rcd or mcb?

Discussion in 'Electricians' Talk' started by trowel head, May 27, 2004.

  1. trowel head

    trowel head New Member

    please bear in mind i'm only a humble brickie and my following question(s) might seem daft to a qualified sparkie! 1)what is the difference between an rcd and mcb? 2)is one safer than the other? 3)could i,in theory,change (not by me i might add!)whatever i have in my cu for the "safer" option? 4)would it be worthwhile? 5)how can i tell what i have in my cu at present? my(wylex) cu consists of a 100 amp "main" switch on the right and 7 mcb/rcd? to the left,made up of nsb06(lights) and nsb32(power).the above might sound stupid to you but after reading about other posts regarding rcd's these do seem to be the way "you lot" prefer to go.last point what is,and what is the purpose of a split load cu? i hope you understand this is only really for my personal knowledge and not to start rewiring my house!!!! thanx chaps.
  2. plugwash

    plugwash New Member

    rcds and mcbs server different perposes and both need to be used in an install

    a rcd detects an imbalance between live and neutral current (which alsmost always represents earth leakage)

    a mcb detects overcurrent

    a rcbo combines the functions of a rcd and mcb however they are rarely used domestically for cost reasons

    all cuircuits from the consumer unit must be protected by appropriately rated mcbs

    all sockets that can feasiblly supply equipement outside must have rcd protection

    most sparkys agree that electric showers should be on rcd even though no regs require it

    some cuircuits should not be on rcd theese include lights, alarms and some would say freezers (though most electricians don't bother with a dedicated freezer cuircuit and the kitchen ring generally needs to be on rcd because it can feasiblly supply outdoor equipment)

    other cuircuits its largely a matter of preference

    normal practice on current domestic installation work is to use a split load CU this has an isolator which switches off everything and a rcd which covers everything that needs rcd protection
  3. unphased

    unphased Screwfix Select

    trowel head

    Been asked before. An RCD (Residual Current Device) is a generic term for several types of devices which protect a circuit against hazards from earth leakage faults. RCD's monitor imbalance in phase and neutral and "trip" when such an event occurs. An MCB (miniature circuit breaker) is a device which protects circuits from overcurrent and short circuits (phase to neutral faults and direct shorts phase-neutral/phase-earth/neutral earth). The modern version of a fuse but not necessarily "better than" a fuse. The fuseboard you describe has a main switch and mcb's but no RCD. There are certain situations where the regs demand the use of an RCD but do a search on the subject and it should be covered.

    Roy
  4. Rabbit Rabbit

    Rabbit Rabbit New Member

    Hiya Trowel Head...

    La that Plug says applies adding...

    MCB = Miniature Ciruit Breaker. RCD = Residual Current Detector. By what you say you have NO RCD's in your current installation only a 2-pole 100 amp main and several 1-pole MCB's. In general terms a 1-pole device switches phase (live) only and a 2-pole device switches both phase (live) and neutral. Your main 100A switch will be 2-pole and the MCB's 1-pole. The nsb06are 6 amp MCB's and the nsb32 are 32 amp MCB's.

    MCB's monitors current flow through the device and 'breaks' if this value should exceed a the specified value (in your case 6 amps or 32 amps).

    And RCD monitors current flowing inphae (live) and neuttral cables. Based on Kirchoff's Second Law what goes in should come out and if it does not - like your fingers on live or neutral! then the RCD will trip. BUT RCD's do not trip if your fingers were between live and neutral, a sort of problem with RCD's

    RR
  5. trowel head

    trowel head New Member

    thanx for such a quick response guys.one last point,could i swap my mcbs to rcds and would this make anything safer?or am i just confusing myself and talking bo**ocks?(moderated by me!)
  6. plugwash

    plugwash New Member

    no you still need the mcbs to provide overcurrent protection

    is your board a wylex standard (plug in breakers) or a modenr din rail based one?

    you could swap the mcbs on the relevent cuircuits for rcbos but it seems wylex only make type C rcbos which raise disconnect time issues

    if its a din rail board has two neutral bars joined by a link and has at least 2 spare ways it may be able to be adapted to a split but i don't know enough about wylex gear to say if this applies to thier boards

    get an electrician in to see you particular installation and get his advice on the matter since you have said you won't be doing the actual work yourself anyway
  7. trowel head

    trowel head New Member

    thanx plugwash,its modern(ish) and the breaker does connect to 2 rails and there are no spare ways,thats about my knowledge!i was just thinking after reading so many posts on rcds that this would be the safest way to go,and assuming you could, do a simple safe swap from mcb to rcd.i thinks i'll stick to bricks! couldn't do the work myself either,been working on the trowel too long,my fingers are like sausages! thanks everyone for there help and advice,much appreciated.
  8. HappyDayz

    HappyDayz New Member

    MCBs, in fact any type of fuse, are only there to protect the installation. Let's face it it only takes a few milliamps to kill. "It's the volts that jolt and the mills that kill". Given that any breaker or fuse in a CU will pull 20 times, say, the current needed to kill before it will operate, you can see that this means there is no intended protection offered by breakers/fuses of any description. Fuses and breakers will protect the users by disconnecting the supply when there is a PN or PE fault but this is only coincidental. With a PN or PE fault something will go bang. The overcurrent devices are designed to be the weakest part of the chain. If they weren't there it would be one of the cables. The real danger is exposed conductive parts that are live.
    Touching an exposed conductive live part will not make any breaker or fuse operate (other than breakers incorporating RCBs) - we are not that good a conductor!

    RCBs were specifically designed to protect users. They operate, as has been said, by detecting when there is only a slight difference between the current in the P & N cores. If there is a difference this means that some current is going directly to earth, possibly through someone's body. This difference in current, typically 30mA, is less than that required to kill someone in ordinary circumstances.

    So if you want to protect yourself, definitely put in a system incorporating RCBs. If you are not too concerned about the people in the house then don't bother.

    Incidentally, other than in a TT situation, where the RCB is really a surrogate main fuse, what is the purpose of RCBs with high (e.g. 300mA) tripping currents? 300ma passing through a person will kill them. Anyone know?
  9. plugwash

    plugwash New Member

    high trip usually time delay rcds are for situations where earth fault disconnect cannot be provided by mcbs alone generally due to a high impedence earth

    could also be usefull for preventing fire when there is a live-earth fault that is not a short
  10. trowel head

    trowel head New Member

    now i'm even more confused happydayz,ta!what is an rcb? this sounds like what i was asking about in my initial post(albeit in a roundabout and confused way!)what i am asking is there a convenient (and easy) way of changing my mcb's to something safer to protect my little 'uns wandering fingers?(i dont give a monkeys about the installation,thats what insurance is for!) or is my set up basically the safest i can get without a full cu change?
  11. plugwash

    plugwash New Member

    rcbos could be fitted if you can get one module wide ones for your brand of CU

    but there is no space for a split connection

    another option is to henly a second small rcd CU into the tails and take downstairs sockets kitchen sockets and a shower if present off it
  12. trowel head

    trowel head New Member

    thanks plugwash,i'm even worse off now,rcbo's!firstly ,what is the benefit of a split connection? secondly,if a potential customer rang you up and asked "what would be the ultimate ,(within reason!)domestic ,safe electrical supply i could have," (main reason to protect my little inquisitive ankle biters)what system would you recommend? ps.these "alphabet" letters are going over my head a tad,but i must reiterate i have no intention of doing any electrical work myself,this is simply a question for advice so (if i need to employ a sparky,dependant on your replies) i will have an idea what i am asking for.thankyou in advance.
  13. The safest... I would have everything over than the lighting on a 100ma RCD in the CU.

    So a new split load CU:
    Main ----> Lighting
    RCD ----> All sockets / showers etc.

    Then I'd put those little plastic plugs in all the sockets that arn't in use.

    Sound good??
  14. HappyDayz

    HappyDayz New Member

    Given that we are talking personal safety here the most effective thing would be to replace the CU with one having an RCD built in, such as Screwfix D60251. You already have the MCBs.

    The incomer on these units is divided into two sections. Half of it is protected by the RCD and half isn't. The half that is protected by the RCB will have put into it MCBs for sockets, shower, cooker etc.. This is the most dangerous stuff; people plugging things in, splashing water about etc..

    It might seem odd to have the other half not protected but RCDs are very sensitive and can sometimes trip for no (apparent) reason. Into this non RCD section goes things that are a) unlikely to be damaged by people messing around and b)where their being out of action, through the RCD tripping for some spurious reason, would be a danger in itself. Into this category go lights, alarms and also sockets dedicated to computers.

    The reason for the lights being in this section is that the dangers through lack of light are greater than the risk of electrocution from lighting circuits. People die from falls everday (not the same people every day [sorry about that, it's getting late]) but very rarely by electrocution from lighting circuits. If a smoke alarm were on a circuit that might trip for no apparent reason, you wouldn't know it was out of action - so not much point in having it! Personally I think they should be slaved off light circuits, if they are not interlinked. The reason for dedicated computer circuits being on the non RCD side of the board is that the switched mode power supplies can cause significant earth leakage which could cause spurious tripping of the RCD. A dedicated computer circuit is only worth doing where there are a number of computers in the same place.

    Lastly, it isn't always plain sailing when you put in an RCD system. Old appliances can cause nuisance tripping, particularly fridges and freezers. My favourite though was caused by a currant (current - gedit!) stuck in a toaster; sometimes it would be damp enough to conduct sufficiently and sometimes it wouldn't. The owner had put up with this for months without realising what was going on!
  15. Lectrician

    Lectrician Screwfix Select

    Agree with most of that, except cookers. These are really 'earthy' 'leaky' items due to the amount of elements etc in them, these inheritly trip the RCD.

    Best keep the cooker on the non RCD side.

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