Table 41.1 Assumed Touch Voltage

ProMbrooke:

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ProMbrooke:

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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:

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.