One instance was a fully insulated piece of equipment, with *no* realistic chance of leakage to earth, and the installation was relatively new and fully tested. In fact, replacing the equipment with an identical item from the same manufacturer produced the same result. It must in essence be a defective design of RCD, the makers have probably sorted that out now, but it's not the only time I've come across this phenomenon.
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
But surely if the line & earth conductors enter & leave the toroid in slightly different paths, they would have slightly differing effects on the secondary? As I said, it's a suggestion that was given as a possible explanation for the problem of surge currents causing the RCD to trip. It seemed plausible to me, not much else did.
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
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.
Actually it's the Phase and Neutral conductors that pass through the coil, not the CPC (earth), but leaving that aside it's a fundamental principle that if you pass a wire through a core you get the same affect on the core what ever you do with the wire, at least at the low frequencies we're talking about. But I can well understand why that sounds wrong since I've come across design engineers who don't understand it 8-).
By the way I used the official terms Phase and Neutral here to avoid confusion since officially both the brown and blue wires in a cable are called 'live' conductors which can cause utter confusion 8-).
Sorry, I meant Line (deliberately not 'Live' ) and Neutral. Don't know why I put Earth. (hurrying to go out!)
On whether the disposition of the primary 'coils' can affect the outcome, my gut feeling would be that if the primary conductors were close to the secondary coil they might have some effect other than through the core but that is no more than a gut feeling. I used to know a little bit about these things but it's all a bit faded now :-(
'Officially' has a lot to answer for
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
It's true that any two conductors have mutual inductance and also capacitance so in principle yes, they can affect each other, but an iron core increases the inductive coupling between two conductors by such a huge amount that it swamps these direct effects at mains frequencies and all that matters is how many times a conductor passes through the core. It's a different story at higher frequencies though, the layout of the transformer then becomes critical.
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