How Could I Implement a Fusable Safety Switch

Question

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.

Answer 1

Unless this switch can be reset under certain conditions then you are effectively creating a fuse that cannot be replaced. A remote button blows an inline fuse that possibly cannot be replaced. Use the button to create an overload that pops the fuse. There is no resetting this. There is no switch.

It could work by placing a relay of the proper voltage across the fuse to be blown. If it does blow then the relay gets power to switch to another circuit. You are still talking about a constructed circuit and not this '3-way fuse' idea.

Answer 2

Your diagram shows a single-pole double-throw switch. Maybe you can simply use a single-pole, double-throw switch with the required electrical rating. The non-reset-ability can be implemented by a mechanical latch on the switch itself. Once switched, the switch cannot be changed back to its original position until it is mechanically dis-assembled, after the situation has been rendered fully safe and de-energized.