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Old 03-17-2019, 08:35 PM   #1
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Default 480v explosion damage underground PVC conduit

Hypothetical question ,

If 480v shorted out in underground PVC, would the explosion be large enough to damage conduit unusable ?
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Old 03-17-2019, 08:43 PM   #2
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Quote:
Originally Posted by mpcxl View Post
Hypothetical question ,

If 480v shorted out in underground PVC, would the explosion be large enough to damage conduit unusable ?
For the Hypothetical answer .,,

all it depending on the AIC rating and size and numbers of conductors are involded in this mess.,, but my answer is oui it will if large enough a expolison.

and which type of OCPD you are using and if that faulted by uility side it will be much worst than load (customer side) fuse useally can clear the fault pretty quick but not always. but breakers that is depending on the setting.
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Old 03-17-2019, 08:55 PM   #3
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#6 or 8 on load side of breaker but before fuse. It’s possible circuit is on a oversized breaker and uses fused over current protection to protect site lighting fixtures

All signs point to short/damaged circuit and damaged/broken pvc

Plan on cutting road , but just wondering if even possible to investigate further
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Old 03-17-2019, 09:18 PM   #4
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#6 or 8 on load side of breaker but before fuse. It’s possible circuit is on a oversized breaker and uses fused over current protection to protect site lighting fixtures

All signs point to short/damaged circuit and damaged/broken pvc

Plan on cutting road , but just wondering if even possible to investigate further
Okie .,, now this getting some good details coming out on this one.,,

first of all what size breaker did ya see it ?

what total load for this circuit and what type of luminaire it is running ?

Fuse size at the luminaire post ?

I know you mention 6 or 8 awg main conductor size but EGC what size it is ? I betcha it was #10 awg if that was the case you will know why real quick espcally with long run for parking lot luminaires.

how long the run it is ? ( in feet or meters )

The more info you posted the more easier we can give you the answer.
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Old 03-17-2019, 09:35 PM   #5
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Don’t have any load calcs atm. Fuses are rated for 20A. Typical 400W MH ballast combo fixture

The egc was the same as conductors so #6 or 8. I think they’re #8. It’s the end of line for the run. So between light poles it’s ~100’.

I should add Ends of wires are charred and I cant get a fish through.
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480v explosion damage underground PVC conduit-a4288b57-6a79-4714-9cb4-81efee695ffa_1552872759965.jpg  

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Old 03-17-2019, 10:05 PM   #6
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Hypothetical question ,

If 480v shorted out in underground PVC, would the explosion be large enough to damage conduit unusable ?
We use 347V for lighting up here, and no... it doesn't explode.

If you had a short hot enough, it could melt the pvc and screw up your duct. But for the most part, if you can't remove the conductors or fish in new ones it's just because the duct is collapsed.
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Old 03-17-2019, 10:39 PM   #7
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Had a 3 phase 480 WYE fault underground blew up a toilet (would have killed anyone on the can, knocked down 4 walls 3-drywall metal stud 1-cinderblock 25' tall.

SO YES

Shut the gym down for a month.
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Old 03-17-2019, 10:59 PM   #8
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Had a 3 phase 480 WYE fault underground blew up a toilet (would have killed anyone on the can, knocked down 4 walls 3-drywall metal stud 1-cinderblock 25' tall.

SO YES

Shut the gym down for a month.
All on #8 or #6 ??
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Old 03-17-2019, 11:42 PM   #9
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All on #8 or #6 ??
3-sets of 750 kcmil al.
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Old 03-18-2019, 12:23 AM   #10
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Is it possible the conduit was damaged years ago. Maybe put a nick in the wire. Took all this time to finally blow. Probably had mud seaping in the whole time.
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Old 03-18-2019, 06:04 AM   #11
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The inside of the conduit will be sticky from the smoke and the conduit will have a real rough area where the wires burnt in the best scenario. In the worst the conduit will have collapsed.

Like hertzhound pointed out the wire burning up may have been the result of a failed conduit which would have made a re-pull impossible before the wire added to the problem.

I would give it my best shot to get a fish tape soaked in lube through the conduit then call it a day and consider replacing some if not all of the conduit. No one wants to cut up a road so i would probably dig up the conduit at the edge and see if the section under the road is usable.
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Old 03-18-2019, 08:12 AM   #12
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If they’re unfused service conductors in PVC conduit, you’re trenching.
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Old 03-18-2019, 08:31 AM   #13
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Possible yes...probable no...I pulled shorted out 15kV cables many times and pulled in new cables no problem.

Blow and Rod the conduit...

For parking lot lights i wouldn't think twice about just trying to pull new in the same conduit.

You could always just lay burnt conductors along conduit path and just dig that section you are worried about.
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Old 03-18-2019, 09:17 AM   #14
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Used a locator and fish tape. Found the break where the fish gets jammed up. Unfortunately, it’s rightunder the road.

Going to try a few more things, mainly vac to suck and blow, before we go ahead and cut road.
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Old 03-18-2019, 11:44 AM   #15
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Used a locator and fish tape. Found the break where the fish gets jammed up. Unfortunately, it’s rightunder the road.

Going to try a few more things, mainly vac to suck and blow, before we go ahead and cut road.
If you haven't already, try fishing from the other end ... sometimes you get lucky.
We've had 50/50 luck with hydrovac trucks too. But if their hose gets stuck, you just bought it !!
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Old 03-18-2019, 11:45 AM   #16
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3-sets of 750 kcmil al.
Yep, that would make the 'earth shattering ka-boom ! '
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Old 03-18-2019, 01:43 PM   #17
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lots of steamer trucks have cameras they can send down if the conduit is 1" or bigger.
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Old 03-18-2019, 05:17 PM   #18
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Don’t have any load calcs atm. Fuses are rated for 20A. Typical 400W MH ballast combo fixture

The egc was the same as conductors so #6 or 8. I think they’re #8. It’s the end of line for the run. So between light poles it’s ~100’.

I should add Ends of wires are charred and I cant get a fish through.
My guess is that something happend to the conduit and it injured the insulation.
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Old 03-18-2019, 08:19 PM   #19
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Before and after. Charred pipe underground
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480v explosion damage underground PVC conduit-d58036ac-35ff-458b-bc8b-a3d7a03c310b_1552954680489.jpg  

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Old 03-18-2019, 08:54 PM   #20
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Before and after. Charred pipe underground
Welp.... Turns oit that electricians path asphalt better than road crews too. Is there anything we can't do?
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