One of the nice or nasty things about 150Hz, is that is looks the same on all 3 phases, so if the 3f zero crossing is say 10 degrees after the 50Hz one, on one phase it will also be on the other 2 phases. So if the loads on all 3 phases were exactly equal, the 3 F, 9F components etc would all cancel. However this does not mean that you need a neutral to have distortion to the line waveform that gives a 150Hz repeating feature. In fact the 3 phase/ 6 diode bridge does it depressingly well because in reality the 6 diodes are never quite equal. So the answer is that the 3 F current in any one phase is imperfectly cancelled by the nearly corresponding 3F currents in the other two, and when the 150Hz current is   is 5-10 percent of the 50Hz RMS current then that 3F component is  is 30-20 dB down and is about the sort of thing you get with no special precautions taken.  I would expect any 3F currents flowing in the CPC to be a lot lower.

Mike.

but also prevent them from tripping.

I'm not quite following the train of thought here. We usually expect circuit breakers (or fuses) to trip in three main circumstances - during faults (to protect conductors), during overloads (also to protect conductors) and for ADS (to give protection from shock). For faults before the equipment (i.e the ones BS 7671 is concerned with) the fault current will flow just as normal (and additional harmonic components won't detract from that current flow) so provided normal loop impedances etc are OK, then then circuit breaker should open at least as quickly as normal. For overload the thermal element of the MCB will see the total heating effect of the current which is a decent representation of the heating effect within the cables it's protecting - whether it's a steady sine wave or something contorted doesn't really matter - they effectively get integrated just the same. If the MCB trips for overload it should mean that the cable its protecting has already reached an undesirable conductor temperature -  so that's correct operation regardless of the waveform. ADS should be like other faults (other than it involves the c.p.c. rather than another live conductor) - so again the normal 50Hz fault current should flow (regardless of any additional harmonics) so the MCB should trip within normal times.

For faults inside appliance (e.g after diodes in Mike's diagrams) then fault currents may well be affected - more due to to additional impedance of those components rather than harmonic currents per se (even a simple resistor would be a problem for ADS if using overcurrent protective devices for disconnection). That's really outside of BS 7671 territory (a problem for the particular appliance standard) but traditionally the "escape of the magic smoke" was considered to provide the answer (i.e. given a short circuit the diode would rapidly go 'pop - resulting either in an open circuit (i.e. providing disconnection) or a short (in which case normal earth fault current can then flow and then ADS performs as normal (if a little delayed). These days we'd probably incorporate a small residual current RCD to make sure.

  - Andy.

So therefore, harmonics can cause nuisance tripping or premature aging of the circuit breakers only?

So, to answer the question about the 30mA RCD device upstream going off when it shouldn’t. Yes, you could have this problem if you don’t use harmonic filters. I realise this is not the only problem that harmonics can cause. They can also damage other parts of the power system and make it less efficient. But the circuit breaker not working is not one of them.

inclined to agree - I cannot see a situation where a peaky waveform causes something to not trip when it should, but a higher peak to average may lead to both breaker and circuit being more stressed than the nameplate rating based on averages or RMS values suggests. And there lies one of the reasons that large loads without power factor correction either by L-C filters or switch mode active methods are not permitted ;-)

More on those active methods is described here.

https://www.st.com/resource/en/application_note/an523-understanding-power-factor-stmicroelectronics.pdf

Mike

A 300 mA type B 4 pole 100 Amp RCD that was protecting an industrial batch washer tripped unexpectedly after 2 days of operation today. The RCD was faulty and could not be reset even with the load disconnected. I replaced it with a new one today and it resumed normal operation. There was no clear indication of what caused the tripping. It could have been harmonics? Probably more likely a short circuit or a voltage spike.

Given that the RCD was faulty, did it need anything to cause the trip?

Would not reset after the trip had occurred 

without opening it it would be hard to say - but an RCD that size probably has electronic amplification between the sense coil and the trigger mechanism, which is probably firing a fairly bosky solenoid actuator  with a triac or power transistor. I'd bet, but not very much that most likely a mechanical issue, followed as next likely by the trigger electronics having failed dead short, so the actuator fires as soon as power is applied. The latter sort of fault is more likely to be spike induced.
Mike.

or premature aging of the circuit breakers only?

Or perhaps it might be fairer to say that the circuit design should take account of any and all effects so that all components are employed within their capabilities? (like we do with conventional power factor - circuits are sized for the apparent current not just the real current).

  - Andy.

I agree with you. You have stated a very important and sensible engineering principle. It is essential that the circuit design considers all the possible effects and ensures that all the components are used within their limits, a good way to achieve a reliable and efficient system.

I agree with you. You have stated a very important and sensible engineering principle. It is essential that the circuit design considers all the possible effects and ensures that all the components are used within their limits, a good way to achieve a reliable and efficient system.

However, I also realise that the circuit design is influenced by other factors, such as cost, quality, and safety. These factors are based on the client’s needs and expectations, who may have different objectives and options. Therefore, I consider it important to balance these factors and communicate clearly with the client to ensure that the circuit design satisfies their needs and expectations.