You are quite right, but I wonder how many products actually use High
Breaking Capacity (HBC) fuses, rated at 1500 A or so.
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK
On 2017-11-12 03:37, Ted Eckert wrote:
I would like to attempt a response to question 2 regarding adequate
breaking capacity.
Fuses should have an ampere interrupt capacity (AIC) rating. A more
extensive discussion can be found on an application note from Bussmann
Fuse at this
link<http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Interrupting_Rating.pdf>.
Finding this rating for a fuse is easy. Figuring out what the
potential fault current in your circuit would be is not easy. It will
depend on the impedances within your product, your products power
connection, building wiring and the step-down transformer providing
power to your circuit. The rating of the branch circuit breaker
usually doesn’t matter as much as the circuit impedances. If there is
a fault leading to a dead short, the circuit breaker will take a cycle
or two before it opens. The current surge during that period can be
significant. I’ve seen available fault currents in the kiloampere
range in residential applications and 50 kA in some commercial
application, all on 20 A circuits.
This is a problem in power supply design and the fuse is often placed
after the common mode choke. The choke provides enough impedance to
limit the rise time of the current under fault conditions. This give
the fuse the chance to open before the current exceeds the fuse’s AIC
rating.
Long ago, I was working on a power supply that failed testing. Under
fault conditions, the fuse would open catastrophically. The fuse had
an AIC rating of 50 A and the current was hitting over 100 A before
the fuse opened. The metal filament would vaporize and then condense
across the circuit board leaving a thin metalized plating. The
solution was to use a higher impedance choke. The designer had used
the same common mode choke that was used in other designs. However,
this was a smaller power supply. It turned out that a higher impedance
but lower current rated choke was available in the same footprint. The
current rating was still sufficient for the design and the new common
mode choke solved the problem. Its impedance was high enough to limit
the rise time of the current. Under fault conditions, the rise time
was slow enough to allow the fuse to open before the current exceed
the 50 A limit.
It’s hard to make accurate assumptions about the impedance of the
circuits into which your product will be connected. Assume the
worst-case based on electrical construction methods, transformers and
other factors for your customer’s installations. Then you can
calculate the breaking capacity your fuse will need. If you fuse is
after impedance in your own product’s design, the calculations are
simplified.
You can run testing on your product, but that can be more difficult to
do. If you happen to insert a fault at the current’s zero crossing,
you may not see the same instantaneous fault current you would see if
the short is applied at the peak of the waveform. If you are using a
laboratory AC supply, you may find that it has a lower available fault
current than a wall outlet. If you have a good high-frequency current
meter, you can watch the fault current to see if it is getting close
to the AIC rating of your fuse. As with any measurement at hazardous
voltages and with these currents, care should be taken to ensure that
the setup does not create a hazard for the laboratory staff.
Ted Eckert
Microsoft Corporation
The opinions expressed are my own and do not necessarily reflect those
of my employer.
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