Ken Finney wrote:
"Jeff Wisnia" wrote in message
...
Ken Finney wrote:
snipped
I knew I was too close to the problem, and that there had to be a simple
solution. Your solution is likely what we will use.
As for the why, we are testing a switch to ensure it won't set off an
explosive atmosphere. The test method isn't too clear other than we
need to
test the switch at maximum current into an inductive load. Rather than
try
to justify why this really means "a partially inductive load", we'd
rather
worst-case the test conditions.
Now, even more than I posted to this thread an hour ago, I think you
really need to find out just what inductance the authority governing the
certification on your "switch" requires for that test, and also the
composition and temperature/pressure conditions of the "explosive
atmosphere".
It sounds like a job for UL, Factory Mutual or CSA to me.
Years ago I used to design "intrinsically safe" monitoring circuits for
use in hazardous locations like the inside of gasoline storage tanks and
had to get my stuff certified by actual tests at those three agencies.
They had pretty involved standards and procedures for testing such
things and used carefully controlled mixtures of gasses and air they
tried to get our stuff to set off. The same kind of gas test cells along
with standardized loads were used for testing "explosion proof"
equipment such as switches.
So forgive me if I'm misunderstanding your requirements, but the answer
to your question may not be as simple as you hope it is, particularly in
today's litigeous world.
The test method we are testing to (MIL-STD-202, method 109, available at
http://www.dscc.dla.mil/Downloads/Mi...section100.pdf)
isn't the best I've seen. As for the inductance, it states "Proper
precaution shall be taken to duplicate the normal load in respect to torque,
voltage, current, inductive reactance, etc. In all instances it shall be
considered preferable to operate the specimen as it normally functions
during service use." However, when you look at MIL-SPEC switches that are
already rated for use in explosive atmosphere, that are always tested at the
maximum inductive current "in accordance with MIL-STD-202, method 109", so
that is what we are trying to do.
And as far a litigation, we are covered by "The Government Contractor
Defense". ;^)
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From: "Ken Finney"
Subject: (OT) Need inductive load
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Date: Thu, 31 Mar 2005 17:28:30 GMT
"Leo Lichtman" wrote in message
news
I'm wondering whether you may have it BACKWARDS. Could it be that they
want
you to run 12 amps through an inductive load, and then OPEN the circuit?
That condition will tend to make the switch arc, which could set off an
explosive atmosphere. If that's the case, the amount of inductance needs
to
be specified, since this will determine the amount of arcing.
Sorry I wasn't clearer, yes, we have to close and open the switch under
test. And no, the inductance isn't specified (even though it should be).
Well, it does now sound like you've got a problem on your hands and if
it were me I'd jump up and down screaming that the inductance should be
specified or at the very least, the specific piece of equipment being
controlled by that switch, such as a motor, a big solenoid actuator or
whatever be defined as the expected (and only) load your swich will be
rated to work with.
Is this perhaps an "explosion proof" switch? i.e. one mounted inside a
strong metal housing so that even if a hazardous atmosphere inside the
enclosure gets touched off by a spark from the switch it won't "leak
out" enough hot gas to ignite a hazardous atmosphere outside of the
enclosure. IIRC one operating principle of such housings is that the
path length of the cover flanges on them is long enough so that any hot
gasses from an explosion inside them which squirt through the joints get
cooled by contact with the metal path so that they aren't hot enough to
ignite the atmosphere outside the enclosure.
IMHO breaking 12 amps of dc with even a tiny amount of inductance in the
circuit is quite liable to set off the atmosphere specified in the
document you referenced, I'd run like hell if the switch isn't an
"explosion proof" design.
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"As long as there are final exams, there will be prayer in public
schools"