Hi

Could someone clarify the difference between an RCD & a RCCD or is it just
different terminology?

I'm also a little confused about MCB's. For example;

Regular customer just around the corner to me asked me to look at why her
washing machine causes a 'trip' to operate.

Socket checked out OK (with one of the plug in testers), plug & fuse fine so
obviously a fault with the WM. Advised the customer to get the WM checked
out.

It was the RCCD tripping, not the MCB for the kitchen circuit.

AIUI an RCD/RCCD reacts to current leaking from live to earth and an MCB
reacts to excessive current.

In the case above the RCCD tripped turning off the kitchen circuit but
leaving all the others in the house on.

So, does an RCCD only protect a circuit where there is a high risk - like a
kitchen, or would it trip if any circuit in the house developed a live to
earth fault? And how do you know which circuit its protecting?


--
Dave - The Medway Handyman
www.medwayhandyman.co.uk

"The Medway Handyman" wrote in message
om...
Hi

Could someone clarify the difference between an RCD & a RCCD or is it just
different terminology?

I'm also a little confused about MCB's. For example;

Regular customer just around the corner to me asked me to look at why her
washing machine causes a 'trip' to operate.

Socket checked out OK (with one of the plug in testers), plug & fuse fine
so obviously a fault with the WM. Advised the customer to get the WM
checked out.

It was the RCCD tripping, not the MCB for the kitchen circuit.

AIUI an RCD/RCCD reacts to current leaking from live to earth and an MCB
reacts to excessive current.

In the case above the RCCD tripped turning off the kitchen circuit but
leaving all the others in the house on.

So, does an RCCD only protect a circuit where there is a high risk - like
a kitchen, or would it trip if any circuit in the house developed a live
to earth fault? And how do you know which circuit its protecting?

There is also a device called an RCBO which combines the features
of an RCD and an MCB.
It is possible to have a consumer unit containing 100% RCBOs
and no MCBs

--
Graham.

%Profound_observation%

The Medway Handyman wrote:
Hi

Could someone clarify the difference between an RCD & a RCCD or is it
just different terminology?

I'm also a little confused about MCB's. For example;

Regular customer just around the corner to me asked me to look at why
her washing machine causes a 'trip' to operate.

Socket checked out OK (with one of the plug in testers), plug & fuse
fine so obviously a fault with the WM. Advised the customer to get
the WM checked out.

It was the RCCD tripping, not the MCB for the kitchen circuit.

AIUI an RCD/RCCD reacts to current leaking from live to earth and an
MCB reacts to excessive current.

In the case above the RCCD tripped turning off the kitchen circuit but
leaving all the others in the house on.

So, does an RCCD only protect a circuit where there is a high risk -
like a kitchen, or would it trip if any circuit in the house
developed a live to earth fault? And how do you know which circuit
its protecting?


Have a look at this page http://www.tlc-direct.co.uk/Book/5.9.1.htm for
help on this.

The Medway Handyman wrote:

Could someone clarify the difference between an RCD & a RCCD or is it just
different terminology?

ITYM RCCB - Residual Current Circuit Breaker. The same thing as RCD but
perhaps slightly more specific. Compare with RCBO Residual Current
circuit Breaker with Overload Protection (combined RCD / MCB)

I'm also a little confused about MCB's. For example;

Regular customer just around the corner to me asked me to look at why her
washing machine causes a 'trip' to operate.

Socket checked out OK (with one of the plug in testers), plug & fuse fine so
obviously a fault with the WM. Advised the customer to get the WM checked
out.

It was the RCCD tripping, not the MCB for the kitchen circuit.

Not uncommon with washing machines - water and electrcity in close
proximity and mineral insulated metal clad heating elements etc.

AIUI an RCD/RCCD reacts to current leaking from live to earth and an MCB
reacts to excessive current.

Yup, or to be more exact it reacts to an imbalance between current
measured in the live and neutral. The implication of an imbalance is
that some current must be flowing through another path.

In the case above the RCCD tripped turning off the kitchen circuit but
leaving all the others in the house on.

Could you mean RCBO here? A device that from the outside of the CU may
look very much like a normal MCB, but which also includes RCD functionality?

Was this a split load board with one or more double width RCDs in it?

So, does an RCCD only protect a circuit where there is a high risk - like a
kitchen, or would it trip if any circuit in the house developed a live to
earth fault? And how do you know which circuit its protecting?

The granularity of the protection (i.e. so called "discrimination") can
vary hugely depending on the age of the install and how it was designed.
Worst case is the so called "whole house RCD" - one device that protects
all circuits (quite often a separate box patched in before an older CU),
and leakage above the trip threshold on any one, or any combination of
circuits, will cause it to trip and cut power to the whole house.

Better is an older style split load CU - some circuits not protected by
RCD at all (lights, fixed equipment, high leakage stuff), and all socket
circuits (or at least high risk ones) protected by one shared RCD.
Leakage on any socket circuit will thus trip all socket circuits but
leave the lights on etc,

Better is a 17th edition CU with multiple RCDs (the simplest have just 2
and are no better discrimination wise than the older split load type),
better examples have three or four RCDs. Here fewer circuits are
allocated to each RCD and finer discrimination is achieved.

Best is full RCBO protection for every circuit that requires it. Hence
it has its own RCD as well as an MCB (in a single combined device). With
this you get full discrimination - only the circuit(s) with the high
leakage current will trip. This used the be the Rolls Royce solution
since RCBOs were very expensive. However recently they have become
cheaper, far more common, and readily available in single module widths
(older ones took at least two slots each in the CU)



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
The Medway Handyman wrote:

Could someone clarify the difference between an RCD & a RCCD or is it
just different terminology?

ITYM RCCB - Residual Current Circuit Breaker. The same thing as RCD but
perhaps slightly more specific. Compare with RCBO Residual Current
circuit Breaker with Overload Protection (combined RCD / MCB)

I'm also a little confused about MCB's. For example;

Regular customer just around the corner to me asked me to look at why
her washing machine causes a 'trip' to operate.

Socket checked out OK (with one of the plug in testers), plug & fuse
fine so obviously a fault with the WM. Advised the customer to get
the WM checked out.

It was the RCCD tripping, not the MCB for the kitchen circuit.

Not uncommon with washing machines - water and electrcity in close
proximity and mineral insulated metal clad heating elements etc.

AIUI an RCD/RCCD reacts to current leaking from live to earth and an
MCB reacts to excessive current.

Yup, or to be more exact it reacts to an imbalance between current
measured in the live and neutral. The implication of an imbalance is
that some current must be flowing through another path.

I.e. RCD also trips on neutral to earth leakage. With such a fault even
if the relevant MCB is turned off the RCD will still trip. Never rely on
1 RCD to protect everything.

In the case above the RCCD tripped turning off the kitchen circuit but
leaving all the others in the house on.

Could you mean RCBO here? A device that from the outside of the CU may
look very much like a normal MCB, but which also includes RCD
functionality?

Was this a split load board with one or more double width RCDs in it?

So, does an RCCD only protect a circuit where there is a high risk -
like a kitchen, or would it trip if any circuit in the house developed
a live to earth fault? And how do you know which circuit its protecting?

The granularity of the protection (i.e. so called "discrimination") can
vary hugely depending on the age of the install and how it was designed.
Worst case is the so called "whole house RCD" - one device that protects
all circuits (quite often a separate box patched in before an older CU),
and leakage above the trip threshold on any one, or any combination of
circuits, will cause it to trip and cut power to the whole house.

Better is an older style split load CU - some circuits not protected by
RCD at all (lights, fixed equipment, high leakage stuff), and all socket
circuits (or at least high risk ones) protected by one shared RCD.
Leakage on any socket circuit will thus trip all socket circuits but
leave the lights on etc,

If the power supply is TT then RCD protection is required to cover all
circuits.

Better is a 17th edition CU with multiple RCDs (the simplest have just 2
and are no better discrimination wise than the older split load type),
better examples have three or four RCDs. Here fewer circuits are
allocated to each RCD and finer discrimination is achieved.

Best is full RCBO protection for every circuit that requires it. Hence
it has its own RCD as well as an MCB (in a single combined device). With
this you get full discrimination - only the circuit(s) with the high
leakage current will trip. This used the be the Rolls Royce solution
since RCBOs were very expensive. However recently they have become
cheaper, far more common, and readily available in single module widths
(older ones took at least two slots each in the CU)

The Medway Handyman wrote:
Hi

Could someone clarify the difference between an RCD & a RCCD or is it just
different terminology?


There are various British and European Standards for the design of
electrical control gear. One of them (don't ask me which, I can't
remember) in about 1980 (possibly earlier - don't quote me) defined the
term RCD:-

RCD is an umbrella term to cover all Residual Current Devices.

RCDs are sub-divided into RCCBs and RCBOs.

So, an RCCB is a Residual Current Circuit Breaker.

and an RCBO is a Residual current Current Breaker with Over-current
protection.

However, at that time, when the contracting industry and the IEE were
pushing the use of RCDs in domestic environments, there was very little
call for RCBOs (which hitherto were mainly used in factories).

So, the term RCD (which includes RCCB's and RCBO's) came to be
synonymous with RCCB only and we have (incorrectly) ended up with the
terms RCD and RCBO in common use.

The situation is not helped by the fact that MK and Wylex label their
RCCBs as RCDs, nor is it helped by the fact that Whitfield incorrectly
uses the term RCCD in lieu of RCCB (he doesn't explain why the
abbreviation RCCD for Residual Current Circuit Breaker doesn't work!)

The use of RCCD is wrong on this page also:

http://www.tlc-direct.co.uk/Book/5.9.1.htm (extracted from Whitfield)


I'm also a little confused about MCB's. For example;

Regular customer just around the corner to me asked me to look at why her
washing machine causes a 'trip' to operate.

Socket checked out OK (with one of the plug in testers), plug & fuse fine so
obviously a fault with the WM. Advised the customer to get the WM checked
out.

It was the RCCD tripping, not the MCB for the kitchen circuit.

AIUI an RCD/RCCD reacts to current leaking from live to earth and an MCB
reacts to excessive current.


RCCB
====

In a correctly operating circuit, the current flowing in the live wire
should exactly equal the current flowing in the neutral wire. An RCCB
detects any imbalance between the current in the live wire and the
current in the neutral wire. This imbalance gives rise to a "residual
current" (i.e. the difference between the current flowing in the live
and neutral). The residual current must be flowing to earth (there's no
where else for it to go). Now it might be flowing from live to earth and
it might be flowing from neutral to earth depending on the nature of the
fault.

Once the residual current reaches a certain level (anywhere between 17mA
and 27mA for a 30mA RCCB), the residual current is strong enough to
operate a solenoid which trips the mechanism in the device and cuts the
power.


MCB
===

An MCB or Miniature Circuit Breaker reacts to over-current. It is called
a miniature circuit breaker because it's a lot smaller than an MCCB
(Moulded Case Circuit Breaker) which preceded the MCB by a number of
years and was at the time the size of a house brick (and only used in
industrial fuse-boards).

The MCB is a thermal-magnetic circuit breaker and has two distinct
operating principles.

The thermal part is a bimetallic strip with a heating coil would round
it. Current flows through the heating coil and warms the bimetallic
strip. When the strip gets warm enough (directly related to the circuit
current), it bends sufficiently to trip the mechanism. This is quite a
slow process (an MCB rated a 6A which has a current of 6.1A might take a
couple of hours to trip).

The magnetic part is a solenoid. When the current gets high enough, the
solenoid pulls in and trips the mechanism. The magnetic part is quite
insensitive to small or moderate over-currents.

So, the thermal part is carefully calibrated to trip at the nominal
current rating of the MCB and the magnetic part protects against dead
shorts. For each characteristic type of circuit breaker (B, C, D, etc)
the thermal part is pretty much the same; it is sensitivity of the
magnetic part which varies.

For a fault current which is (say) twice the rated current then either
the thermal or the magnetic element might operate first to trip the
mechanism.

RCBO
====

This device is a combined RCCB and MCB nad therefore has three ways it
can trip, viz thermal overcurrent, magnetic overcurrent and residual
current.

In the case above the RCCD tripped turning off the kitchen circuit but
leaving all the others in the house on.

So, does an RCCD only protect a circuit where there is a high risk - like a
kitchen, or would it trip if any circuit in the house developed a live to
earth fault? And how do you know which circuit its protecting?



There is a requirement in the IEE Wiring Regulations that, in the event
of a fault, the over-current protection device (fuse or MCB) operates
within a certain maximum time. This time varies between different types
of circuits and between different editions of the regulations.

This is usually achieved by making sure that if a fault current flows,
it will be a big one (by making sure the size of the cables is large
enough and the source impedance of the supply is low enough) and that
therefore the fuse/mcb will trip very quickly.

However, most faults which lead to the risk to life are earth faults and
therefore you could use an RCD to give acceptable disconnection time in
lieu of upgrading the cable/earthing size.

Now it should be said that the Wiring Regs were originally written for
the sole purpose of minimising the risk of your electrical installation
causing a fire. Since the introduction of domestic electrical
installations in the 19th century, fire and smoke suffocation and
collapsing buildings caused by electrical faults has killed many, many
more people than has electrocution.

However, having got the fire protection thing sorted, the IEE have, over
the past 30 years or so really turned their attention to going overboard
with protecting people against electric shock. This in spite of the
simple fact that there are many millions of houses with electrical
installations nowhere near as good as modern specification, but you
don't hear on the news of people being electrocuted every five minutes.
A good house-fire will always make the local paper, often make the
local TV news and sometimes make the national news.

So, the RCD is a good thing in the sense that it directly detects small
faults to earth that would not cause an MCB to trip.

As regards which circuits the RCD is protecting, this should be labelled
on the consumer unit or otherwise, you should be able to infer it from
context.

Sorry about the long post.

On Thu, 13 Aug 2009 11:58:13 +0100, Dave Osborne wrote:

Sorry about the long post.

Not at all! Thank you - that's the clearest explanation that I've ever
seen; also shows that some sources can't be trusted, which in itself is
useful.
--
Peter.
The head of a pin will hold more angels if
it's been flattened with an angel-grinder.

Thanks Dave, I found that very informative.

--
Graham.

%Profound_observation%

In article ,
Dave Osborne writes:
The Medway Handyman wrote:
Hi

Could someone clarify the difference between an RCD & a RCCD or is it just
different terminology?


There are various British and European Standards for the design of
electrical control gear. One of them (don't ask me which, I can't
remember) in about 1980 (possibly earlier - don't quote me) defined the
term RCD:-

RCD is an umbrella term to cover all Residual Current Devices.

It came about due to a campaign by Which? and That's Life!
This was the point where RCDs became consumer products, and
every manufacturer and IEE was calling them something different,
leading to much consumer confusion and avoidance of the
products. Which? and That's Life! pressed the industry to
use one common name - RCD (which probably wasn't the best choice
but that's not important) - and the industry fell into line.

RCCB was one of the old names -- sort of hung on as some other
EU countries adopted it (even though it had no direct local
language meaning).

RCDs are sub-divided into RCCBs and RCBOs.

So, an RCCB is a Residual Current Circuit Breaker.

and an RCBO is a Residual current Current Breaker with Over-current
protection.

However, at that time, when the contracting industry and the IEE were
pushing the use of RCDs in domestic environments, there was very little
call for RCBOs (which hitherto were mainly used in factories).

RCBO's didn't appear until later -- until RCD's themselves
could be made small enough to make them worth combining with
an MCB, around 1990 from memory. They were too pricey for
domestic use at that point though.

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

Andrew Gabriel wrote:
In article ,
Dave Osborne writes:
The Medway Handyman wrote:
Hi

Could someone clarify the difference between an RCD & a RCCD or is it just
different terminology?

There are various British and European Standards for the design of
electrical control gear. One of them (don't ask me which, I can't
remember) in about 1980 (possibly earlier - don't quote me) defined the
term RCD:-

RCD is an umbrella term to cover all Residual Current Devices.

It came about due to a campaign by Which? and That's Life!

Ah, I don't remember that. We didn't watch That's Life! or read Witch?
in our house.

This was the point where RCDs became consumer products, and
every manufacturer and IEE was calling them something different,

I don't remember that there were many different names. I remember that
there was confusion between old-style voltage-operated ELCBs (Earth
Leakage Circuit Breakers) and new-fangled current-operated ELCBs - which
became residual current circuit breakers.

leading to much consumer confusion and avoidance of the
products. Which? and That's Life! pressed the industry to
use one common name - RCD (which probably wasn't the best choice
but that's not important) - and the industry fell into line.

RCCB was one of the old names -- sort of hung on as some other
EU countries adopted it (even though it had no direct local
language meaning).

Hmm. I'm not sure that's true. More likely that the manufacturers made
product to UK spec. and then sold it on to whoever wanted it. Most of
the technical info printed on an RCD is symbolic not natural language
anyway, so each country would need its own crib sheet to interpret the
symbols. Easy to have a local translation of RCCB or RCD - or just
ignore it.


RCDs are sub-divided into RCCBs and RCBOs.

So, an RCCB is a Residual Current Circuit Breaker.

and an RCBO is a Residual current Current Breaker with Over-current
protection.

However, at that time, when the contracting industry and the IEE were
pushing the use of RCDs in domestic environments, there was very little
call for RCBOs (which hitherto were mainly used in factories).

RCBO's didn't appear until later -- until RCD's themselves
could be made small enough to make them worth combining with
an MCB, around 1990 from memory.

You may well be right on this point.

They were too pricey for
domestic use at that point though.


I don't think there was any call for them for domestic use and they
wouldn't have fitted in consumer units of the day anyway - you still
have to be careful to choose a consumer unit/RCBO combination that
physically works.


It's possible that the "standard definitions" were retrospective. I'm
pretty sure I've seen definitions as I have stated (in BS something or
other). I don't have wide access to BS any more, so I can't check
further. Perhaps someone can do some digging.

Dave Osborne wrote:
SNIP

Sorry about the long post.

Thanks ever so much Dave, really informative & much appreciated.


--
Dave - The Medway Handyman
www.medwayhandyman.co.uk

On Thu, 13 Aug 2009 11:58:13 +0100, Dave Osborne wrote:

snip

Sorry about the long post.


Long, maybe, but excellent, definitely!


--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.