The document discusses busbars, which are the backbone of low voltage switchgear assemblies. It covers topics such as busbar material selection criteria, sizing calculations, installation practices, and good practices for bending, punching holes, making connections, and applying anti-corrosion treatments. Key factors in busbar selection include rated current, short circuit withstand capability, ambient temperature, and enclosure protection level. Proper sizing ensures correct operation without overheating.

1. Setraa Al-Amal Manufacturing Co.
Busbars in LV
Switchgear Assembly
By Nooruddin Khan (09/09/2019)

2. Busbar: “Backbone” of LV
Switchgear
 The busbars constitute the real “backbone” of every LV
Switchgear Assembly.

3. Busbar
 Busbar is a conductor of electricity e.g. strip or bar of copper, brass
or aluminum that conducts electricity within a switchboard,
distribution board, substation, battery bank or other electrical
apparatus etc.
 Specification: Cu-ETP (Electrolytic-Tough-Pitch) R240 grade of bus
bar is specified in switchboard application as per EN 1652

4. Power is distributed in switchboards through the following means:
 Main busbar that distributes power horizontally between the various
switchboard columns. It may be installed on the top, middle or
bottom of the switchboard depending on the type of switchboard,
customer specifications and/or local practices.
 Distribution busbars connected to the main busbar. They provide
power to outgoing devices.
Busbar

5. The following must be taken into account when choosing a power
busbar:
 Environmental characteristics of switchboard (ambient temperature, IP
degree of protection, pollution),
 Type of switchboard installed regarding to validation test,
 Characteristics of client's power supply: on top, middle or bottom,
 The rated current of the short-circuit: Icw.
The installation of a power busbar consists in the following steps:
 Select the busbar material,
 Size it (busbar section, number of busbars per phase) and define its
position in the switchboard based on the client's incoming devices,
 Install it in compliance with the creepage and clearance distances of the
standard,
 Fasten it according to good practice.
Busbar Selection

6. Busbar: Selection Criteria
 The type and cross-section of the busbars should be enough to
carry the required current for a given temperature rise, thus ensuring
that the electrical switchboard functions correctly.
 Why Copper or Aluminum are widely used? (a) Easy to use (b)
Excellent conductivity (c) Surface (coating) guaranteeing electrical
contact (d) good resistance to corrosion.
 Two important Factors affecting busbar sizing are (a) Ambient
Temperature (b) Degree of Protection (IP Rating) of enclosure.
 Two important parameters considered during selection of busbar &
busbar supports are (a) Rated current (In) & (b) Rated short time
withstand current (Icw)

7. Busbar: Selection Criteria
 Thumb Rule: 1.5A/mm2 (Not Accurate & just for rough estimation)
 Select suitable size from tables developed by CDA.
 Tables Developed by switchboard manufacturers
 Using Formula
 Using Software

8. Busbar: Selection Criteria

9. Diversity Factor
Generally, not all the devices connected to a busbar are used at full load or at
the same time. It is therefore not necessary to size the busbar to transport the
sum of rated currents of all the devices continuously.
The rated current "In" in the busbar must be calculated:
 by adding up the rated currents of all the connected devices,
 by multiplying the result by the diversity factor (RDF) defined by IEC 61439-
1. The value of the diversity factor depends on the number of devices
connected to the busbar:

10. PE Protective Conductor
The PE protective conductor contributes to bringing all the switchboard
conductive parts to the same potential. It must be sufficiently sized and secured
in the switchboard to withstand heat and electrodynamics stresses generated
by a fault current of a duration of between 0.2 s and 5 s.
IEC 61439-1 standard gives a method for calculating the cross-section of the
PE conductor:
Where:

11. Earthing
All accessible conductive parts of the switchboard must be connected to the
earthing circuit:
 by construction. Continuity is provided via the fastening systems (plate,
relevant fasteners, claws),
 or via a protective conductor with a cross-section chosen from the table
below:

12. Definition of Parameters:
 Rated current (In) : Maximum current that the device can carry
continuously without abnormal temperature rise.
 Rated short time withstand current (Icw) : Maximum short time
withstand current that the equipment can carry without damage.
 Rated Insulation Voltage (Ui) : The highest operating voltage that
will not cause a dielectric strength failure. The rated insulation
voltage is used as a parameter for dielectric strength tests and for
the creepage distance. The rated insulation voltage must always be
higher than the rated operating voltage (Ue).
 Rated Impulse Voltage (Uimp) : The peak value of an impulse
voltage of which the circuit of an assembly is capable of
withstanding without failure.

13. Clearance Distance:
 Clearance Distance : Clearance is the shortest distance in the air between:
(a) two live conductors (b) a live conductor and an exposed conductive part.
The standard provides a table giving the minimum clearance to comply with in order to
observe the rated impulse withstand voltage Uimp declared by the manufacturer for a
circuit.
Interpretation of the below table:
 since a power busbar has a rated clearance voltage Ui of 1000 V, the
declared rated impulse withstand voltage Uimp is 12 kV. The clearance
must therefore be at least 14 mm,
 since a splitter has a rated clearance voltage Ui of 750 V, the declared rated
impulse withstand voltage Uimp is 8 kV. The clearance must therefore be at
least 8 mm.

14. Creepage Distance:
 Creepage Distance : It is the shortest distance along the surface of
an insulation between: (a) two live conductors, (b) between a live
conductor and the bounding surface.
IEC 61439-1 standard defines the requirements applicable to creepage distances inside
a switchboard. These requirements are based on the principles of IEC 60664-1 standard
and enable the coordination of clearance inside the switchboard.
The standard defines creepage depending on:
 - the rated insulation voltage Ui of the table,
 - the type of insulating material (which defines the material group),
 - the degree of pollution in the environment. Creepage: 16mm for a rated clearance
voltage of up to 1600 V.

15. Busbar manufacturers usually provide the information required for
sizing power busbars.
These information must be used in calculating the cross-section of the
busbar. The busbar cross-section depends on:
 the rated current "In" to be conveyed in the busbar,
 the rated current of the short-circuit (permissible short-time
withstand current): "Icw",
 the ambient temperature around the switchboard,
 the IP protection index of the enclosure,
 the rated diversity factor (RDF),
 the space reserved for future enhancements,
 the constraint defined by the trigger time of the protective device.
Caution: never under-size a busbar (risk of overheating).
Busbar Sizing: Good Practice

16.  Always wear gloves when handling copper bars to prevent chemical
deterioration resulting from contact with the skin (perspiration).
 Install the bars edgewise when possible to favor heat dissipation by
convection.
 When the bars are installed flat, the permissible currents must be reduced
(derating). Use the calculation elements defined for the edge bars by
applying the derating coefficient recommended by the manufacturer.
 When using flat bars, e.g., derivations of incoming devices on busbars, SE
recommends a derating factor of 0.8.
Busbar Sizing: Good Practice

17. Eddy Current: The current circulating in the bars generates induced currents
(eddy currents) in conductors and surrounding metal.
Given the high intensity of the current circulating in the bars, this phenomenon
may lead to significant heating of these materials.
To avoid this phenomenon, you must break the induced current loop. Use one
of the solutions below:
 supports in insulated materials,
 aluminum counter-supports,
 non-magnetic stainless-steel fixing screws.
Non-compliance with these recommendations will result in:
 vibrations that could break the supports.
 the over-heating of the supports, counter-supports and fastening screws.
Busbar : Good Practice

18. Busbar Overlap Rule
 For copper bars up to 10 mm thick, you can apply the following rule:
the overlap distance r must be between 3 and 5 times the thickness
e of the secondary bar.
 This is a general rule. Its main objective is to maintain the same
temperature rise as that of a full bar, all the while ensuring the
electrical connection over time.

19. Busbar Contact Pressure
The contact pressure depends on:
 the number of tightening points.
 the type of fasteners used (quality, diameter).
 the tightening torque applied to the fasteners.
Adapt the number of tightening points of the bar to the size of the
contact surface, knowing that the optimum contact pressure is exerted
mainly around the head of the screw.
 The quality of the electrical connection obtained with the class 8.8
fasteners has been tested and validated by lab tests.
 Use a calibrated and certified torque wrench to guarantee the
contact pressure. The tightening torque depends on the diameter
and quality of the fasteners.

20. Busbar Contact Pressure
Caution: an excessive tightening torque or an insufficient number of
tightening points can result in:
 a creep in bars that will lead to an uneven distribution of contact
pressure,
 an abnormal temperature rise of the connection,
 the going beyond of the elastic limit of the screw and therefore a risk
of elongating the screw thread or even breaking it.

21. Busbar Bending
 Comply with the minimum bending radius to avoid cracking or
tearing the metal.
 Do not straighten copper bars in order to re-use them. Do not bend
bars where there are holes.
 Copper quality allows bends with a minimum radius of curvature
equal to bar thickness (5 or 10 mm).

22. Busbar Hole Punching
 Holes should be made if possible using a punch fitted with a spring
loaded stripper plate that allows the punch to be raised without
deforming the bar.
 Check regularly the cutting edge of the punch and the space
between the punch and the die (0.5 mm) in order to avoid flash or
deformation of the copper by creep.

23. Connections & Hardware
The quality of the electrical connection is related to:
 the contact pressure between the bars
 the overlap area (size and quality).
Class 8-8 zinc-coated steel (Zn8C) nuts and bolts are used.
 Once tightened to the appropriate torque, the nuts and bolts retain all their
mechanical properties over time (elasticity), without deforming the material (creep),
regardless of the temperature conditions inside the switchboard.
 Assemblies should be tightened using class 8-8 corrosion-resistant steel nuts and
bolts, fitted with a contact washer on either side. If they are unscrewed, the contact
washers must be replaced.
 Bolt length should correspond to the total thickness of the bars + the washers
(uncompressed) + the nut + two threads.
 check for compliance with electrical clearances
 in certain cases (risk of oxidation), bolts may be set flush with the nut.

24. Anti-corrosion treatments

25. Tightening Torque
 Use a calibrated torque wrench in order to guarantee contact pressure (20
to 30 N/mm2).
 Tightening torque depends on the diameter and quality of the nuts and
bolts.
 tighten the nut in the case of a bolt or the screw for a tapped hole
 mark the bolts when tightened to the specified torque to allow operator
inspection
 tightening tools should be checked at regular intervals
 after dismantling a busbar or its connections, new nuts and bolts should be
used for subsequent mounting.

26. Tightening Torque

27. Questions
Thank You