Use 240V household type MCBs in low voltage circuit?

[WA1ICI]

I think this has been hashed-out successfully - use a breaker with a DC rating.  DC is harder to break, since there are no zero crossings to extinguish the arc.

However, I thought I would relate a relevant incident that happened at my first job at Intel, around 1978.  I was a design engineer on the 8089 I/O processor which followed the 8086 design by about a year. (It never became very successful, but the 8086 - that's another story).  At that time we prototyped microprocessors in a TTL breadboard to test & debug functionality before first silicon came back.  This was in the days before logic simulation.  I got called into the lab by the 8086 lab technician, who wanted me to look into a problem with the 8086 TTL breadboard.  This was built with multiple Augat wire-wrap boards that were mounted in a tall rack cabinet.  At the bottom of the cabinet was a shoe-box sized 5V 200A switching power supply with big (I guess about 8 gauge) insulated wires running to each Augat rack assembly.  The complaint was that the breadboard had stopped working.  Putting a scope on one of the bus lines showed a logic high of about 1.5V.  The VCC was about 3V.  As I was looking around for the problem, I started smelling something hot.  I looked down towards the power supply at the bottom and saw that the +5V wires coming up had no insulation (it was burned off) and were glowing a deep red color.  Tracing the problem, one of the +5V wires had shorted to the rack cabinet due to an over-enthusiastic cable clamp.  At the lowest rack position was a panel with a single household circuit breaker in line with the +5V, rated at 30A, if I remember correctly.  It was somewhat charred, but was otherwise happily passing hundreds of amps from the massively over-sized power supply.  I asked the tech about this, and he thought that a 30 Amp breaker would work just fine.  Two problems here:  Don't use a power supply that is almost an order of magnitude bigger than needed (the bread board used less than 30 Amps), and no, a 60Hz 120VAC circuit breaker doesn't work on 5V DC.  Putting in a smaller power supply with current limiting and fixing the short brought the breadboard back to life.

- John Atwood

[richard.cs]

It was buried deep in the documentation somewhere but I remember Hagar specified theirs for either 48 or 60 V d.c. - I have a feeling we had to ask them specifically, it wasn't in the device datasheets.

One thing to note when using MCBs on DC - the magnetic trip operates on the peak of the AC waveform so a B type with the magnetic trip that is intended to be 3-5 times rated current but will actually be sqrt(2) higher on DC, almost turning it into a C type.

[necessaryevil]

Wait... household style breakers are less than 15 UK pounds ($20 USD)? Or do you have them laying around?


On topic:
Look at a electronic wholesale such as Digikey, Farnell or Mouser. A quick look around tells me you can find some cheap ones there. Another solution might be the use of a PTC fuse.

[mikeselectricstuff]

Wait... household style breakers are less than 15 UK pounds ($20 USD)? Or do you have them laying around?

More like £2-3
http://cpc.farnell.com/click/db216/mcb-c-curve-6a-16ka-sp-type-c/dp/PL10147
http://cpc.farnell.com/pro-elec/aub1-1p-c40a/mcb-single-pole-c-curve-40a/dp/PL11596
http://cpc.farnell.com/crabtree/s61-c16/starbreaker-16a-sp-type-c-curv/dp/PL12119