Request for info

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K8MHZ said:
Are you talking about an SSU? In essence you could have a breaker and a fuse in series with an SSU installation.
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What is an SSU? :confused:

I have occasionally put a "current'limiting fuse" in series with a breaker. The breaker's job is to protect the conductors. The fuse prevents the fault current from exceeding the rating of the breaker, or of the downstream equipment.

Under what circumstances did this question arise?
 
It was inside a 3 phase TVSS, maybe as part of a failsafe config.

Sorry, I don't know what an SSU is, or abbreviations beginning with MW, but I do know HO, poco, cb and mccb.
I don't guess there is a list of acronyms on this Forum somewhere. . . (?)

Thanks for your answers. I doubt I would have thought of these reasons on my own (never worked on more than 1 phase, 100 watts).
 
We also used to put fuses in air conditioner disconnects because the label on the unit said maximum FUSE size. This was interpreted that they needed a fuse to protect them in addition to the breaker in the main panel.
 
What is an SSU?
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Safety Shut-off Unit.

Essentially a single device with a snap switch and a fuse holder primarily used as a disconnect for a residential furnace.

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There might be

There might be

Thank you. Can anyone add to it (after appropriate editing)?
 
and, this just in from some TVSS people

and, this just in from some TVSS people

Most suppressors use parallel connected MOVs (usually with multiple MOVs in parallel with each other). That part is usually fairly straightforward with marginal degree of difficulty. Failure modes of MOVs tend to be severely underestimated and that is where most of the engineering and R&D is (and UL & NEC!).

Why a fuse in series with a breaker?

One is a thermal disconnector and the other is overcurrent protection
(OCP). Why?? MOVs fail towards a short, but not necessarily a hard
short. (MOV operate by varying impedance and failures can have
different impedances too.) If the failed MOV draws enough current to
clear the OCP - great. But if the MOV(s) draw less current than the
operating threshold of the OCP, then the MOV(s) will continue conducting
& drawing current. (Once the fail, they keep conducting, they do not
revert back to non-conducting.) In that case, they will overheat and
stand a reasonable chance of catching fire. Since OVERcurrent
protection does not work in a LOW current situation, another clearing
mechanism is required - i.e., thermal disconnector.
There is a lot more to TVSS/MOV safety. For example, suppose a fuse clears in 5 cycles, but
the MOV(s) can rupture in 3 cycles - explosion, despite having fuses!!
This turns into an exhaustive testing exercise. Sticking MOVs in a
circuit is really easy; making sure failures are fully under control is
an entirely new ballgame. (We OEM Siemens integral TVSS where
non-passive failures inside panelboards are simply not an option.)
 
Fuses are also used in breakers for higher fault current interuption ratings. An AKR has a 65,000A rating, a AKRU (Fused) has a 200,000A rating.
 
higher fault current interuption

higher fault current interuption

That makes sense since a fuse, by its nature, rapidly melts. A CB is somewhat more complicated and so can fail in unexpected ways.
But, I suppose a fuse could also explode under extreme conditions.
 
Lxnxjxhx said:
That makes sense since a fuse, by its nature, rapidly melts. A CB is somewhat more complicated and so can fail in unexpected ways.
But, I suppose a fuse could also explode under extreme conditions.
Click to expand...

There are circuit breakers rated for 200kAIC and fuses rated for only 10kAIC. Fuses only melt rapidly when the current is relatively high (current limiting regions beginning at 12-15x are not unusual). Molded case circuit breakers are constructed differently than power circuit breakers.

It is almost impossible to make a single statement about advantages and disadvantages between fuses and breakers that is universally applicable.
 
When it comes to breakers and fuse the selection is very good and there are several types..I believe in the OP's original question the fuses were used to be a better type of protection..
Now if you are engineering a design there are several engineers here that would love to earn some cash from you..:D

















Hey Barney do you think sucking up to the engineers will help me get date with Wilma..:grin: ..
 
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