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RCD's and RCBO's...Whats the difference you may ask?

Discussion in 'Electrics UK' started by Big_Spark, 21 Feb 2004.

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  1. Big_Spark

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    I read on the other Electrical forum an old discussion about RCD's and American GFI's. In this discussion it was stated that RCD's do not give Overload protection, however I would like to point out that this is wrong, RCD's DO give overload protection, only basic, but it is there.

    For example, a 100A 30mA RCD will trip if the difference between the load on the Phase conductors (Live and neutral) exceeds 30mA or if a 30mA or higher current is detected on the CPC (Earth). Further, if the total load being drawn through the RCD exceeds it's maximum rating, then the RCD will trip and disconnect the circuit.

    However, and this is where many people get confused by this, a standard RCD does this by electronic determination of the load current via the detection circuitry unlike the MCB's and RCBO's. The torus forming the heart of the RCD has a small detection circuit attached to it via an op amp. The magnetic field detected by this circuit is carefully measured and if it exceeds a certain thresh-hold, as determined by the maximum load rating, then it will disconnect the circuit

    To clarify this, RCBO's combine the operation of the MCB and the RCD into one unit. The RCD part operates in exactly the same way as the standard RCD, however for overload protection the RCBO contains magnetic or thermal trips, in good quality units, such as MK, Merlin-Gerin and Hager, the RCBO contains BOTH thermal and magnetic trips, just as an MCB does.

    It is these differences that dictate the use of such devices. RCBO's are used to protect individual circuits that need that extra level of safety, but overload protection is still a vitally important aspect, whereas RCD's are useful, in certain circumstances, for replacing Isolators in Consumer units, Distribution Boards etc where effective disconnection may be required in an emergency or for maintenance etc, but overload protection is not required.

    I hope this clarifies the difference between these devices, a proper understanding of the devices will certainly help to determine their proper use.
     
  2. il78

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    I thought you were telling us a joke.......

    Any-hoos

    Two Neutrons in a bar, one of them asks the bar tender for pint of lager then changes his mind to a pint of bitter, then changes his mind again to lager, bar tender asks him if he is "Positive".....

    I crack myself up....... :confused:
     
  3. Big_Spark

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    il78 I think you need to get a more interesting job :D
     
  4. securespark

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    Aren't RCBO's DC sensing, whereas a lot of RCD's are not??
     
  5. Big_Spark

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    Generally I would say you are correct, to comply with British Standards, all RCBO's must be DC sensing, however with RCD's the picture is two fold as they can be manufactured as DC sensing or Non-DC sensing.

    You need to look at the installation carefully to determine the exact type you want, there are many factors to take into account, not least the environment the circuit will operate in.

    For what it is worth, I have noticed lately that a number of manufacturers have started refering to Larger DC Sensing RCD's also as Main RCBO's, I suppose this is an attempt to clarify the whole situation and to ensure that Installers pick the correct protective device.
     
  6. ellk9203636nib

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    What a lot of crap. since when did an RCD give overlaod protection.
    An RCB on it own will give earth fault protection. The magnetic coil senses an imbalance due to the live conductor increasing its current due to an earth fault and trips.
     
  7. ColJack

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    2004 dude.... check the last date stamp next time...

    and welcome to the forum...

    :D

    and in line with the above poor joke..

    Two neutrons walk into a bar.. one of them asks the bartender "how much for a pint of vodka?", the bartender replies "to you, no charge..."..

    :rolleyes:
     
  8. hairyben

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    RCD's DO NOT provide overload protection.

    Whether or not this circuit you speak of will operate the device under excess load, it's not recognised or listed in 7671 as an overload device, and an RCD should be used in conjunction with suitably rated fuses or mcbs.

    Don't mean to shoot you down mate but this is a forum for DIYers, don't want anyone being mislead ;)
     
  9. LiamPope

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    Out of interest, was that OP true (in 2004, or even now) or a load of bollarks?
     
  10. holmslaw

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    He was wrong.

    I typed a long explanation but some how deleted it.
     
  11. Risteard

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    It was wrong on several counts.

    He referred to "live and neutral" as "phase conductors". That should have been phase and neutral being live conductors. (Now line and neutral since the 17th Edition came out.)

    He also referred to difference between RCDs and RCBOs. RCBOs ARE RCDs, and as such it follows that some RCDs (namely RCBOs) provide overload protection.

    RCCBs do not.
     
  12. ricicle

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    An RCBO is an RCBO not an RCD (although it does incorporate an RCD)

    The RCBO has two parts an RCD and and MCB.

    It is the MCB part providing overload protection, not the RCD part - hence some RCDs DO NOT provide overload protection and to say such will only confuse those trying to understand the difference.....
     
  13. Risteard

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    An RCBO most certainly IS an RCD.

    Look up Definitions (Part 2).
     
  14. ban-all-sheds

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    But that does not mean that an RCD provides overcurrent protection.

    Look up Definitions (Part 2).
     
  15. Risteard

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    An RCBO is an RCD and provides integral overcurrent protection, ergo certain types of RCD do provide overcurrent protection like I said. This does not imply that it is the RCD function of the RCBO providing it, of course.
     
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