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

[Paul]

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