OT : Cheap motor trips RCD

wrote:


A lot of domestic 'RCDs' are actually RCBOs. They will trip on over current as well as residual current. Unless specified differently they will also trip in 200 milliseconds at rated tripping current or less than 40 milliseconds when 5 times rated current is applied. This means that if the motor is ok but on the large side for the DCD, it may well take it out on starting anyway. may not be the OP's problem, but it is one that got me until I went to a re-rated solution in the garage.
Mark Rand RTFM
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The RCD part of the RCBO will indeed trip within the times you quote in response to a residual current, i.e. at 30mA and 150mA respectively, but a motor surge is not a residual current which is the point I've been trying to get across, if the motor insulation is not leaking then the surge is identical in the live and neutral so will not trip an RCD that is working properly.
The MCB part of the RCBO could well trip on the motor surge but at much larger currents and times. A typical 32A RCBO as fitted to domestic rings is the equivalent of a type B MCB and will not trip within several hours at 45A !, even at 100A it will take about 35 seconds to trip, you have to take it over 160A to get the magnetic part to operate and guarantee to trip in less than 5 seconds, though it's allowed to be as short as 0.1 seconds at that point.
If you do have a genuine problem with a motor tripping you can fit a type C or even type D which have the same thermal characteristic as a type C to protect the cable but higher magnetic trips to allow for such surges, but if you're going to fit one of these there are other considerations so you should seek advice.
Greg
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On Wed, 14 Jun 2006 19:34:15 +0100, "Greg"

Single phase, start coil, run coil. 1 + 1 = 2 -- Regards,
John Stevenson Nottingham, England.
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On Wed, 14 Jun 2006 21:11:10 GMT, John Stevenson

Ah, I was talking about the RCD coils, John's obviously thinking of the motor coils. Not sure which coil (singular) Greg is talking about <G>
Cheers Tim
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Show me an RCD that has a start coil and a run coil...I think you must be confusing it with something else. An RCD is a Residual Current Device, previously reffered to as an ELCB Earth Leakage Circuit Breaker, it's not a motor starter or anything like that.
If you're refering to the motor in question having start and run coils, yes it may well do, or may just have two run coils and a phase shifting run capacitor. An imbalance between these coils will definitely not trip an RCD, the only thing that will is if some of the current that flows from the live does not flow back into the neutral which in practice means it has to flow through a leakage path to earth.
If the said motor has such leakage of sufficient magnitude to trip an RCD then it is faulty and the leaking insulation is likely to degrade further if you continue to use it, with the potential to start a fire. This is the reason, for example, that the regs require RCD protection on farms and other industrial places where combustible materials are to be found.
If John has such a faulty motor he should return it, not blame it on the supply and bypass the RCD which may be warning him of a potentially dangerous situation. An electrician will be able to tell in seconds by the use of an insulation tester, usually known as a Megger.
Greg
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Get an electrician to do an insulation test on the pump, it's most likely a leak from the windings to the frame which is exactly what an RCD is supposed to detect.
Greg
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