Re: control panel

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

> > 1.  Fuses (or heavy duty circuit breakers) for input power.
>
>
> I would NEVER rely solely on circuit breakers for a large coil.
> Circuit breakers are prone to not tripping reliably. I have a 32A
> breaker in my box that easily holds off 50A for over a minute. Also,
> circuit breakers can weld shut.

50 Amps divided by rated 32A is 156% overload... a 1 minute trip time might
not be out of spec.  Most circuit breakers have a triptime that is inversely
proportional to the amount of overload (i.e. 100A through a 15A breaker
trips real fast..)  In any case, most breakers will support (per spec) a
115%-120% overload forever.

Also, that rating is just nominal... a breaker may say 20A on it, but
actually be 19A or 21A or somewhere in between.

Circuit breakers can be purchased in various trip time/overcurrent
configurations, as well (something that you don't get a lot of choice on
when buying surplus).  Big breakers sometimes have a couple of settings on
them to allow the user to customize the trip behavior, which is handy when
driving, e.g. big motor start loads.

Circuit breaker trip points are also somewhat influenced by their
environment. Ones based on thermal heaters and bimetallic elements are,
obviously, sensitive to the temperature.

For what it's worth, fuses, too, have various trip characteristics, hence
Fast Blow and Slow Blow designations.

As for contacts welding on a breaker... That's what the Amps Interrupting
Current rating is all about... Typical household panel breakers are 10,000
AIC.  One of the design tasks that an electrical engineer designing panels
has to worry about and analyze is the maximum available fault current.  If
you have a high power load, and you're close to the distribution
transformer, and you're working at 480VAC you might have fault currents as
high as 100 kA or more. In such cases, series inductances (ballast?) is put
in the lines to limit the fault current, OR, you go buy (specify) breakers
(overcurrent protective devices) that can interrupt that 100 kA.  As with
everything, it's a tradeoff between some voltage drop under load and more
expensive downstream components.

By the way, this is why the DC rating is MUCH lower than the AC rating for
most mechanical switches.



>
> I DO have breakers in my cabinet, but these are also protected by a
> fuse. The system is designed to trip the breakers under a "normal"
> fault (me just removing a bit too much ballast inductance), but a
> large sudden unexpected fault (say a short in the ballast) will trip
> the fuses ~instanty. I have the breakers set at what the wires can
> handle continuously. The fuses are set at nearly twice this rating.
> This way I should bever blow a fuse unless something really goes
> wrong and the breakers don't trip first. But the fuses offer that
> final bit of protection.

A decent strategy...

>
> Use good quality HRC (high rupture capacity) fuses. These use a
> silver wire in a silica powder. When they trigger, the silica powder
> makes arcing almost impossible. Some other types of fuses can arc
> across if the current is high enough (kind of like a welding arc -
> the wire melts at one end, a small arc forms and spreads across the
> fuse when the rest of the wire melts).

Fuses have AIC ratings too... (and voltage ratings, as well... )

>