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?

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

... 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