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.

thanks for your response Mike.

forgive my ignorance, but does this depend on how things are connected, and specifically whether that PE is just a case earth? I imagine that this issue arises all the time with industrial motors and drives, which are generally 3-wire with a PE connection somewhere

Hi Mike, Im glad you know a lot about the topic of 150Hz harmonics and how they affect the 3-phase system. You’ve explained how the 3F, 9F, and other odd multiples of 3F harmonics are made by the 3-phase/6-diode bridge and how they’re partly cancelled by the other phases. You’ve also said that the 3F currents in the CPC are likely to be much lower than the line currents. I have a few things to ask and say if you don’t mind:
• You said that the 150Hz current is 5-10 percent of the 50Hz RMS current, but how did you get that number? Is that a normal or a bad case? How does that change with different kinds and sizes of loads?
• You said that the 3F component is 30-20 dB down, but what does that mean for the power and voltage distortion? How does that change the power quality and efficiency of the system? How can you measure and reduce this distortion?
• You said that you’d expect any 3F currents in the CPC to be a lot lower, but how low is low enough? Is there a limit or a rule for how much harmonic currents you can have in the CPC? What are the risks or problems of having too much harmonic currents in the CPC? Cheers

What are the risks or problems of having too much harmonic currents in the CPC?

One of the main risks with any kind of protective conductor current in normal service is the shock risk that can arise if  the c.p.c. develops a continuity fault (i.e. goes open circuit) - anyone touching an exposed-conductive-part connected to the c.p.c upstream of the break and anything else that's still solidly earthed risks that current running through their body. 50mA can easily be fatal - 10mA really isn't desirable and even as little as 0.5mA can be felt - which could be dangerous if the natural reaction to pull back could result in a fall from a height or create some other danger. Normally we try to arrange things so at least two separate faults are needed for normal shock protection to fail, so a single fault in a c.p.c. posing a danger falls below normal standards. BS 761 demands extra precautions be taken if the protective conductor current exceeds 10mA - e.g. duplicate c.p.c.s so that no single fault can cause the danger or larger more physically robust c.p.c.s so the likelihood of an open circuit is much lower.

   - Andy.

Thanks Andy. From one Andy to another, I have a good knowledge of the theory and practice of high protective conductor currents and how to comply with the relevant standards and regulations. I'm no expert but do harmonics have A effect of on the impedance of the CPC ? I have read that harmonics can increase the impedance of the CPC, which can affect the fault current and the disconnection time. This can reduce the effectiveness of the protective devices and compromise the safety of the installation. Therefore, it is important to check the impedance of the CPC at different frequencies and ensure that it meets the requirements of BS 7671

Thanks Andy. From one Andy to another, I have a good knowledge of the theory and practice of high protective conductor currents and how to comply with the relevant standards and regulations. I'm no expert but do harmonics have A effect of on the impedance of the CPC ? I have read that harmonics can increase the impedance of the CPC, which can affect the fault current and the disconnection time. This can reduce the effectiveness of the protective devices and compromise the safety of the installation. Therefore, it is important to check the impedance of the CPC at different frequencies and ensure that it meets the requirements of BS 7671