SheelPandey wrote:
Why one should bother to buy a copy of 1584 from IEEE spending another $800, just to figure out if equipment at 240 volts and below need to be assessed or not for arc flash danger ?
No need necessarily to buy it except for background inforrmation. All the formulas are given in the appendices of NFPA 70E for 10% of the cost.
Quote:
70E table method recommends Category 1 PPE for equipment at 240 volts and below. Why should not adopt this recommendation and label panel boards and other equipment at 240 volts and below for PPE Category 1 ?
It is all about safety. it will remove the confusion from labeling of equipment. Most electrical worker carry Category 2 PPE so it will not be an added cost to safety. Please extend your thoughts. Thank you.
You can't really calculate incident energy below around 250-300 V because the arcs tend to be weak or not self-sustaining. All kinds of strange things happen so there are no good formulas. The best approach is to use test data that's out there or use the standards as backing, which is what you are espousing.
In indusries where FR clothing is already being worn for other hazards (utilities, foundries, steel mills, petro-chemical plants), adopting "arc rated" PPE as standard work uniforms is easy to do.
But outside of those, watch out for unintended consequences. Consider the 120/240 V lighting panel or other 120 V circuits. Does this mean that a secretary has to put in arc flash PPE to plug in the coffee maker? Does the copier repair man have to wear arc flash PPE when working on a copier? Is turning on a light switch a hazardous activity?
I've recommended it before as follows based on standards:
1. For 208 V 3 phase fed by a single tranformer rated 125 kVA or less (IEEE 1584 rule), rating is <1.2 cal/cm^2 (no PPE required).
2. For 240/120 V single phase test work has shown that single phase arcing is close to three phase arcing so even though it is not addressed by IEEE 1584, use the same rule.
3. For DC, rating is under 1.3 cal/cm^2 based on test work at Kinetrics at 130 VDC and up to 20 kA (covers most 125 VDC substation battery systems). Arcing is impossible below 28 VDC based on Ayrton's model and can be extended up to around 100 V depending on available fault current using that model. Above this point use the model from Doan or Ammerman but recognizing that they exceed real world conditions by a factor or 200% or more (based on Battcon study).
4. For all other circuits rated 250 V or less, rating is 4 cal/cm^2 based on NESC (IEEE C2) Table 410-1. IEEE in turn relies on test data generated by EPRI for the most part and some PG&E data. Either way, it's a standard.
This avoids the "unintended consequences" problem and gives a simple way to come up with values that are based on standards without having to do a calculation that is suspect at best. I haven't seen any evidence from public EPRI documents and similar sources that ever pointed to an arc that exceeded 4 cal/cm^2 for these low voltage conditions.
Some test work has shown that arcs exceeding 1.2 cal/cm^2 is possible in these conditions so it can't be ignored but millions of homes with 120/240 V distribution demonstrates that the circumstances under which incidents occur is exceedingly rare.
