Fuse in neutral blows when machine turned off

[vsynth]

Hi all, a strange fault for you I hope, something to get those little grey cells working

A PLC power supply that has a 2A fuse in the live and a 2A fuse in the neutral...an Italian design.

Sometimes when the machine is turned off the 2A fuse in the neutral will blow.

Anyone come across this before?

 

[plugwash]

IF

the switch is double pole and
the switch is turning off stuff other than the PLC PSU and
the switch happens to sometimes break neutral before live and
there is a neutral to earth fault in the PLC PSU

then it would explain the neutral fuse to the PLC PSU blowing.

That is the only explanation I could come up with for why it's always the neutral fuse...

 

[KingboyD]

When the Italians design electrics this happens!

Draw your own conclusions

 

[bernardgreen]

Two possible causes

[1] current flowing in or out of the filter capacitor between neutral and earth / ground as the PSU shuts down. Neutral and earth being connected together in the mains supply network means there is a circuit for the capacitors to discharge

If there is pre-regulator in the PSU ( 300 + volts DC ) then this may dump via the neutral when the mains live is removed and the regulator is shutting down the outputs in a controlled way.

[2] The neutral in the PSU is connected to or has a low impedance path to the 0 volt of the field wiring thus creating a low impedance path between supply neutral and earth / ground. This could be intentional or a wring fault which does not noticably affect operation of the equipment. That connection would then be a route for current from a potential on the neutral of an un-balanced 3 phase supply to ground.