[TheForge] OT - fuses in a 230V (US) circuit

[Bruce Freeman]

Thanks.  I didn't know about circuit breakers failing closed (i.e., conducting).

I did laboriously learn about I2t.  I found two such fuses at ratings
higher than for the normal fuses, but which have clearing I2t
substantially less than the SSR rating.

As to fancier protection schemes -- the SSR just isn't worth enough to
bother.  I can replace this with an identical unit for <$6!  That
doesn't mean I don't need the fuses, it just means that the
consequences of blowing the SSR are minimal.

On Thu, Feb 16, 2012 at 9:36 AM, Paul <forge at wi.rr.com> wrote:
> On 2/16/2012 12:50 AM, Bruce Freeman wrote:
>> I have a question regarding proper fusing of a 230V circuit.  Now,
>> assuming this is two hot lines with no common (earthed at fuse box)
>> line, then having a fuse on each line makes sense for fire safety, if
>> nothing else.  (A dead short to ground, with corresponding safety
>> hazards, could occur on EITHER line, so fusing one line is not
>> sufficient.)  No problem there.
> The actual circuit for the 230v system does not involve the neutral. The
> load is connected to the 230v legs, and the failure of either fuse would
> protect the device. The extra fuse is there to protect against damage
> from unintentional paths from either leg to ground ( neutral.. they have
> different uses electrically, but the lethal potentials are available at
> either 230v leg referenced to neutral or ground.)
>
>>
>> (I think circuit breakers are a better choice.  If either leg shorts,
>> it throws the breaker, shutting off both legs.  Fuses don't offer that
>> protection.  But that's neither here nor there for the question at
>> hand.)
> Circuit breakers are superior, but some times they can fail shorted
> instead of open if they are used as a switch instead of a over current
> device. Fuses don't do that.
>
>>
>> In my case, in addition to the two normal fuses, I need an extra
>> fast-blow fuse in the circuit to protect sensitive electronics (a
>> solid-state relay).  However, I see no reason to use two of these
>> expensive and difficult-to-find fast-blow fuses.  If a current spike
>> occurred that didn't blow the normal fuses, then the fast-blow fuse
>> would blow.  Once it blew, power is off, SSR is protected.  AOK.  Why
>> could I possibly need a second fast-blow fuse on the other leg of the
>> same circuit?
> You might have to find a SSR with a high enough current rating to
> survive until the fuse clears.
> Did you find a quick blow fuse that has a clearing speed higher that the
> SSR?
>
> Current spikes are voltage related. If you have a voltage spike on the
> supply side which will cause excess current to flow, you need an over
> voltage protection. In a high current power supply that I built, we used
> a 'crowbar circuit'. It is a voltage detector that triggers a SCR to
> short out the supply, this gives enough time for the fuse to clear. It
> often results in the SCR being destroyed due to the HIGH current pulse,
> but all that was cheaper than replacing the electronics that was
> connected to the power supply.
>
> If the failure is high current flow due to a short circuit or
> malfunction of the load that would cause a high current, you need to
> sense the current very quickly. You might look into using a modern motor
> starter with solid state current protection, which typically is
> adjustable. Just a thought...
>
> You may be able to do the same thing with a SSR or a Triac using a
> current sensing circuit.
>
> All of this only makes sense if the device you are trying to protect
> costs more than the SSR or the protection circuit.
>
>
>>
>> This is a real headache to me right now, and I can't seem to find an
>> on-line text to explain fusing such circuits.
>>
>
>
> --
> Paul
> WB9HCO
> My Grandfather WAS a blacksmith...
> and it didn't do me one damn bit of good.
> All opinions are the personal beliefs of the author, and are based on decades of experience... so use your best judgement, I'm just a lowly crafstman.
>  "The difference between genius and stupidity is that genius has its limits." -Albert Einstein
> "Life is hard...it's harder if you're stupid" John Wayne - Sands of Iwo Jima
>
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-- 
Bruce
NJ