Clearing Times from 70E Table 130.7(C)(9)

[GMIEE]

I'm looking at basing some PPE on this table, but I am having an issue.
I've got a 480V panel that refers to Note 1: "maximum of 65kA short circuit current; maximum of 0.03 sec (2 cycle) fault clearing time". Well I meet the short circuit portion of this statement, but 2 cycle clearing time?? This is a service entrance panel on the secondary of a large transformer. Clearing time will probably be nowhere near .02 seconds. I need to hook up some monitoring equipment in this panel though. What to do?

 

[zog]

I'm looking at basing some PPE on this table, but I am having an issue.
I've got a 480V panel that refers to Note 1: "maximum of 65kA short circuit current; maximum of 0.03 sec (2 cycle) fault clearing time". Well I meet the short circuit portion of this statement, but 2 cycle clearing time?? This is a service entrance panel on the secondary of a large transformer. Clearing time will probably be nowhere near .02 seconds. I need to hook up some monitoring equipment in this panel though.

You are right, it wont be anywhere near 2 cycles, so you cant use the tables. Depending on the transformer size and breaker settings there probally dont make a flash suit that will protect you (Blast pressure limitations).

What to do?

Thats easy, shut it down, dosent matter what the HRC is, you should be shutting this down, unless it is "unfeasable".

 

[GMIEE]

Thats easy, shut it down, dosent matter what the HRC is, you should be shutting this down, unless it is "unfeasable".

This is what I thought too, but I keep running across qualified people that tell me it's fine to hook up a PQ monitor in this panel hot. The transformer is a 750kva.

 

[zog]

This is what I thought too, but I keep running across qualified people that tell me it's fine to hook up a PQ monitor in this panel hot. The transformer is a 750kva.

Well I suppose you could call that troubleshooting, but to hook them up you still need to do an arc flash analysis to determine the proper PPE required, becasue as you said you cant use the tables. And if the analysis says >40cal/cm2 you will haev to shut it down anyways.

Now that being said, for such a small transformer, you might be <40cal/cm2 depending on the protectove device settings.

 

[jim dungar]

If you are on the utility side of service entrance equipment, there is no protective device. If you are on the load side of the main protective device, unless there is isolation between the load and line side sides of the device, a fault on the load side will usually propagate to the line side. Except for compartmentalized equipment it is unlikely, although not impossible, that any service equipment will not be Dangerous.

You mentioned 65K, are you looking at the amount of available fault current or the short circuit current rating of the equipment?

 

[ron]

Are the short circuit criteria in the Notes of Table 13-.7(C)(9)(a) based on the short circuit symmetrical ampacity from a bolted 3-phase fault calculation (per Annex D.3 or ANSI method), or are they based on arcing short circuit current? I believe arcing short circuit current instead of bolted short circuit current, which means you have to make that calculation before using the notes.

Annex D.8.2. indicates that: to determine the operating time for protective devices, find the predicted three-phase arcing current. That would be significantly less than the bolted fault current current, sometimes 38% less (per Annex D.6(1).
When I calculate arc flash hazards using the IEEE methods I get similar ratios of bolted/arcing fault current too, depending on the voltage and conductor gap.

 

[GMIEE]

If you are on the utility side of service entrance equipment, there is no protective device. If you are on the load side of the main protective device, unless there is isolation between the load and line side sides of the device, a fault on the load side will usually propagate to the line side. Except for compartmentalized equipment it is unlikely, although not impossible, that any service equipment will not be Dangerous.

You mentioned 65K, are you looking at the amount of available fault current or the short circuit current rating of the equipment?

I am getting the 65kA from Note 1 in the table. It lists this as "short circuit current", although I suppose it is possible they are referring to a 3-phase bolted fault.

 

[jim dungar]

I am getting the 65kA from Note 1 in the table. It lists this as "short circuit current", although I suppose it is possible they are referring to a 3-phase bolted fault.

They are absolutely referring to a 3-phase bolted fault.

If you do not know the available fault current is definitely less than 65kSCA you can not use the tables. Simply knowing the AIC or SCCR of your equipment is not sufficient.

 

[ron]

Jim,
Are you sure the note is looking for 3 phase bolted short circuit current or could it be arcing short circuit current?
I agree though, it is definitely not the AIC or withstand rating of the equipment itself.

 

[GMIEE]

They are absolutely referring to a 3-phase bolted fault.

If you do not know the available fault current is definitely less than 65kSCA you can not use the tables. Simply knowing the AIC or SCCR of your equipment is not sufficient.

I know for SURE that the available fault current is less than 65kA. It is the 2 cycle clearing time that I was questioning. It seemed half useless to make a table based on an assumption that clearing time is less than 2 cycles. I almost never see clearing times that fast.

 

[zog]

I know for SURE that the available fault current is less than 65kA. .

So what is your available fault current, and how did you determine it?

 

[jim dungar]

Jim,
Are you sure the note is looking for 3 phase bolted short circuit current or could it be arcing short circuit current?
I agree though, it is definitely not the AIC or withstand rating of the equipment itself.

The foot note says "short circuit current available". It does not say arcing faut current.

All NFPA70E and IEEE 1584 formulas for determing Arc Flash Incident Energy begin with the 3-phase bolted fault current. Determining arcing fault current is then done as part of the process.

 

[ron]

I agree that the process starts with 3 phase bolted.
In order to figure out clearing time for an arc fault, it should involve knowing the arcing current and how it applies to the TCC for the OCPD.
If the notes to the table did not involve arcing short circuit current, then how would you know what the clearing time is?

 

[GMIEE]

So what is your available fault current, and how did you determine it?

I calculated the fault current to be 17kA based on the point to point method.

 

[zog]

I calculated the fault current to be 17kA based on the point to point method.

Ok, now we are getting somewhere, what are your breaker settings and ratings?

 

[GMIEE]

Ok, now we are getting somewhere, what are your breaker settings and ratings?

I've got a 1600 A Siemens Series C breaker with a 1200A plug. The short delay time is set to instantaneous and the short delay pickup is 8x (so 9600A). Looking at the TCC, the instantaneous portion of the curve extends all the way up to 0.05 seconds, which I believe makes the table useless for me.