ETAP FAQ # 23 - What are the most common roadblocks to finding Fault Clearing Time in arc flash calculations?

ETAP FAQ # 23

What are the most common roadblocks to finding Fault Clearing Time in arc flash calculations?

Description: Seven most common reasons why the Arc Flash program cannot automatically determine the Fault Clearing Time. Version: ETAP 5.5.0 Published: June 30, 2006 ETAP eliminates the manual work related to determining the protective device(s) that clear arc faults and the Fault Clearing Time (FCT). In some situations, the program provides a message that reads: “FCT not determined.” This message indicates that ETAP did not have enough data to find a protective device in the path that energizes the equipment, or it was missing data for determining FCT.

This article summarizes the seven most common reasons why ETAP will display this message and how to troubleshoot the system to pinpoint the reason(s).

Please note that when ETAP can’t determine the FCT from the upstream protective devices due to the seven roadblocks presented in this article, it will use the bus user-defined FCT to calculate the incident energy with a flag in the output report indicating this action. This article assumes that the user-defined FCT in the bus editor has been set to zero.

1. There are no source protective devices configured to protect the arc fault location. If you have not added the protective device which actually de-energizes the equipment in the event of a fault, the program may display the message “FCT not determined.” In the image below, the utility connection does not have a protective device. If there is a fault on the line side of the medium voltage breakers, there is no physical protective device that can clear the fault. If the bus “user-defined FCT” is also set to zero, you will see “FCT not determined” displayed on the one-line diagram, as well as on reports and arc flash labels.

2. The circuit breaker interlock for the relay is missing or there is no data selected from the library. Some protective devices require a relay to trip them, like high-, medium- and some low-voltage circuit breakers. ETAP determines automatically which relay will trip and in which order, but it requires that users specify which breaker is interlocked to the relay (which breaker will be tripped by the relay selected by ETAP). If you did not select the protective device from the library, a message will appear alerting you that the FCT cannot be determined. This applies to LVCB, Relays, Fuses, etc.

ETAP FAQ # 23 - What are the most common roadblocks to finding Fault Clearing Time in arc flash calculations?

3. Arcing current is too low to trip the protective devices. The arcing current is typically less than the available bolted short-circuit current for some equipment, especially for low voltage systems. According to IEEE 1584 equations, the arcing current may be less than the short circuit current for systems up to 15 kV. Because of this phenomenon, the protective devices may not trip at all under an arc fault. ETAP includes the arcing current variation automatically to further reduce the calculated current. If ETAP detects that the source protective device does not trip, then it will display the “FCT not determined” message.

ETAP FAQ # 23 - What are the most common roadblocks to finding Fault Clearing Time in arc flash calculations?

In the case of a fault at “Bus30,” the arcing current is too low on the primary side of Transformer “T9.” To determine the arcing current on the primary of T9, take the ratio of the SC contribution over the total SC current and multiply the total arcing current by this ratio.

(34.33 kA/35.3 kA) * 24.27 kA = 23.59 kA

Then convert to the primary base kV.

23.58 kA * (0.48kV/13.2kV) = 0.857 kA

If the calculated arcing current on the primary is too low for “Relay28” to trip, you can confirm this by plotting the relay on a Star view TCC. In the TCC shown below, the red arrow indicates the arcing current value at the beginning of the fault. This clearly shows that the relay does not trip.

Sometimes the protective device will still trip, but it will do so in the overcurrent protection section of the relay. In this case, ETAP will calculate the potential incident energy released and will flag it on the one-line diagram with a message stating: “Exceeds Max. PPE Arc Rating.” This message indicates that the FCT was determined, but is too long, and thus the energy released exceeds the maximum value specified by NFPA 70E Category 4. This is the case for a fault at “Bus14.”

ETAP FAQ # 23 - What are the most common roadblocks to finding Fault Clearing Time in arc flash calculations?

4. The Source Protective Device is outside the Search Zone. To reduce the calculation time and optimize the computer system requirements, a user-defined limit is provided by ETAP for finding the source protective device. This limit indicates the number of bus levels away from the faulted bus that ETAP will search to find the source protective device. The source protective device is then used to determine the FCT.

The default value for the bus levels is 3. In some systems, the protective devices could be placed more than three bus levels away and thus a bigger search zone is required. If the program does not find all the corresponding source protective devices within 3 bus levels, it would again show the message “FCT not determined.”

In ETAP 5.0.3, the way to increase the search level (bus levels away from the fault) is to add the following entry to the ETAPS.INI file, below the [Etap PowerStation] header:

MaxSourceIfLevel=6

In ETAP 5.5.0, the same can be accomplished from the new “ETAP Preferences” tool. You can launch the ETAP Preferences from Tools menu bar and then locate the entry:

Bus Levels Away To Find Source PD

You can modify this entry to 10 or higher (up to 20 bus levels away).

ETAP FAQ # 23 - What are the most common roadblocks to finding Fault Clearing Time in arc flash calculations?

The following image illustrates the concept of bus levels and the search zones for finding the source protective device for a fault at the indicated bus.

5. The source protective device is completely outside the search area of the program. For some very special cases, ETAP Arc Flash will not be able to determine the FCT if the source protective device cannot be located. This means that the system does not have protection within the searchable area of the system. The following image highlights the location of the non-searchable part of the network for a fault at the indicated bus:

ETAP FAQ # 23 - What are the most common roadblocks to finding Fault Clearing Time in arc flash calculations?

As you can see, there are several protective devices which would clear the fault, and the situation where all the PDs are outside the searchable area is quite unlikely.

6. The bolted 3-phase fault current is outside the range allowed by IEEE 1584. There are certain limitations in the IEEE model. One of them is the limitation on the available short-circuit current. The bolted fault current must be within the range of 0.7 to 106 kA. If the fault current falls outside this range, then the arc flash results are not valid based on the available equations. If the program detects this situation, the “FCT not determined” message will be displayed as shown in the image below:

According to IEEE 1584, it is very difficult to sustain an arc at voltages nearing 0.208 kV and thus for lower values it may not be necessary to calculate the incident energy. If the transformer feeding this fault is higher than 125 kVA it may be desirable to estimate the incident energy even if the nominal voltage is less. For these situations, assuming a 0.208 kV with the

ETAP FAQ # 23 - What are the most common roadblocks to finding Fault Clearing Time in arc flash calculations?

same available short-circuit current may yield conservative results. This is an engineering decision that must be made for each specific situation and OSHA requirements.

7) The bus nominal kV is outside the range allowed by IEEE 1584. The empirical method is only valid for voltages higher than 0.208 kV (kV LL). The arc flash results are not valid for voltage levels lower than this value. ETAP detects this situation and it will show the “FCT not determined” message. The following image illustrates this point: