The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

   
Topic Options
Rate This Topic
#150030 - 08/04/04 06:50 PM NFPA 70E Table 130.7(C)(9)(a) Hazard/Risk Category Classifications
Ron Offline
Member
Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
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
Top
Arc Flash PPE Clothing, LOTO & Insulated Tools
#150031 - 08/07/04 04:31 AM Re: NFPA 70E Table 130.7(C)(9)(a) Hazard/Risk Category Classifications
Trumpy Offline


Member
Registered: 07/05/02
Posts: 8560
Loc: SI,New Zealand
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.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin
Top
#150032 - 08/07/04 05:30 AM Re: NFPA 70E Table 130.7(C)(9)(a) Hazard/Risk Category Classifications
Ron Offline
Member
Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
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
Top
#150033 - 10/20/04 01:17 AM Re: NFPA 70E Table 130.7(C)(9)(a) Hazard/Risk Category Classifications
Trumpy Offline


Member
Registered: 07/05/02
Posts: 8560
Loc: SI,New Zealand
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.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin
Top
#150034 - 10/20/04 08:38 PM Re: NFPA 70E Table 130.7(C)(9)(a) Hazard/Risk Category Classifications
Ron Offline
Member
Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
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
Top

Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box

Recent Posts
License exam
by TRUE POWER
48 minutes 0 seconds ago
Looking for older post
by sparkyinak
Today at 12:27 AM
Members: Non-Members: Did you know?
by Admin
Yesterday at 02:34 PM
Safety at heights?
by HotLine1
01/21/17 08:51 PM
International Wire Colour Codes
by Texas_Ranger
01/21/17 03:29 PM

Who's Online
1 registered (Admin), 85 Guests and 11 Spiders online.
Key: Admin, Global Mod, Mod
New in the Gallery:
Desk-mounted "power-board"
Top Posters
gfretwell 9066
Trumpy 8560
pauluk 7693
HotLine1 6833
sparky 5545
Member Spotlight
Member Since: 03/13/07
Posts: 167

ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals