Bus Bar basics and advatages in electrical system.

Bus Bar basics

For selection a Busbar must consider the following facts:

01.  Rated current at 35 deg.cen.

02.  Rated operating voltage

03.  Rated insulation voltage

04.  Protection degree

05.  Allowable rated short-time withstand current (t = 1 s),unit KA

06.  Allowable rated peak current

07.  Maximum thermal stress I2t (t = 1s)

08.  Average resistance at an ambient temperature of 20°C

Bus bar trunking is sized to operate at an ambient air which does not exceed + 40 °C and its average over a period of 24 h does not exceed + 35 °C. Above this value, the busbar trunking must be derated. If  ambient temperature derating coefficient= k1,then for Busbar trunking installed indoors Temp=35 k1=1, Temp=40,  k1=0.93, Temp=50  k1=0.90, Temp=55  k1=0.86.

Calculation of the total current (It) is equal to the sum of the currents absorbed by all of the loads. The loads do not all operate at the same time and, as they are not continuously at full load, a stacking or simultaneity factor Ks has to be taken into account It = Σ It load * Ks. Stacking factor Ks depending on the number of loads according to IEC 60439- 1.For number of load from 10-40 Ks=0.6,again 40 and more number of loads Ks is 0.5. For industrial purpose consider 20% more capacity for future expansion.

Thumb rule is, for proper selection of Amp capacity with considering more safety just multiply copper bus bar cross-section by 1.5 and aluminum bus bar cross section by 0.8 .

BBT Advantages over cable:

1.      BBT system savings material with respect to cable i.e. cables trays, multiple fixings and also labor costs associated with multiple runs of cable.

2.      Reduced installation time since bus bar trunking requires less fixings per meter than run cable.

3.      Multiple tap-off outlets allow flexibility to accommodate changes in power requirements subsequent to the initial installation.

4.      System is easily extendable.

5.      Energy efficient.

Bus Bar Protection

To protect busbar from overload must set a circuit breaker having current rating same as max capacity or something less of busbar.

           

                                                    

Short time Withstand capability of Bus bar Trunking

The asymmetrical short-circuit current value is obtained by multiplying the symmetrical short-circuit current value (Isc) by a standardized asymmetrical coefficient (k).It is the first value of the 1st transient asymmetrical peak of the short-circuit which is taken into account.

Example:

For a circuit with a prospective short-circuit current of 50 kA rms, the 1st peak reaches 105 k (50 kA x 2.1). The short-time withstand value (Icw) of the busbar trunking must be greater than the current flowing through the installation during the duration of the short-circuit (Isc) .

According to the WHO (World Health Organisation), exposure to radiated electromagnetic fields above 0.2 micro Tesla can be dangerous causing a risk of cancer over the long term. Some countries have standardized the limit: Sweden = 0.2 µT, at a distance of 1 meter. All electrical conductors generate a magnetic field, the strength of which is proportional to the distance between them. The Canalis busbar trunking concept (metal casing and conductors near together) helps to considerably reduce radiated electromagnetic fields.

Effects of harmonics on  busbar  trunking

Harmonic currents are caused by non-linear loads connected to distribution systems, i.e. by loads that draw current with a waveform different that that of the voltage that supplies them. The most common non-linear loads are equipment including rectifiers, fluorescent lighting and inverters etc.

Harmonic order is the ratio between the harmonic frequency fn and the fundamental frequency (generally the power frequency, 50 or 60 Hz) :n = fn / f.

H3 is heavier among harmonics, the THD is close to the valueof H3.

Fundamental frequency: ih1 (50 Hz)

No current in the neutral.

The conductors are correctly sized.

The only effective solution is 

Fundamental frequency: ih1 (50 Hz) and 33% of

H3

Abnormal temperature rise in the conductors caused

by current at a higher frequency in the phases (skin

effect) and current in the neutral caused by summing

of the H3 harmonics.

 

Now how can assume amount of harmonics. In Workshops Mix of disturbing loads (computers, UPSs, fluorescent lighting) and non disturbing loads (motors, pumps,heating).Low probability of harmonics THD ≤ 15%.

But in office Numerous disturbing loads (computers,UPSs, fluorescent lighting).High probability of harmonics 15 % < THD ≤ 33%.

Aluminium Vs  Copper busbar trunking

When it comes to performance and resistance many designers consider copper to offer a higher performance than aluminum for the same rated bar. However, the differences are marginal as Lee Jones from Schneider Electric explains:

“Installing aluminum bus bar trunking with an equivalent performance means that it is typically between 16% and 25% larger in volume than copper bus bar

trunking. However, it is 40% lighter, offering contractors a trade-off as it’s easier to handle, more manageable, requires less people to install it, offers a faster installation and could eliminate the need for special lifting equipment – all saving time and money for the contractor and the customer. In addition, the use of aluminum trunking over copper means the building’s electrical support structures can be lighter.”

The final argument relates to cost. Even going to a higher rated aluminum trunking over copper, it still offers financial savings. In cost comparisons between 3,200A copper versus 4,000A aluminum, there is a saving of 23%.

Lee Jones comments: “There are many benefits for choosing aluminum bus bar trunking, which means designers and contractors shouldn’t be put off from using it. On shorter runs the performance difference between the two materials is marginal and on longer runs where volt drop may be an issue, then designers can choose a higher rated aluminum bar.” In addition, no special tap-offs are required on our range because of the use of silver on the joints.