Exploding MV motor weatherheads?

[electromike]

I have seen ground faults inside junction boxes at MV motors do a good deal of damage to the enclosures.(There have even been reports of fatal injuries to people near the motors when such happens.) Lots of "arc" damage will be present but not with my newest MV fun! In the most recent incident some sort of fault occcured inside of the motor (2300v, 300HP, 4pole)that caused a cast iron weather head to fragnment into about 10 pieces with enough velocity to put 1/4" dents into a sheet meatal building about 20 ft away. No arc damage visible to anything in weather head. No blown fuses or GF tripps. Motor meggs good so I am guessing some kind of phase to phase fault but the cast weather head exploding seems odd to me and would like any feedback others may have on this matter. Thanks.

 

[JohnWaa]

It sounds to me like it was a magnetic pulse that took out your weatherhead.

The old "opposites attract" and "likes oppose" can really put a hurting on anything nearby.

Ever see a welding cables slap together? Or hear wires "ringing" in a conduit during a dead short? The magnetic pulse going through the wires cause them to vibrate (that's too light of a word for it) violently.

 

[electricpete]

No blown fuses or GF tripps.

The motor continued to run?

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[electricpete]

One thought - if there were fault creating presure inside a TEFC motor, it would look for a way out which might be the term box. (but I would have expected the motor to trip.

Another thought, if you have a good protection scheme working properly (you could test it...) and didn't trip, then one has to wonder if there is can be some cause that does not involve electrical fault. Cast iron can be a brittle material. Perhaps bolting configuration was putting it in stress? Unusual thermal stresses?

It's a headscratcher.

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[electromike]

It looks as though the motor continued to run and as for uneven torque on the weather head I doupt that was an issue because the nearest conduit body (3") also blew its cover. This cover was sheetmetal and was bent to hell. Again no carbon or signs of arcing. It will be interesting what the motor shop finds inside the old motor.

 

[fsck]

Can you get a metallurgist to look at the remains? I know nothing of the field, but am regularly amazed when I read NTSB reports on aviation/rail events. They can tell where where why and how a landing gear strut failed, or the turbine blade first cracked, etc.

 

[waross]

Any chance of a buildup of explosive gas in the connection box.
A combination of moisture and insulation leakage current may have produced enough Brown's Gas to do the damage you describe, and not leave much trace other than the damage. I once had my car battery blow the top off from a Brown's Gas explosion and I have cleaned up after a number of other such explosions. A small volume of Brown's Gas will do a lot of damage and the product of combustion is water vapor.
(I no longer spec, install or connect battery chargers on standby diesel sets. There is too much drama when the battery explodes.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JimCook]

Terminations in MV motor boxes need to be very carefully made. Our plants use putty, cambric and lots of #88 tape. Failure to properly wrap the T-leads and motor leads can result in phase-to-phase or phase-to-ground faults through the wrap that could easily destroy a cast iron termination box.

Not wrapping properly is a little like using duct tape to fix a leak in a garden hose. It might work for a while but eventually you're going to get wet.