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 Post subject: Contradiction with OSHA + 70E
PostPosted: Wed Jun 13, 2012 4:30 am 

Joined: Tue Feb 17, 2009 11:04 am
Posts: 37
Please help!

OHSA 1910.333(c)(3)(i)(A)(2)
For voltages to ground over 50kV - 10 feet (305 cm) plus 4 inches (10 cm) for every 10kV over 50kV.

Informative Annex C paragraph C.2.1.2 states that “Column 2. The distances in this column are based on OSHA’s rule for unqualified persons to maintain a 3.05 m (10ft) clearance for voltages up to 50 kv (voltage to ground) plus 102 mm (4.0 in) for each 1 kv over 50 kv”.

Is this a typo. If so which one is correct. I am being told that since 70E references OHSA that the OHSA value is correct. Your assistance on this is appreciated

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PostPosted: Wed Jun 13, 2012 4:50 am 
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It's the other way around. OSHA used 70E as the basis for Subchapter S way back.

Both reference IEEE 516, which changed in 2009 for voltages roughly over 25 kV due to a mathematical error. As I recall, OSHA Subchapter S and Subchapter R are or were going to be updated. From a pure safety standpoint I would use IEEE Std. 516 as the basis for determining a safe distance at that voltage. OSHA and 70E may have different values but they will eventually both update to match IEEE 516. IEEE C2 is the basis for OSHA Subchapter R so I would defer to looking there as well.

That's from a purely safety standpoint. NFPA 70E in the introductory material makes it clear that it is a guidance document, and referring to an Annex is even worse because those are all additional informative sections to a document that is already nonbinding. By strict regulatory defintions, you go by OSHA. By strict safety definitions for this particular case, go by IEEE 516. And to go by 70E as a guidance document, you go by the table and not the Annex.

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PostPosted: Wed Jun 13, 2012 5:37 am 

Joined: Tue Jun 12, 2012 11:45 am
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The NESC (ANSI C2) uses 0.4" per 1 kv, I think NFPA 70E forgot the decimal point. The NESC has been recognized as the standard to use for years with respect to clearance distances to high voltage systems.

Also this clearance that OSHA is discussing is being used for unqualified persons and equipment, whereas NESC uses this adjustment factor as the equipment vertical clearance requirement to overhead lines (the 0.4" per 1 kv), similairly to how OSHA 1910.333(c)(3)(iii)(A) uses is. Reference to NESC section 232.C.2

The point that OSHA fails to identify is that the clearance distance to live parts of people for HV systems is based upon BIL or switching surge not just operating voltage. my information may be a bit old but reference table 124-1 collumn 2 & 3 (2002).

Correction to that, OSHA 1910.269 Appendix B has been updated providing information developed from NESC.

NESC section 124.C.1 provides clearance distances on table 124.1

Live parts in compliance with the Rule 110A2 Safety Clearance Zone requirements and having
clearances equal to or greater than specified in Table 124-1 are guarded by location. Parts are
guarded by isolation when all entrances to enclosed spaces, runways, fixed ladders, and the like
are kept locked, barricaded, or roped off, and safety signs are posted at all entrances.

I interperet that as being the unqualified worker distance. I havn't had time to fully digest the diferences between the two but I think that the OSHA may provide a more conservative clearance distance to that of NESC section 124.

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