FCT Not Determined

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Arc Flash – Fault Clearing Time Determination

6/17/2014

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S.NO CONTENTS

1. FCT not determined message

2. Cause – 1 : No Protective device

3. Solution – 1

4. Cause – 2 : Interlocks are missing 5. Solution – 2

6. Cause – 3 : Missing Library information

7. Solution – 3

8. Cause – 4: Magnitude of arc current is low

9. Solution – 4

10. Cause – 5: Bus Levels and Source current contribution.

11. Solution – 5

12. Symptoms and diagnosis

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Definitions

• FCT – Fault Clearing Time in seconds or cycles • LVCB – Low Voltage Circuit Breaker

• HVCB – High Voltage Circuit Breaker • Ibf – Bolted fault current

• Ia – Arc current

• PD – Protective device • OCR – Overcurrent Relay

• TCC – Time Current Characteristics • CT – Current Transformer

• PT – Potential Transformer

• SQOP – Sequence 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 single energizing 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 Not Determined 1. No Protective device in energizing path 2. Interlocks are missing 3. Missing Library information 4. Magnitude of arc current is too

low

5. Bus levels away to find Source PD

and Multiple source

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CAUSE – 1

NO PROTECTIVE DEVICE IN ENERGIZING PATH

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Cause – 1

• 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 “Main Bus” without 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 to zero.

• 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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SOLUTION – 1

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. Fuse

3. 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 Relay

7. 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 a fault at the bus, with FCT as 0.236 seconds, which includes the relay and breaker trip time.

• 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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CAUSE – 2

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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SOLUTION – 2

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 interlocked

with 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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CAUSE – 3

MISSING 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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SOLUTION – 3

SELECT LIBRARIES

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Solution -3

• Select the model from the library from the “OCR” page of the Relay editor to provide TCC information for the determination of 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 the library.

• Relays – Definition of CT and Breaking Device 1. TCC curves for the trip signal

2. CT – Turns Ratios and Burden

3. LVCB – TCC curves from the Trip Unit

4. Recloser - TCC curves from the Controller

5. Fuser – Melting curves from the manufacturer rating 6. Overload Heater - TCC curves from the Heating Unit 7. In-line overload – TCC curves from the Thermal library

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CAUSE – 4

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Cause-4

• The arc current “Ia” is typically less than the available bolted short – circuit current; especially in low voltage systems (Typically less than 1kV).

• ETAP displays “FCT not 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 in

order to clear the

fault.

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SOLUTION – 4

CONSIDER ALTERNATIVE PROTECTIVE DEVICE SIZES

OR INCIDENT ENERGY MITIGATION SCHEMES

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Solution – 4

• 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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CAUSE – 5

BUS LEVELS AWAY TO FIND SOURCE PD AND

MULTIPLE SOURCE CONTRIBUTION LEVELS

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Cause – 5

• The search for source protective devices has some definable capability limits. This section describes how those limits are applied.

• The main reason for these limits is to “optimize the calculation time by reducing memory consumption by excluding unnecessary data”

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

number of bus levels away from the faulted location that ETAP will search to find the source PD. The source PD is then used to determine the FCT.

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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.

Cause – 5

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Cause – 5

• Multiple Source Contribution - This value indicates the number of multiple sources contribution levels to be considered. 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 is set 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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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 Cause – 5.

• 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)

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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 contribution is meshed, the program will require a higher level number as shown in the image (which requires Level 5)

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Symptoms

1. Main PD

Isolation

2. Decay

Method

3.Single

Phase

Breaker

4. Back feed

from Tie-

breaker

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Main PD Isolation

Symptom

– The program reports “FCT Not Determined” for a particular bus, yet the main protective device has been properly 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 this section. 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.

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Decay Method

Symptom

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

-FCT is no longer determined when using the decay method

• The FCT for decay method is obtained by integrating the currents 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

• In this example, The decay method is used on “Bus 20” • 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

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• To correct this situation, protective device settings adjustments which cause relay operation need to be implemented

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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

• Adding a source protective device in the 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 fails to 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 current may 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 a conservative 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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•In this example , The upstream breakers clear the fault but there is a small arc current that is back fed from the source which prevents the fault from being completely de-energized

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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

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TOOLS AND DEBUGGING TECHNIQUES FROM ETAP

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Faulting only one bus

Faulting only one bus is a great way to troubleshoot the “FCT not determined” condition.

• 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.

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Tools

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Using the AF SQOP Tool

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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