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.