fault in a water filled handhole

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mshields

mshields

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Boston, MA
Got called out to take a look a look at the aftermath of a fault on a 200A, 480V feeder. The fault occured in a water filled handhole between two phase conductors. None of the three upstream breakers tripped. Instead, the water actually came to a boil and the steam alerted people to a problem.

Naturally, the whole run will be replaced. But I'm wondering if the Instantaneous setting on the 200A MCCB, if set to something less than Max (as it was) would have at least cleared the fault.

I imagine the only sure way to catch such a fault is with ground fault which in this case was only on the 3000A main upstream and it didn't trip either.

Would setting the Instantaneous lower have done anything in this instance? How much current will flow through water. I guess it's a function of the distance so let's say 4 or 5 inches apart? Anybody know?

Thanks
 
Not odd at all, if I looked on my hard drive long enough I have pictures of a boiling hand hole (well it had just stopped boiling) and that was just a 30 amp 480 volt lighting circuit.

I have also see 20 amp 120 volt circuits supplying floor outlets boil so long that the cement slab heated to the point the VCT tile was lifting.
 
Not sure how much current will flow thru water. Depends on it's chemical makeup, how dirty it is, etc. I would not expect your breaker to trip. I have seen the steam come up from the ground when a metal fence post was driven into a 480v direct bury feeder with a 100 amp fuse ahead of it. It was cold and snow was melting around the post.
 
480x1.73x200=166080 watts

I guess it's possible to have an accidental 166080 watt water heater before a normal breaker would trip, if the conditions are right. And I have also seen this on a 277 volt 20 amp circuit also, but not 208.

I wonder how many watts vs time it takes to boil water? Maybe time to google?
 
defears said:
I wonder how many watts vs time it takes to boil water? Maybe time to google?
Click to expand...

Since 1 calorie raises 1 gram of water 1 degree C, it follows 1000 calories = 4184 Joules raises 1kg of water 1 degree C

If we say a kg of water is about a quart and want to raise the temp 70 degrees from say 30 to 100C, then we need 4184 * 70 = 293kJ and if we have say 10kW of heating power to get it done, the time required is ~ 30 seconds. (If I didn't make any stupid math errors, that is)
 
I've got one more for you while we're on the subject of water and electricity

I've got one more for you while we're on the subject of water and electricity

Last year at a hospital I do work for, they had an underground electrical vault fill to 5 feet above the floor, completely submerging a dry type transformer and the bottom part of the bus bars for multiple panelboards and nothing tripped off. They had to de-energize the feeder to the room manually!
 
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