RCBOs are remarkably constant in that respect.
The fist record I have picked - 7 RCBOs
1 at 0........all 7.6 to 8.9ms
1 at 180....all 18.2 or 18.4
5 at 0........all 8.1 to 8.7
5 at 180....all 18.0 to 18.4
RCCBs less so.
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The fist record I have picked - 7 RCBOs
1 at 0........all 7.6 to 8.9ms
1 at 180....all 18.2 or 18.4
5 at 0........all 8.1 to 8.7
5 at 180....all 18.0 to 18.4
RCCBs less so.
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Thanks. That's exactly what I would expect (and corresponds to what I've seen in my limited experience) - as I said, it's primarily a question of how quickly the mechanical mechanism can 'go clunk', and that's not going to be influenced by the magnitude of the fault current. Higher currents may 'activate' the trip mechanism slighly more quickly, but I doubt that's a very significant effect.
Kind Regards, John
Seems I have to stand corrected. A quick look seems to show that most devices these days don't even mention trip times (or max trip times) - though I'm sure they used to. Perhaps it's one of those things that's covered by some standard and so no-one feels the need to quote it any more - a bit like stating nothing more than "B curve" ?
The principal still stands, for discrimination you need differing trip times - such that the upstream device won't initiate a trip cycle for a fault duration less than that required for the downstream device to have opened it's contacts and disconnected the fault.
What is more interesting about the numbers is the asymmetry. Makes me wonder about the nature of the circuit that's doing the measurement since the sort of circuit I'd be thinking about would naturally be symmetric.
The principal still stands, for discrimination you need differing trip times - such that the upstream device won't initiate a trip cycle for a fault duration less than that required for the downstream device to have opened it's contacts and disconnected the fault.
Very similar to what I've seen with the RCBOs I've used.
What is more interesting about the numbers is the asymmetry. Makes me wonder about the nature of the circuit that's doing the measurement since the sort of circuit I'd be thinking about would naturally be symmetric.
Possibly, although I really don't know what trip time they would quote as a 'rating'. Despite what you said about "pretty much every domestic RCD you'd seen" having a 30ms trip time, I really find it very hard to believe that any would ever have been 'rated' at such a trip time. Having said that, I've got some pretty old ones in my drawers, and (having had a quick look) I can't see any which bear any reference to trip times (any more than do MCBs) - so I'm not convinced that they ever did have a 'trip time rating'.
Sure, but that's totally different. You're now presumably talking about a time-delay deliberately introduced so as to provide discrimination (as in a Type S, or 'TD' RCD), regardless of the magnitude of the fault current. Particularly given that the magnitude of 'fault currents' (particularly those passing through human beings) is widely variable and unpredictable, you could not usually get useful/reliable 'discrimination' between RCDs in series by using devices of different IΔn (e.g. 30mA and 100mA).
What 'asymmetry' are you referring to?
Kind Regards, John
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Look at the trip times EFLImpudence gave. There's a very distinct difference between trip time for a fault that starts at 0˚ and one that starts at 180˚. It occurred to me last night while trying to get to sleep, that the difference is around 10ms, which is the period of half a cycle. Looks very much like these devices only trip on one half cycle - meaning that if the fault starts in the "wrong" half cycle (ie after 180˚
then there's an extra delay until you get into the right half cycle where the device can operate.Possibly the sensing works over the full cycle but the energy to operate the trip mechanism isn't stored and that's unidirectional.
OK, I understand your point now. This is an issue which has been discussed at length in this and/or other forums.Look at the trip times EFLImpudence gave. There's a very distinct difference between trip time for a fault that starts at 0˚ and one that starts at 180˚. It occurred to me last night while trying to get to sleep, that the difference is around 10ms, which is the period of half a cycle. Looks very much like these devices only trip on one half cycle - meaning that if the fault starts in the "wrong" half cycle (ie after 180˚
In the case of non-electronic RCDs/RCBOs (if any are still being made – I’m still not sure), it makes no sense that there could/should be any systematic difference in trip times in response to fault currents which starts at the low-to-high zero-crossing point of the cycle (“0°”) as compared with one which starts at the high-to-low zero-crossing point (“180°”). One would therefore not expect to see any systematic difference between “0°” and “180°” trip times.
With ‘electronic’ ones, it has often been observed that trip times in response to fault currents starting at the ‘low-to-high’ crossing point (i.e. “0°”) are generally about 10ms (i.e. as you say, half a cycle at 50Hz) shorter than the trip times for fault currents starting at the high-to-low ("180°") transition point. As you say, the most likely speculative explanation is that the electronics ‘ignore’ the negative half cycle and only respond during the positive half cycle.
If that’s true, why would it be? One can speculate that it’s probably fractionally cheaper to produce electronics which only look at the positive half cycles. If, as they do, these devices achieve trip times well within the required limits even with a fault current starting at the high-to-low transition point, the manufacturers may therefore ‘accept’ that fractionally cheaper approach. If so, it’s a little naughty, since I’m sure that a person through whom current is flowing would welcome as short a shock duration as possible on every occasion (even if their shock starts at a ‘bad point’ in the cycle!), even if the duration is already within acceptable limits.
Is there, I wonder, perhaps a difference in this respect between expensive and ‘budget’ realisations of these devices? EFLI, do you ever see RCBOs which do not show an appreciable difference between 0° and 180° trip times?
Kind Regards, John
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