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Fuse-link and circuit breaker selectivity in low voltage power distribution systems

In the radial distribution systems normally found in commercial and public buildings and industrial plants, selectivity between protective devices is highly desirable. This is where only the upstream device closest to the fault operates and the remainder of the system continues to function as before. Circuit currents will decrease from the supply end of the installation to the load end and so the rated current of the protective devices will decrease correspondingly throughout the system. In the case of a short circuit, however, the same fault current will be experienced by all the devices which are connected in series back to the incoming supply. Cascading of the fuse-link blowing and circuit breaker trip characteristics from the smallest overload to the maximum available short circuit current for the system will ensure selectivity rather than of all the devices operating at once causing significant loss of power.

Selectivity between devices connected in series requires that the operating or tripping time of the lower rated downstream device is always less than the operating or tripping time of the upstream device. For short circuits where current limitation occurs and the energy let-through as the device clears the fault is significant. For an upstream fuse-link, the total I2t (energy let-through) of the downstream fuse-link or circuit breaker must be less than the pre-arcing (melting) I2t of the upstream fuse-link. Likewise, for an upstream circuit breaker, the maximum cut off current of the downstream device must be less than the current causing release of the instantaneous magnetic trip.

Selectivity between IEC 60269-2/BS 88-2 industrial fuse-links is relatively straightforward as they will discriminate at a ratio of 1.6:1 based on current rating. For example a BS 88 125A fuse-link will always operate before a 200A fuse-link from the minimum fusing current (1.6In = 200A) to the maximum breaking capacity of 80kA.

Fuse-links have the advantage of inherently high breaking capacity, low cut off current, low total energy let-through, and due to the lack of mechanically operated parts, very short pre-arcing times at high current. For these reasons, fuse-links are commonly used as back-up protection for circuit breakers.

European wiring regulations state “The rated breaking capacity of a protective device shall be not less than the maximum prospective short circuit or earth fault at the point at which the device is installed unless back up protection is provided. A lower breaking capacity is permitted if a back-up protective device having the necessary breaking capacity is installed on the supply side and the characteristics of the devices are suitable co-ordinated such that the energy let-through of the upstream device does not exceed that which can be withstood without damage by the downstream device.”

Selectivity between the upstream fuse-link and downstream circuit breaker is relatively easy to ensure.

Selectivity between an upstream fuse-link and downstream circuit breaker  

Figure 1 Selectivity between an upstream fuse-link and downstream circuit breaker

Looking at figure 1, the fuse-link time/current characteristic (2) lies to the right of the circuit breaker overload release curve (1) and only intersects the short circuit release curve (3) at very high fault currents where the circuit breaker contacts are unable to move quickly enough to open and the fuse-link operates. The total breaking time of the circuit breaker including overload curve settings, short circuit trip settings and tolerances must always be less than the fuse-link minimum time/current characteristic which is 10% less in terms of current than the nominal curve at any particular time.

Selectivity between an upstream circuit breaker and downstream fuse-link is similarly easy to achieve as shown in figure 2.

Selectivity between an upstream circuit breaker and a downstream fuse-link

Figure 2 Selectivity between an upstream circuit breaker and a downstream fuse-link

Provided that the fuse-links time/current characteristic (2) including the 10% tolerance on operating time lies to the left of the circuit breakers overload and short circuit release curve (1), then selectivity will be assured if the cut off current of the fuse-link is also smaller than the trip value of the circuit breaker short circuit release.

For these reasons, fuse-links still have an important role to play in protection of electrical circuits

 

 

ARCM

Revision 1           23/01/2013

Revision 2           06/02/2013

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