OT : Cheap motor trips RCD

John it sounds like a bad supply to me, there should be no reason for the unit to trip out the breaker.

I picked up a £37 submersable pump from Machinemart all plastic with a float vavle for on off and it works fine.

Satisfy yourself all is ok with the trip and report the unit back to the suppliers.

Adrian

It may have some form of filter which maybe fooling the RCD that there is earth leakage.

Some RCDs will trip with large surge currents, I believe it may be something to do with a slight imbalance of the two coils which is not enough to cause a problem at the rated current. Single phase induction motors have a starting surge of several times the rated full load current. If you can try it in another supply with a different make RCD of the same trip rating, it might prove something. Or not.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

They shouldn't if they're working properly.

They don't have two coils, they have a single one with both live and neutral passing through.

Greg

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

There's no reason to believe that the supply is in any way at fault from this report, by far the most likely scenario is that the pump is faulty since badly made or old motors are very prone to leakage.

Greg

Impossible shurely?

Anyway, the phenomenon I described, of RCDs tripping with surge currents, does sometimes occur. That's the only halfway reasonable explanation for it that I've heard.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Single phase, start coil, run coil. 1 + 1 = 2

-- Regards,

John Stevenson Nottingham, England.

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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

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Bad wording but I meant he had been supplied a bad pump!

Not only possible but also the only practical way to measure a residual 30mA on conductors carrying tens of amps. Even in big industrial installations they can pass all the bus bars through a single sense coil.

An RCD contains a contactor that's closed against spring pressure by the user and latched, a trip solenoid releases the latch under the control of a circuit board. A single coil of several hundred turns is wound on a (usually) toroidal core through which pass the phase and neutral conductors (all 4 on a three phase type). When the conductors are balanced there is no current in the coil, when unbalanced due to a leakage path to earth in the load there is a current in the coil which is detected by the circuit board which operates the trip solenoid if the threshold and time delay is met.

I've run welders from RDCs on several sites with no problems, the MCBs can trip but an RCD should not and is suspect if it does.

However, there is such a thing as a combined MCB and RCD, called a RCBO, which will trip on surges like any other MCB that's taken past it's magenetic threshold. They're typically used when it's necessary to upgrade a single circuit to have RCD protection in an existing installation, the latest regs require this on downstairs ring mains for example. These look confusingly like a RCD to the untrained eye so that might explain your experiences.

Greg

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

While you are quite right that some equipment has EMC filters that produce leakage current to earth, the law requires any that are designed to be plugged in to have a leakage many times less than that required to trip an RCD. Only equipment that is designed to be wired in can exceed this limit and then it must have very clear warnings about the dangerous leakage. In practice only a huge piece of industrial equipment would ever exceed 30mA, though a room full of smaller equipment such as a computer suite could do in an extreme case.

Greg

That's certainly the most likely scenario, though it wouldn't hurt to get an electrician to test the RCD at the same time as the motor, they have a piece of kit that will fully exercise it.

Greg

Whether it constitutes a 'coil' or not, the toroidal 'core' you describe is the primary (well, multiple primaries) of a transformer. It's an imbalance of these primaries (coils) to which I was referring.

You gave the impression that the line and neutral were passing through the same coil in as much as the current went through the same wire. That's what I said was impossible

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

No.

Tim Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

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

Actually the toroidal core is not the primary, it's the CORE as I said. The thick wires from the the live and neutral terminals can be considered as primaries, but as they are only one or two turns there is no possibility of them being unbalanced since it's a fundamental fact that you can't have a fraction of a turn on a toroidal cored transformer. The coil, or secondary, consists of several hundred turns of enamelled copper wire wound around the same core, evenly spread around it, thus the live and neutral wires do indeed appear to pass through the coil.

Yes that would be impossible!

Greg

Then I would suggest that you get the equipment which causes these trips checked out because it could in fact be faulty with a borderline leak and the RCD could be doing it's job.

Greg