|Posted: Mon May 28, 2018 9:09 am Post subject: Is the OVP 5 A CB supposed to trip when I switch off the
| If you want to protect the relay contacts from back EMF generated in an inductive LOAD put a diode across the LOAD or across the relay contacts. Either place will work. |
Relay and switch contacts are almost never fitted
with prophylactics against wear-n-tear. When you
read design life specs for the various products
you virtually never see recommendations for adding
external devices intended to extend service life
of the contacts.
The reason being that any 'arc snubbing' device
must be tailored to the application . . . the
relay/switch manufacturer cannot anticipate hou
YOU will used the product . . . hence, they are
what they are and any failure to meed design
goals for service life generally involves a
feature in the application that is out in left
field . . . a once in a million encounter with
the immutable laws of physics.
Good case in point was the rash of relay failures
we experienced in the B400 that took a couple years
to track down . . . and the relay manufacturer's
had no clue . . . hence not helpful.
I've designed perhaps 4-5 products wherein it
was useful to add arc suppression directly
to contacts. The most recent being an OV
relay addition to a specific airplane with a generator
where the arc suppression was 'tuned' to
generator field inductance.
But for the most part, relays as switches are
selected to task with no adjuncts intended to
enhance service life.
Diodes and other solid state devices generally
serve no useful purpose across the contacts
of a switch. The Ceaans/FAA got caught with their
pants down around their ankles on the Great
ACS-Gerdes, OFF-L-R-Both-Start Keyswitch fiasco
I captured the corrected documents here
The first issue of these documents placed the
diode across the starter control switch contacts
in the ACS keyswitch assembly . . . only to
discover that the diode had no benefit. The
physics is described in the second page of
Bob . . .
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