For example, 110% of 230= 253 volts. Assuming L and PE are of the same size and material, indirect contact touch voltage is 126.5 volts. Would 0.33 seconds not appear more realistic?
From IEC 61200-413
mapj1:
come on, roll one 6 on the dice, 1 in six chance, throw 2 dice, 36 possible outcomes, of which double occurs once - its got to be a product...
Mike.
The related one to think about is to throw one coin, and you cannot say anything about heads or tails, but if you throw a bucket of identical coins, the odds of anything more than a few percent off from having half of each, is very unlikely, you just do not know which ones..
So how many sides does a possibly faulty R.C.D. have then Mike?
Z.
ProMbrooke:Zoomup:
R.C.D.s have saved many lives. If they are not totally reliable then use two in series to half the perceived risk. Are M.C.B.s a hundred per cent reliable?
Z.Why can't the MCB be the backup? A series RCD would either have to be in literal series, or a delayed trip type main with a higher ma threshold.
Cos the M.C.B. is a greedy beast that requires much current to operate it, and sometimes very sluggishly. Whereas a R.C.D. needs a very small current to operate it and at a very fast speed indeed. If our main concern is touch Voltages and shock protection then the R.C.D. has better credentials. There is no reason not to have two 30mA instant R.C.D.s in series if safety is the main concern.
Obviously this does not preclude the use of a 100mA S type R.C.D. supplying a 30 mA instant type R.C.D.
Anyway, I try to avoid touching any Voltages, nasty possibly hurtful tingly stuff. Avoidance is better than a cure.
I have found that the few M.C.Bs that have I have seen become faulty, have normally failed open circuit or would not re-latch on. I suppose that they could get their contacts welded closed on rare occasions.
Faulty M.C.B. Faulty British General circuit breaker autopsy. - YouTube
Well, perhaps bugs or dust getting inside a protective device could cause danger due to slow or non operation of the device. I think I will remove all M.C.B.s and fit B.S. 3036 protection instead, or B.S. 1361 cartridge fuses. Much more robust and reliable.
Of course the old Crabtree P60 type E.L.C.Bs were of very high build quality and made like a brick outhouse. They would not fail if a bit of light dust or the odd bug got inside. They are the true benchmark of quality, unlike the cheap, vulnerable, plastic, miniaturised stuff of today.
Z.
Zoomup:ProMbrooke:Zoomup:
R.C.D.s have saved many lives. If they are not totally reliable then use two in series to half the perceived risk. Are M.C.B.s a hundred per cent reliable?
Z.Why can't the MCB be the backup? A series RCD would either have to be in literal series, or a delayed trip type main with a higher ma threshold.
Cos the M.C.B. is a greedy beast that requires much current to operate it, and sometimes very sluggishly. Whereas a R.C.D. needs a very small current to operate it and at a very fast speed indeed. If our main concern is touch Voltages and shock protection then the R.C.D. has better credentials. There is no reason not to have two 30mA instant R.C.D.s in series if safety is the main concern.
Obviously this does not preclude the use of a 100mA S type R.C.D. supplying a 30 mA instant type R.C.D.
Anyway, I try to avoid touching any Voltages, nasty possibly hurtful tingly stuff. Avoidance is better than a cure.
I have found that the few M.C.Bs that have I have seen become faulty, have normally failed open circuit or would not re-latch on. I suppose that they could get their contacts welded closed on rare occasions.
Faulty M.C.B. Faulty British General circuit breaker autopsy. - YouTube
Well, perhaps bugs or dust getting inside a protective device could cause danger due to slow or non operation of the device. I think I will remove all M.C.B.s and fit B.S. 3036 protection instead, or B.S. 1361 cartridge fuses. Much more robust and reliable.
Of course the old Crabtree P60 type E.L.C.Bs were of very high build quality and made like a brick outhouse. They would not fail if a bit of light dust or the odd bug got inside. They are the true benchmark of quality, unlike the cheap, vulnerable, plastic, miniaturised stuff of today.
Z.
Sluggishly? With proper loop impedance an MCB will directly unlatch and open in 1.5 cycles. An RCD on the other hand must rely on unlatching a magnet, which then triggers a larger opening mechanism. Rather complex. It can jam. Any electronics can fail.
High currents are a very good thing, especially when they reduce touch voltage.
Fuses do not clear as fact as breakers during moderate fault currents. For example, Table 41.1 requires 0.4 seconds, and with a fuse that is possible. However, with an MCB the thermal portion typically can not respond fast enough thus a solenoid is employed. Instant pull in, no need to wait for anything to heat up first.
ProMbrooke:AJJewsbury:
I suspect it's a mix of history and standardization with a bit of finger in the air approximation thrown in. I believe that 0.4s was agreed on for continental 220V supplies (110V touch voltage if equal sized line/c.p.c.) and we in (then) 240V land adopted the same - arguing that our per-installation bonding would likely reduce the touch voltage inside the building to well below 110V levels. There's a similar argument for permitting reduced c.s.a. c.p.c.s. which otherwise can similarly result in higher touch voltages. Portable equipment outdoors should be covered by a 30mA RCD so have a faster disconnection time anyway.
There are so many unknowns - body resistance, resistance of contact with the general mass of the earth, actual supply voltage, droop in supply voltage due to the short circuit that occurs during a L-PE fault on a TN system, exact effect of main bonding, not to mention variation between individuals - that it's far from being an exact science. There's likely some approximation gone into that table above - nice round whole numbers seem rather unlikely for real life.
- Andy.I'd say so by the looks of it.
Agreed, too (that it's give or take a tens of percent of more, and much about convention).
A thread from about a year ago ( 5 s disconnection times ) has a plot showing the time/voltage curves that IEC TR 1200-413 1996 considers acceptable for normal conditions or for 'particular' (non-dry) conditions. IEC TR 1200-413 is a report that gives explanations of the thinking behind the shock-protection part of the IEC wiring regulations at that time. The voltage for 0.4 s disconnection is claimed to be based on 80% of 50% of the source voltage, with some rounding. The 80% factor comes from assumptions about bonding and the relative influence of internal and external cabling; as you mention, the protective conductors are assumed to be as big as the live conductors, which may be true for most of the countries involved in IEC committees but is not common in the UK.
So - as discussed in that thread - the IEC reasoning behind 0.4 s can easily be severely wrong in UK conditions. Yet people still work rigidly to 0.4 s. Fortunately the times will often be much quicker anyway, particularly when there are RCDs. And there's plenty of conservatism behind the chosen voltage/time curves, at least as long as dry conditions are true. I haven't heard of any death resulting from a standard-compliant 0.4 s disconnection of e.g. a 4 mm2 FTE cable (with its much thinner cpc). During much of the 1900s the wiring regulations appear to have been concerned more with getting a reliable disconnection (e.g. 5 s), rather than with aiming at a good chance of survival even for someone who's unlucky enough to be in contact with an object when it has a fault; so the shorter times based on best-guesses about bodies were an increase in safety anyway.
ProMbrooke:Zoomup:ProMbrooke:Zoomup:
R.C.D.s have saved many lives. If they are not totally reliable then use two in series to half the perceived risk. Are M.C.B.s a hundred per cent reliable?
Z.Why can't the MCB be the backup? A series RCD would either have to be in literal series, or a delayed trip type main with a higher ma threshold.
Cos the M.C.B. is a greedy beast that requires much current to operate it, and sometimes very sluggishly. Whereas a R.C.D. needs a very small current to operate it and at a very fast speed indeed. If our main concern is touch Voltages and shock protection then the R.C.D. has better credentials. There is no reason not to have two 30mA instant R.C.D.s in series if safety is the main concern.
Obviously this does not preclude the use of a 100mA S type R.C.D. supplying a 30 mA instant type R.C.D.
Anyway, I try to avoid touching any Voltages, nasty possibly hurtful tingly stuff. Avoidance is better than a cure.
I have found that the few M.C.Bs that have I have seen become faulty, have normally failed open circuit or would not re-latch on. I suppose that they could get their contacts welded closed on rare occasions.
Faulty M.C.B. Faulty British General circuit breaker autopsy. - YouTube
Well, perhaps bugs or dust getting inside a protective device could cause danger due to slow or non operation of the device. I think I will remove all M.C.B.s and fit B.S. 3036 protection instead, or B.S. 1361 cartridge fuses. Much more robust and reliable.
Of course the old Crabtree P60 type E.L.C.Bs were of very high build quality and made like a brick outhouse. They would not fail if a bit of light dust or the odd bug got inside. They are the true benchmark of quality, unlike the cheap, vulnerable, plastic, miniaturised stuff of today.
Z.
Sluggishly? With proper loop impedance an MCB will directly unlatch and open in 1.5 cycles. An RCD on the other hand must rely on unlatching a magnet, which then triggers a larger opening mechanism. Rather complex. It can jam. Any electronics can fail.
High currents are a very good thing, especially when they reduce touch voltage.
Fuses do not clear as fact as breakers during moderate fault currents. For example, Table 41.1 requires 0.4 seconds, and with a fuse that is possible. However, with an MCB the thermal portion typically can not respond fast enough thus a solenoid is employed. Instant pull in, no need to wait for anything to heat up first.
Do faults of "negligible impedance" really reliably exist? If not, then your high current requiring device like an M.C.B., which also has mechanical moving parts, may be slower to operate than you think.
Your proper loop impedance and high current to operate the overcurrent protective device relies on perfect connections, large conductors and no loose screws or corroded connections. Our friend R.C.D. operates at a tiny current in comparison and operates very quickly. even with relatively high loop impedances.
411.3.3
Table 41.5
Z.
ProMbrooke:
Any idea as to why IEC TR 1200-413 is retired?
No. I've wondered too. I was surprised even when I first saw it, as I've not come across other cases of such a detailed 'technical report' devoted to explaining why certain choices were made in a standard. Perhaps it's not fashionable now to bother with public explanations. The IEC and national standards bodies appear happy to carry on selling this retired publication at a hefty price. (IEC says 'withdrawn').
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