Polymeric circuit protection devices are a kind of Positive
Temperature Coefficient (PTC) thermistor. They have a really steep
non-linear tempco, so that their resistance climbs rapidly after they
reach a certain temperature. This process is reversible, so they
conduct normally after cooling down. However, it's important to
understand a couple of things about them:

1) Their 'trip current' depends on anything that affects their
temperature.

2) They don't remain 'tripped' if you don't allow some current to flow
to keep them hot. With some types of loads, they will oscillate off
and on at a rate that depends on how fast they can cool down. With
other loads, they may deliver enough current to the load while hot
('tripped') to cause damage.

The manufacturers web sites have lots more info on this. My
experience has been that polyswitches are good for protecting
batteries, wiring, motors, speakers and other relatively rugged
components. They are less useful for protecting electronics.

Surplus DC circuit breakers are frequently offered on ebay. They have
the advantage of incorporating a switch into the breaker, simplifying
wiring.

Paul Mathews

"Evan Gatehouse" wrote in message ...
Hi,

I've been considering the use of Polyfuses (made by Raychem among others)
instead of circuit breakers in an electric panel on my boat. They are a lot
cheaper (like $0.50 each) and smaller than a breaker. The specs say "100A
maximum current" for a typical 5-10A fuse. This is the maximum fault
current that can be used to trip such a device.

The typical C series Carling hyd./magnetic circuit breaker has a
interrupting capacity of 7500A @ 80VDC. This is the toggle type circuit
breaker that you see on most new boats.

My question: is 100A interrupting enough? If there is a short in a typical
wire, will fault currents exceed that? I don't know enough electrical
engineering to determine if this would be a safe application for these
fuses. I do know of one kit plane builder (who is an EE) who thinks they
are o.k. And one maker of boat parts who is offering them:
http://www.lalizas.com/products.asp?S0=5&S1=13&S2=37

This is a kit plane builder who uses them:

http://www.expbus.com/pages/avionics_expbus.htm