PFTL201DE 10KN/20KN/30KN ABB Tension sensor measuring system

PFTL201DE 10KN/20KN/30KN ABB Tension sensor measuring system

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Definition of discrimination

The over-current discrimination is defined in the Standards as “coordination

of the operating characteristics of two or more over-current protective devices

such that, on the incidence of over-currents within stated limits, the device

intended to operate within these limits does so, while the others do not operate” (IEC 60947-1, def. 2.5.23);

It is possible to distinguish between:

• total discrimination, which means “over-current discrimination such that, in

the case of two over-current protective devices in series, the protective device

on the load side provides protection without tripping the other protective device”

(IEC 60947-2, def. 2.17.2);

• partial discrimination, which means “over-current discrimination such that,

in the case of two over-current protective devices in series, the protective

device on the load side provides protection up to a given over-current limit

without tripping the other”(IEC 60947-2, def. 2.17.3); this over-current threshold

is called “discrimination limit current Is” (IEC 60947-2, def. 2.17.4).

Current discrimination

This type of discrimination is based on the observation that the closer the fault

comes to the network’s feeder, the greater the short-circuit current will be. We

can therefore pinpoint the zone where the fault has occurred simply by calibrating

the instantaneous protection of the device upstream to a limit value higher than

the fault current which causes the tripping of the device downstream.

We can normally achieve total discrimination only in specific cases where the

fault current is not very high (and comparable with the device’s rated current) or

where a component with high impedance is between the two protective devices

(e.g. a transformer, a very long or small cable...) giving rise to a large difference

between the short-circuit current values.

This type of coordination is consequently feasible mainly in final distribution

networks (with low rated current and short-circuit current values and a high

impedance of the connection cables).

The devices’ time-current tripping curves are generally used for the study.

This solution is:

• rapid;

• easy to implement;

• and inexpensive.

On the other hand:

• the discrimination limits are normally low;

• increasing the discrimination levels causes a rapid growing of the device sizes.

The following example shows a typical application of current discrimination based

on the different instantaneous tripping threshold values of the circuit-breakers

considered.