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Low Voltage BusbarTrunking Systems

Performance under Short-Circuit Conditions 


Busbar trunking systems to BS EN 61439-6 are designed to withstand the effects of short-circuit currents resulting from a fault at any load point in the system, e.g. at a tap-off outlet or at the end of a busbar trunking run. The short-circuit current rating for busbar trunking, for a particular installation,should match the prospective fault current available at the feeder unit.There is no advantage in specifying a higher figure for short-circuit rating. 



(Photo Credit Siemens Website)



Rating under Short-Circuit Conditions 


The withstand ability will be expressed in one or more of the following ways: 

 a) rated short-time withstand current and corresponding time. 

 b) rated peak withstand current. 

 c) rated conditional short-circuit current when protected by one or more short-circuit protective device(s) (SCPD(s). 

These ratings are explained in more detail: 


Rated short-time withstand rating (Icw) 


This is an expression of the value of rms current that the system can withstand for a specified period of time without being adversely affected such as to prevent further service.Typically the period of time associated with a short-circuit fault current will be 1 or 3 seconds, however other time periods may be applicable. The rated value of current may be anywhere from about 10 kA up to 50 kA or more according to the construction and thermal rating of the system. 



Rated peak current withstand current (Ipk) 


This defines the peak current, occurring virtually instantaneously, that the system can withstand, this being the value that exerts the maximum stress on the supporting insulation. In an a.c.system rated in terms of short-time withstand current, the peak current rating must be at least equal to the peak current produced by the natural asymmetry occurring at the initiation of a fault current in an inductive circuit.This peak is dependent on the power-factor of the circuit under fault conditions and can exceed the value of the steady state fault current by a factor of up to 2.2 times. 


Rated conditional short-circuit current (Icc) 


Short-circuit protective devices (SCPDs) are commonly current-limiting devices; that is they are able to respond to a fault current within the first few milliseconds and prevent the current rising to its prospective peak value.This applies to HRC fuses and most circuit breakers in the instantaneous tripping mode.Advantage is taken of these current limiting properties in the rating of busbar trunking for high prospective fault levels.The condition is that the specified SCPD (fuse or circuit breaker) is installed upstream of the trunking. 

Each of the ratings above takes into account the two major effects of a fault current, these being heat and electromagnetic forces.The heating effect needs to be limited to avoid damage to supporting insulation.The electromagnetic effect produces forces between the busbars, which stress the supporting mechanical structure, including vibrational forces on a.c.The only satisfactory way to establish the quoted ratings is by means of verification to the British Standard. Guide to LowVoltage Busbar Trunking SystemsVerified to BS EN 61439-6 13 


Verification 

Busbar trunking systems are verified in accordance with BS EN 61439-6 to establish one or more of the short-circuit withstand ratings defined above. In the case of a short-time current test a current is applied such as to achieve the let-through energy and peak current corresponding to the rated value. In the case of a conditional rating test with a specified SCPD, the test is conducted with the full prospective current value at the busbar trunking feeder unit and not less than 105% rated voltage,since the SCPD (fuse or circuit-breaker) will be voltage dependent in terms of let through energy. 



Application 

It is necessary for the system designer to determine the prospective fault current at every relevant point in the installation by calculation, measurement or based on information provided, e.g. by the supply authority.The method for this is well established, in general terms being the source voltage divided by the circuit impedance to each point.The designer will then select protective devices at each point where a circuit change occurs, e.g. between a feeder and a distribution run of a lower current rating. The device selected must operate within the limits of the busbar trunking shortcircuit withstand rating.The time delay settings of any circuit-breaker must be within the specified short time quoted for the prospective fault current.Any SCPD used against a conditional shortcircuit rating must have energy limitation not exceeding that of the quoted SCPD. For preference the SCPD recommended by the trunking manufacturer should be used. 


Voltage Drop 


The requirements for voltage drop in an installation are given in BS 7671: 2008 (2011) Regulation 525. For busbar trunking systems the method of calculating voltage drop is given in BS EN 61439-6 from which the following guidance notes have been prepared. 

Voltage Drop Values


 • Figures for voltage drop for busbar trunking systems are given in the manufacturers’ literature. 

 • Figures are usually expressed in mV/A/m or mV/A/100 metres, for various power factors, allowing a simple calculation for a given length of run. 

 • Figures are usually given as line-to-line voltage drop for a 3 phase balanced load. 

 • Figures take into account resistance of joints, temperature of conductors and assumes the system is fully loaded.


 Voltage Drop Calculation from Basic Data 


BS EN 61439-6 requires the manufacturer to provide the following data for the purposes of calculation, where necessary: 

R20 – the mean ohmic resistance of the system, unloaded and at 20°C, per metre per phase. 

X – the mean reactance of the system, per metre per phase. 

R – the mean ohmic resistance when loaded at rated current and at an ambient temperature of 35°C, per metre per phase. 

In general the voltage drop figures provided by the manufacturer are used directly to establish the total voltage drop on a given system; however this will give a pessimistic result in the majority of cases. 

Where a more precise calculation is required (e.g. for a very long run or where the voltage level is more critical) advantage may be taken of the basic data to obtain a more exact figure. 


Resistance 


The actual current is usually lower than the rated current and hence the resistance of the conductors will be lower due to the reduced operating temperature. 

 Rx = R20 [1+0.004(Tc - 20)] ohms/metre and Tc is approximately Ta + Tr where Rx is the actual conductor resistance. 

 Ta is the ambient temperature 

 Tr is the full load temperature rise in Kelvin (obtained from the manufacturer) 

Power Factor 


The load power factor will influence the voltage drop according to the resistance and reactance of the busbar trunking itself. 

The voltage drop line-to-line (Δv) is calculated as follows: 

 Δv = √3.I (R cos Ø + X sin Ø) volts/metre 

Where: 

 I is the load current and Rx is the actual conductor resistance (Ω/m) X is the conductor reactance (Ω/m)

 Note: For BTS rated below 100A the reactance X is deemed to be negligible. 

 cos Ø is the load power factor (PF) 

 sin Ø = sin [cos-1 (PF)] 



Distributed load 


Where the load is tapped off the busbar trunking along its length this should also be taken into account by calculating the voltage drop for each section. 

As a rule of thumb the full load voltage drop may be divided by 2 to give the approximate voltage drop at the end of a system with distributed load. 

Frequency 

The manufacturers’ data will generally give reactance (X) at 50Hz for mains supply in the UK.At any other frequency the reactance should be re-calculated as follows;

Xf = X.f / 50 

where Xf is the reactance at frequency f in Hz 


Electromagnetic Fields (EMF) 


Any electrical conductor carrying current will generate a magnetic field.This effect is of importance in some applications to ensure safety of personnel and trouble-free operation of equipment installed in the vicinity. 

BS EN 61439-6 provides a method of test to establish the field strength surrounding a busbar trunking system to enable the determination of distances for safe levels of exposure. This test is not mandatory and is not a condition of compliance with standard. 

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) publishes guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields in the frequency range from DC up to 300GHz. 

Busbar trunking systems (BTS) are better suited for power distribution than cables when a low magnetic induction is required, as the BTS construction facilitates the optimum arrangement of conductors to keep magnetic interference to a minimum. In many cases the conductors of a busbar trunking system are totally shielded by the equipotential metal casing of the system. In the case of sandwich-type construction the conductors are closely packed together and the induced magnetic fields cancel one another to a large extent,resulting in an extremely low external magnetic field. The material of the conductors in a BTS (e.g. copper or aluminium) has negligible effect on the magnetic field. 

Often, less attention is paid to EMF when installing cables.As a result, neither the distance between the conductors nor the sequence of the conductors is considered, which results in high magnetic fields compared to the equivalent BTS.


This article has been extracted from "Guide to Low Voltage Busbar Trunking Systems" by BEAMA


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