Will standard mcbs work on 12v AC?

Yes standard types of MCB will work on 12 volts AC or other much reduced voltages.

The relatively low short circuit current from a 300 VA transformer may be insufficient to operate the magnetic trip which means waiting for the thermal trip.

An estimate of the short circuit circuit current from the transformer would be about 10 times full load, or say 250 amps. Even a short length of intervening cable will substantially reduce the fault current.


I would be inclined to use 6 amp MCBs rather than 10 amps to give closer protection. 6 amps is still 20% in excess of the calculated load current.


Alternatively, consider a "like for like replacement" rather than altering the design. Fit new fuse holders, perhaps with soldered connections or decent screw terminal connections rather than crimped push on types.

If the heaters are PTC types rather than a simple resistance, then altering the design might be best avoided.

The MCBs will be fine - the whole ppoint is the that they are a series element that does not drop much voltage.


They do come unstuck on DC however, or rather they don't - without the zero crossing of the AC waveform , they can weld - not an issue at 12V, but certainly a problem by you reach arc welding sort of voltages (50-80) .

Some designs do not work at anything far from 50Hz, so the 'electronic transformers' that are really a switching supply at a supersonic frequency, as used to generate "12V AC"  for some light fittings, can cause them to trip at the wring setting.


I'd be tempted to look at the wiring to the fuse holders first - this is almost automotive quality

regards Mike.

Thanks for the replies.

                      Regards,Hz

The manufacturer is permitted to assign the voltage(s) an mcb will operate at, and associated rated short-circuit capacity.


According to BS EN 60898-1, there is no obligation on the manufacturer to guarantee operation at lower voltages than 230 V (nominal).


So ... check with the manufacturer's data.

True if you go by the letter of the standards, but not if you actually ask the electrons - clearly any MCB that introduced  even a fraction of a volt drop resistively would soon fail, as the consumer unit is was in would catch fire - in this case a 10A MCB dropping 1 volt at 10A would be dissipating 10W - in reality dissipation has to be far less than that.


And to have such a voltage drop inductively in that volume would requires a closed magnetic circuit, but the trip has to be such that the magnetic circuit does not form a complete iron 'loop' until after the contacts have de-latched, or there would be no force to actuate the mechanism -  if you like to think of it that way, it is the desire of the system to seek a low energy state, that causes it to be favourable for the magnetic parts to move together.

Yes you are may be using them outside their guaranteed range but only because the makers never foresaw this case and never tested for it.

However, in the interests of size and sanity, I'd still suggest looking at automotive fusing techniques first, but because this is the voltage and current range they are intended for, and they are smaller and neater, and are clearly not going to be confused with mains,  not because of the MCB maker's data.


regards,

Mike.

example - takes the medium sized blade fuses.

The issue is indeed one for conformity, and mcb's can be found that are stated as suitable for ELV. They are used in many applications.


Also ignores the fact that mcb's also heat up to activate the thermal element.


Whilst I'd agree that, if a 6 A mcb were used, they would offer closer protection, they would operate at a higher temperature than 10 A mcb's because of the thermal element.



Does any of this explain why the fuse holders are overheating? It could well be that the fuse holders or fuses are subject to conducted heat from the transformer in close proximity operating at, or perhaps even slightly above, its ideal rating with real-world loads.


Would a 500 VA transformer provide the solution?