Circuit breaker and fuses

[Neale]

Simple question, but the answer's a bit more complicated...

First of all, fuses/MCBs are not really there to "protect" devices downstream. They might, under certain circumstances, but in general overcurrent devices like fuses are to protect against the risk of fire in the wiring when there is already a fault downstream of them. For example, motor wiring is shorted where it runs round a sharp corner; the driver (if it does not have effective overcurrent control) pops its output transistors, too much current drawn from the supply, wiring overheats - then fuse blows before it gets out of hand.

On the plus side, the PSU to which you have linked has built-in overcurrent/short circuit protection - it says. That might be enough to protect against any downstream faults of any significance.

On the down side, a switch-mode PSU is not ideal for stepper drivers Iwhatever the ebay ad says). Stepper drivers take a modest average current but it tends to be very "peaky" in that the steppers are driven with a series of high-current pulses which average to a modest value. SMPS do not like high-current pulse loads and tend to go into "protection" mode if they see them. You have to stay well within the ratings to avoid this, remembering that your drivers will all be demanding pulse currents at different but sometimes overlapping moments. It's not easy to do the sums, which is why most CNC builders (and I note that this is not a cutting machine that you are building!) use "linear" power supplies - wihich are bigger, heavier, a bit more costly, but easy to build and relatively bullet-proof.

How big an MCB? Your power supply is rated at 350W output. Let's be conservative and say it needs 500W input (it will be less in practice). That's only 2A from a 240V supply (will, near enough). Maybe a bit more to cope with switch-on surge (which is a problem with linear supplies but not much with a SMPS). So, 5A MCB? And make sure that the wiring from MCB to power supply, etc is rated at at least that - not difficult - and you will be fine.

Others may, and almost certainly will, disagree...