RCDs in all their glory

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John Rumm

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May 26, 2007, 10:46:23 PM5/26/07
to
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!


--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
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Andy Burns

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May 27, 2007, 3:48:06 AM5/27/07
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On 27/05/2007 03:46, John Rumm wrote:

> 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?

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

The Medway Handyman

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May 27, 2007, 5:05:17 AM5/27/07
to
John Rumm wrote:
> 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!
>
Excellent John, thank you - I am now a wiser man.


--
Dave
The Medway Handyman
www.medwayhandyman.co.uk
01634 717930
07850 597257


dennis@home

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May 27, 2007, 6:11:32 AM5/27/07
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"John Rumm" <see.my.s...@nowhere.null> wrote in message
news:4658f10c$0$8725$ed26...@ptn-nntp-reader02.plus.net...

> 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!

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.


John Rumm

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May 27, 2007, 8:10:27 AM5/27/07
to
dennis@home wrote:

> RCDs don't really protect you from shocks.. they protect you from the effect
> of shocks.

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)

> You will still know you have had a shock IME.

Indeed. (although I can't say I have ever tried it)

Rumble

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May 27, 2007, 8:26:26 AM5/27/07
to
John Rumm said the following on 27/05/2007 03:46:

> 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!
>
>
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

Andrew Gabriel

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May 27, 2007, 9:16:49 AM5/27/07
to
In article <465978d1$0$647$5a6a...@news.aaisp.net.uk>,
Rumble <nospam.@invalid.invalid> writes:
> Hi John, Excellent article.

Agreed, although I haven't had time to read it all yet.

> 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.

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.

> 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.

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.

> 4. "A standard RCD does not offer any overcurrent protection". An RCCB

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.

> 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

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.

> 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!

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.

> 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.

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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

John Rumm

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May 27, 2007, 9:49:08 AM5/27/07
to
Rumble wrote:

> Hi John, Excellent article.

Thanks

> 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.

I do actually include a section on RCBOs later, it might be worth
placing them in the "types" table as well, and expanding the definitions
slightly.

> 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.

Yup agreed. I did specify the other uses in the descriptive stuff, but
was sloppy with the "executive summary" description.

> 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.

Agreed, again was trying to keep it perhaps a little too simple.

> 4. "A standard RCD does not offer any overcurrent protection". An RCCB
> does not offer over-current protection. An RCBO does. See 1 above.

That should be more obvious if I move the RCBO def.

> 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.

I have described that circumstance elsewhere, however it is not the only
way this can be achieved. For example in my own TT installation I used
two separate CUs - one protected at 100mA and the other at 30mA. Thus
achieving the required level of protection for all circuits, but not via
a cascaded RCD setup.

> 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

I was debating on how much detail to include on this... there are a
number of other tests that are doable with the correct equipment as well.

> 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!

;-) but not all in the same axis

Assuming the CU is tall enough the single module ones are no "larger"
than the MCBs in the direction that counts (i.e. the one that limits the
number of devices in the CU).


> 8. Appliances that typically exhibit high leakage currents.
>
> Heater elements: "hydroscopic" should read "hygroscopic"

well spotted.

> 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.

Will mention that

> 10. Insulation Resistance. Missing ) after "...connected to it."
>
>
> 11. Series Earth Current Measurements: "Suitable teat lead" ?

I Like the sound of that, wonder where I could get one? ;-)

John Rumm

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May 27, 2007, 9:56:44 AM5/27/07
to
Andrew Gabriel wrote:

> 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.

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.

>
>> 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.
>
> 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.

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

>> 4. "A standard RCD does not offer any overcurrent protection". An RCCB
>
> 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.

BS7671 does not use it either....

>> 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
>
> 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.

Will add that to the design section at the end.

>> 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!
>
> 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.

tis indeed what I was getting at.

Rumble

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May 27, 2007, 11:32:24 AM5/27/07
to
John Rumm said the following on 27/05/2007 03:46:
> 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!
>
>
OK,

On the subject of RCD/RCCB/RCBO naming convention:

I have Googled extensively, but cannot find a definitive source. There
may be a definition in a BS somewhere.

It is clear that there is an intention within some standards body or
other for the following definitions (courtesy of the MEM website):


MEM Website:
============

What do the abbreviations mean?

R.C.D.: Residual Current Device is the generic term covering the range
of devices incorporating sensing of residual current and includes within
the scope R.C.C.B. and R.C.B.O. type products.

R.C.C.B.: Residual Current Circuit Breaker is an RCD which will cause
disconnection of electrical supply should a residual current passing
through the device exceed a specified level.

R.C.B.O.: Residual Current Circuit Breaker with Overload protection is
an RCD which will cause disconnection of electrical supply due to
residual current exceeding specified limits together with integral
overload; overcurrent and short circuit protection associated with a
miniature circuit breaker.


Now, some manufacturers adopt this distinction and some don't. Some
that do are:


ABB Website:

"Besides, depending on the type of construction, residual current
devices (RCDs) may be classed as RCBOs (magnetothermic with overcurrent
protection), RCCBs (without overcurrent protection releaser
incorporated) and RCD blocks."


Merlin Gerin Low Voltage Distribution Products Catalogue:

"Function
Residual current circuit-breakers ensure:
The control and isolation of electrical circuits
The protection of persons against direct and indirect contacts
The protection of installations against insulation faults.
They conform to both the residual current device standard BS EN 61008 and to
switch standards BS EN 60947-1 and BS EN 60947-3.
Residual current circuit-breakers are used in the housing, commercial
and industrial
sectors."


Hager Electric Website:

Hager has launched the Modular Protection and Control product range.

The range consists of single and multi-pole MCB's, RCCB's and RCBO's.
They have been developed to meet customer demands for innovative
products that meet requirements of modern installations."


Baco Electric:

A Range of auxilliary devices for the 2- pole and 4-pole RCCB's:
auxilliary changeover switch, fault signalling changeover switch, shunt
trip release unit

A range of 2-pole and 4-pole RCCB's in 25-40-63 and 80 Amp 30mA with
high protection against nuisance tripping.


Crabtree

224/030 40A 30mA 2 Pole rccb Lifestar

These units are not interchangeable with the RCCBs now available in the
Starbreaker range
* BS EN 61008; IEC 1008.
* Terminal capacities: - 50mm².
* Type A rccb#s with 100mA sensitivity available on request.


Other manufacturers (e.g. MK, Wylex, Contactum) do not make this
distinction and refer to RCD's (but *meaning* RCCB's) and RCBO's

I agree that the general public have picked up on RCD as a catch-all,
but if a comprehensive Wiki article is to be writ, then I think we
should ensure that any ambiguity or uncertainty is cleared up.


Cheers,

Rumble

Andrew Gabriel

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May 27, 2007, 11:54:58 AM5/27/07
to
In article <46598e2a$0$8733$ed26...@ptn-nntp-reader02.plus.net>,

John Rumm <see.my.s...@nowhere.null> writes:
> Andrew Gabriel wrote:
>
>> 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.
>
> 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.

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.

Andrew Gabriel

unread,
May 27, 2007, 12:12:57 PM5/27/07
to
In article <4659a467$0$644$5a6a...@news.aaisp.net.uk>,

Rumble <nospam.@invalid.invalid> writes:
> OK,
>
> On the subject of RCD/RCCB/RCBO naming convention:
>
> I have Googled extensively, but cannot find a definitive source. There
> may be a definition in a BS somewhere.

See my earlier post -- I have explained what the names mean and how
the names were derived. RCCB is just one of the old names for RCD,
today sometimes implying an RCD in a CU. RCCB is (rightly) not
mentioned at all in current BS7670.

The Wiring Regs (BS7671) is about as definitive as you are likely
to get for the UK, but as I explained, you have to go back through
old editions to see how we ended up where we are now.

I don't think there's any confusion over RCBO.

John Rumm

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May 27, 2007, 12:16:44 PM5/27/07
to
Andrew Gabriel wrote:

>> 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.
>
> 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.

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.

Andy Wade

unread,
May 27, 2007, 2:04:55 PM5/27/07
to
John Rumm wrote:

> 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.

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.

--
Andy

John Rumm

unread,
May 27, 2007, 3:17:27 PM5/27/07
to
Andy Wade wrote:

>> 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.
>
> 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."

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
>
> is also very relevant, and offers supporting evidence for my arguments.

Yup, remember that thread.

(perhaps I ought to leave it to Andy 1 and 2 to start a wiki edit war ;-)

> 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.

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

> 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.

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

Owain

unread,
May 27, 2007, 11:43:29 AM5/27/07
to
John Rumm wrote:

> Rumble wrote:
>> 11. Series Earth Current Measurements: "Suitable teat lead" ?
> I Like the sound of that, wonder where I could get one? ;-)

I think teat lead has probably been banned along with leaded solder.

I'm told cabbage leaves are good.

Owain


Andrew Gabriel

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May 27, 2007, 3:50:17 PM5/27/07
to
In article <4659c848$0$19247$da0f...@news.zen.co.uk>,

Andy Wade <spamb...@maxwell.myzen.co.uk> writes:
> John Rumm wrote:
>
>> 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.
>
> 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

Sorry Andy, but on this one you're wrong.
Please go back and look at the wiring regs of that time.

> 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."

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.

John Rumm

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May 27, 2007, 4:13:23 PM5/27/07
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Andrew Gabriel wrote:

>> 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."
>
> 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.

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)

John Rumm

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May 27, 2007, 4:15:46 PM5/27/07
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Owain wrote:


>>> 11. Series Earth Current Measurements: "Suitable teat lead" ?
>> I Like the sound of that, wonder where I could get one? ;-)
>
> I think teat lead has probably been banned along with leaded solder.

It sounds much more interesting if you read that as lead (as in dog)
rather than lead as in metal.

As in what you may find on the end of a teat lead... ;-)

> I'm told cabbage leaves are good.

Not with my ability to produce digestive gas!

Andrew Gabriel

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May 27, 2007, 5:01:11 PM5/27/07
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In article <4659e671$0$8737$ed26...@ptn-nntp-reader02.plus.net>,

John Rumm <see.my.s...@nowhere.null> writes:
> Andrew Gabriel wrote:
>
>>> 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."
>>
>> 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.
>
> Which are you taking as the sense coil in this case?

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 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)

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.

ARWadsworth

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May 27, 2007, 6:40:51 PM5/27/07
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"Owain" <owain...@stirlingcity.coo.uk> wrote in message
news:11802940...@proxy00.news.clara.net...

> John Rumm wrote:
>> Rumble wrote:
>>> 11. Series Earth Current Measurements: "Suitable teat lead" ?
>> I Like the sound of that, wonder where I could get one? ;-)
>
> I think teat lead has probably been banned along with leaded solder.

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

Message has been deleted

John Rumm

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May 27, 2007, 10:31:33 PM5/27/07
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m...@privacy.net 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.

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.

Andrew Gabriel

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May 28, 2007, 4:30:06 AM5/28/07
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In article <465a3f14$0$8734$ed26...@ptn-nntp-reader02.plus.net>,

John Rumm <see.my.s...@nowhere.null> writes:
> m...@privacy.net 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.
>
> 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

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.

Andy Wade

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May 28, 2007, 5:58:42 AM5/28/07
to
Andrew Gabriel wrote:

> Sorry Andy, but on this one you're wrong.

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.

> Please go back and look at the wiring regs of that time.

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.)

> 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.

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.

--
Andy

Andy Wade

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May 28, 2007, 6:25:35 AM5/28/07
to
Andrew Gabriel wrote:

> 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).

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

> put across the earth impedance, which includes all parallel
> earth paths (there's a diagram showing parallel earth paths
> are permitted).

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.

> 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.

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 m...@privacy.net, or somebody, could provide a scan.

--
Andy

Andy Wade

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May 28, 2007, 6:44:42 AM5/28/07
to
Rumble wrote:

> On the subject of RCD/RCCB/RCBO naming convention:
>
> I have Googled extensively, but cannot find a definitive source. There
> may be a definition in a BS somewhere.
>

> [...]


>
> I agree that the general public have picked up on RCD as a catch-all,
> but if a comprehensive Wiki article is to be writ, then I think we
> should ensure that any ambiguity or uncertainty is cleared up.

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.

--
Andy

Rumble

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May 28, 2007, 7:03:57 AM5/28/07
to
Andy Wade said the following on 28/05/2007 11:44:

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


Cheers,

Rumble

Message has been deleted

John Rumm

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May 28, 2007, 2:02:46 PM5/28/07
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Andy Wade wrote:

> 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 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.

John Rumm

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May 28, 2007, 2:27:18 PM5/28/07
to
Andrew Gabriel 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
>
> 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.

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.

> 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.

Yup, with you up to there.

> 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.

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)

John Rumm

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May 28, 2007, 2:33:02 PM5/28/07
to
m...@privacy.net wrote:

> Looking at my 14th edition (1964 with 1968 qmendments*), the test is 750 va

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

> 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.

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.

Message has been deleted

John Rumm

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Jun 5, 2007, 7:01:11 PM6/5/07
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Did this conversation ever have a conclusion? It seems to have died half
way through.

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