RCDs in all their glory

Anyone fancy having a read of a the RCD entry on the wiki that I have been hacking to death over the last few days?
There is a draft here:
http://wiki.diyfaq.org.uk/index.php?title=RCD
Feel free to post comments here or on the "Talk" page for the article, or edit it yourself[1] and save me the bother! ;-)
[1] Its handy if you log in first so I know who I am talking to!
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On 27/05/2007 03:46, John Rumm wrote:

the section on testing http://wiki.diyfaq.org.uk/index.php?title=RCD#DC_Resistance_tests has two tests entitled "Live Neutral Resistance"
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John Rumm wrote:

Excellent John, thank you - I am now a wiser man.
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RCDs don't really protect you from shocks.. they protect you from the effect of shocks. You will still know you have had a shock IME.
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dennis@home wrote:

If you want ot be precise then they don't really do either, they limit the duration of shock (and hence the total energy let through)

Indeed. (although I can't say I have ever tried it)
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John Rumm said the following on 27/05/2007 03:46:

Hi John, Excellent article.
A few comments:
1. RCD is a generic term for Residual Current Device and in modern terms encompasses two categories, viz RCCB and RCBO (without and with overcurrent protection respectively). Strictly, you should only use RCD as an umbrella term to reference RCCBs, RCDs and older devices (e.g. ELCBs). All of the "Types of RCD" you have shown are types of RCCB.
2. "A Residual Current Device or RCD is a circuit protective device designed to protect users from electric shock."
Not strictly true. Some RCDs (which have suitable operating parameters) offer the benefit of electric shock protection. Others don't.
3. "A RCD detects a fault condition which would typically only be seen when a person is receiving an electric shock from the circuit."
This is not true. An RCD "typically" detects an earth fault which occurs when a piece of electrical equipment gets wet or when the power cord to a piece of portable equipment (e.g. lawnmower) is damaged.
4. "A standard RCD does not offer any overcurrent protection". An RCCB does not offer over-current protection. An RCBO does. See 1 above.
5. "To maintain discrimination between different classes of circuit,[...] and a higher 100mA trip threshold device protects all the other circuits."
The 100mA trip should also be time-delayed to allow the 30mA trip to open in the event of a single fault generating an imbalance in excess of 100mA.
6.Trip characteristics. The standards for testing RCD's are (basically) as follows:
The RCD must not trip at 0.5x the rated trip current The RCD must trip within 40ms at 5x the rated trip current
7. RCBO's
The disadvantage of RCBOs [...], and secondly many of them are physically larger than a standard MCB.
They are _all_ physically larger than a standard MCB!
8. Appliances that typically exhibit high leakage currents.
Heater elements: "hydroscopic" should read "hygroscopic"
9. Faulty RCD
It is worth noting that the "test" button on an RCD introduces an internal current imbalance within the RCD and does not introduce a genuine earth fault. If you are in any doubt about whether an RCD is working properly, it should be checked with a commercial RCD tester.
10. Insulation Resistance. Missing ) after "...connected to it."
11. Series Earth Current Measurements: "Suitable teat lead" ?
Cheers,
Rumble
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Agreed, although I haven't had time to read it all yet.

ELCB (Earth Leakage Circuit Breaker) was the generic term covering all types.
The original Voltage Operated ELCB's go back to at least the 1950's (possibly earlier), and as John correctly stated, they measure the voltage between the CPC and real earth, and were required to trip before it reached 50V (and in practice, tripped much lower than this).
Current Operated ELCB's appeared in the 1970's AFAICR. These measure the difference between live and neutral current, and trip before it reaches the trip rating. Current Operated ELCB was the official name in the 14th Edition regs. The name was too cumbersome for consumer products, and each manufacturer made up their own name, causing much confusion. A campaign by "Which?" and "That's Life!" persuaded the industry to adopt a new name, RCD, to replace the previous names, and to be an instantly recognisable name from the consumer products point of view. This name change worked amasingly well and very quickly replaced all the old names.
So, one correction to the Wiki is due here, and that is than a Current Operated ELCB _is_ and RCD -- it was simply a uniform name change agreed throughout the industry in response to pressure from consumer organisations.

RCD's <= 30mA trip rating are considered to offer protection against electric shock.
All RCD's provide protection against high earth fault loop impedance, but where protection against high earth fault loop impedance is required and electric shock protection isn't required, RCD's of >= 100mA trip current are generally used.

I don't believe there is any accepted definition of an RCCB, other than being one of the old names for RCD which hasn't quite died.

Another useful figure which used to be in the regs but which has vanished is that the design leakage (i.e. the normal leakage expected) in a circuit should not exceed 0.25x the rated trip current. Although no longer in the regs, it's still a very good guideline and worth bearing in mind when constructing a circuit for IT equipment, and can be the limiting factor on the number of PC's which can be run from a circuit.

I suspect the issue here is that some can replace an MCB in a CU, whereas some take the space of more than one MCB.

The test button usually generates a fake imbalance which is much higher than the trip rating, so it's not good for checking accurate operation.
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Andrew Gabriel wrote:

I did go through a couple of versions of wording here (in fact NT's original wording was probably better than what I change it to!).
The reason being that I have memory (possibly erroneous, or possibly one of those theoretical things they teach for physics O Level) of a current operated ELCB that was not a residual device, but one that sat in series with the earth conductor and measured current flow rather than voltage differential.

Yup, much better explanation... consider it pinched! ;-)

BS7671 does not use it either....

Will add that to the design section at the end.

tis indeed what I was getting at.
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It's a very common misconception and you might have been taught that, but it would be good to get it right in the FAQ. If you can dig out a copy of the 14th edition wiring regs, it clearly describes the two types, and you will recognise a Current Operated ELCB as an RCD. What you describe above is a Voltage Operated ELCB, and again the 14th Ed test procedure for a Voltage Operated ELCB makes this very clear.
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Andrew Gabriel wrote:

Yup, was just trying to cover the bases with the original description (gone now, I updated the page).
I have never seen any type other than VO ELCBs and modern style RCDs, but was not sure if this so called "earth leakage current" detecting ELCB was mythical or not. I strongly suspect it is.
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John Rumm wrote:

Not mythical at all, IMHO: all household ELCBs that weren't what we'd now recognise as RCDs were of the current sensing type. More detail in http://groups.google.co.uk/group/uk.d-i-y/msg/c42e6415af841c2c?dmode=source&hl=en
Prior to the advent of the RCD principle, these devices were just referred to as "earth leakage trips". I don't recall the "current operated" / "voltage operated" terminology being used at all until RCDs appeared. At that point the RCD type was christened "current operated" and the older type (also current operated, in reality) suddenly became "voltage operated."
http://groups.google.co.uk/group/uk.d-i-y/msg/650ad9a6bc6a198d?dmode=source&hl=en is also very relevant, and offers supporting evidence for my arguments.
Otherwise, that's an excellent article, John. Do remember though that "circuit protection" with > 30 mA RCDs is also shock protection (indirect contact), as well as thermal (& fire) protection. This point is well made at one point, but seems to get lost elsewhere in the article.
Also the symbol for the SI unit of time is a lower-case "s" (the upper-case letter being reserved for the unit of conductance, the siemens, fomerly known as the mho). A space is preferred between a numerical quantity and the symbol.
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Andy Wade wrote:

http://groups.google.co.uk/group/uk.d-i-y/msg/c42e6415af841c2c?dmode=source&hl=en

Ah, that explains some of the confusion - its not my memory going completely barking then ;-)
I recall seeing circuit diagrams for what were probably just described as "earth leakage trips" or circuit breakers. (we are talking 1983 ish here). Two types were described - one which is recognisable as today's idea of a current balance RCD, and another with the CPC fed through, inductively coupled to a sense coil, which via a high gain DC amp would activate the solenoid to open the switch.
I had erroneously assumed (without thinking through the implications) that the so called VO ELCBs were actually a third class of device that was potential difference sensing... which when you think about it, would actually be quite difficult to achieve with a "straight through" earth connection - you would need a second independent earth to measure the voltage rise of the "real" one against.

http://groups.google.co.uk/group/uk.d-i-y/msg/650ad9a6bc6a198d?dmode=source&hl=en

Yup, remember that thread.
(perhaps I ought to leave it to Andy 1 and 2 to start a wiki edit war ;-)

Yes that is a good point. Its easy to get tunnel vision and focus too much on direct contact.

Hmmm, odd that. Not sure why I wrote it that way since I would normally write "msec" anyway.
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John.

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http://groups.google.co.uk/group/uk.d-i-y/msg/c42e6415af841c2c?dmode=source&hl=en
Sorry Andy, but on this one you're wrong. Please go back and look at the wiring regs of that time.

They are called "voltage operated" because that's how they're specified to work. They are required to trip before 50V appears across the sense coil.
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Andrew Gabriel wrote:

Which are you taking as the sense coil in this case?
I have not taken apart one to look at, but if the diagrams I have seen represent reality, there is a coil in series with the earth in and out connections, and a separate sense coil inductively coupled with the first one. So stick enough current through the first, and you get enough induced in the second to fire the trip)
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The coil connected between the two earth connections.
Obviously the coil has a resistance/impedance so there's going to be a current at which the coil trips too, but as there are typically multiple earth paths only one of which is through the sense coil, that's really rather meaningless.

I've taken several apart. There are minor differences but basically the sense coil is part of a relay. When energised and the relay contacts close, these operate a separate solenoid which opens the main breaker contacts. In some of them the solenoid just releases a latch and the main contacts spring apart, and some use the solenoid to fire the contacts apart faster than the spring would and they open with quite a bang. The solenoid sometimes also shorted out the sense coil when it operated, presumably to protect it against burning out, and some sense coils have spark gaps across them too. The sense coil relay contacts are very sensitive, and several of them could be tripped by tapping on the case enough to make those contacts touch.
I had about 6 of them taken out of mostly houses, and one commercial building. Most had no visible trip rating marked on them, but two of them were marked with a trip voltage rating which was significantly less than the 50V max in the regs (IIRC it was 20V or 25V). None had any trip current rating marked on them. If you applied DC across the sense coil, they all actually tripped at around 5V. I didn't experiment with AC on the sense coil. I think I kept two of them when I had a clearout ~15 years ago -- the oldest which was a Henley IIRC from the commercial building, and a Chilton (sp?) which looked to be the most recent -- probably one of the last made. Not sure where they are now, but I still have them somewhere.
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Andrew Gabriel wrote:

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 http://groups.google.co.uk/group/uk.d-i-y/msg/650ad9a6bc6a198d?dmode=source&hl=en
A 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.
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Andy Wade wrote:

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.
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On 27 May, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

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.
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snipped-for-privacy@privacy.net wrote:

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.
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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.
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