25A slow blow circuit breaker?

I'm replying to myself to bump my question up the list - it had been blocked due to some of the words contained therein being in the moderators banned list but has now been resolved - thanks Lisa!

So, what is this slow blow CB?

F 

I suspect that they mean a type C MCB as these are relatively slow acting if compared to the more common type B items. Not that common in a 25 amp rating.

As protection is also provided by a 13 amp fuse, then in my view the 25 A MCB should be regarded as a MINIMUM, and not as a requirement. 32 amps would in my view be fine, with cable to suit.

I suspect that the equipment has a significant inrush current, that a 13 amp fuse should withstand, but that might trip a standard type of circuit breaker of 20 amps or less. Hence the requirement for a minimum of 25 amps.

I think it is a Type D MCB, as that’s what I was taught at college.

www.consumerunitworld.co.uk/hager-ndn125a-mcb-type-d-single-pole-25a-10ka-441-p.asp

I would have assumed that also but the data sheet states a type B.

if the same X ray rig is sold in other countries the instructions may be assuming unfused plugs.  I sometimes find a perusal of the instructions for other places to  be informative. I agree the load is likely to have current bursts, and a C or D will ride that out much as a hot wire fuse would have done.

If you have a 13A fuse in line then any larger breaker will also work, after all you are using the fuse you just want the breaker not to trip before the fuse blows. Mind you if it really draws 12 the 13 A fuse will run hot However,  I suspect that is only during the Xray of some seconds, and most of the rest of the  time it is a few hundred watts at most.

Mike.

Yes, that is right. The dental x-ray exposure time is very short, less than a second, so the load is really very small. I suggest a type D and lose the 13A fuse, it is likely to blow after a number of "shock" type use and it would be a serious problem for the dentist as he probably wont have a clue as to why it doesn't work. Common problem with FSUs!

A further thought. You say switch fuse asked for, I presume this is for isolation during any servicing. Personally I would fit a BS4343 plug and socket, as I would not like to be working on EHT supplies like this without positive isolation from the supply. A 16A one would be perfectly satisfactory, and probably a 16A trpe D in the CU, but I wouldn't worry about a larger breaker as BS4343s are never a point of overcurrent failure, unlike BS1363 stuff. The Xray tube probably operates at about 100 kV and 30-50 mA, a very dangerous supply.

I agree - "slow blow" is a description of some types of fuses - not ordinary circuit breakers. MCBs all trip in the same sort of time once their tripping level is reached - the difference between B, C and D types is just the level at which they trip instantaneously, rather than the speed of tripping. Below that level they might trip by the thermal mechanism - but then you're talking of much longer times - way beyond the second or two that's likely to be a concern here. Sounds like the instructions have lost something in translation.

The other possibility is that the "B" is referring to the characteristics of an RCD rather than overcurrent device (presumably the machine generates d.c. internally...).

   - Andy.

the installing engineer asked for a 20/25A type B CB, along with the switch spur.