Circuit Breakers, more
To: mail11:;-at-msd26.enet.dec-dot-com (-at-teslatech)
Subject: Circuit Breakers, more
From: "I am the NRA." <pierson-at-msd26.enet.dec-dot-com>
Date: Tue, 29 Aug 95 08:55:02 EDT
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(I was on vacation, so first some olde bizniss:
Richard Q asked if he should write a book:
I think the book review of the other book answers that....
1) Circuit breakers, like other electromechanical devices can age, and their
characteristics change. In critical circuits _testing_ them from time to
time is a good idea.
1A) Testing requires nominal current, but not nominal voltage at the same time.
A circuit breaker, a meter and a variac, and maybe some series R to limit things
allow testing to be done at low voltage. If a panel style breaker, some
monkey wiring to get to the terminals may be needed. Cycling by hand, under
modest load at full voltage will verify the voltage interrupting ability.
2) Circuit breakers come in various styles, with various sorts of sensing &
mechanism. Each style comes in various delays. "fast" ones try to trip
instantly on overload. "delays" (there are whole hosts of curves of time
vs current) will hold modest overloads, to avoid nusiance trips on, ferisntance
motor starting loads, or large transformers (anyone here with large
transformers????). ACTUALLY, as i read the typical large transfromer set up
here, the control gear(variac, welder, ballast resistors should damp the turn
on transient nicely.
3) Back to mechanisms
a) One basic mechanism is thermal. A small heater is operated by the load
current. If the load current gets too high, a mechanism (commonly a bimetallic
strip or disk) changes mechanically and trips the contacts. The size of the
heater and the size of the strip control the trip time.
b) The second family, commonly more sophisticated, uses a coil of wire as a
solenoid. The load current generates a mag field, which pulls on some linkage.
When the springs in the linkage are overcome, it trips. Trip current is set
by the coil design, and by the mechanical linkage. Springs can set the
delay, or it can be set by "dashpot" using air or hydraulics.
Arguably the thermal design is "better" in a Tesla-type application, as the
coil of the other may be subject to funny effects. Each has some mechanical
linkage, fairly delicate, which can get sticky/grungy/corroded with age.
Contacts can weld shut, if overloaded(!). (it is not usually relavant to
application in Tesla-type circuits, but the circuit breaker DOES have a small
series drop in it.)
The current rating seems obvious, except it has a time associated with
The voltage rating has to do with how much voltage it is designed to
interupt. A contact opening sufficient for a 12v automotive breaker
would not be trusted on a 240Vac tesla-type power supply.
The interrupting rating has to do with the max fault current possible.
That is, a circuit which operates at 10A, trips at 15A may be trying
to intrrupt 100 or 1000A into a fault.
Load type refers to the fact that a resitaive load is (typically)
eaiser to interupt than an inductive load. The inductive load will
"try" to keep the current flowing. A breaker rated for AC may not
properly interrupt a DC load.
A tesla-type application can have odd effects, since the current
being interrupted may (depending on breaker location and circuit design
details) have some HF/LF "contamination" which can lead to odd
Ever see a big draft horse pulling? One of the Budwieser Clydsdales?
THAT is a Horsepower. 756 watts is a horsepower. Every 756 watts
in a coil is ONE CLYDESDALE, running around the lab. If it gets