# How Could I Implement a Fusable Safety Switch

I am looking to design a low-level safety override. This override can be represented as a three terminal device that is electrically a switch.

simulate this circuit – Schematic created using CircuitLab

In normal operation, INPUT and OUTPUT are shorted together and OVERRIDE pin is floating. It is important that there is complete isolation (Up to 1KV) between OVERRIDE and OUTPUT as the voltages present on INPUT is 400VDC. A blocking diode may not be sufficient.

During a rescue/safety event, the user applies an override pulse to the override port when the device is turned off. This should permanently connect OVERRIDE to OUTPUT. Allowing an external device to take over the OUTPUT device, and not allowing any further normal function. It is important that there is total non-resetable isolation of INPUT and OVERRIDE/OUTPUT, as the circuit may interfere and damage the rescue box. (However, a resetable solution can be considered if it is a significantly simpler design)

Is there a circuit that I can use as an example for this type of functionality, or is there a discrete device that serves this fusable purpose. I would appreciate any input on how to approach this design.

• If the override pulse comes from a terminal that normally requires 1kV of isolation, then the pulse itself must be at least 1kV. So how do you get a 1kV pulse on purpose, and safely? And what does the output do when it receives the tail of that pulse? – AaronD Sep 1 '18 at 3:11
• Could you provide some context? E.g. why wouldn't a bi-stable relay do it? – Dmitry Grigoryev Sep 6 '18 at 9:07