Installing conduit under a slab

[mshields]

I have an outdoor pad on which sits a MV switchgear lineup and three pad mounted transformers. We're replacing the MV switchgear and have determined that the only place it can go is directly in front of the existing switchgear. So for an interim period, we'll have the new switchgear lineup, behind that we'll have the old switchgear lineup and behind that the three transformers. I'm trying to determine how best to refeed those transformers. The work will be done cold but it has to be done within a 24 hour or less shut down.

What I'm wondering is this, is there a method for digging under the slab such that we can slide the new conduit(s) in and up into the window under the transformer. I'm imagining some kind of support provided to take the place of the ground you'd be removing. I don't know???? Any thoughts???

Oh by the way, I already know one part of the answer - very carefully!!!! Just thought I'd take that one before someone else did!!!

thanks,

Mike

 

[BryanMD]

I suppose someone will come up with a more artful suggestion but the key factor I read in your post was the time issue. Therefore...

I'd pretend it was a new job and have a new precast pad of the required configuration/size etc and rent a crane for the day.

Kill the power.
Disconnect all the conductors.
Lift all the old gear out with the crane.
Demo (or lift/remove?) the existing pad.

Do whatever below grade pipe work is needed.
Compact/grade as needed.
Set the new pad.
Set the new gear.
Set the new transformers.
Connect.
Call for pizza and beer.

Hope this helps.

 

[mkgrady]

Why not just pluck the old gear off the pad and place the new. That way the conduits feeding the three transformers could stay as is. I think Bryanmd's suggestion would take longer than 24 hours.

 

[BryanMD]

I think BryanMD's suggestion would take longer than 24 hours.

EVERYTHING takes longer than proposed.

My concern (aside from the time constraint) is that undercutting the soil below the existing pad with gear weight on it (and possibly still live conductors at that stage) and then sawing it to create the new windows sized/placed to suit the new gear and transformers is an engineering and safety nightmare. The OP focused his concern on this aspect.

Again, my concern is that when all of that is said and done, it would still likely result in even larger problems that (50/50?) would have you needing to deal with replacing the pad anyway.

 

[Smart $]

...it would still likely result in even larger problems that (50/50?) would have you needing to deal with replacing the pad anyway.

Your post #2 proposal requires the replacing of the three xfmrs. I don't believe he stated they are to be replaced. Additionally, it sounds as though the exisiting pad is for all four pieces of equipment.

Why place the new gear in front of the existing? Does it not match up with the existing gear? Why and in what ways does it not match up?

 

[mshields]

clarification

clarification

thanks for all your input. Here's some more info:

The existing gear has a single input. The new will have two incomming sections to accomodate secondary feeder service from the utility. One more additional section is required by the utility after the two incomming sections to act as a disconnect to the 3 feeder sections.

So, if it is indeed practical to remove the gear and replace it in kind in a 24 hour period, we could extend the pad to accomodate the additional sections and have the three feeder sections sitting precisely where they sit now. I'm intrigued at the idea of being able to do this. But do you really think so???


Mike

 

[Smart $]

The big question is how existing under-slab (-pad) work lines up with the new switchgear... and can modifications, if necessary, be made to the existing to accommodate. Modifications to existing, if any?and for the time being I will assume there will be?have to be evaluated in the manner 'can be done pre-shutdown' vs. during shutdown. A dimensional layout of existing xfmrs and switchgear, the new switchgear, and both existing and new conduit routing would help tremendously from my and other participants point of view

Regardless of all the details involved, yes it is entirely possible to change out the switchgear (in same location) in less than 24 hours. Personally, I don't believe there would be a time loss vs. relocating the gear...