Yes common sense must be applied to "out of spec uses" or indeed any use.
Having repaired a 110 Volt DC powered 100 ton crane (ship mounted, Russian)
The relays controlling the motors were a good education on the sparks to be
expected and unbelievable in use
Sparks up to 100 mm (diverted from the main contact area) and an opening ga=
p
of around 8"=20

12V on a 240V MCB (magnetic circuit breaker) would not cause me to loose
sleep,=20
not fitting a fuse because they are too expensive/hard to obtain would.
MCB do fail, they arc and destroy themselves on AC too, failure to properly
tighten terminals can also cause an arc=20
Fire risk IMHO very very low. This sort of failure has been allowed for in
the MCB design, self extinguishing plastics used etc=20

PC

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Sean Breheny
Sent: 25 November 2013 01:03
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Low current breaker

What you say is MOSTLY true but there are some significant caveats.

Firstly, there is a big difference between AC overcurrent protection device=
s
and DC ones. This is mainly because AC current goes to zero 100 ot 120 time=
s
per second, so arcs can be extinguished more easily. Almost all AC-rated
current interrupting devices (switches, relays, fuses, circuit breakers,
etc.) either are not rated for DC operation or have a significantly lower
set of ratings for DC. Another reason for this is that DC circuits can have
unlimited inductance without it placing an inherent limit on the current
flow. With AC circuits, for a given voltage and current, there will be a
maximum circuit inductance. Higher inductance makes arcs harder to stop.

Household circuit breakers will work for DC, but they will have different
characteristics and they will have a much lower maximum voltage and lower
maximum fault interrupting current (this is NOT the trip current bur rather
the measure of how high the fault current can be before the CB can no longe=
r
stop the arc that happens when it opens).

All of this is likely irrelevant for the original poster's application sinc=
e
the available fault current is low. However, when you get into the realm of
lead acid batteries of more than 5 Amp-Hours capacity, it can make a big
difference.

I just wanted to post a warning to people that they need to be careful in
selecting overcurrent protection devices for DC circuits where the
short-circuit current can be 100s of amps or more. Many AC fuses, switches,
relays, and CBs will just arc over and destroy themselves, maybe causing a
fire, in such an application. I have seen this first hand several times wit=
h
48V, 28 Amp Hour lead acid battery systems.

Sean




--=20
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