R.C.D. Immobilising Electrical Appliances.

Q. What exactly is 6mA when pulsating D.C. is involved? What does it refer to? How is it measured?

Z.

Q. What exactly is 6mA when pulsating D.C. is involved? What does it refer to? How is it measured?

Actually this a a good question, traditional RCDs are based on a transformer, where it is the rate of change of current that sets the voltage seen on the secondary (and primary, but we normally ignore the one turn voltage drop)

The old sweats used to refer to continuous current or interrupted direct current, and that is very descriptive. Modern parlance is to use “DC” for both ( arguably a backward step, but then other things are in the right direction - try and use jars as the unit for for capacitance or any pre-metric magnetic units for a day…)

For the RCD what matters is the peak-to-peak amplitude (p-p) of the pulsed DC (or the p-p of hte ripple on the back of a constant value) and its duty-cycle or mark-to-space ratio, but what most  meters will show is the mean.

In terms of blinding currents the smooth DC part is the offset from a zero-centred waveform - a shift of the baseline if you will by adding a constant to it.

For testing A RCDs an unsmoothed but rectified waveform is assumed, as if a single diode is in the fault path - this is credible, as many common items based on  SCR/triacs such as lamp dimmers can create this sort of waveform. A fault from the load side of a lamp dimmer to earth is credible, though such things normally blow dead short or dead open after a few cycles of more than the rated current.

A smoothed DC fault requires deliberate effort to arrange, and neither a supply or load side fault on a VSD will do it.

Mike.

Thanks Mike.

Z.

A surprising number of appliances draw DC current from AC mains. Examples include.

Hair dryers, mains on the element for full heat, mains via a silicon power diode for half heat.

Rice cookers, as above.

Slow cooking pots, mains via a diode for lowest heat setting.

Multi voltage travel kettles. Mains onto element for 110/120 volt supply. Mains via power diode for 220/240 volt supply.

Some Christmas lights, negative mains half cycles operate some lamps, positive half cycles operates the others. Gives various patterns and chases with fewer wires.

Some old style Belisha beacons at road crossings. Thermal lamp flasher shunted by a silicon power diode. Flashes dim/bright rather than on/off. The 50 cycle flicker made the lights more eye catching.

Some old style traffic signals. Straight mains onto the signal lamps during the day. At night mains via a diode shunted with a resistance. Diming at night reduces glare and prolongs lamp life.

Vintage valve radio sets. HT obtained by half wave rectification of the mains.

Some older TV sets that used some valves and also transistors. Valve heaters in a series chain connected to the mains via a silicon diode. Lower heat production than a dropper resistance.

Crude battery charging circuits for 240 volt nominal batteries. Half wave rectifier from the mains, with a 110/120 volt lamp to limit the charging current. Change the lamp for one of a different wattage to adjust the charging current.

But all of those examples are not a smooth DC, but rather an AC with a DC offset or if you prefer to call it that, a “pulsed” DC. And a short to earth within such a device would still operate a type A RCD, and as the diode is likley to blow short or open pretty smartly, the fault is then removed - either by diode failure or a type AC RCD.

Mike

R.C.D. blinding and its prevention, seems to be a very complicated subject.

A guide.

Page 18 shows the many various waveforms.

Z.