My war against dual rcd boards

Three days ago I said:

”The question that Graham asked was quite specific:

”However, if the lamp is damaged, and the user is being protected against accidental contact with live parts, say after the rectifier, would the type AC RCD operate is perhaps another?”

in that he said “after the rectifier”, so assume there are some diodes charging a capacitor, which discharges to supply the DC current, and the input current is 25 mA at 230 volts, what is the DC output current and voltage?”.

So presumably after the rectifier the current and voltage is peak, not RMS? So if the current and voltage is 25 mA at 230 volts AC measured RMS, then after the rectifier it should be 35,35 mA at 325 volts pulsed DC unless there are a few more electronic components to drop the voltage?

I think not. The "average" rectified current must be lower than the upstream a.c. because there is none for 10 ms in every 20 ms cycle. The RMS current of the pulsed DC is only 17.7 mA.

Some maths to follow. :-)

Some misunderstanding there. If it's a single loop its the same current all the way around the loop. And superposition.. 

Lots of careful misuse of apparently similar terms! Worse than trying to find a true earth..

Philip Oakley said:

If it's a single loop its the same current all the way around the loop.

Quite so. If the (earth) fault is after the rectifier, the "pulsed d.c." flows through the RCD, which is where the problem lies. If the type AC RCD does not trip, lives could be at stake.

Philip Oakley said:

And superposition

OK, so you have a 30 mA RMS half-wave rectified (pulsed d.c.) current and superpose 6 mA of smooth d.c. What is the combined RMS current?

Chris Pearson said:

If the type AC RCD does not trip, lives could be at stake

Which is the point.. (why we don't like RCDs that become 'blinded' by average DC flows)

The simplistic assumptions (e.g. no inductive/capacitive energy storage/filtering) produce worst case figures. 

The very sharp corners between half cycles aren't that realistic and, if Fourier analysed, imply a high frequency harmonics content, which tends to be filtered by the general wiring etc (without additional components).

BigClive did some video on the workings of diode bulbs - worth looking at. I can't remember if the ones he looked at had a half or full wave rectification.

There are problems of nomenclature in the attempted mixing of the two signals, in that often the first includes the second (commonly because of an arbitrary zero marking).

A pulsed DC is a smooth DC and a pure AC (i.e. 'centred' about that smooth DC level)

Philip Oakley said:

A pulsed DC is a smooth DC and a pure AC (i.e. 'centred' about that smooth DC level)

Oh no it isn't (do we still have pantomimes?).

BS EN IEC 61557-6:2021 4.2.1 Note: A smooth direct current is a current with less than 10% AC ripple (peak to peak).

So-called, pulsed d.c. could have no smooth component.

Philip Oakley said:

problems of nomenclature

Exactly. Given that definition, the half wave rectified shape can never contain any DC component (which by most definitions is the 'smooth' component).

Probably drop into the 'superposition' of components discussion here. "Clearly defined areas of Doubt & Uncertainty"

I agree that there is a problem with nomenclature.

BS EN 61008-1:2012+A12:2017 hardly helps. Under, "definitions" at 3.1.3: pulsating direct current current of pulsating wave form which assumes, in each period of the rated power frequency, the value 0 or [my emphasis] a value not exceeding 0,006 A d.c. during one single interval of time, expressed in angular measure, of at least 150º

Please forgive me if this seems pedantic, but a definition cannot contain the word, "or".

BS EN 61008 specifies 3 groups of tests for type A RCDs: sinusoidal a.c.; pulsating d.c. (assuming a value of 0 for at least 150º of the cycle); pulsating d.c. superimposed by a smooth direct current of 0.006 A.

So I think that we need to be careful what we mean when we say, "pulsed d.c."

Ross C said:

I'm not sure of the exact derivation, but the IET Wiring Matters article concurs:

'When the Type A setting is selected on the instrument, a half wave pulsating residual test current superimposed on a smooth direct  current of 6 mA is produced, which effectively applies a 1.4 multiplier to the rated residual current (I_Δn). For example, if the 30 mA setting is selected, the RCD will be subjected to a test current of 42 mA (30 x 1.4 = 42 mA)'

That may well be an error.

The manual for Megger 1700 series instruments states:

'Type A' RCDs are sensitive to pulsed DC as well as AC fault currents, and are tested with a pulsed waveform. The RMS current is √2 x the rated operating current of the RCD.

There is no mention of a smooth component.

but a definition cannot contain the word, "or".

I think it's just a wordsmiths way of saying between 0.0 and 6.0mA

   - Andy..

Then say so!

I think that you are correct (as ever) and the clue may be the "EN" in the document reference, but if you delete, "the value 0 or a", the meaning of the definition does not change.

Of course, if wordsmiths got it right first time, every time, there would be no amendments and no need of appellate courts (of which I have experience).

but 

but for a half-wave pulsating current, it is half the peak value.

Is also not true I'm afraid, - its only about 33%, as half sine waves are bottom heavy, not a triangle...  and yes we do have pantomimes.

But actually the whole thing could have been solved by actually showing, and annotating the waveform intended, by providing the existing oscillogram with labelled axes. Sadly that was not done.
Luckily perhaps we don't need to worry about it, as we only have to test RCDs at 250mA RMS with a sinusoidal  AC. But it does rather beg the question of what happens with other waveforms, and if makers are in fact all testing in the same manner.

Mike.

but 

but for a half-wave pulsating current, it is half the peak value.

Is also not true I'm afraid, - its only about 33%, as half sine waves are bottom heavy, not a triangle...  and yes we do have pantomimes.

But actually the whole thing could have been solved by actually showing, and annotating the waveform intended, by providing the existing oscillogram with labelled axes. Sadly that was not done.
Luckily perhaps we don't need to worry about it, as we only have to test RCDs at 250mA RMS with a sinusoidal  AC. But it does rather beg the question of what happens with other waveforms, and if makers are in fact all testing in the same manner.

Mike.

I don't have a problem with the word "or". 

For 150 360ths of the cycle period it is at 6mA, for the rest, it is zero,

The problem is, that is not describing a DC, by any stretch of the imagination. It may be unipolar - i.e. non-reversing, but the one thing is is not is steady over time.....

They meant, and should simply have said, a square wave with an asymmetric  duty cycle.

Mike.

mapj1 said:

but for a half-wave pulsating current, it is half the peak value.

Is also not true I'm afraid, - its only about 33%, as half sine waves are bottom heavy, not a triangle...  and yes we do have pantomimes.

Oh yes it is.

Mike, you appear to be thinking of the average current, which is indeed about a third.

An alternating current reverses direction periodically. a direct current is one which does not. So long as the value is always zero or positive, it is d.c. It does not have to be smooth.

it is d.c. It does not have to be smooth.

hmm..

Would you consider a unipolar square wave, or a triangle wave where the lower crest is at zero, as examples of DC?

I would not, because in addition to a DC term (0Hz, non changing, there for 'ever' or at least for the duration of the measurement) , there is a clear fundamental frequency, and some harmonics...

I agree there is a de-minimis case where the 0Hz term dominates - a DC with a bit of ripple on it is a common situation and makes sense described as such.

I might even consider the output of a 3 phase bridge to be in that class, but I'm not sure I'd consider something that fell to zero for a non-trivial part of the time to be quite the same.

I'm not sure how well that fits with the more 'power electrical' definitions, it is certainly open to more than one interpretation.

Mike

Perhaps we should re-introduce the term "continuous current"....

   - Andy.

mapj1 said:

Would you consider a unipolar square wave, or a triangle wave where the lower crest is at zero, as examples of DC?

I think that I can see Mike's point. Whilst square-wave d.c. might be useful, e.g., for a flashing lamp, I suspect that the usual aim is to obtain smooth d.c.

Save that, for example, your 42 mA half-wave rectified d,c, superimposed upon 6 mA of smooth d.c. would be continuous.

Here is a Boxing Day question for all of you. It may be unlikely to occur, but if you had one device on a circuit with a fault to ground of 15 mA r.m.s. a.c. and another one with a fault of 15 mA r.m.s. half-wave rectified pulsating d.c., would a 30 mA type A RCD trip?

We need to take care with the peak, average (mean) and rms values of such signals.

The 'pi' value keeps cropping up in a lot of situations (e.g. see also noise bandwidth vs signal bandwidth factors).

As an aside, what is truly missing, is the explanation as to how/when these half sine [pulsed DC] style currents actually 'blind' the RCD. The pragmatic engineering/ material/physics are just (if not more) interesting than the mad/bad maths definitions .

... re-introduce the term "continuous current"....

maybe we should, but I think we'd have to agree what we meant by it first, which is kind of where we came in.  If we assign a single figure to something that is changing, are we refferring to  its mean, its RMS or its peak ?

I'm not sure..

Mike

fault to ground of 15 mA r.m.s. a.c. and another one with a fault of 15 mA r.m.s. half-wave rectified pulsating d.c., would a 30 mA type A RCD trip?

haha - maybe only  half the time ;-) 

you have defined exactly what you want, The sum of these upper 2 current traces, making the lower one

We do not, however, as we have just discussed,  know for certain what the RCD spec actually means.

Mike

PS for those interested, this is a simulation in LTspice, a tool I find helpful for this sort of stuff.

This is the circuit being simulated.