RCDs in all their glory

Leaded solder banned? I asked for some solder last week at a plumbing merchants and it was lead solder, I had to ask for lead free.

Adam

Reply to
ARWadsworth
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Yes, I've just dug out my 14th edition. The test was to put a voltage across the VO elcb, and ensure it trips before 48V is reached. The earthing terminal on the VO ELCB had to be outside the zone of any other earthing path. It limited the indirect contact voltage wrt earth to 48v.

Reply to
<me9

That does not really tell you much though does it? It could equally say check it trips on 35mA current or less. It would amount to the same test. I bet if you fed a typical VO ELCB with a 100V high impedance test voltage it would not trip due to the inability of the test supply to provide enough current to trip it.

ISTM that all that is going on is someone has designed the device with a suitable insertion impedance such that when added to the maximum supported earth rod resistance (500 ohms many seem to claim), you get enough trip current flowing to operate the device at a leakage voltage below 50V. That would suggest however that with a lower impedance earth rod the device will trip at lower leakage currents/voltages than with a high resistance one.

Reply to
John Rumm

The test is of the installation, not the VOELCB by itself. Can't find my 14th ed regs, but the test is done with a

48V/50V supply at 1kW or 2kW (can't recall which). This is put across the earth impedance, which includes all parallel earth paths (there's a diagram showing parallel earth paths are permitted). If the earth impedance is high enough to allow 48V to develop across it from a 1kw or 2kW supply without the VOELCB tripping, then the test fails. If the earth impedance is low enough that a 1kw or 2kW supply can't generate 48V across it, the test also passes (and the VOELCB probably isn't required). The amount of the current which flows through the VOELCB as opposed to the any parallel paths is irrelevant, except it shouldn't be so much that its sense electrode's voltage rises too much or it will fail to detect the 48V difference.

So in operation, the VOELCB is monitoring the voltage across the earth impedance, and if that gets too high, due too high a current leaking through too high an earth resistance, it trips. It doesn't know what the earth resistance or the leakage current is, it just knows what voltage is developed across it, and hence if the current is too much for the earth resistance. Hence the device's name.

Reply to
Andrew Gabriel

Well, I'll readily admit that my experience is limited and knowledge may be incomplete. Always interested in learning more, I hope we can continue the discussion...

The fundamental point of disagreement, I think, is that you think the normal mode of installation was to have the installation main earthing conductor directly connected to the primary earth electrode(s), and then to have the trip coil connected between that and a second 'monitoring' electrode, with non-overlapping resistance area. That would of course give a tripping characteristic related to the 'earth voltage lift' - a very worthy idea, and one certainly used in more technical installations, BUT my experience is that I've never seen that in an ordinary domestic TT installation. The latter, IME, always have the main earth terminal connected to the one earth electrode via the device coil, and nothing else.

I've nothing earlier to look at than the 15th edition, and these devices were still permitted when that edition was first published. I reproduced all relevant 15ed. regulations in the previous article at

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key one is:

413-11 Where protection is afforded by fault-voltage operated protective devices, all exposed conductive parts and associated extraneous conductive parts protected by any one such protective device shall be connected by protective conductors to an earth electrode via the voltage-sensitive element of that device.

which seems to support my observation about use of only one earth electrode. Regs 544-1 to 544-5 go on to make it clear that the earth electrode must be independent of any other parallel earth path. (This is easier said than done of course, which is one reason the devices are no longer used.)

That certainly seems to be the intent, and the test method uses 50 V from a low-impedance source applied between neutral and earth. (Full wording and link to associated diagram are in the above Google archive ref.)

Interesting though that "50 V lift" does not actually appear as a requirement in the body of the regulations: 531-9 merely says "the characteristics of every fault-voltage operated protective device shall be such as to comply with Regulation 413-3 for automatic disconnection in the event of a fault of negligible impedance between a phase conductor and exposed conductive parts [...]"

As to the devices themselves, you do seem to have more experience than me of the various types. I only have an example of the black Crabtree one now (photos linked from above ref.), although this was one of the most common, in houses at least. As I said before this has no external trip marking, but under one of the terminal covers it says 35 mA (no voltage). I'm sure I can recall seeing 'trips' in the 60s & 70s marked

30 mA, but the memory does play trick and I accept this might be wrong. I certainly don't recall seeing one with any voltage trip rating either, but I note that you have; maybe that's something that came in later in the life of these devices, with a change in the relevant standard.
Reply to
Andy Wade

Interesting: it's 50 V at 750 VA min. in the 15th edition

In the 15th it's applied between installation N & E, so the current path includes both the installation's own intended earth and the supplier's substation earthing. Any parallel paths at the installation will appear partly in parallel with the device's coil, desensitising it.

Again, you're assuming the coil is between the main earth rod and a separate monitoring electrode, which I continue to maintain was not usually the case. So what it was was really monitoring is the current leaking to earth from the installation CPCs through its coil. Whichever mode applies it's ultimately ampere-turns in the coil that causes the trip.

It would be very interesting to see the diagram from the 14th ed. I wonder whether snipped-for-privacy@privacy.net, or somebody, could provide a scan.

Reply to
Andy Wade

Well, I've just looked up the BS EN 61008 series, which is the relevant harmonised standard for "RCDs", derived from IEC 61008. The standards refer to them as RCCBs throughout, supporting your original notion that the RCCB is but one type of RCD.

The former BS 4293 refers to them as "residual current-operated circuit-breakers" (I can't help thinking they've got the hyphenation wrong there) but doesn't use an abbreviation.

Reply to
Andy Wade

Thanks Andy. I picked up on this a while ago but never got to the bottom of it.

Cheers,

Rumble

Reply to
Rumble

Looking at my 14th edition (1964 with 1968 qmendments*), the test is 750 va capable supply between neutral and earth continuity conductor (which may have paralell paths through plumbing etc). The VO ELCB has a separate earth electrode connected with an /insulated/ conductor. This earth electrode should be outside the zone of any other earth electrode (to provide discrimination - often impossible in practice). The test ensures that with

750VA the voltage of the CPC wrt this separate earth electrode can't exceed 45 volts. This may cause the ELCB not to trip, as the cross bonding rectifies the deficiency in the earth loop impedence. This is quoted as being a Good Thing. I think thinking has changed since then.
  • I kept this edition as it is the last one before the introductiom of metrication, and thus had tables for the imperial sizes of conductors -- probably no longer relevant.
Reply to
<me9

I can't add much to this debate since I don't think I have seen more than four or five of these in use (and I will plead to not being old enough to have paid that much attention to wiring in the 70's!). However all of them that I have seen were wired with this "loop through" configuration. This includes the one that was in this house on our arrival (the photo of the ELCB on the wiki is of it). The connections were main earth terminal in CU connected to ELCB, ELCB connected to gas pipe (gas pipe being used as a main earth electrode). No other main bonds or equipotential bonds.

Reply to
John Rumm

Ah ok, did not realise you were describing an installation test rather than a device one.

If you look at the "big picture" then I accept that there is some justification for calling the arrangement "Voltage Operated", since that is the parameter that you are seeking to limit, and the number of potential parallel paths make the absolute trip current of the device taken in isolation not relevant.

Yup, with you up to there.

That is the bit I don't follow... ISTM the only thing that it can apply a threshold test to is the current passing through it. You have a circuit which is notionally two series impedances:

Earth Rod ELCB Coil (300 ohm) --------/\/\/\/\-------/\/\/\/\----------> Main earth terminal | |

----- --- -

One of known value (the coil in the ELCB), one unknown (the earth electrode), but assumed to be under 500 ohms. With that knowledge you can now safely decide on a trip current to design the device to operate at, and know that it will limit the voltage rise to something under 48V as you describe in the above test scenario.

I(t) = 48 / 800 = 60mA

So a trip current of 60mA or less will achieve the required result.

However you can't say what voltage it would trip at without knowing the earth rod impedance. Say you set it to trip at 35mA like the Crabtree one, that should give you a trip at 28V with a 500 ohm earth rod. Best earth rod I have seen recently I measured at 7 ohms, so with that it would trip at 11V

(this is assuming that the wiring is configured as it was in my house)

Reply to
John Rumm

Pre dates me then (if you ignore the last amendment)! ;-)

Indeed. The "fortuitous" reduction in earth impedance from main bonds and any other parallel paths is deliberately disconnected when testing the earth rod impedance these days.

I suppose the chances of someone ripping you gas pipe out and giving you a plastic one back was not considered in the 60's so it was considered fair to include their contribution to the the overall result.

Reply to
John Rumm

I have a feeling you weren't allowed to connect to a gas pipe in those days. Water pipe was accepted as an earth electrode, certainly n the late 50s. I'm not sure (probably the introduction of alkathene in the 60s) when it became verboten.

Reply to
<me9

Did this conversation ever have a conclusion? It seems to have died half way through.

Reply to
John Rumm

Sorry, I meant to reply, but been rather busy at work. I'll try and find the time...

Reply to
Andrew Gabriel

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