# 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? Sep 1, 2018 at 3:11
• Could you provide some context? E.g. why wouldn't a bi-stable relay do it? Sep 6, 2018 at 9:07

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.

• I will update my question to make it clear this in intended to be an inline solution, a three terminal device with a form factor of a wire clamp is the ideal , used as a last resort rescue feature of a failed embedded device to put the external environment into a safe state before decommissioning Sep 1, 2018 at 2:38
• I have considered it, what I am looking for is a "three way" fuse, if such a thing exists. Burning it opens one connection and closes the other Sep 1, 2018 at 2:41
• That 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. Is that closer to what you want?
– user105652
Sep 1, 2018 at 2:47
• You will need multiple parts to implement what you want. As far as I know there is no 3-way fuse. It would be a constructed device.
– user105652
Sep 1, 2018 at 3:04

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.