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ounce of prevention #12593 08/12/02 09:39 PM
Joined: May 2002
Posts: 68
Eandrew Offline OP
I always megger phase to phase at the feeder breakers (after termination) just as a triple check to make sure that someone did not swap phases on parallel runs.

However, I'm told that you cannot do this on transformers because you can't send voltage through the windings. Would I be able to use a continuity tester between phases? In otherwords, lets say instead of AA,BB,CC the terminator was daydreaming and you had AB,AB,CC.
I would think you would read continuity between A lug and B lug. Or would you read continuity anyway due to the transformer windings. since I cant use a megger, is the continuity test a valid one to check for swaped phases?

I'm just looking for a quick check which would prevent a massive explosion.

Work Gear for Electricians and the Trades
Re: ounce of prevention #12594 08/13/02 12:04 AM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline
Microohmmeter with Kelvin probes. [Rental?]

Re: ounce of prevention #12595 08/17/02 05:58 PM
Joined: Oct 2000
Posts: 2,721
Scott35 Online Happy
Broom Pusher and
Any test you do with the Transformer connected to the Feeders will come up with a near zero Resistance reading.

One other thing to consider is when the tester is disconnected from the Feeders, this will create an EMF equal to the Transformer's winding which will flow for a short amount of time. It will also burst a current on the Primary side of the Transformer.

Keep this in mind when working on Transformers.

The only thing to do for a proper test would be to open the Feeders at the Transformer while testing.

Scott S.E.T.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: ounce of prevention #12596 09/13/02 09:33 PM
Joined: May 2002
Posts: 68
Eandrew Offline OP
thanks scott, good information.

I was also wondering what would happen if you energized a transformer with swaped phases on parallel runs.

Would would something blow up or is there protection for this built in.

I know that OCPD's protect against shorts (phase to phase) and its supposed to clear maximum short ckt without causing extensive damage. Do you think the feeder breaker would trip (if properly sized per nec) or would you have a bomb? Assuming no load on it.

Re: ounce of prevention #12597 09/13/02 10:55 PM
Joined: Apr 2001
Posts: 449
Fred Offline
Funny you should ask today of all days. I just installed a new 800A 1Ø service in a new church. Today the utility came to hook us up. They set a 100kva pad mount transformer yesterday and ran 2 runs of 500MCM aluminum URD tri to my 800A MDP. They had marked both ends of each set of conductors as follows: L1-Red, L2-Blue, neutral-unmarked(has a yellow stripe). The URD sets entered the panel via (2)3" PVC LBs nippled to the *00A panelboard. I took a Red(L-1) from each set and connected to the L-1 buss lugs and a Blue(L-2) from each set and connected to the L-2 buss lugs and connected both of the neutrals to the neutral buss which was bonded to the enclosure and grounded via (4) 5/8"x8' ground rods cadwelded with #4 bare CU. The utility thoroughly inspected my connections to make sure they suited them before I closed up my panelboard. Well, today they came back to connect the transformer to the primary. They closed the cut-out on the pole and told me to check voltage. I had nothing. They opened and closed the cut-out a couple more times and I still had nothing. They opened the cut-out and opened the transformer and discovered that they had on each stud block(L-1 and L-2)one red and one blue marked conductor! Luckily the transformer had a fused secondary. The secondary fuse was vaporized! This was a Kohlman 2400V primary, 240V secondary 100Kva pad mount shorted phase to phase. Thank God it had a fused secondary. I saw a 67Kva pad mount without secondary fusing become shorted when a trencher cut the secondary a few years ago and it blew the bushings out. Kind of as if someone tossed a grenade in it.
Side note: The two guys who energized the transformer today were not the same two who installed the secondary and terminated the transformer yesterday but all four were veteran utility workers with 20+ years of experience. EVERYBODY makes mistakes!

Re: ounce of prevention #12598 09/14/02 04:47 PM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline
Fred — Some telco folks refer to excavation digins as "backhoe fade."

Realize that it’s perfectly normal for molded-case breakers to experience a degree of "external plasma venting" when closed into bolted faults. Sounds can be memorable, too.

Re: ounce of prevention #12599 09/14/02 05:28 PM
Joined: May 2002
Posts: 68
Eandrew Offline OP

That is really something. Good thing for that fuse. I think a fuse would provide better protection (higher AIC) that a breaker for the bolted fault.

yEA, WE are all human, thanks for the response.

Re: ounce of prevention #12600 09/14/02 07:48 PM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline
Eandrew — Back when building industrial-control panels, I subscribed to UL Standard 508. For 600V {and under} fuses in control panels, one 508 test was to "blow" them packed in surgical cotton, where success would yield no discoloration of the cotton. Molded-case thermal-magnetic breakers or motor-circuit protectors [and maybe insulated-case and air-frame versions] under the same conditions would cause the cotton to burst into flame. Id est, for that reason I agree with your opinion favoring modern fuses versus breakers.

[The Bussmann guy that posts here didn't spend a nickel on me. The same applies to Ferraz-Shawmut and Littelfuse.] [[Dan, send f.c. tickets for "blood-and-guts" Gubany tour at your earliest convenience.]]

[This message has been edited by Bjarney (edited 09-14-2002).]


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