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Type A rcd . EICR coding ? etc

aligarjon
Hi Guys.   Not been on for a long time, just had a bit of a search and couldn't really find anything so thought i would ask and see what you all thought.


1.  Are we or will we be coding type AC rcd's if there are LED's or induction hobs, lots of electronics  etc  present.

2. How much DC leakage does it actually take to saturate an rcd and cause  problem?

3. How much does a standard LED lamp or induction hob  leak ?

If we test an AC RCD with no load and it's fine then re-test it with all LED lights, induction hobs etc turned on and it operates correctly could we then say that it is ok with a note on EICR  OR EIC if installing any of the above.  


Obviously also on an EICR if the RCD then doesn't operate with it all on it becomes a C2 ?


Any thoughts



Gary
  • 0
    davezawadi (David Stone):

    The interesting effect is that the leakage seems to cause an increase in trip time, rather than a failure to trip when using my MFT to test.


    Smoking gun? Increase in trip time could well mean not tripping in a time (for additional protection) that saves someone's life. The trip times for RCDs are based on roughly a P90 demographic ("protects about 90 %") of adults.


    Of course, with most MFT it's perhaps not possible to generate a fault of negligible impedance (resulting in the machine recording a trip time) downstream of the rectifier's output? If I can think of a way of doing this, I'll post back on that.


  • 0
    I have spent a couple of hours yesterday experimenting with RCDs, rectifiers, and various degrees of leakage from the rectifier DC to Earth. The interesting effect is that the leakage seems to cause an increase in trip time, rather than a failure to trip when using my MFT to test. A different RCD tester shows much less effect although they both give similar results with a normal unloaded circuit and RCD. I am using a 1kW lamp load on the rectifier output, so the RCD transformer is actually having to do some work, rather than the unloaded condition. This is an attempt to demonstrate a real circuit rather than the unusual test condition of zero load.

    Well done that man!

    5k resistance increases trip time to a second or so, but does no prevent a trip.

    Although for additional protection 1s is a lifetime...

    Of course, with most MFT it's perhaps not possible to generate a fault of negligible impedance (resulting in the machine recording a trip time) downstream of the rectifier's output?

    I presumed Dave had the MFT connected to the a.c. supply side (emulating the situation where additional protection is needed on the cables supplying the boiler) rather than on the d.c. side which had a fixed 5k (or whatever) fault.

       - Andy.
  • 0
    Andy, I think Dave was trying to see what level of "rectified AC" might muck up a Type AC RCD? Which I think is a valid point ... not all fault in electronic equipment will be faults of negligible impedance, although faults on the output of controlgear (such as inverters, higher current power converters, VSDs or other power controllers) are a different issue.
  • 0
    Such tests are very interesting - its a pity the manufacturers do no do something similar and stick to the prescribed tests only. I'd be very interested if you ever get time to write them up.


    As regards synchronising/ triggering and mis-using an MFT,

    If you return the earth core of the 3 wire tester to neutral, it will never trip the RCD, looking instead like a normal load, but you could use the current in that lead as a  trigger signal to fire an SCR or similar to create a different sort of fault further on.


    This is really a job for a dual channel  oscilloscope and some suitable rated voltage dividers however, as the other problem is that the tester looks for falling L-N voltage to decide the RCD has tripped and once you start adding reservoir capacitance or external DC sources, the voltage may be held up from the load end,  and that  very simple 'has the power gone' detection is not to be relied upon any more.

    regards Mike
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    As far as test gear goes Mike, I have a whole room full, and 3 various digital oscilloscopes, with 8 channels between them and enough bandwidth to see your local TV transmitter. I am not at the scope stage yet, and I will probably video the results when I have found what there is to know. No point in doing it now as otherwise I just get hours of video which is hard to edit properly.


    The comment about additional protection from Andy is a point, but how does a person contact anything inside the boiler, an Earthed metal box? If the boiler leaks it does trip so far, but that is just normal Earth leakage protection which is not personal protection from a class 1 appliance, and time is much less important. I am expecting that leakage of more than an amp or so should pop the appliance's internal fusing, as it is clearly an internal fault. Please don't mention the heating man working live, that is his problem, not the consumer and clearly he should be using the correct PPE, and of course be fully trained to do this. I am still very much at a loss as to how this kind of fault under discussion occurs, the LV directive makes it pretty much impossible unless there are multiple problems. Think clearance distances, insulation, etc.


    My tests are rather different as I said above, I use the tester on the mains supply after the RCD, 3 wire as normal, and a DC 4.5A load and a resistor from the DC+ to Earth. Nothing unusual from the case where one would expect the RCD to operate. I will look at the current waveforms later, and check what happens with leakages from + to Earth greater than about 45mA when I get some lower value resistors. The test bench supply is direct from a TNC-S supply without an RCD.


    An interesting point about this internal pump, have you a circuit diagram of how it is connected? Any external wiring should either be mains voltage and Earth, or a SELV supply either isolated to class 2 or one side Earthed.
  • 0
    The pump in the Worcester 2000 is mains fed Dave. It has an LED display on it to display fault codes. it must have an AC/DC converter within it since the motor is DC apparently. Cant find an internal wiring diagram
  • 0
    Leaky boiler ?
  • 0
    The comment about additional protection from Andy is a point, but how does a person contact anything inside the boiler, an Earthed metal box?

    I think the worry about additional protection is that the boiler could remove additional protection from everywhere downstream of the RCD - so picture nail into a cable upstream of the boiler (or elsewhere in the house if the RCCB is covering many circuits) and additional protection might be ineffective.

       - Andy.
  • 0
    AJJewsbury:

    I think the worry about additional protection is that the boiler could remove additional protection from everywhere downstream of the RCD - so picture nail into a cable upstream of the boiler (or elsewhere in the house if the RCCB is covering many circuits) and additional protection might be ineffective.


    I see three issues:


    (1) The boiler, or at least it's DC modulating pump will trip a type AC RCD in normal use. I assume the electronics are rather like the front half of a VSD, so that is perfectly possible. That would be obvious during commissioning.


    (2) A fault in the boiler might not trip a type AC RCD. I think that we have covered that one. If the boiler does not require RCD protection, then this possibility isn't a problem.


    (3) The boiler might blind an RCD, either in normal use, or under some fault condition which does not trigger ADS.


    I have asked Schneider whether their type AC RCDs are compatible with DC modulating pumps and I have asked Worcester to explain - i.e. to answer my original question.


    All this does seem to be a bit beyond the call of duty. Perhaps the householder should have told the plumber to fit a boiler which can cope with a type AC RCD? ?


  • 0
    The Worcester Bosch 2000 installation instructions don’t seem to to be explicit when it comes to the choice of RCD.


    How do you know it doesn’t need a type B or F RCD?


    4.2 Mains supply

    4.2.1 Electrical supply

    • Supply: 230V AC - 50 Hz

    • This appliance must not be connected to a three phase supply.

    • The wiring between the appliance and the electrical supply must

    comply with the latest IET wiring regulations that apply to wiring a

    fixed appliance for Class 1 equipment.

    • The correct type of RCD must be employed where additional

    protection is required that is suitable for a low energy DC modulating

    pump according to IET wiring regulations.

    • External 3 A fuse to BS1362.

    • The appliance must be earthed.

    • Cable: PVC insulated 0.75mm2 (24 x 0.2mm) temperature rated to

    90 °C, to BS EN50525.

    • Any additional components that are connected to the appliance with

    mains electrical supply must not have a separate supply.

    • Additional equipment wired to the appliance m comply with the

    latest IET wiring regulations.

    • Appliance protection rating - IPX4D


    https://www.worcester-bosch.co.uk/professional/support/literature/worcester-2000-installation-instructions-1


    It seems to me the answer is to omit RCD protection by hard wiring it on its own circuit installed in a manner that doesn’t require the inclusion of a RCD.


    However potentially from next year onwards a new boiler circuit will need an AFDD, which may incorporate a Type A RCD as well as a MCB, so installing a boiler supply may cost several hundred pounds rather than a few quid for installing a simple SFCU to an existing circuit supplied by an obsolete consumer unit with a Type AC RCD that cannot be upgraded.


    There’s a lot to be said for specialising in installing 40 amp circuits for resistive loads 

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