Category: Siemens Barduct Busbar Specification
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Siemens Barduct
Busbar Specification
Siemens Barduct Busbar Technical Specifications
Performance under Short-circuit Conditions
Busbar trunking systems to BS EN 60439-2 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 point or at the end of a feeder
run.
Rating under Short-circuit ConditionsThe withstand ability will be
expressed in one or more of the following ways:
a) short-time withstand rating (current and time)
b) peak current withstand rating
c) conditional short-circuit rating when protected by a short-circuit
protective device (s.c.p.d.)
These ratings are explained in more detail:
a) Short-time Withstand Rating
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 second,
however, other time periods may be applicable.
The rated value of current may be anywhere from about 10kA up to 50kA or
more according to the construction and thermal rating of the system.
b) Peak Current Withstand Rating
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.
c) Conditional Short-circuit Rating
Short-circuit protective devices (s.c.p.ds) 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 many
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
s.c.p.d. (fuse or circuit breaker) is installed up stream of the
trunking. Each of the ratings above takes into account the two major
effects of a fault current, these being heat and electromagnetic force.
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 way to verify the quoted ratings
satisfactorily is by means of type tests to the British Standard.
Type Testing
Busbar trunking systems are tested in accordance with BS EN 60439-2
to establish one or more of the short circuit withstand ratings defined
above. In the case of short-time rating the specified current is applied
for the quoted time. A separate test may be required to establish the
peak withstand current if the quoted value is not obtained during the
short-time test. In the case of a conditional rating with a specified
s.c.p.d. the test is conducted with the full prospective current value
at the trunking feeder unit and not less than 105% rated voltage, since
the s.c.p.d. (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 short-circuit withstand. The time
delay settings of any circuit breaker must be within the specified short
time quoted for the prospective fault current.
Any s.c.p.d. used against a conditional short-circuit rating must have
energy limitation not exceeding that of the quoted s.c.p.d. For
preference the s.c.p.d. recommended by the trunking manufacturer should
be used.
Voltage Drop
The requirements for voltage-drop are given in BS 7671: Regulation
525-01-02. For busbar trunking systems the method of calculating voltage
drop is given in BS EN 60439-2 from which the following guidance notes
have been prepared.
Voltage Drop
Figures for voltage drop for busbar trunking systems are given in the
manufacturers literature.
The figures are expressed in volts or milli-volts per metre or 100
metres, allowing a simple calculation for a given length of run. The
figures are usually given as line-to-line voltage drop for a 3 phase
balanced load.
The figures take into account resistance to joints and temperature of
conductors and assume the system is fully loaded.
Standard Data
BS EN 60439-2 requires the manufacturer to provide the following data
for the purposes of calculation, where necessary:
R20 the mean ohmic resistance of the system, unloaded, at 20ºC per metre
per phase
X the mean reactance of the system, per metre per phase
For systems rated over 630A:
RT the mean ohmic resistance when loaded at rated current per metre per
phase
Application
In general the voltage drop figures provided by the manufacturer are
used In general the voltage drop figures provided by the manufacturer
are used 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.
i) 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 ºC (assume say 55ºC)
ii) 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 ( ∆v) is calculated as follows:
∆v = √ ³ I (R x cos φ + X sin φ) volts/metre
where I is the load current
Rx is the actual conductor resistance (Ω/m)
X is the conductor reactance (Ω/m)
Cos φ is the load power factor
sin φ = sin (cos-1 φ )
iii) Distributed Load - where the load is tapped off the busbar
trunking along its length this may 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.
iv) 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.
Xf = X F
50
where Xf is the reactance at frequency F in Hz
Note : Information taken from EIEMA (The Electrical Installation
Equipment Manufacturers’ Association)
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