DC on AC supply

You are making another error Andy.

Nathaniel:

A generally accepted definition of 'dc' appears beyond what humans can agree on!

Any current which is not alternating?


For some reason, I am reminded of the brothers Jacob and Esau who were smooth and hairy respectively (Genesis 27:11). I quite like the idea of "hairy DC". ?

Weirdbeard:

What I have wondered with regards this situation is if an installation has equipment that can apparently disable commonly found RCDs, would this disablement also apply to other installations that share a supply, for example with looped supplies? 

This is a question I got elsewhere a year ago, mainly about worries with solar PV installations and EV chargers affecting other customers on the network by their possible dc, and so a need to have type-B RCDs as default regardless of whether the installation included such devices. The answer: practically, no.    [edit: note that - in view of the thread's topic - this is about  disablement by dc currents, not e.g. by high frequencies]


Put more generally, it's a question of whether dc(residual)-causing apparatus could affect RCDs that have that apparatus [edit, for clarity: load->apparatus] on their supply side instead of their load side. Various complicated situations can be found where it could matter, but they involve multiple faults or bizarre situations to the extent that it's not worth considering compared to plenty of other risks (like simple RCD failure, or a pure-dc-causing fault on the load side).  And generally only a small proportion of the original dc current would go through the 'victim' RCD.


The main points include that: a healthy circuit on the load side of an RCD has plenty of insulation resistance against dc; a sudden fault even with dc voltage present on the ac supply would trip a type-A RCD except in remarkable conditions; even a gradual fault (condensation) would in most cases pass much more ac than dc due to the voltages involved; and the low resistance of supply cables and transformers prevents significant dc voltage forming between supply conductors.

The above is a bit biased toward the country where the question arose, which has TN* supply so most currents pass through metal unless e.g. to a body or non-bonded electrode.  For a situation possible in the UK, with a chance of causing a problem, consider a TT installation where pure dc is passing from live conductors to earth (we'll ignore how it arises ... and note that it would have to be an awful lot more than just a few mA), in a supply system with rather high neutral-earthing resistance so that significant voltage could arise from neutral to earth. Then consider an RCD on another circuit (or in another TT installation) in which a N-PE fault happens at a time of low load (so the N-PE ac voltage doesn't cause a trip even with e.g. 6 mA of dc flowing through that RCD and the fault). And then a pulsating dc fault in the same direction also happens in that circuit, such as someone poking about in an electronic device; the RCD could have been made insensitive by the first two faults (dc current into the earth, and N-PE fault in the RCD's circuit) so it fails to detect the third fault at the right level of current.  This would require some rather extreme values of current and earth resistance, besides the combination of faults. Plenty of other hypothetical situations can be given, involving external earth contacts instead of a TT system, or funny arrangements with inductors, 3-phase-connected resistors that cancel the ac but add the dc, etc, etc.  But in view of the probabilities, reasonable cost-benefit (plenty of guesswork there, of course!) and so on, my conclusion - particularly for the TN* situation and the low level of expected dc injections - was to ignore the risk.  I think there are more convincing reasons than this one to have type-B RCDs (considering the range of modern appliances) but even those reasons have trouble justifying the cost in present regulations. 


 

Chris Pearson:

Nathaniel:

A generally accepted definition of 'dc' appears beyond what humans can agree on!

Any current which is not alternating?


For some reason, I am reminded of the brothers Jacob and Esau who were smooth and hairy respectively (Genesis 27:11). I quite like the idea of "hairy DC". ?

That sounds one very reasonable definition.  Do we even agree on what alternating  means ... the word makes me think it must change direction regularly, not just fluctuate. In that case, half-wave rectified is not alternating, so it's dc. That fits with the 'pulsating dc' terminology, and with the need to specify 'smooth dc' if really meaning a steady value.  I don't have any strong opinion on what's best, and the above might be the most suitable or accepted within power or electrical installation terminology.  However, we've already seen here some different opinions of whether 'pulsating dc' is dc or ac. Spectrum-wise it's both - dc and multiple ac frequencies, added. In other electrical subjects there could be very different ideas: in electrical insulation studies 'dc' is sometimes seen as the situation of steady values, only approximately reached in practice.

I like the hairy idea. It reminds me of descriptions of oscilloscope waveforms that have small hf noise on them.

 

Nathaniel:

Weirdbeard:

What I have wondered with regards this situation is if an installation has equipment that can apparently disable commonly found RCDs, would this disablement also apply to other installations that share a supply, for example with looped supplies? 

This is a question I got elsewhere a year ago, mainly about worries with solar PV installations and EV chargers affecting other customers on the network by their possible dc, and so a need to have type-B RCDs as default regardless of whether the installation included such devices. The answer: practically, no. 

 

Thanks for the reply, are you aware of any common consumer appliances that are likely to defeat a typical 30mA RCD and render the RCD useless as additional protection against electric shock?

 

Weirdbeard:

Nathaniel:

Weirdbeard:

What I have wondered with regards this situation is if an installation has equipment that can apparently disable commonly found RCDs, would this disablement also apply to other installations that share a supply, for example with looped supplies? 

This is a question I got elsewhere a year ago, mainly about worries with solar PV installations and EV chargers affecting other customers on the network by their possible dc, and so a need to have type-B RCDs as default regardless of whether the installation included such devices. The answer: practically, no. 

 

Thanks for the reply, are you aware of any common consumer appliances that are likely to defeat a typical 30mA RCD and render the RCD useless as additional protection against electric shock?

The answer: no. 

I don't usually manage such short answers, as there are so many ifs and buts in this sort of subject. But you helped me by including the word 'aware', and then further helped with 'typical' and 'useless'.  And - please note - I'm assuming that the appliance is in its intended state, not having a fault.

aware:

I've had several reasons to study RCD standards and actual behaviour, particularly with passive RCDs and dc, so I'm quite confident about that situation. But I haven't done so much with high-frequency disturbance or different types of voltage-dependent RCD. Many modern RCBOs are voltage-dependent in order to make them easily fit in 1 module.

For appliances and inverters I've not looked thoroughly at what extremes the product standards would permit for protective-conductor currents at dc and different frequencies. Also, if each device is permitted some small level of dc in the PE, then when dozens are connected and by bad luck the currents mainly sum in the same direction, they could anyway exceed a threshold like 6 mA. In the case I mentioned earlier, the question I received assumed a worst case of 6 mA dc from each inverter or EV, so it was enough to show that there wasn't a practical problem (for other customers) in their type of system even with many times this current.

I've checked PE-currents for only a few dozen varied household electronic loads (IT equipment, induction hobs, heatpumps, washer/dryer), and found only low dc current (<1 mA but without good measurement in this range) in the PE, and at most about 10 mA rms 'noise' at higher frequencies during some parts of a wash cycle. The L-N dc wasn't considered: RCDs can have a little influence from L-N currents when these aren't well balanced in the way the wires are wrapped, but they're tested to not trip at 6*In. So 96 A in a 16 A device shouldn't give an imbalance as much as the residual trip level of e.g. 15 mA to 30 mA for a 30 mA RCD. Taking the same ratio for dc, tens of amps L-N dc would be needed in order to get 6 mA imbalance.

typical:

The standards for RCDs are based on passing a few tests. RCDs tend to be made in quite similar ways governed by familiarity, physics and economics. Performance outside the specified tests can then be quite well guessed within some range. But if we go by the letter of the standard, then a type-A wouldn't (from what I remember of IEC61008) even have to work with superimposed 6 mA dc and full-wave 30 mA ac, as the dc test is only done with half-wave ac. So situations that are typically safe may differ from situations that must be safe within the requirements of a standard.  A month or two ago I linked to the Australian state that requires a further test not included in the IEC standard, to check that voltage-dependent RCBOs wouldn't simply burn out their tripping coil on the first operation in the event that they're fed with the load terminals connected to the line (source).  Weak connection to VW and diesels, regarding standards and reality.

useless:

if the RCD can still work for some possible waveforms of the fault that needs additional protection, or its threshold is only mildly increased, then I'd argue it's not useless, but just not so useful...

 

Nathaniel:

A generally accepted definition of 'dc' appears beyond what humans can agree on!

Any current which is not alternating?


For some reason, I am reminded of the brothers Jacob and Esau who were smooth and hairy respectively (Genesis 27:11). I quite like the idea of "hairy DC". 


DC is a "new" name for what was originally called Continuous Current.................


Regards


BOD