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Joined: Mar 2002
Posts: 582
R
Ron Offline OP
Member
I will start by saying that many times this table is misused. It is shown in the standard as an option for determining appropriate PPE, but the notes
1. 25 kA short circuit current available, 0.03 second (2 cycle) fault clearing time.
2. 65 kA short circuit current available, 0.03 second (2 cycle) fault clearing time.
4. 65 kA short circuit current available, 0.33 second (20 cycle) fault clearing time.
and
5. 65 kA short circuit current available, up to 1.0 second (60 cycle) fault clearing time.
are often ignored and make the table inappropriate for use.

In order to use the table, you must determine that the notes apply for your condition. So you have to calculate the arc flash fault current anyway and plot it on the OCPD characteristic curve to see the clearing time.
I have found many times that the clearing time exceeds the time permitted for OCPD clearing time in the notes.
Have others experienced this too?


Ron
Arc Flash PPE Clothing, LOTO & Insulated Tools
Joined: Jul 2002
Posts: 8,443
Likes: 3
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Hey Ron!,
An interesting, yet understated point you make there.
Disconnection times are one thing that PoCo's here are trying to get into line.
We have a Standard disconnection time for all Installations of 0.4 second, regardless of the type of protection, be it HRC fusing or Circuit Breaker protection.
Hence the way we are going now, we are ripping out old Panels that use the old Porcelain fuses, some of which, depending on who last loaded the fuses, can have an Arc time of up to 4 hours, in some cases, they never blow and we get there in our Red Trucks with the flashing lights on top.

Joined: Mar 2002
Posts: 582
R
Ron Offline OP
Member
The key is that each type of overcurrent protection has a curve associated with it, in that if there is a high fault current, the device operates quicker than if there is a low fault current (or for that matter an overload). The arc fault current is 1/3 to 1/2 the value of the bolted fault current, and that must be applied to the curve to see how fast it will clear before you can try to use the table. Most don't know how long the device will take to clear before using the table.
They should perform the detailed calculations in NFPA 70E or IEEE1584


Ron
Joined: Jul 2002
Posts: 8,443
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Just re-reading your original post Ron,
60 cycles sounds like a rather long clearing time for a fault of that sort of magnitude (65kA).
Also, could someone please tell me what a bolted fault is, I think we may call it something else over here, like a lot of things. [Linked Image]

Joined: Mar 2002
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Ron Offline OP
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A bolted fault is the somewhat theoretical condition when two or more conductors/buses are connected when they shouldn't (metal to metal contact). An arc fault is similar except the air in between two or more conductors/buses has its insulating capabilities broken down and acting as a conductive path (not a direct metal to metal contact).


Ron

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