If the total Zs of the circuit is within the specified limits, the fuse / MCB will go within the time required (0.4s or 5s generally). For a 30A BS3036 fuse, the max Zs is 1.14 ohms, for an equivalent BSEN60898-B MCB rated at 32A, it is 1.50 ohms. If your circuit is within the BS3036 figure, changing to MCBs is going to really achieve anything, certainly not on safety grounds. It just makes it more convenient to reset.
If you have a TN-C-S supply, with theoretically maximum Ze of 0.35 ohms, this allows you to have R1+R2 of 0.79 ohms. Working backwards:
P+CPC = 3.16 ohms (assuming a proper ring circuit)
CPC = 2.5P
Gives P resistance of about 0.90 ohms, or about 120m of cable. Should be possible to wire the average sized house on one ring and still fall within the limits for a fuse, let alone an MCB, to meet the 0.4s disconection time.
There was thread the other day, kicked off by somebody concerned about the presence of old rubber wiring in his property; the general consensus of the replies was that he was virtually advised not to enter the death-trap of a house until it was sorted out.
Now, I'm thinking there must still be thousands of rubber-wired properties around the country - I've certainly come across plenty in my time - and while I'm not for a minute arguing against the desirability of rewiring ASAP under such circumstances, I'm just wondering how many fatalities/injuries/house fires can be attributed to this ancient wiring? Are there stats available? How dangerous *really* is it?
This is a very good question, and it would be nice to have some data. It could tell us more about the safety of both ancient rubber installs and more modern installs, when all we have is combined data - which I gather isnt that accurate anyway.
I suppose also the risks would vary by person too. I suspect some would be more vulnerable to shock than others.
On Wed, 08 Feb 2006 10:18:36 +0000 someone who may be Ben Blaukopf wrote this:-
Far too sweeping a generalisation to be correct, though it is a common misconception often expressed with absolute certainty. There are fuses and fuses, there are MCBs and MCBs and there are faults and faults.
Never forget that an MCB takes an irreducible minimum time to operate. The magnetic coil has to attract a bit of iron, this has to release a mechanism, springs have to go boing, things have to pivot and contacts have to move some distance apart before the circuit is opened. No matter how high the current this minimum time is irreducible for a particular design. In contrast with all this mechanical stuff, all a fuse wire has to do is melt. Rapid melting is something HBC fuse links do very rapidly at high currents, faster than the fastest MCBs. In fact even rewirable fuse wires melt rapidly at high currents, though (like many ranges of MCB) they are unable to break the fault current.
With a small overload of long duration the position is reversed. The irreducible minimum operating time of the MCB is now irrelevant, as the controlling factor is how quickly the thermal part of the mechanism activates the contacts. HBC fuses will take longer to operate (though one can argue this is an advantage in a properly designed circuit). Rewirable fuses are rather uncertain in such circumstances.
MCBs do have advantages of ease of operation and resetting, though if one operates more than once in a blue moon then there is probably something wrong with the circuit or something connected to it. They are thus particularly suitable for those who don't know much about electricity, provided that the correct range is selected to break the prospective fault current (or adequate backup provided).
On the other hand some people sleep a bit better at night knowing that they live near a substation, but if a serious fault was to develop in their wiring a HBC fuse will react incredibly quickly to disconnect the particular circuit concerned in far less than a quarter of a cycle and limit the fault current. With typical MCBs the same fault would probably have caused the main "electricity board" fuse to operate, cutting off all electricity, hopefully before the MCBs had caught fire because they were unable to break the fault. Some MCBs are a lot better than typical MCBs, but there are still limits on how fast a series of mechanical components can move.
This is true, though not the whole story. The time it takes to replace a fuse wire at a guess 2 or 3 times in the life of an installation is less than the time taken to go get some mcbs and retrofit them. Oft overloooked in the rush to '''upgrade.'''
Those are the maximum impedances to achieve the disconnect time, but not to avoid thermal damage to the cable.
t = k^2S^2/I^2
Zs is 1.14ohms, so I is 240V/1.14ohms == 210A t is therefore 0.299s, and if the disconnect time is any longer, the cable may be damaged by heat. A type B MCB will disconnect much faster than that because I/In > 5, but looking at
formatting link
BS3036 30A fuse will just meet the 400ms disconnect requirement, but certainly not 299ms.
If we required that Zs was no more than 0.69ohms (plucking a figure from the OSG), then t becomes 100ms, and S is 350A. At that point, a BS3036
30A fuse still doesn't disconnect fast enough - but a BS88 does - just. Which is, as far as I can see, why BS88 is okay with 1mm cpc, but BS3036 isn't.
On 9 Feb 2006 02:02:26 -0800 someone who may be snipped-for-privacy@care2.com wrote this:-
Agreed. It is even easier to take the spare HBC fuse out of the carrier and put this in the correct part of the carrier, after one has located the fault. If one has a "wylex style" board then one may have to locate the spare fuses, if they are not in a handy place.
Rewirable fuses suffer from the person who thinks they know what they are doing. Such people can fail to put the wire in the right places, even if they get the right sized wire.
On 9 Feb 2006 07:59:51 -0800 someone who may be snipped-for-privacy@care2.com wrote this:-
Different carriers have different routes for the wire. If one doesn't follow this route then the melting wire may melt parts of the carrier and/or consumer unit.
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