NFPA 70E - Arc flash analysis (What do you need)
NFPA 70E - Arc flash analysis (What do you need)
(OP)
I work for a large POCO and I'm currently revising/developing a procedure for handling information request related to NFPA 70E arc flash analysis. Depending on who's performing the customers analysis the type and amount of information requested varies, with the most common items listed below. Except for the transformer impedance all of the other items are highly variable and based current system configuration. Several firms are OK with only receiving available 3-phase fault current or a transformer impedance while others want significantly more.
I'm trying to determine what the minimal amount of information necessary to perform an analysis and properly categorize a site is. I don't want to restrict information or provide overly conservative numbers that would negatively affect a customers ability to perform maintenance or up-rate a hazard zone unnecessarily. However I don't want to spend a lot of time tracking down and providing information which is time sensitive and possibly may not reflect worst case scenarios if our protection changes or the system is reconfigured.
It would be fairly easy to publish on our website minimum transformer impedance, or a maximum available fault current which would be based on 1. minimum transformer impedances, 2. minimum source impedances, 3. minimum secondary length (20 ft). Would this information be suitable for an analysis of most site?
Positive and zero sequence impedance and or X/R ratios
3-phase and 1-phase available fault current
First upstream protective device
Transformer impedance
Breaker settings
Description of any nearby DG sources
Secondary conductor, conduit type






RE: NFPA 70E - Arc flash analysis (What do you need)
If they are taking service at 480 V, you can give them the fault duty at 480 V, if you define where in the system the fault was calculated. But it needs to be based on the actual transformer impedance and secondary conductors for that service. The maximum arc-flash energy is not necessarily related to the maximum possible fault current, because the fault clearing time is a factor.
RE: NFPA 70E - Arc flash analysis (What do you need)
The worst arcs energy will often occur when there is a low arc current which results in a long clearing time.
The customer has to be able to calculate the arc energy at the service entrance which requires knowing the utility protection since there is no customer protection upstream of the utility connection to the first protection device.
RE: NFPA 70E - Arc flash analysis (What do you need)
Then, on your side, you need cable sizes and lengths, protective devices, settings and time overcurrent curves all the way from the source to each bus and from each bus to each power panel and end device. It gets quite involved, especially when you add cases for differnet system configurations where such things are possible, i.e., transformers tied, buses tied, facility running on emergency generator, etc.
As others say, knowing "worst case" maximum fault current won't work sometimes because a strong source may provide enough fault current to operate a device in and instantaneous region, clearing fast, but a weak source may allow a fault to go on for much longer at a time element times out.
old field guy
RE: NFPA 70E - Arc flash analysis (What do you need)
How to say so ?
RE: NFPA 70E - Arc flash analysis (What do you need)
Take a look at a time-current curve for any molded-case circuit breaker, or fuse. You'll figure it out.