nuckolls.bob(at)cox.net
Guest
|
 Posted: Tue Aug 26, 2008 6:23 am Post subject: High resistance pops breaker? ("High resistance" defined) |
 |
|
I've received a number of private e-mails on the
"high resistance" joint thing that illustrated a
disconnect between the various participants in the
discussion with respect to the likely characteristics
of a "high resistance" joint.
We've discussed on these pages (and in my seminars)
the need for gas-tight joining of two materials for
the purpose of making long lived connections. The
easiest way (non destructive) to deduce the quality
of a joint is with a device called a "Bonding Meter".
We have one here that I've used many times both for
my work in the OBAM aircraft community and at RAC/H-B.
It's a 4-wire ohmmeter operating on principals similar
to those devices described in . . .
http://www.aeroelectric.com/articles/grnding.pdf
http://www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf
The later article was crafted and published for the
benefit of technicians working RAC/H-B products attempting
to find degraded microswitches in landing gear position
indicating systems without pulling the devices off
the airplane.
My all time favorite tool is the T477W shown here
http://www.avtron.com/pdf/ate/t477w.pdf
Bought one of these for myself at a Boeing surplus
sale a few years back. This device uses an AC
signal to stress the joint under investigation.
It's MUCH easier to signal condition and measure
microvolts of AC than DC which is what makes this
instrument and it's close cousins so practical/
attractive.
When we speak of a "low resistance" connection in
"the business" . . . we're talking 500 MICRO-OHMS
max. It's not possible to get these values without
having pressing cleaned surfaces into intimate
contact with each other . . . i.e. gas tight. When
we speak of a "high resistance" joint, the bonding
meter says 600 uOhms or perhaps even 1-2 milliohms.
For most folk's way of thinking, this is still a
dead short (no such thing really - except in a
superconducting world). For those of us tasked with
crafting and maintaining a gas tight joint, loss of
bonding integrity is manifested in a 2x or greater
increase of normal resistance readings that are still quite
small in the grand scheme of things.
Now, how much resistance might one expect to see in
a joint where the two conductors are simply touched
but without a lot of pressure? It's still measured in
tens of milliohms . . . but for all practical purposes
a "good" but intermittent connection from the perspective
of troubleshooting things like retracting landing lights.
When a mechanic reports a "high resistance" connection
in some made-up joints (fast-on connection, ring terminal
over a screw, two pieces of brass bolted together) the
image I see in my head is a joint once measured in
micro-ohms having been elevated to the tens of milliohms.
In a cranking circuit, this joint would heat up, smoke,
and perhaps set things on fire when hit with 100+
amp loads.
http://aeroelectric.com/Pictures/Failures/6041_Contactor_Failure.jpg
100 milliohms in series with the motor/lamp circuit
of our landing light example would have NO immediately
observable effect on landing light operations. Assuming
a 100W landing light (and I think many of those things
were 250w) the running current of 8A x 100 millohms =
6.4 watts of HEAT being dumped into the joint. This kind
of stress causes localized heating that can begin to degrade
the materials locally over time. Not unlike the situation
that caused the switch failure discussed here:
http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/Anatomy_of_a_Switch_Failure.html
So when we speak of "high resistance" joints, I'll suggest
that the increase in resistance is not great enough to
produce readily observable effects. One MIGHT deduce a
high resistance joint by measuring voltage drop under load
(an adaptation of the 4-wire milliohmmeter technique)
but while a 100 milliohm joint will eventually smoke something
over time, it's certainly not the proximate cause of increased
current flow that opens breakers, ect.
I mentioned an "intermittent" joints . . . a joint that makes
transient good/bad connection. These joints will degrade over
time due to heating effects of arcing but for the most part,
they manifest themselves in the form of flickering lights,
upset radios, and get fixed before the failures at the
connection become catastrophic. The term "high resistance"
to describe the intermittent joint would not be expected to
produce the same mental images on the part of both writer and
reader.
An intermittent joint offers a whole new thought process that
explores different dynamics than the slow-moving effects of
localized heating. It's sad that both writers and publishers
of the kinds of articles that started this thread haven't a clue
as to the nature of microscopic, simple-ideas that drive the physics
of the situation they're discussing. Further, they may use words
that in their mind describes apples while their readers perceive
oranges.
The end product often generates and propagates more mis-
understanding than if the article had never been written.
Unfortunately, much of what passes for sage advice
in the revered documents of our craft do not pass muster as
lucid words of understanding. Caveat emptor - without warranty
a buyer assumes all risks. You are a buyer of ideas when you
spend $time$ to read, assimilate and apply notions proffered
by the words and illustrations. Critical review is NEVER a
bad idea for anything you read from the wild . . . from ANY source
(including aeroelectric.com).
Bob . . .
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
