# ESD protection for power input with several fused branches

I'm designing an industrial board that will be powered with a +24V DC. On the PCB there are several fuses to protect each branch against over-current/short-circuit. Example:

simulate this circuit – Schematic created using CircuitLab

Of course the resistors simulates the actual loads: MCU, relays, LEDs, ... The 24V is applied through screw terminals.

I need to protect the circuits ("loads" in this examples) from ESD. As far as I know the ESD protection should be placed very close to the input connector.

So the obviously solution is to add an unidirectional TVS at the input:

simulate this circuit

Usually I put a fuse before the TVS so it can also be effective on persistent over-voltages or inversion of polarity. But here I don't think it's worth to add another fuse!

Is a single TVS enough for ESD protection? Is there a better (= safer, more reliable) circuit to use in this scenario?

By the way, I don't have a real "ground", so I need to close the discharge to the common voltage (0V).

• If you don't have a ground (i.e. an earth) then that doesn't mean the ESD discharge magically uses your 0 volts. That wouldn't be correct unless your 0 volts is earthed. In all unearthed scenarios the ESD discharge flows through your exposed terminals via parasitic capacitance to real ground (earth). Jan 21, 2021 at 15:14
• re: no earth -- make an "FE" terminal on your board, and if it goes into an insulated box with only an unshielded cable connecting the box to the outside world, that terminal simply won't get hooked up. Jan 21, 2021 at 20:35
• Generally, fuses is mostly a thing for high current electronics. If you add a 1A fuse to protect a PCB drawing some 100mA, it will achieve absolutely nothing. 1A short circuit current will fry the whole board long before the fuse blows. That's enough current to burn off traces, bond wires, everything. Feb 2, 2021 at 7:21
• @Lundin, where did I write that a put a 1A fuse to protect something that draws 100 mA?
– Mark
Feb 2, 2021 at 7:46
• You didn't. But in case your device draws 950mA the fuse would be equally pointless, for other reasons. Feb 2, 2021 at 7:58