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.
1. Tighter Protection Coordination
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.
2. Selective Protection Coordination
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.
3. Install a Fuse / Breaker
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.
Conclusion
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.
About Jeff