Hi,

I am trying to understand the process of doing an arc flash study. Our company is planning to outsource the work and I am asked to at least draft a scope of work document. Have anyone here have a template they can help me get started. I am a control engineer with basic electrical training from my college day, however, most of my experience has been in the controls area. My understanding is that to properly conduct an arc flash study we will need to collect data, short circuit study using utility data and single line model, coordination study and then arc flash. My question is more for information and education. Our company had purchased a copy of Etap but noone had learned to use it. Thus it makes sense for us to have the contractor do the work in Etap so we can maintain the model once the project is completed. I have been trying to use the software building some mock model and work with it. I do have some generic questions:

1. The arc flash module seems to be able to use the results from the model short circuit study and able to pick up the protective device that would trip in case of an incident. If this is the case, (my exp is limited to my small 5 buses model) why is coordination curves have to be studied if the software could flag issues if any?

2. We have a fairly large size system (radial) with at least 35 feeders and buses, if we are to do the full coordination study, do we need to look at the coordination curves for every path from the utility down to every bus to the final end device (motors)? or is there some rule of thumb depending on motor size or load?

3. Is loadflow analysis add any value to arc flash?

4. At a quick glance it looked like one of our utility transformer 2500KVA have roughly 38KA of available fault current on load side of a 800A breaker which is rated for 35KA IR. This looks to be a problem from a code standpoint, plant management said that the plant is very old and code is grandfathered in. This does not sound right to me. Is there any safety factor build in to the 35KA? my worry is that we won't be able to change these breakers within the next 3-4 months. What can we do in the meantime to mitigate this risk if a risk at all.


Thank you all for your help in advance.

The software is picking up the device performing the tripping but if there is a lack of coordination this may not be the device closest to the fault. In addition, coordination study will look at the main device on a bus and the feeder devices to see if they coordinate. Another area is if the device on the primary side of a transformer is above the transformer inrush current. Just a few examples of what coordination study does.

To perform a detailed study, yes.

No. The loadflow would be valuable to look at voltage drops and transformer loadings.


Depending on the available fault current from the utility, this device may not be overdutied. The 38kA would be for an infinite source which is proper to size equipment withstand ratings to.

Hi,

Thank you for your response. From your experience with arc flash study, is a detailed coordination study required? I saw a sample from a local contractor, on one 2500KVA service, they only did one TCC from utility to main switchboard breaker, the largest feeder reaker and largest branch breaker in the largest bus duct. The rest of the switchboard and breakers were not studied. Is this common practice.? I wanted to put some parameters around the actual study so that the cost would not get out of hand. Basically balancing cost/quality of the study.

For cable, wires contractor suggested to ignore cable 10ft or shorter. Is this acceptable practice?

Also, they would model SOW cord as THHN? Not sure if this is correct, i supposed copper is copper of equal AWG.

VFD are ignored in terms of fault current contribution. Transformers smaller than 112KVA are ignored as well as motor smaller than 50HP? Not sure why this is?

Details like these are what I am looking for to make sure we balance cost/time to conduct study and quality of the study.

Thank you.

It all depends on how coordinated you desire your system to be. Bear in mind with MCCBs there tends to be lack of coordination especially in the instantaneous region.


That short of a conductor will have a negligible effect on fault current. I know the software program I use will not accept a length less than 10 feet.



That should be okay. I am curious on your wording of "they would model". Sounds like you may already have a bid in hand and trying to evaluate?



VFDs can only be ignored for fault contribution back to the bus if they are non-regen type. The equipment serviced by transformers less than 125kVA and below 240V are exempt from analysis per IEEE 1584. Motors 50hp and less do not contribute significant short circuit currents to a faulted bus.



You may want to get a copy of IEEE 1584.1, IEEE Guide for the Specification of Scope and Deliverable Requirements for an Arc-Flash Hazard Calculation Study in Accordance with IEEE Std 1584 to help with your scope and review.

Another source to give you a good idea on what is involved in a study and the steps to do a study is the book, "Complete Guide to Arc Flash Hazard Calculation Studies" by Jim Phillips, P.E.

Thank you again for your help. I started some conversation with an electrical contractor to look at the work. I do not think they have the expertise to do the work. My supervisor recommended to solicit bids from companies specialize in arc flash study. I will procure a copy of the book.

Regards,

SR.