With AC mains heated beds... safely...

... Is it best practice to limit the duty cycle and fit fuses on both the live and neutral connections?

I'm thinking, if the controller goes fully open due to failure, or there either cable gets shorted to something, either fuse should blow and stop any further damage / heating.

I was also thinking, hmm,. recycled cooker hob... ideal for Deltas. Probably OK with square build plate too.

I have a mains-driven heat-bed on my Core-XY printer: The metalwork of the whole machine is connected to protective earth, including the moving metal plate of the heat-bed itself. This is the first line of defence should either of the power lines come loose and touch the frame. I also have a thermal fuse between the heat-bed plate and its heater pad, chosen to fail at around 125C (my chosen max print bed temp is 110C) - this will disconnect the power if it goes over-temp (the software will also raise an alarm at this point, with a message along the lines of "I'm feeding power in, but the temperature is dropping").

The neutral side of the line is normally not fused because it connects to the ground wire at the breaker box.

---

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Quote
the_digital_dentist
The neutral side of the line is normally not fused because it connects to the ground wire at the breaker box.

That's the US practice, I believe. In the UK, the neutral is earthed at the nearest sub-station (usually somewhere in the local district), which means that it may carry some voltage, but nowhere near full live potential. Earth is always connected within the house, usually via incoming water pipes (if metal), the sleeving of the incoming mains cable (again, if metal) or, in difficult environments, a long earth spike driven into the ground. All electrical installers will check the quality of the earth connection whenever they do work within the house (ours did!).

Most UK houses these days are fitted with residual current devices that will trip if there's any imbalance in live and neutral, or any unexpected current to earth in either leg - I'm sure that's common in the US as well. Coming back to the original question - with my house wiring, if the printer's own fuse (in the live side of the circuit) doesn't catch a fault, then the RCD will shut everything down.

But I'm probably telling you stuff you already know!

In the US, what you call an RCD we call a GFCI (ground fault circuit interrupter), this function is also included in AFCI (arc fault circuit interrupter) devices which additionally trigger if they recognize symptoms of arcing as additional fire protection. GFCIs are required only for certain areas (e.g. wet areas like kitchens, bathrooms, and outdoors). Most US GFCIs I have seen are integrated into outlets and protect that outlet and downstream outlets (though it is also possible to get GFCI breakers, they are less commonly deployed). AFCIs are now required at least for bedrooms for new construction and are integrated into the general circuit breakers. Within a house, neutral and earth ground should be bonded in exactly one place, the primary distribution panel.

The voltage on neutral is still there in a house and will be proportional to current in that circuit because the wires are long, low-value resistors. But it should be lower voltage than a neutral that is earthed at the substation, due to the shorter run.

To the point here: if you have a US GFCI on the circuit, like the RCDs in the UK, it should trip if ground touches neutral, at least under any load. In practice, my printers are behind a GFCI, and when I had a stray wire strand bridge ground to neutral, the GFCI tripped instantly.

Going back to the OP: It is very unusual to fuse the neutral side; I've never seen it.

Always put the thermal fuse on the high ("hot") leg of the heater, and of course the printer must have a current-limiting fuse on the high side as well. That's two fuses in series.

IIRC, the neutral should never be fused - if the neutral fuse should blow before the live one, the entire circuit would be at live potential.