Induced currents and heating in foundation rebar 1

[electrickiwi]

Greetings,

I'm looking at cable ducting arrangements for a 415 V, 600 kVA generator where the single core power cables feed out through a concrete foundation slab in which the rebar is bonded to the earthing network.
Cables are single-core and connected 2 per phase (carrying around 400 A each).
Rebar is 20mm OD but unfortunately the tight spacing and bend requirements makes it difficult to get a set of 3 conductors through a single rebar "loop".

This is for the UK. Is there any UK-specific guidance as to what minimum current level (rebar size, spacing, conductor size, etc) requires consideration of EM induction and heating??
If I need to go down the route of insulating between rebar joins so as to break the current loop, how is this normally achieved? Is there a specific product for this purpose?

On a related note, the generator comes with a steel terminal enclosure that we would punch individual holes through for the single core cable glands. The comment from the manufacturer was that in only a tiny percentage of cases are they ever asked for a stainless alternative, and that for 400 A per core (800 per phase), they don't deem it necessary to worry about induced heating.
Does that sound reasonable??

Thanks

 

[racookpe1978]

Would there be any at only 415 volts?

 

[waross]

I generally punch a group of holes, A, B, C and neutral and use a reciprocating saw to cut a slot between the holes so that in effect it becomes one oddly shaped hole. Do the same for the second group. Try to use aluminum glands and locknuts if possible. I did this on a job and my supervisor was so impressed that he had another crew do the same on another project. He didn't listen when I told him 8 times to be sure to use aluminum locknuts. The job was finished when the AHJ called him on the ferrous locknuts. He burned up a lot of labour disconnecting everything to change out the locknuts.
A more common but more expensive practice is to cut a large rectangular hole in the junction box and install an aluminum plate. The cable glands are then landed on the aluminum plate. The use of nonferrous connectors and locknuts holds throughout.
If the cables are screened, the screens should be insulated at the load end. This holds true for armour also. This is often done by installing a sheet of micarta rather than aluminum at the load end. On occasion I have been able to insulate the armour with varnished cambric tape and use a dry type connector. Holes and slots were then used in the ferrous enclosure. This is not always possible.
The re-bar worries me. I have seen too much heating and in some cases paint burned off by a ferrous encirclement of unbalanced currents. Whatever method you choose to eliminate encirclement, make sure it is done well by someone who understands the implications of encirclement.
You don't want to be breaking concrete to remedy hot re-bars.

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

 

[ScottyUK]

At 400A per phase I think you'll see some heating. The generator OEM likely wants to do it cheap, not do it right so they'll try to get away with a ferrous glandplate if you let them. I would choose a pre-drilled brass plate with brass glands, unless you're in an environment where an austenitic stainless material is needed. Stainless is a hard work to drill using standard holesaws, so get it punched or drilled at the fab shop.

If you have time to modify the bar bending schedule, work with the structural design engineer to form the cable entry. You don't really want to put a core cutter through rebar if you can help it. I'm not aware of any specific UK guidance or legislation.

 

[electrickiwi]

Thanks gents,

I do have an Al gland plate on the switchboard and use brass or PVC glands. Was a bit surprised by the generator manufacturer's response considering we'd be paying for the special gland plate anyway... their rationale was that the current was too low to cause any significant heating in the ~2mm steel plate. I'm less worried about the generator than the rebar because the former is easily retro-fixed, but could a non-ferrous gland in the ferrous plate reduce the magnetic field to an insignificant level?

Anyway, I was hoping there might be some equivalent guidance on concrete-encased rebar that said 400 A through a loop is too low to worry about... Does anyone know if there's a particular UK regulation that dictates anything about single cores through ferrous loops? Or an international equivalent?

Cheers

 

[ScottyUK]

In partial answer, if you form a closed ferrous loop around a conductor you will get heating. A non-ferrous gland in a steel glandplate will not contribute to the heating problem, but the steel glandplate will still get hot because of currents circulating within the glandplate itself.

Is this a welded mesh for light duty concrete or a full structural reinforcement made from many discrete rods with tie-wires? You won't get particularly good connection between discrete rods so joint resistance will limit the magnitude of any circulating current, but welded mesh will have a fairly low loop resistance and you may well get a few amps circulating. Gut feeling is that the current will be nowhere near what you'd get in the glandplate.

Waiting with interest to see if anyone knows of a specific regulation covering this.

 

[electrickiwi]

It's structural bar, 20 mm dia, bent rods tied together.

 

[waross]

Canadian Electrical Code Part I
Section 12
Wiring methods
12-106 Multi- and single-conductor cables
sub rule (5)
A single-conductor cable carrying a current over 200 A shall be run and supported in such a manner that the cable is not encircled by ferrous material.

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