Exactly.
Booster Box connected to electrical supply
- Thread starter sophiejazz
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I've come across quite a few central heating installations where after a short circuit, the switching contacts inside a programmer had either been welded together, or destroyed, (usually because someone who didn't know what they were doing was trying to install a new thermostat or programmer) In every instance the FCU had been fitted with a 13A fuse. Never come across it when a 3A fuse was fitted....
I have to say that it would surprise me if the difference between a 13A and 3A fuse were particularly relevant in that context. In the face of a 'short circuit', the (very high) fault current through the contacts would obviously be the same in both cases, and (at the very high currents involved), as for duration of the fault current, I would expect a 13A fuse to blow virtually as quickly as a 3A one.
Fault protection (i.e. in the face of 'short circuits') is likely to be adequate in most cases, with any credible OPD. As far as I can make out, those who feel that 'fusing down' is desirable are thinking in terms of overload current, rather than fault currents.
Kind Regards, John
A 3A fuse has a higher cold resistance than a 13A fuse, so if everything else is unchanged, the fault current is less when the 3A fuse is installed.
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True, but is the resistance of either particularly significant in relation to the total (L-N) loop resistance (hence fault current)? I would have thought not - and I've never really thought of a fuse as a 'current limiting' component!
To save me an experiment, does anyone know roughly what the cold resistances of 3A and/or 13A BS1362s are??
Kind Regards, John
What's more important is the let-through energy, I²t. I've a table somewhere...
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Having measured the resistance of a selection of 3A and 13A fuses:
A 13A fuse resistance approx 5mΩ, 3A fuse approx 45mΩ
40mΩ more for the 3A fuse compared to a 13A fuse.
A typical fault of 1kA, or 0.23Ω at 230V could be expected at a socket outlet or some other similar circuit.
With the 3A fuse instead of the 13A, 40mΩ is added, so it's 0.27Ω, or 852A.
I²t assuming disconnection in 0.01 seconds for both values:
1000A = 10000
852A = 7259
Around 27% less.
There are other factors to consider as the disconnection time is probably not the same for both fuses, and the thinner wire will heat up more quickly which further increases it's resistance during the fault.
A 13A fuse resistance approx 5mΩ, 3A fuse approx 45mΩ
40mΩ more for the 3A fuse compared to a 13A fuse.
A typical fault of 1kA, or 0.23Ω at 230V could be expected at a socket outlet or some other similar circuit.
With the 3A fuse instead of the 13A, 40mΩ is added, so it's 0.27Ω, or 852A.
I²t assuming disconnection in 0.01 seconds for both values:
1000A = 10000
852A = 7259
Around 27% less.
There are other factors to consider as the disconnection time is probably not the same for both fuses, and the thinner wire will heat up more quickly which further increases it's resistance during the fault.
Thanks. I'm a bit surprised that either are as high as that - that amounts to best part of a Watt (~0.85W) for a 'fully loaded' 13A fuse, and nearly half of that for a 3A one. When one considers how many fuses there are dotted around the average installation, I wonder how much energy is being 'wasted' in heating them up!
Fair enough. I've often wondered where these high 'typical' fault currents actually come from (except perhaps in London!). In mu very limited experience, I haven't come across (m)any situations in which even the external loop resistance is as low as 0.23Ω, let alone the figure when measured at a socket/whatever.
That sounds logical but, if the disconnection time was (per the assumption of your calculations) roughly the same for both fuses, wouldn't they have to be heating up at roughly the same rate (since they presumably blow when the wire reaches a certain temperature)?
Kind Regards, John
BS1362 specifies the maximum power loss to be 1W. It also specifies the prospective fault currents, which depend on the fuse rating;
Rated current
of fuse link (A) < 2 > 2 <4 >4 < 6 > 6 < 10 >10 < 13
Prospective
current (A) 100 160 315 500 630
Rated current
of fuse link (A) < 2 > 2 <4 >4 < 6 > 6 < 10 >10 < 13
Prospective
current (A) 100 160 315 500 630
Fair enough. Flameport's figures therefore just about make it (for a 13A fuse).
Are those maximum PFCs, or what??
Kind Regards, John
They are the prospective fault currents for the breaking capacity tests.
The problem now is not the current from a stalled circulator which isn't particularly high for a small domestic one, a little bump with a rubber mallet and everything's running again. Modern versions are a nightmare as they are now fitted with unnecessary electronics, not only has this reduced the life of the unit from 10-15+ years down to around 2-3 the failure mode is not with the motor but with the electronics. All the failed ones I have seen have gone dead short L-N. Unfortunately now many are wired back to the boiler PCB to control pump overrun, and on several occasions I have seen the short circuit blow tracks off of the boiler PCB, not something I have seen with a faulty basic circulator, not that I've come across many of those. I replaced the Grundfos UPS15-50 at my parents a little while back, it was still working but it was getting a bit noisy, it was 22 years old.
I do a lot of heating and appliance fault finding, and 3A fuses are very good at limiting the damage caused to PCBs and switch contacts during faults, even if that is not their main purpose. Cooker hoods are a common one, where a 13A fuse has been fitted damage to the PCB tracks in the switch modules is common when one of the lamps blows. And as mentioned above, programmer and thermostat contacts welding together is much more common when a 13A fuse has been fitted.
Thanks.
Does that mean that a BS1362-compliant fuse cannot be guaranteed to safely break currents higher than those figures? If that were true, it would be pretty 'worrying', since it would seem to imply that 3A & 5A, sometimes 10A and, if one assumes flameport's 'typical PFC', perhaps even 13A ones cannot be guaranteed to be 'safe' in plugs/FCUs connected to typical sockets circuits. What am I missing?
Kind Regards, John
Well, I've certainly learned something - and something which maybe somewhat justifies the fact that, as I often say, I must be one of the more valuable customers of those who manufacture 1A BS1362 fuses
Just as I/we am/are always telling people that RCDs do not limit, or even reduce, fault currents, I have also been in the habit of telling people that the same is true of OPDs (including fuses), since I did not think that the resistance of a fuse was significant in relation to the total fault loop impedance. In view of what flameport and yourself have said, I may have to re-think that!
Kind Regards, John
No, they can break 6kA +10%-0. Those PFCs I quoted are for the test to determine the breaking characteristic. Not sure I fully understand it myself - luckily I don't have to!
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