Dear All,

I would like to ask one important issue related to Arc flash boundary markings in the industries like we have to mark the boundary for wall mounted DB's or not (which can be in Office area). If yes, then what would be the distance in mm or feet. Please advice.
Thanks,

Your arc flash boundary is a calculated value for each piece of equipment based on your arc flash study.

If you don't have a study, you would use the distances listed in NFPA 70E Table 130.7(C)(15)(A)(b).

Additionally NFPA 70E-2015 requires the arc flash boundary to be on the label. The only boundaries I have seen actually marked out are the ones in front of electrical panels to keep people from storing stuff in front of them.

NEC has a suggested list of what has to be marked but keep in mind that the list is a guideline and should not be read as an absolute rule. The key is whether or not the equipment is accessed for maintenance "frequently" which is generally considered to be more than once a year. For instance if you have equipment that is rarely (cycle is more than 12 months) serviced and not used as say a primary disconnect for servicing other equipment, then it would not have a label. This would also mean that some things that are not on the NEC guideline would qualify.

In addition, NEC does not specify that it has to be marked directly on the equipment. 70E gives specific requirements for labels but does not have direct regulatory authority. So some plants have a practice where if it is not labelled, there is an assumed value. This is usually (but not always) 1.2 cal/cm2. Others follow the above example and the rule is that when in doubt, seek out the maintainer of the arc flash study (engineer) to find out the value to use for rarely used equipment.

Personally I don't like the assume "category 0" (<1.2 cal/cm2) because of the possibility of missing something. The industrial plant I am working in now has missing labels on equipment I know is greater than 1.2 cal/cm2. I'd rather the worker come to me and I can look up the arc flash value (and as soon as I get a printer I can print them a label to put on the equipment.)