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
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)
Not uncommon with washing machines - water and electrcity in close proximity and mineral insulated metal clad heating elements etc.
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.
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?
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)
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:
formatting link
(extracted from Whitfield)
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.
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.
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).
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.
Ah, I don't remember that. We didn't watch That's Life! or read Witch? in our house.
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.
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.
You may well be right on this point.
They were too pricey for
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.
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