What you want to know about Arc Flash, NFPA 70E, and more
There are a lot of great videos available showing the affects of an Arc Flash and of an Arc Blast. If you have ever sat through a training course you have seen them.
We will be posting videos that show the awesome affects when things don't go right. Remember, whenever possible work only at a zero energy state.
Near the end of October I was shown a very poorly written Incident Energy Report. The reason that it was given to us, a competing consulting firm, was that we designed the system and on of the action items listed was regarding the size of the transformer protection. They said it was wrong, we checked and they were wrong.
For the record, the secondary circuit breaker of a transformer may be used for the transformer protection as long as it is no greater than 250% of the full load current of the transformer AND the up stream protection from the transformer is no greater than 600% that of the full load of the transformer.
We checked and we meet both of these requirements. Besides that, the breaker settings that we used, and they used in there report, were well within the transformer damage curves.
But that is NOT the reason I am writing this first entry, basically there are some things that every Incident Energy Report must include. Since this is more than can be covered in a single post I am going to write a 6 Part series explaining what I believe must be included in every IE Report.
The parts are:
Each one of these sections must be included in every report that you may write or receive from a consultant. If one is missing when a review report is issued ask why it is missing.
This is not to say that each section will have to be called out within the report, depending on the scope and size of the system they may be simply broken out into line items, but they must be included. However in this one report that I had viewed, this was not the case, and this is why I have decided to write this series of posts. I will be adding the posts through the months of January and February and will have a detailed summary post at the end.
In the past you determined the bus withstand of equipment by the bolted fault. In some cases the system would be designed to limit the available fault energy on any one bus by installing higher impedance transformers, multiple small transformers, etc.
Today there is as much concern regarding the Incident Energy (IE) released in an arc fault as there is regarding if the equipment can withstand a bolted fault. In regards to IE more fault current may actually lower the energy when the protection is in its instantaneous range. The amount of time required to clear a fault decreases as the magnitude of fault current increases. The IE equation, energy is related to the square of time. If the time to clear the fault is halfed, the IE is 25% of the original.
Higher fault energy typically represents a stiffer overall electrical system. This leads to better coordination as the designer will be better able to predict what will happen during an incident.
You have to be more Precise
When an Arc Flash Study, currently being referred to an Incident Energy Study, is completed owners and operators start to look for ways to minimize the workers exposure to high incident energies. One of the ways that this can be done is to ensure that all work is completed in a zero energy state.
This is not always possible, and when operating disconnecting devices the worker may be in danger. Following are some options that can be used to reduce the incident energy, and associated flash protection boundaries.
One of the first steps to lower the incident energy of an arc fault is to review the Protection Coordination Study surrounding the specific buses that a lower incident energy would be attractive at. The traditional philosphy regards to protection coordination is to ensure that faults are isolated in a strictly predictable manner. This means that there is space on the Time-Current Curves to ensure that the protective device that trips is the one that the operator expects.
These studies are coordinated at worse case, a bolted fault, and an arcing fault will always be at a lower value. This results in the required protective device taking longer to operate than is desired. If possible a facility may be able to use a slightly smaller fuse, or tweak the breaker settings to use less space between the protective devices and in turn allow the desired device to trip sooner.
The negative of doing this is that in higher current faults the devices may not be coordinated, this may cause an up-stream device to operate extending the range of the outage unecessairly.
There are new technologies being developed and released to the market everyday. One of the technologies that I have been more impressed with is the ability to have multiple sets of Time-Current Curves installed in a circuit breaker. These curves are selectable with a switch either on the wall of the electrical room or mounted on the breaker enclosure. This will allow the operator to change the characteristics of teh breaker and have it trip in the instantaneously for a larger range of currents. When this happens the clearing time of a fault is significantly reduced and in turn so is the incident energy.
When the breaker is in this instantaneous range it is no longer coordinated with the devices downstream from it. If the worker does not replace the device in its normal operating mode when work is complete there is a significant risk of it operating when there is a fault, which will then shutdown more of the facility than is required.
Another consideration to remember when this method is used is that the breaker that feeds the equipment that is to be worked on must be the one that is altered. If that breaker is in the same enclosure, then in most cases, the next up stream breaker must be moved into the instantaneous range. The reasoning behind this is that there is still a risk of the feeder breaker located in the enclosure faulting which would then require the upstream breaker from it to operate to extinguish the fault.
If only they were this small
I have included both these options in the same line on purpose. If you have had a representative from either of these manufacturers give a presentation on Arc Flash from 2002 til recently I am sure that you have seen the fantastic videos showing how their respective equipment is the "bee knees" with regards to Arc Flash and reducing incident energy.
I believe that the reality of the matter is somewhat more complicated than replaceing your electrical infrastructure with one product and assuming that everything is going to work out fine.
My typical rule of thumb is this, if the available fault current is high, then fuses have the faster clearing time (as little as a ¼ cycle). However if the fault current is lower a breaker may be the better way to go.
What is going to be your road map to safer work conditions
There is never the perfect answer with regards to ways to reduce the Incident Energy of an Arc Flash. Consult with the party that completed your study and with your operations people to determine what if any changes can be made to the system and operation to minimize the required PPE. However always remember that PPE is the last line of defense and all other methods to ensure safe work procedures are employed are first.
If you have any suggestions or questions please let me know in the comments.
An arcing fault, which is the cause of an arc flash is described below.
Arcing faults (an Arc Flash) are defined as high-impedance faults, since any fault current must travel through air, as opposed to the low-impedance path normally associated with a short circuit. A short circuit study of the electrical system is required to determine the maximum available short circuit energy, which in turn may then be used to calculate the potential incident energy available.
There are intense heat and pressure waves associated with these types of faults. This heat and pressure wave will cause shrapnel and molten metal to explode from the point of the fault.
The core temperature of an arc fault can easily reach 5000ºC (source), for comparison the surface temperature of the Sun is only 6000 ºC and the boiling point (not melting point, but BOILING) of copper is 2500ºC.
Assuming that the electrical protective device in the circuit operates fast enough to extinguish the fault so that these extreme temperatures do not cause greater than second degree burns to the operator, the ignition temperature of the typical non-PPE clothing the operator is wearing would have been reached, this burning will cause serious harm to the operator if they are not extinguished quickly
When proper PPE is worn for the calculated incident energy at the fault, the worker should walk away from the incident with a maximum of second degree burns.