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Torque and current ratings

Torque and current ratings

Torque and current ratings

(OP)
So the new European built ANSI rated 1200 A gas breakers have arrived. A design detail has us concerned. The NEMA four hole aluminum pad is attached to the top of the aluminum bushing cap by a single stainless bolt, approximately a 25M. When we put a wrench on it, the bolt extracted way too easy in our opinion. When asked, the manufacturer gave us a torque value of 25 Nm. Still sounds low. In addition, there is no keeper pin so only the bolt clamping force prevents the pad from spinning around the bolt, and no lock or Belleville washer is used. Our concern is that even if the breakers pass initial thermal scans, that over a short time and a few operation the bolt will loosen.

Is 25 Nm on a large bolt sufficient to provide enough clamping force to reliably carry up to 1200 A? Is there a standard that addresses this? Thanks.

RE: Torque and current ratings

Sounds awfully low. But have no reference.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Torque and current ratings

Can you obtain a European manual for this equipment and verify that torque value? It seems very low for a single bolt of that size. Whose breaker is it and what type - I or one of the other contributors might have some information.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Torque and current ratings

(OP)
Scotty,

The torque value is not in the manual, that's why we asked. Breaker is an ABB DTB 72,5 kV. It is a single pedestal breaker built in Italy.
  

RE: Torque and current ratings

There are many papers [or treatises] in the fixed contact field. But there is a consensus in some of the conclusions.
A pressure of 250 kg/cm^2 for aluminum bar bolt stretched is recommended.
If the bolt is 25 mm diam. then the nut pressed area will be 9.8 cm^2[for a usual hexagonal nut].
The force -recommended- will be 9.8*250=2450 kgf =24034 N.
The torque will be 0.2*F*diam=120 Nm.
Maximum permissible torque for stainless steel 1"-14 tpi bolt  is 80% of 3250 inch-lbs =367.3*.8=294  Nm.
If we shall use the R.Holm formula for fixed contacts R=K/P where K=3.5/10^4 and P in kg/cm^2
the contact resistance will be 1.4*10-6 ohm.
In case of short-circuit the contact temperature will be about 100 oC [35 kA, 5 sec].
If the torque will be only 25Nm the pressure on aluminum lug will be only 25/120*250=52 kg/cm^2.
The contact resistance will grow up to 3-4 times and a layer of oxide will be created between contacts.
The contact temperature may reach 300-400 oC and the mechanical properties of aluminum will decrease fast.
My opinion: I don't think the torque could be less than 120 Nm.
See:
R. Holm, Electric contacts – Theory and Applications
Electrical contacts: principles and applications By Paul G. Slade pp 452
http://openlibrary.org/a/OL2870833A/Paul_G._Slade
Fundamental of Electrical Contacts by Milenco Braunovich and others.
Engineers Edge
http://www.engineersedge.com/torque_table_sae.htm
 

RE: Torque and current ratings

(OP)
New info is that the value should be 250 Nm. Wire brushed the aluminum surfaces, coated them with anti-oxidant, and torqued the bolts to 250. A small bit of reefing on the attached conductor in the cc direction will cause the pad to turn and the bolt to back out. Don't see how this arrangement can be expected to survive short circuit forces in the conductors.

Also, I was mistaken, a split washer type of lock washer is used.

RE: Torque and current ratings

(OP)
Update:

Manufacturer has agreed to supply studs instead. Stud screws into the bushing instead of the 24M (size corrected from above) bolt and pad. With the conductor extending straight up, short circuit forces will not cause any torque.
 

RE: Torque and current ratings

I wonder if they will adopt that as a standard design? Sounds like a good solution.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Torque and current ratings

(OP)
Actually, it sounds like it is the standard design. The single bolt NEMA pad was developed for customers like us who specified a NEMA pad.

  

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