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James Frantz wrote:
I like the idea of an essential bus that can be isolated from all the non
essentials with a mechanical switch that closes/opens a relay.
Only as long as the relay is properly sized and protected! I am sure that
most people know that a relay is an inductive load and needs flyback diodes for
protection. Failure to add the flyback may damage the relay or the switch
driving it, resulting in a situation when one is not able to energize the
relay. However, most people do not consider the possibility of the relay
failing in its "energized" state. Once you flip the switch and remove power,
the relay is spring loaded to fall back to its "power off" state. So one would
hope and expect, but that is not always the case.
I have seen several instances of underspecified relays where the relay contacts
welded close. This can happen when a relay with a large capacity power source
on one side (a battery maybe) is connected to a capacitive load (input sides of
a number of avionics devices). The surge current will be several times larger
than the maximum steady state current for the load. If the relay was sized for
the nominal current, the higher current may be enough to spot weld the relay
contacts, leaving it in permanently connected mode. This may not happen the
first time, it may take a few, a few dozen or a few hundred actuations. If the
relay is underspecified, this is the most likely failure mode.
This is not just a theoretical situation. I have personally examined at least
a dozen relays from 3 different designs that failed in this manner. In a
couple of cases, the weld was weak enough that a gentle tap on the relay was
enough to get it to release, but in most cases cutting open the relay revealed
that the contacts were solidly welded together.
The bottom line is to be careful when specifying relays -- they can not be
taken for granted. Talking about this particular failure mode, leads me to my
next point. When you design a system, think about how you will test it. Most
people have means of turning off the main bus and operating only on the
emergency bus to make sure that it is working as expected. But do you have the
means of detecting if the relay or diode isolating your emergency bus has
failed shorted? If you think of these failure modes up front, you can design
for them and design for testing them.
Hamid
LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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