From: Sky2high@aol.com
Message-ID: <70.283f83e.26d9e0d9@aol.com>
Date: Sat, 26 Aug 2000 23:11:21 EDT
Subject: Circuit Breakers for Hydraulic Pump Relays
To: lancair.list@olsusa.com
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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Rob,

The following is the text of an E-Mail I submitted to LML in December 1998:
<LC20 Hydraulic Pump Wiring Considerations:

1.  Lancair wiring provides for two circuit breakers, 50 amp for the pump and 
5 amp (or smaller) for the relays.  They also state that during any testing, 
to pull both breakers.  Disengaging both is not always done -- "I forgot", 
they are not co-located, one is a breaker and the other is a fuse or 
whatever.  If the main power bus is activated and only the pump breaker is 
opened, then the up or down relay will be activated (depending on the 
position of the gear switch) if the corresponding pressure switch is not 
sensing enough pressure because the pressure has leaked down or the gear 
extension valve is open.  The "gear transit" light will not be on because the 
power for it comes from power being sent thru the relay contacts to the pump 
and that circuit was de-activated by pulling the pump breaker.  This means 
the "intermittent use" relay could be engaged for the length of time of the 
testing (in my case, 4 hours).  The  heat build-up in the relay can result in 
melting internal coatings and causing the "selenoid" plunger to become stuck 
in the "picked" position.  Removing power from this relay will not open the 
contacts and the real excitement begins when the pump breaker is reset.  Of 
course, moving the gear switch to the other position is only an attempt to 
run the motor in both directions at once.  Yikes!

The solution (after replacing the screwed up relay) to prevent this is to 
cascade the 5 amp relay power breaker (or fuse) off of the 50 amp pump 
breaker so that when you pull the pump breaker, no power is available for the 
relays either.

2. Locate the 50 amp pump breaker where it is easy to get at -- not on the 
baggage bulkhead.  It may be desireable to stop the pump quickly whilst 
flying solo and reaching behind is not safe (remember John Denver).  Since my 
battery is behind the co-pilot seat, it was not much more wire length to put 
the pump breaker just forward of the glove box cover, on the top of the 
between seat console.  Because I have second-generation pressure switches, 
sometimes the pump will run too long on a gear-up request and pulling the 
breaker for a few seconds helps the pressure switch settle down.

3.  If the wire to the "gear transit" light were shorted, it would surely 
burn before the 50 amp breaker popped.  One should, for safety's sake, put 
isolation diodes at the indicate (I) position of the pump relays and an 
in-line fuse on that wire.>

Do not let somebody's switch panel limitations force the design of your 
electrical system to be something less than safe.

Scott Krueger
N92EX
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