I'm avocating designing the trace for continuous fault current. For example,
for 30 amps through 1 oz copper the trace width can be 2.5 cm for a 50 deg C
rise.
For transient conditions (such as until a fuse opens) an adibiatic
calculation is conservative and will do. For that, calculate the amount of
energy dissipated in the trace (accounting for R increasing as it heats) and
using the specific heat of copper calculate the trace temperature rise.
Dave Cuthbert
Linear Technology
From: [email protected] [mailto:[email protected]] On Behalf Of John
Woodgate
Sent: Monday, February 05, 2007 10:29 AM
To: [email protected]
Subject: Re: Protective Earth through PCB traces
In message <[email protected]>, dated
Mon, 5 Feb 2007, David Cuthbert <[email protected]> writes
>It sounds like you have a trace width calculator. For those who do not,
>here is a link to an online trace width calculator.
It's not likely to be valid for impulsive currents, where the conductor
rises in temperature very rapidly and the heat doesn't get time to flow
away.
On the other hand, you don't need a track that will carry the fault
current continuously, only for as long as it takes the protective device
to operate in the worst case, which may occur at less than the maximum
prospective fault current. You need to look at the device's Isquared-t
characteristics.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of 2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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