They are magnetic and like TNP said the Live and Neutral are woud around the same core when there is suffcient current imbalance the resultant magnetic field pulls the latch set when the device is set to "on" and a spring causes it to trip off. AFAIK they are not held on by power otherwise everytime you had a power cut you'd have to reset them...
The only CU RCD here is the same temperature as the MCBs next to it, at least by touch, ie cold.
Quite simply ohmic losses in the coils.
Ahh - we might be talking about different things. I'll try and clarify.
All the RCDs I have used are not consumer unit mounted ones, but either
a) Unit placed between a 13A plug and socket b) Unit replacing 13A plug
I have found that they can be obtained with a feature of being either
a) latching - if power is restored after a power failure, the RCD will remake the circuit b) non-latching - if power is restores after a power failure, the RCD will NOT remake the circuit
The non-latching variety I have used require a button to be pushed to make the circuit - it seems to arm a spring loaded mechanism. The latching variety I have do not have a spring loaded mechanism, but a pair of microswitches, one to test, and one to make the circuit after testing or detection of a residual current. This latching variety (of which I have many examples) gets warm even when passing no load. The non-latching do indeed have to be reset after a power failure - which would be a right royal pain if such an RCD were feeding a freezer and you happened to be away on holiday.
Now, my understanding of RCDs (which is imperfect) is that the sense coil is wound around the (straight) phase and neutral conductors, and some RCDs electronically process the output of the sense coil to allow features like delayed activation, activation when the current is not sinusoidal, and to prevent activation on normal inrush currents (and possibly ofr other reasons). Some RCDs use the output of the sense coil directly to activate a solenoid which triggers a spring loaded mechanism to disconnect the circuit. The description you have of the phase and neutral being wound round a core may be a variant of the latter - possibly used in consumer unit RCDs.(Idle thought - would it trip if there were an excess of current in the neutral?). AFAIK, they are only guaranteed to work within specification on sinusoidal currents.
So, my imperfect knowledge of consumer unit RCDs may be leading me to make the wrong conclusion. My understanding of consumer unit RCDs is that they are routinely of the latching variety (i.e. will remake a circuit on restoration of power after a failure), and that therefore they will be dissipating power at zero load, as my assumption is that _all_ latching RCDs disspate power at zero load. Either or both of my assumptions could be wrong, and I would be grateful if someone with better knowledge (and hopefully URLs to demonstrate this) could elucidate. Most of my Google searches end up at Wikipedia, which can hardly be regarded as authoritative on this matter.
Cheers,
Sid
I think you'll find that the latching type stay set, like a CU RCD if the power fails rather than make/remake the circuit on power/power loss. The non-latching ones drop out (trip) when the power fails, they may well consume a little bit of power to hold the latch in. I think that all plugin type RCDs these days are supposed to be the non-latching type, it's safer. Means that if are using a portable power tool and the power goes off it doesn't come back on with the power.
All the ones I have are spring loaded set mechanisiums, one I have looks like a small microswitch push button with only a small amount of movement but it still sets a spring loaded latch.
A possibly inaccurate a sense coil around the conductors is much more likely from the enginerring POV, the size of wire required to carry, say
80A, is not conducive to making small compact coils... I've not had dead RCD to disect. B-)They should, any imbalance between the send and return should trip an RCD.
I have three latching RCDs that interrupt and remake the circuit on power loss. They are very useful (but not on power tools, obviously). If all plug-type RCDs are meant to be non-latching these days it explains why I can't find more of them!
Could be. None of mine have died and I haven't succumbed to the temptation to take them apart to see how they work.
That is what I would have thought too, but 'The Natural Philosopher' thinks otherwise in this thread.
'Should' is a great word. I may be being simplistic, but an excess of current on the neutral conductor rather than a deficit will presumably reverse the direction of the current output of the sense coil, which, if is being fed into a solenoid, would reverse the direction of the generated force. This can be accommodated mechanically with suitable mechanism design, but without tearing a few RCDs apart and testing, I don't have an easy way of determining what's going on. I would hope that when people write 'imbalance' that is what they mean, rather than 'excess of phase over neutral', but sloppy thinking and wording are rife (I'm no exception).
Although we are dealing nominally with AC here, the target disconnect time of 30ms not much different to one standard mains AC cycle of 20 ms, so 'steady state' assumptions and calculations are not necessarily valid. The inductance of the sense coil and solenoid, and the effect of any electronics probably needs to be taken into account, and I'm definitely not qualified to work out exactly how it all works.
Thank-you very much for taking the time to write your previous considered reply.
Cheers,
Sid
Ah, yup that does make a difference.
Normally when discussing RCD for circuit protection we are talking about the DIN rail mounting type (last two pictured here):
The NVR type often include a solenoid to hold the device "on" once powered up. I could certainly understand these getting a little warm in operation - even NVR switches without a RCD function can do this under load.
Indeed. The standard CU type latch - but the latching is done with a permanent magnet.
My understanding also...
The solenoid when activated is in effect reducing the mechanical attraction of the permanent magnet, thus allowing the spring loaded mechanism that is working against the magnet to pull the contacts open.
As in greater neutral current than phase? Yes that will trip it.
Depends on the type you specify. MK for example have a number in their range designed for different conditions like pulsing DC etc:
nope, see notes above and also:
Thank-you for your reply, John; and the link to the d-i-y wiki.
Sid
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