3000DB0902 ZSI Application Note - Schneider Electric

Data Bulletin 

3000DB0902 

05/2009 

LaVergne, TN, USA 

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: 

Arc Flash Results for a 100ms ZSI system 

Bus Name 

kH represents 

bolted 3PH 

fault current 

Protective 

Device 

Name 

Bus 

kV 

Bus 

Bolted 

Fault 

(kA) 

Prot 

Dev 

Bolted 

Fault 

Prot Dev 

Arcing 

Fault 

(kA) 

Trip/ 

Delay 

Time 

(sec.) 

Breaker 

Opening 

Time 

(sec.) 

Ground Equip 

Type 

Arc Flash Working Incident 

Gap 

BoundaryDistance 

Energy 

(mm) 

(in) (in) (cal/cm2) 

Required 

Protective 

FR 

Clothing 

4160V_5kA ZSI_0.1_5kA 4.16 5 5 4.91 0.1 0.083 Yes SWG 104 29 36 0.96 Category 0 

4160V_10kA ZSI_0.1_10kA 4.16 10 10 9.71 0.1 0.083 Yes SWG 104 62 36 2 Category 1 



4160V_50kA ZSI_0.1_50kA 4.16 50 50 47.21 0.1 0.083 Yes SWG 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_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: 

Arc Flash Results for a 200ms ZSI system 

Bus Name 

Protective 

Device 

Name 

Bus 

kV 

Bus 

Bolted 

Fault 

(kA) 

Prot Dev 

Bolted 

Fault 

(kA) 

Prot Dev 

Arcing 

Fault 

(kA) 

Trip/ 

Delay 

Time 

(sec.) 

Breaker 

Opening 

Time 

(sec.) 

Ground Equip 

Type 

Arc Flash Working Incident 

Gap 

Boundary Distance Energy 

(mm) 

(in) (in) (cal/cm2) 

Required 

Protective 

FR Clothing 

Category 

A_4160V_5kA 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 

J_13800V_50kA ZSI 0.2 50kA 13.80 49.99 49.99 47.21 0.2 0.083 Yes SWG 153 634 36 19 Category 3 

Most 4160V systems have bolted three phase fault currents less than 20- 

30kA. From the table the incident energies are 4.2 and 6.5 cal/cm 2 , 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 



05/2009 


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: 

• 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. 

© 2009 Schneider Electric All Rights Reserved 3


05/2009 

Figure 3: 

MV application of ZSI within the lineup 

Figure 4: 

Radial ZSI Application for high speed line protection 

52 

52 

4 



05/2009 


Figure 5: 

MV application of ZSI M-T-M 

Figure 5a: 

Alternate MV application of ZSI M-T-M (Non-relayed Tie) 



05/2009 

Figure 6: 

Closed Loop / Ring Bus Application 

6 



05/2009 


Figure 7: 

Low Voltage MicroLogic trip units supply blocking signals to 

“virtual” main with SEPAM relay 



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 



05/2009 


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. 



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: SFT2841 Control Matrix for the Main 

10 



05/2009 


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 



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 

Switchgear Main Bus 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. 

*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 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. 

12 



05/2009 


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. 



05/2009 

Application 3 – Closed Loop System 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 (67- 

1) 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 



05/2009 


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 


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. 



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 



05/2009 


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 


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. 



05/2009 

Figure 15: 

Fault Scenario SC2 

18 



05/2009 


SEPAM SFT2841 Settings for Closed Loop ZSI 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. 



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: Series 40 Feeder in the Closed Loop ZSI – 67 

20 



05/2009 


Control Diagram Figure 22: Control Diagram with “external” block sending and receiving 

A 

PTD 

1 

M7 

M8 

I13 

ZSI BLOCKING 

RECEPTION 1 

DL5C 

MES114 

S4D 

RTO 

13 

RTO 

14 

M1 

M2 

I11 

DL5C 

MES114 

S4D 

M4 

M5 

I12 

DL5C 

MES114 

S4D 

A1 

L(+) 

DL5C 

S40 

POWER 

SUPPLY 

RTO 

1 

RTO 

3 

125VDC 

SOURCE 

RTD 

5 

BLOCKING SIGNAL 

FROM T3-N-01 5069 

PO#24383883-007 

MAIN BREAKER 

RTD 

6 

53 

12 

13 

54 

RTN 

15 

SDL 

52 

/b 

RTN 

16 

56 

21 

22 

60 

RTN 

21 

SDL 

52/d 

RTN 

22 

N(-) 

A2 

0 

GRD 

G 10 

A10 

A11 

DL5C 

S4D 

03 

RTO 

2 


BREAKER 52-M1 

L5 

L6 

DL5C 

MES114 

012 

RTO 

4 


BREAKER 52-T 

B 

PTD 

2 

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. 



Data Bulletin 05/2009 

Schneider Electric USA 

Street Address 

City, State Zip Country 

1-888-SquareD (1-888-778-2733) 

www.schneider-electric.us 

Square D ® is a trademark or registered trademark of Schneider Electric. Other 

trademarks used herein are the property of their respective owners. 

Electrical equipment should be installed, operated, serviced, and maintained only by 

qualified personnel. No responsibility is assumed by Schneider Electric for any 

consequences arising out of the use of this material. 

22

3000DB0902 ZSI Application Note - Schneider Electric

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