Hi Alan:
Here is a short but comprehensive explanation:
http://www.meltric.com/html/screwterminals.html
Here is still more explanation:
http://www.mdsr.ecri.org/summary/detail.aspx?doc_id=8106
>From another source:
"Terminal bodies often are made from a copper
alloy that has the same expansion coefficient
as wire. This prevents loosening between metals
with different thermal expansion rates and lower
contact resistance. Corrosion, normally caused
by electrolytic action between two different
metals, is also eliminated."
Contact resistance exists in any mechanical
connection. Current through the terminal is
dissipated as watts in the contact resistance.
This power heats the mating parts. Depending
on the expansion coefficients of the materials,
the contact area may change, which will change
the contact resistance and change the power
dissipated and the temperature of the parts.
Turning power on and off will cause the terminal
to go through thermal cycling. This leads to
loosening of the terminal.
Best regards,
Richard Nute
Product Safety Consultant
San Diego
From: [email protected] [mailto:[email protected]] On Behalf Of Brewster, Alan
Sent: Tuesday, August 15, 2006 2:22 PM
To: [email protected]
Subject: Failure of electrical terminations
Greetings Listers,
I was looking at EN60204-1, Clause 14.1.1, ‘All connections…shall be
secured against accidental loosening.’, and came to wonder if there are any
published scientific studies on the causes of loosening, in particular of
screwed terminals. It occurred to me that throughout my career I have seen
plenty of terminals that have backed off. Some expensive terminal blocks
failed and some that were cheap ones, worked. I have heard a number of
theories, some even plausible, but have never seen a good, solid, written,
technical explanation as to why it is possible for clamping force to change.
Can anyone out there help me with an answer to this question?
Regards,
Alan Brewster, MIET
Staff Engineer,
Product Safety and Environmental Department
Novellus Systems, Inc.
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