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TECHNI CAL DATA
MOTOR PROTECT I ON & CABL E SPREAD I NG
70
Cable Size Dimension ‘A’
(mm
2
) (mm)
16 103
25 127
35 142
50 167
70 197
95 224
120 245
150 272
185 297
240 333
300 375
A
4 CORE TERMINATIONS, SPREADING ROOM
BS5372 SPREADING ROOM
The scope of BS5372 relates to dimensions which should be observed
in the design of cable terminations, having uninsulated conductor fittings,
for external cables on electrical equipment to enable the cables to be
connected satisfactorily.
The standard is based upon the use of compression type conductor
terminations and allows for the cores to be crossed within the terminal
enclosure because this combination requires more space.
The diagram and table below gives the dimensions quoted by this standard.
l
PRIMARY-SIDE TRANSFORMER PROTECTION
I
The circuit breaker as primary-side transformer protection
When switching on low-voltage AC transformers, the extremely high inrush
current peaks place special demands on the trip unit or on the making-
capacity of the circuit breakers if these are also used to switch the
transformer.
For most applications, an inrush current of 20 to 30 times rated operating
current is expected in practice and must be taken into account when
selecting the circuit breakers.
The maximum short-circuit current
I
k
of the Powerstar circuit breakers
7T2 – 7T5 depends on the frame size. A circuit breaker in the lower setting
range must therefore be operated for primary-side transformer protection.
Example:
A 300kVA transformer with a 430A rated current; 20 times inrush
current
Selected: 7T3 with ETU of I
n
= 630A; setting range 0.4 - 1 x I
n
= 252A to
630A
I
R
can be set at 430A; Instantaneous short circuit protection set at 10.8kA =
25 x current setting
Note
Switching the circuit breaker off
It is imperative to note that the minimum short-circuit current I
KMIN
in
accordance with VDE 0100 is switched off in every case using a protection
facility (e.g. circuit breaker).