The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!

Featured:
   

2017 NEC and Related
2017 NEC
Now Available!

   
Recent Posts
Industrail Control Panel bonding per 409.108
by sparkyinak
Today at 03:17 PM
Calling all Non-US members!! (Non-US only)
by aussie240
Yesterday at 02:39 AM
Photo Upload Tutorial
by DanK
12/06/16 11:35 PM
Sprinklered equipment 26-008
by bigpapa
12/02/16 04:24 PM
On Delay Relay with Auto Reset
by Potseal
12/01/16 09:59 AM
New in the Gallery:
12.5A through 0.75mm flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
gfretwell 13
HotLine1 9
Texas_Ranger 8
sparkyinak 8
Trumpy 6
Who's Online
0 registered (), 139 Guests and 6 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#61107 - 01/16/06 07:05 PM Another brain teaser....480 on a 240 volt delta
WFO Offline
Member

Registered: 09/03/05
Posts: 206
Loc: Cat Spring, TX
I work at a utility. We got called out on a customer complaint to a grocer who had burned up his 3 phase motors.

His service was a 120/240 volt open delta secondary fed from two phases of a 4 wire wye, 12.5/7.2 Kv primary.

When we got there, we read across the "lighting" legs and got 120 volts from A to neutral and 120 volts from B to neutral. A to B read 240 volts, as did B to C.

So far so good. So.....

A to C read 480 volts (instead of 240) and C to neutral read 360 volts (instead of 208).
Neither transformer fuse was blown.

Hint: There had been many outages due to storms a few days before and lots of backfeeding had been going on.

Top
Work Gear for Electricians and the Trades

Work Gear for Electricians and the Trades
Work Gear for Electricians and the Trades
Arc Flash Clothing, Gloves, KneePads, Tool Belts, Pouches, Tool Carriers, etc. etc....

#61108 - 01/16/06 07:12 PM Re: Another brain teaser....480 on a 240 volt delta
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Did the transformers end up on the same input phase?
_________________________
Greg Fretwell

Top
#61109 - 01/16/06 07:14 PM Re: Another brain teaser....480 on a 240 volt delta
WFO Offline
Member

Registered: 09/03/05
Posts: 206
Loc: Cat Spring, TX
Dammit! Dammit! Dammit!
Didn't even get past the first guy.

Top
#61110 - 01/16/06 07:15 PM Re: Another brain teaser....480 on a 240 volt delta
caselec Offline
Member

Registered: 04/14/02
Posts: 557
Loc: San Jose, CA
I don’t know what back feeding has to do with this but you can’t feed a three phase transformer bank with only 2 phases. By feeding both transformers from the same two phases your secondaries are connected in series giving the voltages you see.

Curt
_________________________
Curt Swartz

Top
#61111 - 01/16/06 07:17 PM Re: Another brain teaser....480 on a 240 volt delta
caselec Offline
Member

Registered: 04/14/02
Posts: 557
Loc: San Jose, CA
I’ve got to learn to type faster.
_________________________
Curt Swartz

Top
#61112 - 01/16/06 07:44 PM Re: Another brain teaser....480 on a 240 volt delta
WFO Offline
Member

Registered: 09/03/05
Posts: 206
Loc: Cat Spring, TX
Quote:
"I don’t know what back feeding has to do with this but you can’t feed a three phase transformer bank with only 2 phases. By feeding both transformers from the same two phases your secondaries are connected in series giving the voltages you see."

Right on the second half.
You CAN feed a 3 phase open delta bank with only two phases and neutral. I didn't mean to give the impression that they were going phase to phase on the primary with the common floating. The common is bonded to the neutral.

Quote:
"I’ve got to learn to type faster."

Indeed. The second response was going to win a brand new Porsche.

...oh, wait. I was the second response.

Top
#61113 - 01/16/06 10:08 PM Re: Another brain teaser....480 on a 240 volt delta
mxslick Offline
Member

Registered: 10/24/04
Posts: 785
Loc: Atomic City, ID USA
Oh, man lookit what I started....
_________________________
Stupid should be painful.

Top



ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals