NFPA 70E TABLE MAX FAULT CLEARING TIME

[mikeybmdb32]

So, I know that the tables for NFPA 70E cannot be used if the exact parameters are not met (ie working distance, fault clearing time, max short ckt current, etc). However, I'm confused as to what the meaning of maximum fault clearing time means. Does it mean:

1) the actual maximum time that the upstream device could take to open. This would include the overload regions of the breaker curve and would almost never meet the table requirements.

2) the upstream device's operating time at the maximum short circuit current listed in the table?

3) the upstream device's operating time at the actual short circuit current of the equipment being worked on?

4) something else?

As always, thanks.

 

[don_resqcapt19]

In my opinion it is #3.

 

[wbdvt]

The tables represent at best a compromise to give people something to provide the proper PPE. The problem is that the majority of the people do not know what the short circuit levels are in their facility. In order to determine that a short circuit study needs to be done and if that is the case, it is minimal additional effort to do an arc flash evaluation.

All that being said, the table parameters mean that it is the fault current at the equipment being worked on that the protecting device would see. For example: A panel cover needs to be removed to perform IR scan. The breaker feeding this panel would have to trip in less than the maximum time and the fault current would have to be less than the maximum listed fault current. If the panel has a main breaker, this is not the protecting device as it could be compromised in an arc flash. The maximum current parameter is also not the maximum possible fault current but the actual fault current. The tables also ignore that the arcing fault current is less than the bolted fault current. So, how do you know that the parameters of the table are being met?

Therefore using the tables is a compromise at best. There was a study done by DuPont on arc flash events at their facilities and they concluded:

• having no PPE resulted in injuries 100% of the time
• using the NFPA 70E tables resulted in less than adequate PPE resulting in injuries 50% of the time
• performing a study to determine actual incident energy levels resulted in proper PPE and no injuries

So, I guess the obvious question is how much do you want to compromise the safety of your employees?

 

[mikeybmdb32]

I wrote the original post poorly. Here's what I meant:

1) the actual maximum time that the upstream devices could take to open. This would include the overload regions of the device curves (which can take hours to trip/blow) and would almost never meet the table requirements.

2) the upstream device's operating time at the maximum short circuit current listed in the table (ie table says max available short circuit can be no greater than 65kA, so use 65kA)?

3) the upstream device's operating time at the actual maximum available short circuit current of the equipment being worked on (ie short circuit study shows that max short circuit current is 37kA, so find the operating time for 37kA)?

4) something else?

 

[mikeybmdb32]

wbdvt,

You said exactly what I'm thinking; there's no way to know what the actual fault current will be and, as a result, you can't know if the maximum fault clearing time will be met. 130.7(C)(15) requires that the maximum fault clearing time be met, but this seems to be a CYA for NFPA since there is no way to guarantee the trip time will be less than the table requirement, so it's on YOU if there's an injury since YOU worked outside of the parameters prescribed by NFPA. I understand the need for CYA, but this requirement causes confusion/frustration on the part of the maintenance guys since they are trying to figure out a maximum trip time when it cannot actually be found. They instead should remove this requirement and note that there is a lower probability of injury when the tables are used, but that injury is not completely eliminated because a certain maximum fault clearing time is assumed.

 

[zog]

Exactly, and that's the flaw in the tables. When you do an actual study you model many different fault currents and clearing times and often you find the lower fault currents actually produce the largest hazard due to longer clearing times.

 

[Bugman1400]

If this is in reference to arc-flash for figuring out the proper PPE class then, I think the maximum exposure time is 2 seconds. This comes from the typical reaction time for a worker to turn away or pull away from the flash. The other device trip times should be faster but, if they are not, then the 2 seconds would be the limit.

 

[wbdvt]

If this is in reference to arc-flash for figuring out the proper PPE class then, I think the maximum exposure time is 2 seconds. This comes from the typical reaction time for a worker to turn away or pull away from the flash. The other device trip times should be faster but, if they are not, then the 2 seconds would be the limit.

2 sec can be used under certain circumstances when DOING A STUDY, I.E. AN ENGINEERING ANALYSIS. It cannot be used in the context of the tables in NFPA 70E Those parameters are what need to be met for the tables.

 

[Bugman1400]

2 sec can be used under certain circumstances when DOING A STUDY, I.E. AN ENGINEERING ANALYSIS. It cannot be used in the context of the tables in NFPA 70E Those parameters are what need to be met for the tables.

I'm not sure I follow. Specifically, which Tables are you referring to? Also, can you elaborate when you say that the 2 seconds can't be used in the context of the tables?

Thank you.

 

[wbdvt]

The discussion has been on the tables in NFPA 70E to determine PPE. These tables have parameters associated with them - max fault current and max clearing time. Both parameters must be met to use tables.

I don't have NFPA 70E with me but the clearing time is in cycles, typically 2 to 5 cycles, so far less than 120 cycles (2 sec). The 2 seconds can be used if you are doing a study and know that the person can escape or get blown clear. This value can then be entered as a max time for the software program to use with the actual arcing current.

 

[Bugman1400]

The discussion has been on the tables in NFPA 70E to determine PPE. These tables have parameters associated with them - max fault current and max clearing time. Both parameters must be met to use tables.

I don't have NFPA 70E with me but the clearing time is in cycles, typically 2 to 5 cycles, so far less than 120 cycles (2 sec). The 2 seconds can be used if you are doing a study and know that the person can escape or get blown clear. This value can then be entered as a max time for the software program to use with the actual arcing current.

After a bit more reading and reviewing the Table 130.7(C)(15)(A)(b), I think I understand what you mean.

However, it is easy to estimate the max available short circuit.... even if its not known. The conservative approach is to assume an infinite source. If the xfmr impedances are known and the cable lengths, sizes, and material are obtained, the max available short circuit can be calculated. At low voltage levels, there is often little difference between an infinite source and a lower known source.

Also, am I to assume that the max clearing times are at the estimated max fault level? If so, this could lead to recommended PPE levels that are below what they should be since it is known that a reduced fault level will lead to longer clearing times and higher incident energy. Perhaps the tables take this into account, apply a safety margin, and recommend a higher PPE level, whereas, an arc-flash study would recommend a lower PPE level.

 

[wbdvt]

The max available fault current derived from an infinite bus is not conservative from an arc flash point of view. For example, using the max fault current may allow you to use the tables because at that level, the OCPD trips at less than the time parameter of the table.

But what if the fault current is considerably lower and now the OCPD trip time is 4 seconds. Will the employee that got his/her PPE from that table be adequately protected from an arc flash?

 

[zog]

The max available fault current derived from an infinite bus is not conservative from an arc flash point of view. For example, using the max fault current may allow you to use the tables because at that level, the OCPD trips at less than the time parameter of the table.

But what if the fault current is considerably lower and now the OCPD trip time is 4 seconds. Will the employee that got his/her PPE from that table be adequately protected from an arc flash?

And very often that is the case, the lower fault currents with longer clearing times produce the highest arc flash hazard.

 

[Bugman1400]

The conservative approach can be with using a higher fault current. However, I think using the word 'conservative' in my previous post was for calculating the max fault current, not for incident energy. I agree that a lower fault current leads to longer clearing times and I refer to just that in my previous post. However, sometimes the lower fault current and longer clearing times leads to higher incident energy and sometimes it doesn't. Sometimes, a drastically higher fault current leads to higher incident energy because of the setup of the protection scheme. All that to say, is it just depends on the system under study.

No?

 

[lielec11]

I'm late to this party, but just to be sure are you talking about the 2015 version of NFPA 70E?

 

[Bugman1400]

I'm late to this party, but just to be sure are you talking about the 2015 version of NFPA 70E?

As far as I can tell....yes.

 

[wbdvt]

The conservative approach can be with using a higher fault current. However, I think using the word 'conservative' in my previous post was for calculating the max fault current, not for incident energy. I agree that a lower fault current leads to longer clearing times and I refer to just that in my previous post. However, sometimes the lower fault current and longer clearing times leads to higher incident energy and sometimes it doesn't. Sometimes, a drastically higher fault current leads to higher incident energy because of the setup of the protection scheme. All that to say, is it just depends on the system under study.

No?

No. You need to know the actual bolted fault current and clearing time to use the table in NFPA 70E. This would be obtained from a short circuit study where the actual utility available fault current is used. The term "max fault current" refers to just that. The max fault current delivered by the utility, not a utility that consists of an infinite bus but the actual real world utility.

Not sure how much more clear I can be on this.

 

[don_resqcapt19]

No. You need to know the actual bolted fault current and clearing time to use the table in NFPA 70E. This would be obtained from a short circuit study where the actual utility available fault current is used. The term "max fault current" refers to just that. The max fault current delivered by the utility, not a utility that consists of an infinite bus but the actual real world utility.

Not sure how much more clear I can be on this.

But the maximum available fault current from the utility often changes, even within a given day. You really need to do a number of calculations based on the fault currents that may reasonably be available from the utility.

 

[wbdvt]

But the maximum available fault current from the utility often changes, even within a given day. You really need to do a number of calculations based on the fault currents that may reasonably be available from the utility.

The utility may give you a min and max fault current or they may give you just one. Most utility systems are fairly stable and drastic changes in fault current are usually due to loss of lines requiring switching to different sources or if your facility is located close to a major generation facility that goes off line. Typically utilities will strive to return to normal operating condition as soon as possible.

Studies are required to be reviewed every 5 years as a minimum and part of this reasoning is to capture any utility changes.

 

[Bugman1400]

No. You need to know the actual bolted fault current and clearing time to use the table in NFPA 70E. This would be obtained from a short circuit study where the actual utility available fault current is used. The term "max fault current" refers to just that. The max fault current delivered by the utility, not a utility that consists of an infinite bus but the actual real world utility.

Not sure how much more clear I can be on this.

I just don't see where it indicates in NFPA 70E_2015 that the actual bolted fault current and clearing time is required to use Table 130.7(C)(15)(A)(b). It does says, "The estimated maximum available short-circuit current, maximum fault-clearing times, and minimum working distances for various ac equipment types or classifications are listed in Table 130.7(C)(15)(A)(b)." It then lists the scenarios where you cannot use the table. I can tell you that when the actual bolted fault current from the utility is unknown, an infinite bus is often used. As I said before, when you get down to the lower voltage buses (ie 240 & 120) from an medium voltage service, the actual bolted fault current is not much different than an infinite bus.