PFTL201C 10KN/20KN/50KN ABB Tension controller head

PFTL201C 10KN/20KN/50KN ABB Tension controller head

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The following diagram shows the limit curve for Tmax T2L160, In160 circuit-breaker.

The x-axis shows the effective values of the symmetrical prospective short-circuit

current, while the y-axis shows the relative peak value. The limiting effect can

be evaluated by comparing, at equal values of symmetrical fault current, the

peak value corresponding to the prospective short-circuit current (curve A) with

the limited peak value (curve B).

Circuit-breaker T2L160 with thermomagnetic release In160 at 400 V, for a fault

current of 40 kA, limits the short-circuit peak to 16.2 kA only, with a reduction

of about 68 kA compared with the peak value in the absence of limitation (84 kA)

Considering that the electro-dynamic stresses and the consequent mechanical

stresses are closely connected to the current peak, the use of current limiting

circuit-breakers allows optimum dimensioning of the components in an electrical

plant. Besides, current limitation may also be used to obtain back-up protection

between two circuit-breakers in series.

In addition to the advantages in terms of design, the use of current-limiting

circuit-breakers allows, for the cases detailed by Standard IEC 60439-1, the

avoidance of short-circuit withstand verifications for switchboards. Clause

8.2.3.1 of the Standard “Circuits of ASSEMBLIES which are exempted from

the verification of the short-circuit withstand strength” states that:

“A verification of the short-circuit withstand strength is not required in the

following cases.

…

For ASSEMBLIES protected by current-limiting devices having a cut-off current

not exceeding 17 kA at the maximum allowable prospective short-circuit current

at the terminals of the incoming circuit of the ASSEMBLY.

...”

The example above is included among those considered by the Standard: if

the circuit-breaker was used as a main breaker in a switchboard to be installed

in a point of the plant where the prospective short-circuit current is 40 kA, it

would not be necessary to carry out the verification of short-circuit withstand.