DC on AC supply

It would take a long time to try to answer thoroughly, unless restricting the scope. For example, is it the L-N loop (you say 'into the neutral'), or the L-PE (or N-PE) as would be more important for the influence on an RCD? And is only pulsed-dc (e.g. like half-wave rectified ac) what's meant, or steady dc? 


Input stages of PSUs in modern equipment like a computer or charger do have isolation, but practically never directly on the input any more. Older equipment started with a transformer, then a rectifier and voltage regulator.  (And even older equipment started with the rectifier, then the valves, then the isolation, if any, was on the output.) Modern electronic equipment starts with a rectifier, which might in bigger items be a cleverer type than just diodes in order to reduce harmonics and phase-shift in the input current. Then there's more electronics to switch the resulting ~300V dc quickly into a high-frequency transformer, from which the output is rectified. The advantage is that a high frequency transformer can be much smaller and therefore much cheaper than a 50 Hz one.  That's why modern wall-warts are so light compared to old ones that started with a bulky transformer.  As you imply, it also gives the potential for unbalanced positive and negative parts of current, although most devices try to be balanced.


Some loads can deliberately inject several amps of pulsed dc from L to N. Typically these are hairdryers and electric blankets (ok, not multiple amps for the blankets), with a design where a single diode is put in series for the half-heat setting; I've come across cheapo usb-chargers that also have just one diode. This is permitted in EN_61000-3-2 if the power is below 100 W or the device has only two wires and is intended for short use (a few minutes).  Other loads can unintentionally do a less extreme case if their positive and negative half-cycles aren't balanced. I don't see a problem for RCDs unless there's a fault that causes this current to return through PE instead of N, with a type AC RCD.


I have briefly studied claims of RCD+dc problems in a related matter where different organizations disagreed about the basic principles and the risk. One point l I learned was to (largely) ignore videos. Some example cases were given by references to youtube. Generally there were clear lacks, like measuring only in a place that doesn't confirm the current that actually matters, or with equipment that can't be expected to give useful results. I stopped reacting to them, since they mainly wasted many minutes without firm, useful information. However, if it's a simpler question like L-N currents with a non-zero average then it should be hard to measure wrongly. Just don't trust too much about 'proving' the effects on an RCD unless the setup and measurements are very clear. 

I hope something in the above will be useful for you.

It would take a long time to try to answer thoroughly, unless restricting the scope. For example, is it the L-N loop (you say 'into the neutral'), or the L-PE (or N-PE) as would be more important for the influence on an RCD? And is only pulsed-dc (e.g. like half-wave rectified ac) what's meant, or steady dc? 


Input stages of PSUs in modern equipment like a computer or charger do have isolation, but practically never directly on the input any more. Older equipment started with a transformer, then a rectifier and voltage regulator.  (And even older equipment started with the rectifier, then the valves, then the isolation, if any, was on the output.) Modern electronic equipment starts with a rectifier, which might in bigger items be a cleverer type than just diodes in order to reduce harmonics and phase-shift in the input current. Then there's more electronics to switch the resulting ~300V dc quickly into a high-frequency transformer, from which the output is rectified. The advantage is that a high frequency transformer can be much smaller and therefore much cheaper than a 50 Hz one.  That's why modern wall-warts are so light compared to old ones that started with a bulky transformer.  As you imply, it also gives the potential for unbalanced positive and negative parts of current, although most devices try to be balanced.


Some loads can deliberately inject several amps of pulsed dc from L to N. Typically these are hairdryers and electric blankets (ok, not multiple amps for the blankets), with a design where a single diode is put in series for the half-heat setting; I've come across cheapo usb-chargers that also have just one diode. This is permitted in EN_61000-3-2 if the power is below 100 W or the device has only two wires and is intended for short use (a few minutes).  Other loads can unintentionally do a less extreme case if their positive and negative half-cycles aren't balanced. I don't see a problem for RCDs unless there's a fault that causes this current to return through PE instead of N, with a type AC RCD.


I have briefly studied claims of RCD+dc problems in a related matter where different organizations disagreed about the basic principles and the risk. One point l I learned was to (largely) ignore videos. Some example cases were given by references to youtube. Generally there were clear lacks, like measuring only in a place that doesn't confirm the current that actually matters, or with equipment that can't be expected to give useful results. I stopped reacting to them, since they mainly wasted many minutes without firm, useful information. However, if it's a simpler question like L-N currents with a non-zero average then it should be hard to measure wrongly. Just don't trust too much about 'proving' the effects on an RCD unless the setup and measurements are very clear. 

I hope something in the above will be useful for you.