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


--
Evan Gatehouse

you'll have to rewrite my email address to get to me
ceilydh AT 3web dot net
(fools the spammers)

Evan,

The capacity of the wire has little to do with the maximum current it
can carry. A #18 wire can carry several thousand amps for a second or
two. You have to consider the maximum possible amps that could be
dumped through the device and a 4 battery house bank can dump a bunch of
amps.

Lalizas makes sub-panels so I would assume the panel would be fed
through a heavier capacity breaker at the main. Kit planes have a
single battery and maybe a 35 amp alternator.

Evan Gatehouse wrote:
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



--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

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

"Evan Gatehouse" writes:

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?
snip

What you are asking is really, "What is the short circuit available on this
boat?"

The answer is dependant on the size of the house bank.

5,000 amps DC would not be considered unusually large.

BTW, what you are asking is the answer to a coordination study, something
consulting electrical engineers do for a living for large industrial
facilities.

For the device in question, you need to determine the maximum let thru
current under a bolted fault condition.

The manufacturer can provide this info.

HTH


--
Lew

S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
Visit: http://home.earthlink.net/~lewhodgett for Pictures

"Lew Hodgett" wrote in message news:Zsnvc.19148

What you are asking is really, "What is the short circuit available on
this
boat?"

The answer is dependant on the size of the house bank.

5,000 amps DC would not be considered unusually large.


Thanks Lew and all, I think you're right. I sure wasn't comfortable with
just the 100A interrupting capability.

What about the idea of a 40A or similar size main breaker (like a Carling)
feeding the polyfused panel board? If there is a fault such that the
polyfuse doesn't trip, the main breaker would trip. This would black out
the boat, but I can live with that in the short term, until I could manually
switch off the circuit that was shorting out the main breaker?

With about 15 circuits on my boat, even ebay DC breakers add up in cost
fast.


--
Evan Gatehouse

you'll have to rewrite my email address to get to me
ceilydh AT 3web dot net
(fools the spammers)


--
Evan Gatehouse

you'll have to rewrite my email address to get to me
ceilydh AT 3web dot net
(fools the spammers)