3000DB0902
05/2009
LaVergne, TN, USA
Data Bulletin
Sepam™ ZSI Application Note
Class Number 3000
What is ZSI?
Zone Selective Interlocking (ZSI) is a communication-based protection
scheme built into the firmware of the SEPAM MV protective relays. The
basic idea involves a downstream breaker sending a blocking signal to an
upstream breaker. The blocking signal blocks a high(er) speed tripping
element.
All devices within the ZSI system have backup elements which are
independent of the high speed scheme. The normal result of a ZSI system
for through faults is the high speed elements are blocked and standard time
overcurrent elements are ready to provide backup protection. If the fault is
internal to the ZSI zone, a high speed (typically 100-200ms) trip will occur.
Refer to Figure 1 and Figure 2 below.
Figure 1:
Fault Downstream of F1
Figure 2:
Fault Upstream of F1
Communication is done via inputs and outputs on the SEPAM relay at
control power voltage (typically 125Vdc or 110Vac), and not by low millivolt
signals susceptible to noise. The inputs and outputs represent almost no
burden, blocking signal distances of ½ mile show very little voltage drop
(with 125Vdc control power).
ZSI in its simplest form (Figures 1 and 2) provides the basic idea of the
scheme, however this idea can be expanded to use multiple feeders (Figure
3), multiple mains and a tie breaker (Figures 5 and 5a), and a complex ring
bus arrangement (Figure 6).
The purpose of this paper is to provide a thorough understanding of the ZSI
scheme so an engineer can design, wire and program the SEPAM relays
(with SFT2841 software).
© 2009 Schneider Electric All Rights Reserved
Sepam™ ZSI Application Note
3000DB0902
05/2009
Why use ZSI?
The primary use of ZSI is the ability to trip faster than the normal overcurrent
(ANSI 50/51 elements) coordinating interval. The result of tripping faster is
significant reduction of arc flash energy. This reduction is at the expense of
a couple of control wires per circuit; however it is NOT at the expense of
coordination.
Below is a table that indicates the AF energy for 5kA, 10kA, 20kA, 30kA and
50kA bolted three phase faults at voltage levels of 4160V and 13.8kV.
Table 1:
Bus Name
kH represents
bolted 3PH
fault current
Bus
Prot Prot Dev
Protective
Arcing
Bus Bolted Dev
Device
kV Fault Bolted Fault
Name
(kA) Fault
(kA)
4160V_5kA
ZSI_0.1_5kA
4.16
5
5
4160V_10kA
ZSI_0.1_10kA 4.16
10
4160V_20kA
ZSI_0.1_20kA 4.16
20
4160V_30kA
ZSI_0.1_30kA 4.16
4160V_50kA
ZSI_0.1_50kA 4.16
Arc Flash Results for a 100ms ZSI system
Trip/ Breaker
Required
Arc Flash Working Incident
Delay Opening
Protective
Equip Gap
Boundary Distance Energy
Ground
Time Time
FR
Type (mm)
(in)
(in)
(cal/cm2)
(sec.) (sec.)
Clothing
4.91
0.1
0.083
Yes
SWG
104
29
36
0.96
Category 0
10
9.71
0.1
0.083
Yes
SWG
20
19.18
0.1
0.083
Yes
SWG
104
62
104
131
36
2
Category 1
36
4.2
30
30
28.58
0.1
0.083
Yes
SWG
104
Category 2
204
36
6.5
50
50
47.21
0.1
0.083
Yes
SWG
Category 2
104
357
36
11
Category 3
13800V_5kA
ZSI 0.1_5kA
13.8
5
5
4.91
0.1
0.083
Yes
SWG
153
33
36
1.1
Category 0
13800V_10kA
ZSI 0.1_10kA 13.8
10
10
9.71
0.1
0.083
Yes
SWG
153
70
36
2.3
Category 1
13800V_20kA
ZSI 0.1_20kA 13.8
20
20
19.18
0.1
0.083
Yes
SWG
153
149
36
4.8
Category 2
13800V_30kA
ZSI 0.1_30kA 13.8
30
30
28.58
0.1
0.083
Yes
SWG
153
232
36
7.3
Category 2
13800V_50kA
ZSI 0.1_50kA 13.8
49.99
49.99
47.21
0.1
0.083
Yes
SWG
153
405
36
13
Category 3
Table 2:
Bus Name
A_4160V_5kA
Arc Flash Results for a 200ms ZSI system
Bus Prot Dev Prot Dev Trip/ Breaker
Required
Protective
Arc Flash Working Incident
Protective
Bus Bolted Bolted Arcing Delay Opening
Equip Gap
Device
Boundary Distance Energy
Ground
Fault Time Time
FR Clothing
kV Fault Fault
Type (mm)
Name
(in)
(in)
(cal/cm2)
(kA)
(kA)
(kA) (sec.) (sec.)
Category
ZSI_0.2_5kA 4.16
5
5
4.91
0.2
0.083
Yes
SWG
104
45
36
1.5
Category 1
B_4160V_10kA ZSI_0.2_10kA 4.16
10
10
9.71
0.2
0.083
Yes
SWG
104
96
36
3.1
Category 1
C_4160V_20kA ZSI_0.2_20kA 4.16
20
20
19.18
0.2
0.083
Yes
SWG
104
205
36
6.5
Category 2
D_4160V_30kA ZSI_0.2_30kA 4.16
30
30
28.58
0.2
0.083
Yes
SWG
104
320
36
10
Category 3
E_4160V_50kA ZSI_0.2_50kA 4.16
50
50
47.21
0.2
0.083
Yes
SWG
104
558
36
17
Category 3
F_13800V_5kA
ZSI 0.2 5kA 13.80
5
5
4.91
0.2
0.083
Yes
SWG
153
51
36
1.7
Category 1
G_13800V_10kA ZSI 0.2 10kA 13.80
10
10
9.71
0.2
0.083
Yes
SWG
153
109
36
3.5
Category 1
H_13800V_20kA ZSI 0.2 20kA 13.80
20
20
19.18
0.2
0.083
Yes
SWG
153
233
36
7.4
Category 2
I_13800V_30kA ZSI 0.2 30kA 13.80
30
30
28.58
0.2
0.083
Yes
SWG
153
363
36
11
Category 3
49.99
47.21
0.2
0.083
Yes
SWG
153
634
36
19
Category 3
J_13800V_50kA ZSI 0.2 50kA 13.80 49.99
Most 4160V systems have bolted three phase fault currents less than 2030kA. From the table the incident energies are 4.2 and 6.5 cal/cm2, both of
these results in Cat 2 PPE.
A 10MVA base rated, 6% impedance transformer has an infinite bus let
through of 23,131A at 4160V. A 25MVA base rated, 6% impedance
transformer has an infinite bus let through of 17,432A at 13.8kV. Most
industrial applications have 5kV buses that are fed from transformers
smaller than 10MVA and 15kV buses smaller than 25MVA. The conclusion
is that for most industrial applications, the ZSI will limit the AF energies to
CAT 2 or below.
If AF reduction is the main desire in a normal system design, the
conventional bus differential relay (87B) will typically have an AF category of
2
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
Sepam™ ZSI Application Note
zero. The differential requires dedicated ct’s all sized the same. In this
respect the 87B and the ZSI do not “compete”. However, if the fault currents
are limited to values that are below a desired AF category, or if there is an
existing lineup of switchgear that requires AF reduction, the ZSI becomes a
very cost-effective solution.
The ZSI scheme can be cascaded, without the need for coordination. One
“zone” may include the main bus of a switchgear lineup, another zone may
include a feeder breaker that has a conductor run of 2000 feet. Each of
these zones can have ZSI time delays of 100-200ms. If a fault does occur in
the 2000 foot run of cable, the feeder that feeds the fault would block the
main ZSI from tripping high speed, the feeder would not receive a blocking
signal from the breaker that the conductor terminates into, and therefore
would trip high speed.
Where is ZSI used?
Square D Company began developing low voltage (LV -480 volt) ZSI
technology in 1986, and began using ZSI extensively in 480 volt substations
in 1988. At that time the term “Arc Flash” was virtually unknown, and the
electrical phenomena that dominated the trade magazines was the effects
of harmonic distortion on the power system.
The application of the 480 volt ZSI is almost identical to the MV design.
Most people believe the MV design is easier to troubleshoot. This stems
from the inherent information available in modern day digital relays versus
the low-voltage trip units where virtually none exist.
The most recent application of ZSI is between LV trip units and MV relays.
Refer to Figure 7. A complete technical paper written by Van Wagner is also
included in the Appendix.
ZSI is used in the following MV applications:
© 2009 Schneider Electric All Rights Reserved
•
Only within an MV lineup of switchgear (provides high-speed trip for bus
fault). Refer to Figures 3, 5 and 5a.
•
Between separated radial MV lineups of switchgear (provides highspeed trip for conductor fault). This scheme trips one breaker and
removes service to all loads downstream of a tripped breaker. Refer to
Figure 4.
•
Between separated looped MV lineups of switchgear (provides highspeed trip for conductor fault). This scheme trips two breakers to clear
fault, but maintains service to other loads. Refer to Figure 6.
•
LV trip units to “virtual” 480V device (MV Relay) (provides AF
improvement by tripping MV feeder breaker). Refer to Figure 7.
3
Sepam™ ZSI Application Note
3000DB0902
05/2009
Figure 3:
MV application of ZSI within the lineup
Figure 4:
Radial ZSI Application for high speed line protection
52
52
4
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
© 2009 Schneider Electric All Rights Reserved
Sepam™ ZSI Application Note
Figure 5:
MV application of ZSI M-T-M
Figure 5a:
Alternate MV application of ZSI M-T-M (Non-relayed Tie)
5
Sepam™ ZSI Application Note
3000DB0902
05/2009
Figure 6:
6
Closed Loop / Ring Bus Application
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
Sepam™ ZSI Application Note
Figure 7:
© 2009 Schneider Electric All Rights Reserved
Low Voltage MicroLogic trip units supply blocking signals to
“virtual” main with SEPAM relay
7
Sepam™ ZSI Application Note
3000DB0902
05/2009
MV Applications
Application 1 – Simple Main / Feeder ZSI
Figure shows the control diagram for the main breaker shown in Figure 3, a
single main with two feeders. The main breaker receives two contacts, O3
from the SEPAM relay connected to Fdr 1 and O3 from the SEPAM relay
connected to Fdr 2. If either one of these contacts close, then I13 (the input
on the SEPAM relay connected to the main breaker) would activate blocking
of the high speed ZSI scheme and the main would not trip the ZSI element.
Figure 8:
Main Bkr Control Diagram for a single main and two feeder
system in Figure 3..
Figure 9 shows the SFT2841 Program Logic settings for the Main and
Figure 9a shows the SFT2841 protection settings for the feeders.
8
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
Sepam™ ZSI Application Note
Figure 9:
SFT2841 Program Logic Settings for Main
I13 accepts the blocking contacts from feeders
I11 accepts the 52b contact
I12 accepts the 52a contact
Figure 9a:
SFT2841 Program Logic Settings for Feeders (also see Figure
11a) which only require an output assignment, in this case O3.
Figure 10 shows the SFT2841 protection settings for the Main and Figure
10a shows the SFT2841 protection settings for the feeders.
© 2009 Schneider Electric All Rights Reserved
9
Sepam™ ZSI Application Note
3000DB0902
05/2009
Figure 10:
SFT2841 protection settings for the Main (only Elements 1
and 3 are need for both ZSI and the backup Time-based 51
setting).
Figure 10a: SFT2841 protection settings for the Feeders
Keep in mind that the last relay in the ZSI scheme blocks ONLY. The
tripping for this position in the ZSI scheme is performed by the standard
50/51 elements.
Figure 11:
10
SFT2841 Control Matrix for the Main
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
Sepam™ ZSI Application Note
Figure 11a: SFT2841 Control Matrix for the Feeders
Recommended outputs for ZSI:
Series 20/40 – O3 and O12 (if needed)
Series 80 – O102 (Block Direction 1), O103 (Block Direction 2)
These recommended output assignments are made to avoid a conflict with
other frequently used assignments. Worth mentioning is the “remote close
via communication”, this function is pre-programmed in the Series 20/40 as
O11, in the Series 80 it is O3. This is believed to be an ever increasing
application because it allows circuit breakers to be closed without an
operator standing in front of the switchgear. Presently there is a demand for
the Square D Field Services designed remote open/close and racking
device.
Recommended inputs for ZSI:
Series 20/40 – I13 (Direction 1 – Series 20 only has 1 direction)
Series 40 – I21 (Direction 2)
Series 80 – I104 (Direction 1), I105 (Direction 2)
NOTE: A further discussion on the meaning of “direction” is included in the
application of the closed ring.
General Procedure:
1. Wire I/O
2. Configure Program Logic page
3. Configure Control Matrix
4. Configure Protection Tab which includes
a. ZSI settings
b. Phase Over current settings
c. Ground Over Current settings
© 2009 Schneider Electric All Rights Reserved
11
Sepam™ ZSI Application Note
3000DB0902
05/2009
The ZSI scheme requires specific settings for the main and feeders. It is
common for application engineers to set the ZSI (phase) pickup setting per
Table 3 below:
Table 3:
ZSI “Common” Pickup Settings for Blocking to Occur
Min. PU*
Max PU
1200A
2.4-3.6kA
½ maximum three phase fault current.
2000A
4-6kA
½ maximum three phase fault current.
3000A
6-9kA
½ maximum three phase fault current.
Switchgear Main Bus
*Min. PU assumes that the short circuit contribution from motors is less than
the pickup value. If there is a system that has a significant motor
contribution in the ZSI zone, perform the following steps:
Increase the time delay of the scheme to 200ms.
Use directional 67 element (pointing in the forward direction) to block
only if the fault is downstream of the feeder. The motor contribution
would be in the reverse direction.
Within a given scheme, all relays typically have the same pickup. The
ground fault ZSI pickups are commonly set to 50-87% of the phase settings
for a solidly grounded system. The common time delays are 100ms. As
mentioned above, there may be instances where motor contribution at one
level of the ZSI may cause the time delay to be 200ms and the time delays
near the utility to be 100ms (the SEPAM instruction book shows this
example).
Protection for most industrial feeder breakers typically allow for a ANSI 50
function (instantaneous). In the example above, the ZSI blocking is set up to
2.4kA and the feeder breakers also trip at 2.4kA. The feeder 50 setting is
typically set to1.7 x maximum inrush. So if there is a fault downstream of the
feeder of 3000A, the feeder would send a blocking signal to the main and
simultaneously trip the feeder. Once the trip signal is sent to the feeder, the
blocking signal is released 200 ms later. If the feeder breaker did not clear
the fault (i.e. failed to operate), the main would immediately trip to clear the
fault. Therefore the ZSI scheme has a built-in breaker failure protection.
If a feeder feeds multiple transformers and an ANSI 50 element is not used,
the downstream 3000A fault would be allowed to flow until the feeder 51
element cleared (if a downstream transformer primary fuse did not clear the
fault). If this were the case, the feeder would send and maintain the blocking
signal and the main would not trip on the high speed element. The feeder 51
element would eventually trip (based on its setting).
Application 2 – Main-Tie-Main
12
The normal Main-Tie-Main (M-T-M) is very common to Application 1
covered in detail above. In the M-T-M, the feeders have a second output
contact to block the TIE (O12 refer to Figure 12). All settings for the feeder
are the same. The TIE and Main settings in the M-T-M are just like the Main
in Application 1. TIE is slightly different in that it blocks both Mains (O12
blocks M1 and O3 blocks M2); the ZSI pickup and time delay settings are
the same as the Main.
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
Sepam™ ZSI Application Note
Figure 12:
M-T-M ZSI with I/O labeled for SEPAM Series 20/40 with I/O
assignments.
In Figure 12 assume the TIE is closed and M2 is Open.
Table 4:
SC1, SC2 & SC3 Block and Trip Summary
Short Circuit Event
CB Blocked / Blocked By / Output Contact
Trip
SC1
TIE / F4 / O12 and M1 / TIE / O12
F4
SC2
M1 / TIE / O12
TIE
SC3
NONE
M1
Note that in “SC1 and SC2” the TIE breaker sends a blocking signal to M2
but in these cases this breaker is already open. If the N.O. TIE is closed
then either M1 or M2 must be open. With this scheme, there is no need to
write Boolean logic. Blocking both mains works well for this situation. If the
TIE is NC, then this configuration would be considered a “closed loop” which
is covered in the next application example.
© 2009 Schneider Electric All Rights Reserved
13
Sepam™ ZSI Application Note
Application 3 – Closed Loop System
3000DB0902
05/2009
Refer to Figure 6 and Figure 13.
Figure 13:
Closed Loop ZSI with I/O assignments
The closed loop system is a system that has multiple sources that are
normally tied together. This could be the case with a facility that has two
separate utility feeds (most common), or a system that has one utility feed
and one or more generators (common in paper mills).
The benefits of this scheme are commonly used in the “critical power” area.
This scheme requires a more thorough knowledge of all the power system
disciplines. Special care is required when multiple sources are tied together
to make certain the switchgear interrupting ratings are adequate and that
reverse current relays (ANSI 67 and 67N) elements are properly applied
and tested. This scheme also requires a higher level of safety training for
operators and technicians. When a circuit breaker is opened in the loop, it is
more likely than not that the line and load side of the breaker is still hot.
The basis of the closed loop ZSI is that within the portion of the system that
is closed, each relay has two 67 elements; one looking in the forward
direction and one looking in the reverse (Figure 13). The 67 Element 1 (671) is typically pointing in the reverse direction. The convention for choosing
the direction for the relays in the system is somewhat arbitrary. Following a
14
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
Sepam™ ZSI Application Note
consistent convention is recommended. Below is convention that has
worked well:
1. Choose all 67-1 directional elements in the reverse direction of normal
current flow (into non-polarity, out of the polarity marks on the CT). 67-2
is always in the opposite direction as 67-1.
2. Eventually there are feeder breakers that are not in the “closed loop” (i.e.
they cannot produce a constant source of fault current) these feeders do
not require 67 elements; the non-directional 50 settings work fine
(unless the motor contribution is in excess of the desired pickup value).
3. The I/O are associated with a specific direction.
a. Series 80 Inputs:I 104-Rev; I105- Fwd
b. Series 80 Outputs: O102, O104*, O106* - Rev
O103, O105*, O107* - Fwd
c. Series 40 Inputs: I13 – Rev; I21 - Fwd
d. Series 40 Outputs:O3, O12* - Rev
O13, O14 – Fwd
* = If needed.
Direction 1 corresponds with “Rev”. Direction 2 corresponds with “Fwd”
4. Figure 13 output contacts also have arrowheads indicating which 67
element they work in conjunction with. Since blocking is always
“backwards”, the arrowhead of an output contact never points toward an
input, but instead it will always point away from an input. Study Figure 13
and follow the blocking contacts for a 67-1 element and a 67-2 element.
In the examples for Figures 14-15 it is assumed that the Mains and Ties
have Series 80 SEPAM relays and the feeders have Series 40.
Short Circuit Events (SC1 and SC2 in the closed loop system)
SC1 – refer to Figure 14
F2a does not have dual 67 elements since it has been determined that the
load downstream of F2a does not have either a generator, utility or large
motor contribution. The non-directional element at F2a would block (via O3
and O12) TIE-a and M2a elements that can provide fault current to F2a. F2a
would trip on it’s time based 50/51 setting.
Table 5:
SC1 Block and Trip Summary
Short Circuit Event
CB Blocked / Blocked By / Output Contact
Trip
SC1
TIE-a and M2a / F2a / O12 and O3
F2a
F2 / M2a / O103
M1a / TIE-a / O103
F1 / M1a / O103
TIE and M2 / F2 / O14 and O13
TIE and M1 / F1 / O14 and O13
All the breakers in this example are blocked from high speed tripping except
F1a (which does not see the fault) and F2a which trips.
A 67 element will not trip if either of the following is true:
It is blocked by a downstream relay that also sees the fault in the proper
direction.
The fault is not in the direction of the element.
© 2009 Schneider Electric All Rights Reserved
15
Sepam™ ZSI Application Note
3000DB0902
05/2009
Figure 14:
Fault Example SC1
SC-2 Refer to Figure 15.
When a fault is detected by a directional element, the ZSI blocks backwards
to the source breakers that can provide fault current in that direction. For
fault SC2 the 67-2 element at F1 senses current in “its” direction and closes
the two output contacts that match the forward direction, in this case O13
and O14. These two contacts block the forward flowing element in M1 from
tripping (67-2) and the TIE element 67-1. The 67-1 in M1 or 67-2 in the TIE
does not see fault current in “their” direction. The TIE 67-1 element also
blocks backwards which includes M2 67-2 and F2 67-1.
16
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
Sepam™ ZSI Application Note
Since the fault current coming in from M2 can split, some going through the
TIE and some going through F2, M2a, TIE-a, M1a to fault, this loop must
also be blocked. See the table below for complete blocking sequence.
Table 6:
SC2 Block and Trip Summary
Short Circuit Event
SC2
CB Blocked / Blocked By / Output Contact Trip (Element)
M1 and TIE / F1/ O13 and O14
F1 does not receive blocking from M1a and Trips
F1 (67-2)
M2 and F2 / TIE / O102 and O104
TIE and M2 / F2 / O14 and O13
F2 / M2a / O103
M1a does not receive blocking from F1 and Trips
M1a (67-1)
M2a / TIE-a / O102
So the faulted line segment between breakers F1 and M1a is cleared
quickly by the ZSI scheme and the rest of the load continues to operate.
Note that some breakers are blocked more than once; the TIE is blocked by
F1 and F2. The F2 block is in a direction that the TIE 67-2 element which is
not picked up, so nothing occurs. Faults downstream of F2 could cause
current to flow across the TIE in the 67-2 direction so this blocking is
necessary.
© 2009 Schneider Electric All Rights Reserved
17
Sepam™ ZSI Application Note
3000DB0902
05/2009
Figure 15:
18
Fault Scenario SC2
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
SEPAM SFT2841 Settings for Closed Loop ZSI
Sepam™ ZSI Application Note
Figure 16:
Closed Loop ZSI Main and Tie 51 Settings
In the closed loop system, the 67 elements (see Figure 17) perform the
blocking and tripping functions, therefore only the backup time based 51
settings are needed.
Figure 17:
Closed Loop ZSI Main and Tie 67 setting example.
Figure 18:
Closed Loop ZSI Control Matrix – Logic/Outputs
Figure 19:
Closed Loop ZSI Control Matrix – Protection/Outputs
A subtle but important point is that the blocking assignments are made in
the Logic/Output section of the control matrix, not the Protection/Outputs
page (refer to Figures 18 and 19). The gray outputs are the outputs
controlled by the “Circuit Breaker Control” (ON) setting.
© 2009 Schneider Electric All Rights Reserved
19
Sepam™ ZSI Application Note
3000DB0902
05/2009
Figure 20:
Series 40 Feeder in the Closed Loop ZSI - 51
The feeders in the closed loop have the same setting philosophy, the 67
elements block and trip and the 51 element is for backup (see Figures 20
and 21).
Figure 21:
20
Series 40 Feeder in the Closed Loop ZSI – 67
© 2009 Schneider Electric All Rights Reserved
3000DB0902
05/2009
Sepam™ ZSI Application Note
Control Diagram
Figure 22:
Control Diagram with “external” block sending and receiving
PTD
1
A
M7
ZSI BLOCKING
RECEPTION 1
I13
M8
DL5C
MES114
S4D
RTO
13
RTO
14
M1
I11
M2
M4
DL5C
MES114
S4
S4D
I12
DL5C
MES114
S4D
M5
A1
L(+)
DL5C
S40
POWER
SUPPLY
N(-)
A2
125VDC
SOURCE
RTD
5
53
RTN
15
0
GRD
G 10
RTN
21
A10
21
12
RTO
2
22
13
54
A11
RTN
16
DL5C
S4D
03
L5
L6
DL5C
MES114
012
SDL
52/d
SDL
52
/b
BLOCKING SIGNAL
FROM T3-N-01 5069
PO#24383883-007
MAIN BREAKER
RTD
6
56
RTO
3
RTO
1
60
RTN
22
BLOCKING SIGNAL
BREAKER 52-M1
RTO
4
BLOCKING SIGNAL
BREAKER 52-T
PTD
2
B
What is unique to this scheme in comparison to the M-T-M scheme is that
care must be taken so that the two different lineups of switchgear do not tie
DC power (typically 125Vdc) sources together.
The convention used by the Square D Company switchgear plant in
Smyrna, TN is to power the dry output contacts with the DC voltage from the
“input” location. With this convention, the dry contacts are whetted at the
location of the inputs. Refer to Figure 22.
© 2009 Schneider Electric All Rights Reserved
21
Sepam™ ZSI Application Note
Data Bulletin
Schneider Electric USA
Street Address
City, State Zip Country
1-888-SquareD (1-888-778-2733)
www.schneider-electric.us
22
3000DB0902
05/2009
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