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NFPA 70E - Arc flash analysis (What do you need)

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)

They need the actual fault current (realistic) at the service point. For arc-flash all they really need is the three-phase value and the X/R ratio. Most people want to get the SLG curent and X0/R0 as well. They also need to know, in some form or another, the upstream protective device characteristic curve (TCC). They have to figure out the arc-flash energy for a fault at their main service disconnect. For that, they have to rely, in general, on the utility's protective devices.

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)

No, maximum worst case currents won't be acceptable and you didn't mention providing any fault clearing data.

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)

Start with the power provider, usually an electrical utility. They will be able to provide you with the available fault current at your point of service as well as any protective devices on their side of the line of demarcation.

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)

Low arc current which results in a long clearing time ?

How to say so ?

RE: NFPA 70E - Arc flash analysis (What do you need)

Linspire:

Take a look at a time-current curve for any molded-case circuit breaker, or fuse. You'll figure it out.

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