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)