On 8/18/2020 8:52 AM, Dean Hoffman wrote:
On 8/18/20 9:12 AM, trader_4 wrote:
On Sunday, August 16, 2020 at 7:36:03 PM UTC-4, micky wrote:
In alt.home.repair, on Sun, 16 Aug 2020 16:47:58 -0400,
wrote:

On Sat, 15 Aug 2020 19:47:38 -0400, micky
wrote:

In alt.home.repair, on Sat, 15 Aug 2020 17:07:08 -0500, dpb
wrote:

On 8/15/2020 4:12 PM, micky wrote:
In alt.home.repair, on Sat, 15 Aug 2020 07:53:01 -0700 (PDT),
wrote:

It still might be a good idea to ask the service people to come
out and look the interface over.Â* And be sure to ask if their
fuses are late-model current limiting fuses or not.

What are current limiting fuses.Â* I looked at a couple webpages
and it
didn't say.Â*Â*Â* I thought all fuses were current limiting.

https://www.eaton.com/Eaton/ProductsServices/Electrical/ProductsandServices/Bussmann/MediumVoltageFuses/CurrentLimiting/index.htm


"A current limiting fuse is a fuse that, when its current responsive
element is melted by a current within the fuses specified current
limiting range, abruptly introduces a high resistance to reduce
current
magnitude and duration, resulting in subsequent current
interruption."

I had read that and didn't understand it.

For ordinary distribution panel there's no point.Â* The fuse will open
and protect what it's there to protect.

They're useful for motor starters, etc., ...

So there will still be some current to the motor starter.Â* What's a
motor starter?Â*Â* ;-)

Typically a relay but it can be anything that gets a motor going and
may have some protection built in. A lot depends on what kind of motor
you are talking about and the application.

So why would you want some current to be still going through it if there
had been enough to blow the fuse?Â* There must be a short circuit so even
the small amount of current will be going throughÂ* the short circuit and
it won't be enough to close the relay.

The large relay in a motor starter is called a contactor.

Some 3p starters are pretty
sophisticated electronic circuits that bring them up slowly.

The same question for a starter like this.

That's what I don't get either.Â* Why would you want some current still
goinghe
to something that's part of the circuit?Â* And isn't the load that
caused the
trip still present and part of it?

If you are referring to the "high resistance" in the fuse definition,
the definition is incredibly bad. CL fuses do not any intentional
resistance other than the resistance of the fusible elements (which may
be silver).
See the definition in my other post.

The defining characteristic of a current-limiting fuse is that it melts
and clears in a shorter time than 'ordinary' fuses - less than 1/4 cycle
when the fault current is high enough to cause the fuse to current limit.

Â*Â*Â*Â* This isn't the best picture in the world but might serve the purpose.
http://assets.suredone.com/1599/media-photo/jf06170-allen-bradley-480v-size-1-combination-motor-starter-512-bab-24-4.jpg

Â* Power comes in way at the top, goes through the disconnect, then the
fuses.Â* Everything else is after that.
The starters have 480 volt coils so need power from two lines to keep
them engaged.Â* A fuse blowing on either of those two lines will
immediately kick the starter out.Â*Â* A fuse blowing on the third line
will overload the other two circuits which will cause the motor starter
to kick out.
Â* The motor will be disconnected in either case.

If others are deciphering the picture, the fuses are not installed an go
in the open space toward the middle. The fuses are long because they are
for 480V.

"Motor starters" almost always have an "overload" unit that trips the
starter off (by opening the control circuit that powers the coil). The
overload is at the bottom of the black blob (contactor) at the bottom.
This overload uses "heaters" - one in each phase. There is probably a
table on the door label for selecting the size of the heaters using the
motor current. The black disc toward the bottom of the door pushes a
post on the overload to reset it.

Fuses/circuit breakers don't provide overload protection - they provide
short circuit protection. g posted an example years ago where a
circuit breaker at the source of a motor circuit could be twice the wire
amp rating (and the wire rating is higher than the motor full-load
current). (Fuses sometimes are used - i.e. a 6 1/4 amp fuse may be used
on some small motors.)

You really don't want single fuses to blow in a 3 phase set.