FCT Not Determined

8/9/2019 FCT Not Determined

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Arc FlashFault Clearing Time Determination

6/17/2014

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S.NO CONTENTS

1. FCT not determined message

2. Cause1 : No Protective device

3. Solution1

4. Cause2 : Interlocks are missing

5. Solution2

6. Cause3 : Missing Library information

7. Solution3

8. Cause4: Magnitude of arc current is low

9. Solution4

10. Cause5: Bus Levels and Source current contribution.

11. Solution5

12. Symptoms and diagnosis

13. Tools and debugging techniques from ETAP


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Definitions

FCTFault Clearing Time in seconds or cycles LVCB Low Voltage Circuit Breaker

HVCBHigh Voltage Circuit Breaker

IbfBolted fault current

IaArc current

PDProtective device

OCROvercurrent Relay

TCCTime Current Characteristics

CTCurrent Transformer

PTPotential Transformer

SQOPSequence of Operation

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FCT not determined message

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ETAP eliminates the manual work related to determining the

protective device(s) that clear arc faults and FCT. In some

situations, the program provides a message that reads: FCT

not determined.This message indicates that ETAP did not find

source protective device(s) to de-energize every singleenergizing branch.

This article summarizes the causes of why ETAP will display this

message and how to troubleshoot the system to pinpoint the

reason(s).

NOTE: The problem can be caused by a single item or

combination of any of the issues described in this document


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Causes for FCT not determined

FCT NotDetermined

1. No Protectivedevice in

energizing path

2. Interlocks are

missing

3. Missing Libraryinformation

4. Magnitude of

arc current is toolow

5. Bus levels awayto find Source PD

and Multiplesource

contribution levels

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CAUSE1NO PROTECTIVE DEVICE IN ENERGIZING PATH

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Cause1

If there is no protective device to de-energize the fault, the

message is displayed.

In this example , the utility source is directly connected to

the MainBuswithout any protective device.

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Cause - 1

The message will appear If the option User-defined from

Bus Editor is enabled from the study case AF FCT page (as

shown below), and the bus user-defined FCT value is set tozero.

In the images below, the User defined FCT section has no

values and the study case shows FCT is based on User-

defined from bus editor

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SOLUTION1

ADDING PROTECTIVE DEVICE IN THE ENERGIZING

PATH

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Solution -1

The following protective devices are available in ETAP that

can be added in the energizing path to trip the circuit in the

event of a fault

1. Disconnect Switch interlocked to a Relay

2. Fuse3. LVCB with self trip unit or interlocked to a Relay

4. HVCB interlocked to a Relay

5. Contactor interlocked to a Relay

6. Overload Heater & In-Line Overload Relay7. Recloser

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Solution -1

In this case, CB29 (HVCB) is introduced between Utility and

Main Bus in order to de-energize the circuit in the event of afault at the bus, with FCT as 0.236 seconds, which includes the

relay and breaker trip time.

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Note that the message FCT Not Determined will still be

displayed for a arc fault on the line-side (source-side) of the

newly added protective device.


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CAUSE2

INTERLOCKS ARE MISSING

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Cause -2

Protection schemes with relay devices require an interlock

between the relay and the protective device.

ETAP will not determine the clearing time if the interlock is

missing since the overall fault clearing time is the sum of the

PD opening time plus the relay trip time.

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SOLUTION2

DEFINE INTERLOCKS

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Solution -2

Interlocks are defined in the Output page of the Relay editor.

In this example, the OCR 1 (Overcurrent Relay) is interlockedwith CB29 , such that on the event of the fault, the relay

sends the trip signal to Open the circuit breaker.

The following interlocks are available for a relay

Interlock to HVCB, LVCB, Switch, Contactor.

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CAUSE3MISSING LIBRARY INFORMATION

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Cause -3

If the protective device from the library is missing then the

program cannot find the FCT. See image below. This applies to all protective devices with curves in the library

such as LVCB, Fuses, Relays, Reclosers, etc .

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SOLUTION3

SELECT LIBRARIES

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Solution -3

Select the model from the library from the OCRpage of the

Relay editor to provide TCC information for the determinationof trip time.

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Solution -3

Check that the relays, protective devices, current transformers

that are involved in fault clearing have data defined from thelibrary.

RelaysDefinition of CT and Breaking Device

1. TCC curves for the trip signal

2. CTTurns Ratios and Burden3. LVCBTCC curves from the Trip Unit

4. Recloser - TCC curves from the Controller

5. FuserMelting curves from the manufacturer rating

6. Overload Heater - TCC curves from the Heating Unit

7. In-line overloadTCC curves from the Thermal library

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CAUSE4MAGNITUDE OF ARC CURRENT IS TOO LOW

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Cause-4

The arc current Iais typically less than the available bolted

short circuit current; especially in low voltage systems

(Typically less than 1kV).

ETAP displays FCTnot determined if the arc current is too

low such that it falls below the long-time pick up value of the

trip unit.

In this example, when Bus1 is faulted, the arc current is too

low and it lies on the left-hand-side region of the Fuse TCC

below any possible pickup time

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The magnitude of arc

current is 10.87kA at

0.48 kV and does not

intersect with the

available TCC curve inorder to clear the

fault.

Cause4

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SOLUTION4

CONSIDER ALTERNATIVE PROTECTIVE DEVICE SIZESOR INCIDENT ENERGY MITIGATION SCHEMES

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Solution4

Make sure that the protective device selected is capable of

tripping under the arc fault current conditions. If the fault clearing time of the selected protective device is

high, a message which indicates high incident energy value

may appear on the one-line diagram.

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CAUSE5

BUS LEVELS AWAY TO FIND SOURCE PD ANDMULTIPLE SOURCE CONTRIBUTION LEVELS

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Cause5

The search for source protective devices has some definable

capability limits. This section describes how those limits areapplied.

The main reason for these limits is to optimize the

calculation time by reducing memory consumption byexcluding unnecessary data

Bus Levels Away to Find Source PD - This limit indicates the

number of bus levels away from the faulted location thatETAP will search to find the source PD. The source PD is then

used to determine the FCT.

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C 5


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The default value for this entry is ten(10). The maximum level is

fifty (50). It is recommended that this entry be left as default,

unless the source PD is located more than 10 levels away.

The circled PDs has to be within the Bus levels away to Find

source PD , Otherwise, ETAP cannot determine the FCT

Note that protective devices which are considered as Load PDs

are not considered for determining the bus FCT as they cannot

isolate the fault at the bus as mentioned in the image below.

Cause5

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C 5


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Cause5


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Cause5

Multiple Source Contribution - This value indicates the

number of multiple sources contribution levels to beconsidered. A level is encountered every time two sources

mesh upstream from the fault location.

If the corresponding sources are not seen by the limit that isset up, the program still displays the message. The default for

multiple source level search is two (2) as can be seen in the

following image.

However, there may be some systems in which the source PDs

are located further away in higher levels of multiple source

contributions

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Cause 5


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Cause5

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Cause 5


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Cause5

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SOLUTION -5OPTIONS (PREFERENCES)

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Solution 5


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Solution -5

The Tools Options(Preferences) Arc Flash Section has the

following two entries that can help to determine the FCT for

Cause5.

The number of levels can be changed by modifying the entry

in Tools/Options (Preferences) Bus Levels Away to Find

Source PD = 10 (default).

Multiple Source Contribution Levels : The searchable area of

the program is limited by the number of meshed source fault

current contributions and the number of bus levels away.

The search area can be increased by modifying the following

entry under Tools\Options (Preferences) section: Multiple

Source contribution levels = 2 (default) 34

Solution 5


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Solution -5

Using a very high number for both above options can cause

performance issues with arc flash calculations.

The Highest level of meshed source contribution is level 4 .

Multiple utility sources energize this location.

In a real system, the individual sources could be wind

turbines, PV arrays or synchronous generators.

Whatever the case may be, every time a source contributionis meshed, the program will require a higher level number as

shown in the image (which requires Level 5)

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Solution 5


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Solution -5


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SYMPTOMS AND DIAGNOSIS

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Symptoms

Symptoms

1. Main PDIsolation

2. DecayMethod

3.SinglePhase

Breaker

4. Back feedfrom Tie-breaker

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Main PD Isolation


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Main PD Isolation

Symptom

The program reports FCT Not Determined for a

particular bus, yet the main protective device has beenproperly configured with libraries, interlocks, etc.

The main PD may not be properly isolated from the main bus

or individual compartments (i.e. the line side of the main PD is

exposed or there is chance the arc fault will expand into thissection. If the upstream PD is not properly configured, the FCT

may not be determined.

Note 1: This option is applicable only for systems rated 15kV

or less. Note 2: The main PD isolation option can be removed by un-

checking it from the Study Case editor -> AF FCT tab.

The following link gives more detailed analysis for the same.https://helpdesk.etap.com/cgi-in/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Default

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Decay Method
https://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Defaulthttps://helpdesk.etap.com/cgi-bin/wonderdesk.cgi?do=faq_view_record&faq=1&view_detail=1&faq_id=490&queue=Default


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Decay Method

Symptom

-FCT is determined using either the 1/2 Cycle or 1.5-4 cyclemethods

-FCT is no longer determined when using the decay method

The FCT for decay method is obtained by integrating thecurrents from three different stages (subtransient, transient

and steady-state). The integration using IEEE Std C37.2

equation as shown.

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Decay Method


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Decay Method

In this example, The decay method is used on Bus20

The steady state arc current falls below the relay long

time pickup setting and the magnitude and duration of

subtransient and transient currents were not enough to

cause the relay to trip 41

Decay Method


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To correct this situation, protective device settings adjustments

which cause relay operation need to be implemented 42

Decay Method


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Arc Flash 1-Phase

Symptom

Single-phase system has upstream

protective device in 3-phase region

which is outside the searchable area

when the fault occurs in the 1-phase

system

For 1-phase & panel sub-systems, the

program cannot search the source PD

that is in the three phase system (i.e. it

only searches all 1-phase PDs feeding

the system The image shows Fuse 1 cannot de-

energize the Bus2 as it is single phase.

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Arc Flash Single Phase


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Arc Flash Single Phase

Adding a source protective device inthe single phase system as shown in

the image (Fuse2) will help to de-

energize the faulted bus and determine

the fault clearing time.

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Back Feed from Tie-Breaker

Symptom

- All source PDs appear to be tripping, but the program failsto find the FCT

In case of Meshed or Loop system with tie-PDs involved, there

is a possibility of small arc current flowing back through the

looped path. The magnitude of this back-feed arcing currentmay not be enough to trip feeder relay

ETAP requires that all sources of current be de-energized to

determine the final FCT (regardless of how small of a

contribution they make to the fault location). This is aconservative method since it is not known exactly at what

point an arc fault will self extinguish as multiple sources de-

energize at different times.

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Back Feed from Tie- Breaker


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Back Feed from Tie Breaker

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In this example , The upstream breakers clear the fault butthere is a small arc current that is back fed from the source

which prevents the fault from being completely de-energized

Small Magnitude of Ia

Back Feed from Tie- Breaker


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Back Feed from Tie Breaker

If the relay is set to pick up this small magnitude of Ia that

is back fed, then the total fault clearing time can be

determined.

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TOOLS AND DEBUGGING TECHNIQUES FROM ETAP

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Faulting only one bus

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Faulting only one bus is a great way to troubleshoot the FCTnot

determinedcondition.

It marks the source protective devices by showing the arc

current next to them. This allows you to graphically see which

source PDs lie on the energizing branches.

Only source PDs are marked with current flows (even though all

the contributions are considered) as shown in the image below.

Tools


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Tools

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Using the AF SQOP Tool


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The SQOP tool indicates two very important items for

FCT not determined troubleshooting:

1) It lists only the source PDs which energize the fault

location

2) It also lists the source PDs which did not trip. These

are the PDs which are causing the FCT not Determined

condition



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Us g t e SQO oo



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Documents

FCT Not Determined