3RF2 / 3RF34: Statement about behavior of Solid State Switching Devices 3RF2 and 3RF34 at thermal overload or after short circuit
do the Solid State Switching Devices 3RF2 or 3RF34 behave at thermal overload
or after short circuit?
After an OFF command it is the question whether the thyristors make a closed circuit (ON status) or an open circuit (OFF status).
ANSWER:This is to describe the behavior of Solid State Switching Devices 3RF2 / 3RF34 in fault operation:
When a Solid State Switching Device 3RF2 or 3RF34 is operated under overload conditions and the thyristors become too hot thereby (> approx. 130 °C), then they can temporarily lose their blocking ability. As a consequence, after an OFF command (no control voltage at A1 and A2) the main current path remains conductive as long as the thyristors are still too hot. This case, however, does not lead to an immediate destruction of the semiconductors but often repetitions of such over loads will impact early ageing of the semiconductor which will result in a defect.
After short circuit
(without employing the semiconductor fuses as per documentation of 3RF2 / 3RF34):
Our experience shows that, in 95 % of all faults, thyristors are destroyed by short circuits, whereas they melt through. Melting through means in this case that the current density in the thyristor´s chip is so high that the material is melting. The effect is that a thyristor acts as a permanent conductor and there is no external control possible anymore.
In seldom situations, e.g. at very high short current circuits, the connection line between the thyristors and the main terminals can interrupt due to high dynamic current forces. Adverse to the melting through, this fault could be measured as an interruption. In such cases the degree of destruction is usually so high that the damages are even noticeable at the surface of the 3RF2 / 3RF34 devices.
One shall consider that -principally- a Solid State Switching Device does not provide galvanical isolation between infeed and load, even when there is no fault. A leakage current of up to approx. 10 mA must be assumed at any time. For that, a direct comparison with conventional air or vacuum contactors is not possible.