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I would appreciate if someone would share their thoughts on why someone would put a fusible resistor in series before an NTC? Also in the rectifier diode stage there are two extra series diodes in each branch (L1,L2,L3 & N), I tried to simulate the series diode in LTspice and I could not find any noticeable difference...

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For more information on the circuit for context, this a 3 phase AC/DC power supply used in a reference ATSAM4L electricity meter.

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    \$\begingroup\$ 3-phase, 230v, 460v, 600v? They are there almost certainly to prevent severe damage from arc-flash. At these voltages, an arc for any reason is to be avoided. Adding a fusible resistor helps ensure that should anything go awry, the resistor will hopefully stop it from becoming a fireball. \$\endgroup\$
    – rdtsc
    Commented Mar 24, 2020 at 21:41
  • \$\begingroup\$ 3-phase 230V for each phase to neutral. Thanks for sharing your thoughts \$\endgroup\$
    – Sultanpepper
    Commented Mar 24, 2020 at 22:00

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The MOVs from each phase to ground slowly degrade with incoming surges and transients and eventually fail short. The fusible resistor protects the unit from damage caused by excess dissipation in the MOV, as well as protecting against general overcurrent due to faults in the downstream circuitry.

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The fuse isn't redundant with the NTC. NTCs don't have anything to do with fuses. You are thinking of PTCs.

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