SMART & SAFE PROTECTIVE SHUTDOWN OF LV SWITCHGEAR
From the point of view of the operational safety and reliability of an entire low-voltage installation, it is usually desirable to specifically isolate the part of a system affected by a short-circuit in order to prevent spreading of the fault.
Selectivity is intended to ensure that the protective shutdown is as close as possible to the location of the fault so that unaffected installation components can continue to operate normally.
IEC 61439 standard – The new standard for low-voltage switchgear and controlgear ASSEMBLIES – Applies to enclosures for which the rated voltage is under 1000 V AC or 1500 V DC.
This is often also desired for safety reasons and in IEC 60439-1 (low-voltage switchgear assemblies) addressed for installations that require a high level of continuity in current supply.
In buildings and industrial plants, radial distribution networks are the norm. In radial distribution systems there are several protective devices in series, usually with decreasing rated currents from the supply end to the load end.
While the operational currents decrease from the supply end to the load end, in the event of a short-circuit the same fault current will flow through all the protective devices connected in series.
By a cascading of the trip characteristics it must be ensured that only the respective protective device that is closest to the location of the fault is activated and hence the fault is selectively limited to the smallest possible part of the installation. We saw in one of the previous technical article Simplify Downstream Installation with Cascading – that cascading actually makes protection system cheaper by simplifying the downstream installation (e.g. circuit breakers).