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 Post subject: Utility Meter Sockets
PostPosted: Mon Apr 10, 2017 9:35 am 

Joined: Mon Aug 24, 2015 10:24 am
Posts: 29
On our campus we have meter sockets slapped onto the side of the building utility transformers. We are our own utility on campus. We currently feed the phase conductors into the meter socket without PTs off of the transformer secondary terminals. We feed the current conductors to the meter socket connections via CTs. Since we do not use PTs, the voltage at the meter socket is 480 volts. Since the meter phase conductors are not protected at the transformer secondary terminals, and are literally 5-10 feet away (#12 copper conductors) the arc flash calc. energies at the meter sockets are shown to be higher than the calcs at the transformer itself.

For example, at the secondary terminals of a 1000KVA, 12.5kV:277/408V transformer (5.67%Z) the arc flash energy is calculated at ~17cal/cm. Tapping off of that terminal with 10 feet of #12 conductors to the meter socket yields ~23cal/cm at the meter socket.

I understand why this is the case - the OCPD upstream of the transformer takes over 1.1 seconds to trip! The question is - are there industry exemptions to not going with the high energy levels when it comes to meter sockets? The 125KVA/208V transformer exemption exists due to the practical reality of an arc not being sustainable in the lab at these conditions (I think so anyways...), despite what the calculations show. Is there something similiar for meter sockets? If not, how are utilities dealing with this issue?

NESC has Table 410-1 which provides some guidance if calculations are not done but I can't turn that direction since we have calculations available.

Putting on the bomb suit to plug in a meter into a socket seems very excessive. Any experiences, references, suggestions would be appreciated. Perhaps if we develop an in-house electrical safety protocol for meter installations we could justify PPE that is less restrictive for these specific tasks. That, or we just standardize on installing PTs ahead of all of our meter sockets, a simple enough solution though major headache as that means outages for each and every building on our campus...


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 Post subject: Re: Utility Meter Sockets
PostPosted: Tue Apr 11, 2017 5:25 am 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 863
Location: Rutland, VT
I would say you should use the NESC figures instead of the calculated numbers. I say this because the IEEE calculated numbers are based on certain configurations of gap, bus size and electrode orientation. This most likely is not what you have in the meter socket.

Whereas the values in the NESC table are based on actual testing of meter sockets and therefore more accurate. And since you are essentially a utility, you should be following NESC and OSHA 1910.269

Barry Donovan, P.E.

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 Post subject: Re: Utility Meter Sockets
PostPosted: Tue Apr 11, 2017 8:25 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 579
"Perform a detailed arc hazard analysis, or use Table 410-1..."

Not sure I buy the interpretation that "or" excludes "and". You can always ask for an interpretation. And have you really performed the detailed study at every 480 V location on your system? Don't forget to include alternate feeds for each location, and make sure you redo the study after every transformer replacement and re-conducting job. You still need 20 cal/cm^2 per the table. Since lugs tend to get pulled out along with the meters, and 480 V arcs don't self extinguish, we went to VTs long before arc flash made it into the NESC. VT packs from Two Sockets Two Meters helps where space is limited.

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 Post subject: Re: Utility Meter Sockets
PostPosted: Fri Apr 14, 2017 12:45 am 
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Location: North Carolina
The NESC results come from a study done by EPRI. Their reports for non members run $5,000-$50,000 but you are in luck because after 10 years the reports become public (free) and this is one of them. Head on over to their web site to download for free.

Meter sockets really are quite hazardous. OSHA has documented more than one case where a meter socket blew up. There are major design differences though and EPRIs test work shows that some meter sockets are almost harmless while others exceed IEEE 1584 calculations. So like I said, download the report. This is one area where calculated results probably should not be used. NESC tables for the most part are based on EPRI type testing work.

Don't confuse the 70E table "prohibition" with others. 70E prohibits mixing their task and PPE tables with another method. It definitely does not prohibit the use of tables in other standards and IEEE C2 (NESC) does allow intermixing testing, calculations, and tables because they don't incorporate "risk" into their tables.

Finally, the 1584 "exemption" is based on the fact that at that time they only ever got an arc flash once at 208 V in the initial testing. Among other issues, the fuse wire was too large so subsequent tests show something much different. The exemption will be reduced when the new version comes out. NESC gives an "exemption" without a transformer size up to 250 V again based on actual tests but they also make FR shirts and pants mandatory as basic PPE so in the 70E world, 4 cal/cm2 is the minimum cutoff. I believe the corresponding EPRI tests showed an actual maximum of 3.2 cal/cm2. Some early PG&E results were more in support of the exemption but Mike Langs testing (Mersin, again free) showed why those tests were invalid and why 70E and a future 1584 will look more like NESC.

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