METAL CLAD BUS-BARS AND CONNECTIONS

What Should Consider in Copper Bus-bar Jointing

We can list up the major considering points for copper bus-bars jointing techniques are as below.

·        Bolted Joint;

·        Clamped Joint;

·        Riveted Joint;

·        Soldered Joint;

·        Welded Joint.

There are main two reasons to bus-bar joint; to extension the bus-bars where linear joints are suitable to assemble; and to tap-off for connections where T-joints are perfect.

Two things must consider as important to bus-bar joints; joint be mechanically strong and electrically have a low resistance.

Copper bus-bar uses not any special type of joint but very familiar type of joints like bolting, clamping, riveting, soldering and welding etc.  Bolted and clamped joints are little easy and possible to use at site, on the other hand riveted and soldered/welded joints are uses in factory pre-fabricated for special shape and to achieve low resistance joints.

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Bolted/clamped joints are easy to install and re-installed; but riveted/soldered/welded joints are difficult and need special arrangement to install and re-install or dismantling.  

 What Effect Should Consider a Bus-Bar Design

The Bus-Bar is usually a normal part of the electrical power generation, transmission and distribution network system who carry the continuous rated current like others main equipment- generator, transformer, circuit breaker etc.

Rated value is the basic requirement for any electrical design, but considering the short circuit condition what happen for a short period of time (1 to 4 sec) is very essential obligation.

The short circuit current is not same for every time, every point of the circuit; but very often it becomes ten to twenty times of continuous current rating.

The short circuit effect may leads to dangerous overheating, including bus-bar the weak portion of the circuit mainly affected by hues amount short circuit current.

During Bus-bar design, major considering points are as-

·        Short Circuit Heating Effect;

·        Electromagnetic Stress;

·        Corona Discharge.

When conductor, herein bus-bar carries current in same direction, an attractive magnetic force is produces. The magnetic force due to continuous rated current is negligible, but it become larger for short circuit current; so it should consider for bus-bar design that may destructive factor for bus-bar insulator and other stuff.

When high voltage air insulated bus-bar exposed with minor spacing between phase to phase, phases to earth, other metallic poles or plane then there is chance to happen flash-over or corona discharge. To avoid this unwanted situation must keep free from any sharp edges.  To better performance corona rings should be incorporated.

Select Sizes of Bas-bar for Maximum Working Current

The measurement of copper bar size for bus-bar is directly formulating is difficult. The change of temperature is not remain the proportional to the size of copper bar, but continuous current rating affected by temperature. The following figure will give a basic calculative idea on bus-bar cross-sectional area and maximum current rating.

Width (mm)	Thickness (mm)	Area (sqmm)	Max Current (A)
25 31.5 40 50 63 80 100 125 160	6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3	158 198 252 315 397 504 630 788 1008	530 639 776 932 1127 1390 1662 1987 2433

Case study:

If we consider there is cable or conductor size is 500 sqmm, then what will be the bus bar size?

So, for 500sqmm conductor, the bus bar size must be 500sqmm or near about 500sqmm. From the above table, if we consider 80 mm width cupper bar and thickness is 6.3mm, then total cross section will be 80x6.3= 504sqmm which is near to 5oosqmm.

or,

we can select 40mm width with 6.3mm thick cupper bar, 2x40x6.3 sqmm= 504sqmm, means doubling the 40mm instead of 80mm we can get the same result. Similarly, we can select the different thickness.

What Should Consider in Bus-bar Connection

Metal clad Bus-bars and its connection should comply with British standard BS 159 and should be electrolytic copper, unless otherwise agreed with the Engineer.

The bus-bars, assembles and connections of equipment for services up to 33 kV should be of a type which does not rely solely on air for insulation purposes. The covering material should be non-deteriorating at the rated short-time maximum temperature of the bus-bars and should have such thickness as is required to withstand rated line to line voltage between bus-bar and a conducting object on the exterior of the covering material for a period of not less than 60 seconds.

The bus-bars and their connections, and insulation materials as appropriate should  be capable of withstanding, without damage, the thermal and mechanical effects of a through fault current equivalent to the short-time rating of the switch-gear.

Facilities to the approval of the Engineer should be provided to accommodate thermal extension of the bus-bars and associated components including the insulating medium if appropriate.

Bus-bars should be contained in a separate compartment within the general casing of switch-gear. Bus-bars barriers should be provided between switch-gear equipment to prevent the spreading of ionized gases in the event of a fault.

Access to bus-bars and the connections directly connected thereto should be gained only by the removal of covers secured by bolts or screws. Such covers should be clearly and indelibly marked “DANGER-BUSBARS”.

Bus-bars should be extensible at both ends; such extension should entail the minimum disturbances to bus-bar compartments. Bus-bars should be of uniform cross-sectional area throughout their length.

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