt the supply of
power via (A) and (B)). However, discrimination
is indispensable if circuit breaker (B) is open, to
avoid interrupting the supply of power to the entire
right-hand side of the installation.
b Rated current In at (C):
To ensure a maximum degree of flexibility, the
coupling device has the same rating as the circuit
breakers on the incoming feeders, i.e. In = 2500
A. Given that the Isc is 36 kA, a selective circuit
breaker may be used, thus making possible time
discrimination with (E) and of course with (D)
because it is a current-limiting device.
For example, a Masterpact NW25H1
(In = 2500 A, breaking capacity = 65 kA at 415 V,
Icw = 65 kA/1 s).
Discrimination sytem:
As Icw is equal to the breaking capacity,
the circuit breaker is not equipped with an
instantaneous self-protection trip and time
discrimination is maintained without any
limitations up to the breaking capacity. Circuit
breaker (C) is therefore equipped with a
selective control unit, with the instantaneous
setting turned to the OFF position and the
short-time delay set to 0.2 (because the shorttime delay on circuit breaker (E) is set to 0.1).
Discrimination between circuit breakers
(D) and (C)
The solution selected to ensure discrimination
between (E) and (C) may also be used between
(D) and (C) because (C) is totally selective up to
its breaking capacity.
Discrimination between circuit breakers
(C) and (B) and between (C) and (A)
(A) and (B) are selective circuit breakers,
without an instantaneous self-protection
control unit. Again, time discrimination
is maintained up to the breaking capacity.
Therefore, (A) and (B) are equipped with
selective control units, with the instantaneous
settings turned to the OFF position and the
short-time delays set to 0.3 (because the shorttime delay on circuit breaker (C) is set to 0.2).
Discrimination between circuit breakers
(A) and (B) and the MV fuses
To ensure discrimination, the tripping curves of
the LV circuit breakers and the MV fuses must
be compared.
For this, the curve of the MV fuses must be
converted to low-voltage values by multiplying
the current scale by the transformer ratio, i.e. in
this case, 20 000 / 410 = 48.8.
The tripping curve for circuit breakers (A) and (B)
must be adjusted such that for a given current,
the circuit-breaker tripping delay is less than the
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Industry-Building
blowing time of the MV fuses (the circuit-breaker
curve must remain to the left of the fuse curve in
the figure below).
Discrimination is evaluated for two types of
control units, standard selective control units and
IDMTL-curve control units.
10 000
HVF curve
tr = 2 s
1000
100
80 A MT fuse
converted to BT
I2t curve
tr = 12 s
Isc max = 36 kA
10
1
short time
delay
setting = 0,3 s
short time
pick up = 4 Ir
0,1
0,01
0,100
1000
10 000
100 000
Settings for standard selective control units
b Long-time threshold
The non-tripping current of the fuse is far greater
than the tripping current of the circuit breaker.
The long-time threshold can therefore be set to
its maximum value (i.e. Ir = In).
b Long-time delay and short-time pick-up
The curve of the MV fuses is steeper that that of
the long-time function (I2t curve). The time delay
(tr) or the short-time pick-up (Isd) must be set
to sufficiently low values. In this case, a good
compromise would be Ir = 12 second (at 6 Ir) and
Isd = 4Ir. (A detailed study taking into account the
downstream loads should be carried out).
b Short-time delay
Given that the short-time delay is set to 0.3
(see above), the fuse and circuit-breaker curves
intersect at approximately 11 In. The guaranteed
level of discrimination must therefore be limited
to approximately 10 In, i.e. 25 kA rms for a
maximum short-circuit current Isc of 36 kA rms.
Settings for IDMTL-curve control units
With this type of control unit, it is possible to
adjust the slope of the long-time curve. In this
case, the HVF (High Voltage Fuse) curve could
be selected because it is the most similar to the
fuse curve (I4t curve).
With the same 1-second time delay at 6 In, the
sensitivity to high transient currents (switching or
start-up currents) is reduced and discrimination
with the fuse is improved. In this case, there is
no longer any need to set the short-time delay to
lower values.
Device selection
to ensure discrimination
Principle
Discrimination is ensured by comparing the
characteristics of each circuit breaker with the
protective device (fuse or circuit breaker) located
just upstream. The power circuit breakers
located in the most downstream positions of
the installation must be selected and set to
trip “as fast as possible” in order to limit the
forces exerted on the installation in the event
of an overcurrent. Once the characteristics of
these circuit breakers have been established,
the system designer moves one step upstream
to ensure discrimination between each circuit
breaker pair (each downstream breaker and its
upstream neighbour).
20 kV
TGBT
80 A
80 A
20 kV/415 V
1600 kVA
In 2253 A
Isc 36 kA
20 kV/415 V
1600 kVA
In 2253 A
Isc 36 kA
A
Masterpact
NW25H1
C
Isc 72 kA
D
Masterpact
NT08L1
700 A
Masterpact
NW25H1
cable
B
Isc 36 kA
Masterpact
NW25H1
Isc 72 kA
E
Masterpact
NW25H1
750 A
cable
Isc 50 kA
F
Compact
NSX250H
185 A
Isc 50 kA
G
Compact
NSX400H
330 A
Discrimination between circuit breakers
(F) and (D)
b Circuit breaker (F): In = 185 A, Isc = 50 kA
A 250 A circuit breaker may be used, for example
a Compact NSX250H (breaking capacity = 70 kA
at 415 V).
b Circuit breaker (D): In = 700 A, Isc = 72 kA
A 800 A circuit breaker may be used, for example
a Compact NS800L or a Masterpact NT08L1
(breaking capacity = 150 kA at 415 V).
Discrimination system:
Device (F) is a current-limiting device (maximum
let-through current = 22 kA peak for an Isc of
50 kA), which makes it suitable for “pseudotime discrimination” in conjunction with circuit
breaker (D).
However, circuit breaker (D) is also a currentlimiting device with a low electrodynamic
withstand capacity to ensure strong current
limiting. Consequently, the “SELLIM” principle is
implemented here to obtain total discrimination
between (F) and (D). The “SELLIM” principle
stipulates that the upstream circuit breaker (D)
does not trip on the first current wave.
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Discrimination between circuit breakers
(G) and (E)
b Circuit breaker (G): In = 330 A, Isc = 50 kA
A 400 A circuit breaker may be used, for example
a Compact NSX400H
(breaking capacity = 70 kA at 415 V).
b Circuit breaker (E): In = 750 A, Isc = 72 kA
The same circuit breaker as (D) may be used,
however given that the current-limiting capacity
of the Compact NSX400H is less than that of the
Compact NSX250H, total discrimination would
not be ensured between the two.
A selective circuit breaker may be used, in
which case the current-limiting capacity of
(G) may be called on to provide "pseudo-time
discrimination", if necessary. For example, a
Masterpact NW10H2 (In = 1000 A, breaking
capacity = 100 kA at 415 V, Icw = 85 kA rms/1 s).
Discrimination system:
Icw (85 kA) is less than the breaking capacity
(100 kA). This device is therefore equipped with
an instantaneous self-protection trip (DIN) with
a threshold of 170 kA peak.
With an Isc of 72 kA rms, the maximum peak
current at (E) is 72 x 2.3 = 165 kA peak.
The DIN threshold will therefore never be
reached, i.e. DIN tripping will never occur and
discrimination is maintained.
What is more, in the event of a short-circuit at
(G), the maximum peak current corresponding
to an Isc of 50 kA will be limited to 30 kA peak
for (G).
Discrimination is therefore total, on the
condition, however, that circuit breaker (E) be
equipped with a trip unit with an instantaneous
setting higher than 30 kA peak, i.e. 30 / 2.04 =
14.7 kA rms = 14.7 In, and that the time delay
for the short-time protection function be set to
0.1 (time delay = approximately 100 ms).
Alternative
It is also possible to use a current-limiting device
for (E), with a higher electrodynamic withstand
than (D). For example, a Masterpact NW10L1
(In = 1000 A, breaking capacity = 150 kA at 415
V, Icw = 30 kA/1 s).
Advantages of this alternative
Due to its current-limiting capacity (125 kA peak
at 72 kA rms compared to 165 kA peak without
current limiting), this type of circuit breaker
significantly reduces the electrodynamic forces
exerted on the cables between (E) and (G). This
circuit breaker is equipped with an instantaneous
self-protection control unit set to 80 kA peak, which
will never be called upon in the event of a fault
downstream of (G) because the Isc is limited to 30
kA peak. The result is again total discrimination, of
the "pseudo-time discrimination" type, i.e. ensured
by the current-limiting capacity of the downstream
device.
Note: in the event of a short-circuit, a non
current-limiting device at (G) would let through a
peak current of 50 kA x 2.3 = 115 kA peak, which
would trip circuit breaker (E).