> "If electricity in a wire is like water in a pipe, then why do pipes get
> cold but wires get hot ?"
It depends on how much water is flowing in the pipe and how well it is
insulated. This may be an extreme example, but four-loop Westinghouse
nuclear power plants are brought up to operating temperature by running four
7000 HP reactor coolant pumps to circulate the reactor coolant. The
friction of the pump impellers and the water flowing through the insulated
reactor vessel, piping, and steam generators raises the coolant temperature
to over 500F. We were able to bring the turbine generator up to speed and
briefly synchronize the generator to the system using only frictional heat
from the RCPs at a plant I worked at during pre-operational "hot functional"
testing performed before the nuclear fuel was brought on site...
Back to fuse ratings - fuses are designed to protect wiring from faults, and
are not sized to protect the connected load. Fuse sizes are chosen based on
the amount of current the wiring can carry without overheating. There will
be a voltage drop across the fuse due current flow acting on the resistance
of the fuse element.
I2t represents the thermal energy delivered to the wiring harness and is
calculated by multiplying amperes squared by the time in seconds. This value
denotes the energy to which the object to be protected by the fuse can be
subjected before the fuse blows. It is used as a comparison between fuse
links to judge the speed of operation. A fuse with a high I2t would let more
energy through and take longer to fully operate (that is, blow) than a fuse
with a lower I2t.
The UL 248-14 standard calls for ampere rating tests to be conducted at
110%, 135%, and 200% of rated current. The fuse must carry 110% of its
ampere rating for a minimum of four hours and must stabilize at a
temperature that does not exceed a 75C rise at 100%. The fuse must open at
135% of rated current within one hour. It also must open at 200% of rated
current within 2 minutes for 0-30 ampere ratings and 4 minutes for 35-60
ampere ratings.
British Standard BS 4265 (now IEC 60127) calls for 150% of rated current for
1 hour minimum and 210% for 30 minutes maximum. It also requires an
endurance test whereby a fuse is subjected to 120% of its rated current for
100 cycles followed by one hour at 150% of the rated current. The test is
said to be successful if the fuse still conducts and has a maximum
voltage-drop increase of 10%.
To put it in perspective: The IEC spec for the Lucas 35A fuse (continuous
rating of 17A) calls for the fuse to hold 150% (25.5A) for 1 hour minimum
and 210% (35.7A) for 30 minutes maximum. Replacing the Lucas "35A" fuse
with an AGC 35 fuse will let your wiring harness pass 47.25 amps for up to
an hour, or 70 amps for up to four minutes.
You can't simply substitute an AGC fuse for a Lucas fuse with the same
number on it because the standards used to rate them are entirely different.
I've seen a TR6 wiring harness melted while being "protected" by 35 AGC
fuses. A battery disconnect in the car saved the owner from an electrical
fire at night, hundreds of miles from home. You don't want to experience
this yourself - use the IEC rated fuses designed to protect the IEC standard
wiring in your Triumph.
More information on fuse ratings and a typical set of time-current curves is
available at http://www.ce-mag.com/archive/05/01/018.html
Tim Buja - Rockford, IL
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