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6AWG Fast-ons and VF7 Relays

 
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nuckollsr(at)cox.net
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PostPosted: Tue Feb 21, 2006 6:07 am    Post subject: 6AWG Fast-ons and VF7 Relays Reply with quote

At 10:34 PM 2/20/2006 -0500, you wrote:

Quote:


Jon

FWIW I downloaded spec sheet for the VF7 tyco relay and believe it is
not suitable for that purpose. Figure 1 shows the ability to
disconnect a resistive load and extinguish the arc. At 40 amps (or a bit
more) it can't manage more than about 20 volts. Remember that you aren't
interupting a 12 volt supply but something higher during a runaway and
it will rise further as the contacts start to open and arc. With some
inductive characteristics this is also probably a more severe case than
the resistive situation that the graph refers to.
http://www.snesne.com/vf7.pdf

I found those "integrated" connectors to be problematic even on 1/4"
spade terminal relays. The open barrel connectors don't seem to grip
tight enough to reliably handle much current. I prefer separate PIDG
connectors for each wire. I tried using PIDG connectors in an integrated
connector but then they were too difficult to install or remove without
destroying the relay due to the high forces involved. Most automotive
relays don't interupt more than about 14 volts and I'm having trouble
thinking of any that handle more than about 10 amps except maybe the
anti-skid.

Ken

You stole my thunder sir. I think you're dead-on correct.
First, a b-lead contactor needs to be a real contactor, i.e.
double make-break contacts in series. This greatly improves
the ability of the device to open a rapidly rising source
voltage when the contacts open.

As to the boss-hog relays offered in the automotive world
with impressive ratings, I agree that their usefulness is
limited at the higher ratings and there are other issues
like quality of the fast-on joint that makes them problematic
for use at those ratings.

Even though the fuse blocks are RATED at up to 30A per
branch, I prefer to de-rate them to 15A. Any larger
load should step up in robustness like the Maxi Fuse
in-line holders. Degradation due to electrical heating
is an I(squared)*R effect. Cutting currents to 1/2 drops
stress to 1/4.

I have no doubt that ALL of the manufacturers of these
products have crafted ratings in good faith . . . I wouldn't
suggest that anyone is being dishonest. But we do know that
the highest stresses on most light aircraft electrics are
environmental . . . they degrade with age and ultimately
fail electrically when mate-up forces and finishes don't
carry the current any more.

Hence our preoccupation with longevity for gas-tight
joints, de-rated contacts and doing the best we can with
the $time$ invested in parts selection and installation.

I agree that there are few if any places where I would
want to install the VF-7 with an intention of loading it
to a high percentage of its ratings.

Bob . . .

Quote:
Jon Goguen wrote:

>
<jon.goguen(at)umassmed.edu>
>
>I have several uses in mind. The most demanding is as an OV disconnect
>for the B lead of a 40 amp IR ND alternator, with an 8 or 6 AWG wire.
>The contacts in the relay are rated for 100,000 cycles at 70 amps/14
>volts. Do you have some reason to believe that fast-on style
>connectors don't scale up well to the 0.375 size and won't meet the
>manufacturers specs on these relays with respect to current rating?
>They would be installed as part of an integrated connector that
>includes all four terminals, and would be provided with proper wire
>support. I like them because they're very light, very cheap, and
>replaceable on a plug-in basis.
>
>Jon
>
>Jon Goguen
>
>


--



Bob . . .
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >


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