Clamping a single medium voltage conductor inside a transformer

[Journeyman106]

I have a question. My engineer and I have been going back and forth for the last few days on the selection of a proper cable clamp for a single medium voltage conductor. Here is the situation. We have a pad mounted transformer with 3 single conductors coming in and 3 single conductors going out. The system voltage is 34.5kV. There will be upwards of 300 to 450 amps of continuous current on each conductor. The engineer is requiring that the cable be supported within the transformer. He stated that he is only concerned with the conductors thrashing around in case of a fault current. We did come to an agreement that we could use heavy duty ty-wraps but the owner does not like this idea, he wants something more sturdy like a cable cleat or clamp.

So the dilemma is that I need to get these clamps pronto. I have found numerous clamps in stock, such as the 3M MB-4 thru MB-6, Raychem has them, as well as Nordic. However the ones they have in stock or made of plate steel or galvanized steel. The strap basically resembles a stand-off strap for conduit except that it has a piece of rubber in it that protects the insulation of the cable from the strap. The argument is that since it is made of a ferrous steel that it will heat up and burn through the cable or damage the insulation. I understand the theory or principles behind the thinking, I understand that by completely encircling the conductor with a ferrous metal that it could induce a current from the magnetic field and those heat up. The thing is these clamps are made of steel, but they have a stainless steel bolt that brings the two sides together and there will be an air gap with only the stainless steel bolt connecting the two sides.

My argument is that 3M and other manufacturers have been making these clamps for years for this exact application, but yet my engineer will not sign off on it because the whole clamp is not stainless or non-ferrous. The question is does the stainless steel bolt that brings the two sides of the clamp together break the circle? The engineer is under the impression since the stainless steel bolt is still metal and that since the ampacity is so high, that it could still force current through the bolt because it is metallic.

Any suggestions would be greatly appreciated, or any tidbits of education on the matter would also be greatly appreciated.

 

[GoldDigger]

The current through the wire does NOT induce a current in the clamp or other encircling metal. Or if it does the amount is so small as to be negligible.
The problem is the magnetic field induced in the clamp which heats the metal through hysteresis losses, not I²R losses. It is analogous to an induction heating hob and an iron bottomed cook pot. Or an induction furnace for that matter.

Just as cutting a slot in a ferrous metal plate between two holes breaks the magnetic circuit even if you then put an aluminum plate on top of it, the gap between steel parts interrupts the magnetic circuit even though the stainless bolt bridges the gap. It still prevents magnetic heating.

Just be sure that the bolt is a non-magnetic stainless alloy. Not all stainless is non-magnetic.

 

[Tony S]

Eddy currents will induce heating.

I think if the OP looks at the specification for the CT clamp it will be a broken ring. I hope the specification also mentions grounding the clamp.

D. K. Moriarty has an excellent website which includes information on MV CT’s