No. 16 - Bus joint fundamentals

No. 16 - Bus joint fundamentals
www.usa.siemens.com/techtopics
TechTopics No. 16
Bus joint fundamentals
Proper design of bus bar joints is a necessity for long equipment
life. The objectives that a good bolted bus bar joint must fulfill
include:
Cap screw
Bus bars
Flat washers
It must provide good conductivity, so that the bus system
will meet the temperature rise requirements in the ANSI/IEEE
standards.
It must withstand thermal cycling, so that the low-resistance
joint will be maintained for the life of the equipment.
The joint pressure should be high (for good conductivity), but
not so high that cold flow of the bus material occurs, which
would cause the joint to deteriorate with time.
The joint should have good resistance to corrosion in normal
installation environments.
It must be able to withstand the mechanical forces and thermal
stresses associated with short-circuit conditions.
Lock washers
Figure 1: Anatomy of a bolted bus bar joint
Figure 1 shows a bolted bus bar joint, simplified to show two bus
bars connected using a single bolt. Except in rare situations, the
bus bars are silver plated (standard) or tin plated (optional), to
improve the resistance to corrosion. The bolt is a high-strength,
grade 5 cap screw, while the nut is a grade 2 (heavy wall) nut.
The joint includes a large diameter, thick, flat washer on both
sides of the joint, adjacent to the bus bars. A split lock washer is
installed under the nut to assure that the joint stays tight over the
life of the equipment.
Answers for infrastructure.
Why do we use a grade 2 nut with a grade 5 bolt? The grade 2
nut is more ductile than the grade 5 bolt, so that when the nut
is torqued in place, the threads in the nut will tend to be swaged
down and burnished to a degree, which results in a more equal
distribution of load on all threads. This spreads the force more
evenly and avoids unacceptable stress levels in the bolt and the
nut.
Some users request that special, non-magnetic hardware be
used in bus joints. Historically, particularly in open bus systems
exposed to the weather, difficulties were encountered with
corrosion, and this may be one reason that some still ask for
non-magnetic hardware. Others prefer non-magnetic hardware
because of the perception that it results in a lower temperature
rise. While these reasons may have had merit decades ago,
Siemens feels they are unnecessary today. Non-magnetic
hardware (usually stainless steel or silicon bronze) is expensive
and difficult to obtain. In addition, the tensile strength and yield
strength of non-magnetic hardware is lower than that of highstrength steel, so that tightening torques will generally be lower
with the special hardware. The net effect of lower torque and
pressure may very well counterbalance any slight temperature
rise benefit associated with non-magnetic hardware.
Maximum
Pressure
Pressure PSI
Siemens also specifies that the flat washers are to have larger
diameter and greater thickness than standard washers. The
purpose of the washers is to distribute the clamping force of the
bolts over a wider area. To accomplish this, a washer is needed
that is relatively rigid, with a larger diameter than would be
normal for the size bolt used. If a normal, small diameter, thin
washer (or worse, none at all) is used, the joint will deteriorate
over time because of cold flow of copper from the high-pressure
region directly under the bolt head (or the nut).
Lapped Hole
bars
Figure 2 shows the distribution of forces in a bolted bus
bar joint. To obtain a low-resistance bus bar joint, we must
establish and maintain sufficient pressure, and distribute the
pressure over a large area. Initially, the two bus bars mate at
only a few peaks or high spots. As the bolt is tightened, the
bus conductors begin to deform, bringing more of these peaks
into contact. At the design pressure, there is a relatively larger
contact area, so that there are a multitude of parallel electrical
connections between the bus bars.
As shown in Figure 2, the force is concentrated more heavily
around the bolt hole. Since the pressure is highest in the
vicinity of the bolt hole, the surface irregularities in this area
are flattened out as the mating surfaces are forced into more
intimate contact. The joint resistance in this area will be lower
than elsewhere in the joint. As distance from the bolt hole
increases, pressure decreases and joint resistance increases.
Beyond the area defined by the washer, pressure decreases
rapidly and little effective current-carrying capacity results.
Figure 2 shows how the large diameter washers serve to
distribute the clamping force more uniformly over a wider area
than would be the case with a smaller washer, or none at all.
A properly designed, bolted bus bar joint will allow the bus
system to meet the temperature rise limits imposed by the
ANSI/IEEE standards, and will also have the thermal and
mechanical capability to withstand the heat generated and
forces imposed under the worst-case short-circuit conditions.
Elastic limit of
condictor material
Average
pressure
Lapped
bars
The information provided in this document contains merely general
descriptions or characteristics of performance which in case of actual
use do not always apply as described or which may change as a result of
further development of the products. An obligation to provide the
respective characteristics shall only exist if expressly agreed in the terms
of contract.
All product designations may be trademarks or product names of
Siemens AG or supplier companies whose use by third parties for their
own purposes could violate the rights of the owners.
Thick rigid washers to
distribute the applied
bolt force
Figure 2: Distribution of forces in a bolted bus bar joint
Siemens Industry, Inc.
7000 Siemens Road
Wendell, NC 27591
Subject to change without prior notice.
Order No.: E50001-F710-A305-X-4A00
All rights reserved.
© 2012 Siemens Industry, Inc.
For more information, contact: +1 (800) 347-6659
www.usa.siemens.com/techtopics
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