Siemens Barduct
Busbar Specification
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
Conditions The 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 manufacturer’s 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 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. 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 (
Δv) is calculated as follows: Δv = √ 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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