Exception 2

EXCEPTION 2: For secondary systems below 1000 V, applicable work rules required by this part and engineering controls shall be utilized to limit exposure. In lieu of performing an arc hazard analysis, clothing or a clothing system with a minimum effective arc rating of 4 cal/cm2 shall be required to limit the likelihood of ignition.

So it has been proposed to use three phase 100 kVA as a threshold. 4 cal/cm^2 for all single phase, 4 cal/cm^2 for all three phase 100 kVA and below, and analysis for the rest. Does this meet the rule as you read it?

Thanks.

I have to ask, who proposed 100 kVA as the threshold? Have I missed something in the standard?

Alan

A consultant proposed the threshold.

To clarify my question: Can we (not) invoke the exception on a case by case basis as illustrated above (regardless of where the threshold is drawn)? Or must we make one decision utility wide?

Seems your question is....could you use the exception in selected cases and do an analysis for others. Good question! Since the standard says "in lieu of" it would seem you would need to do either one or the other. As with 70E, the consensus is use the tables if appropriate or do the study, but no mixing of options. Will be interesting to hear from other folks as well.

We have taken the approach that due to the sheer number of devices, different levels of fault duty, and clearing times on a distribution system it is not practical or meaningful to attempt to calculate IE for each device and each possible connection scenario. We have taken "typical" systems and equipment at various locations and sizes to determine the level of PPE for below 1000 volts and use this system wide. Our basic approach is to always de-energize if possible.

The 100 kVA threshold probably comes from IEEE-1584 which says that 208 V systems fed by transformers smaller than 125 kVA should not be a concern.

Exception

The exception was to prevent a huge issue with the NESC by utilities. The committee recognized this would be a larger study by far if they included the secondary side. They also knew that the secondary side usually has more energy BUT they didn't want to overload engineering departments in 2009.

The "should" on engineering controls assumes the utility is doing something to reduce exposure or risk on the secondary side.