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

[Lynn and Susan Lang]

Bruce
What I would do is have a fuse, circuit breaker for the higher amperage
circuit and a lower fused control circuit. The design would be all off
if the controls are not on or a failure.

lynn  

-----Original Message-----
From: theforge-bounces at mailman.qth.net
[mailto:theforge-bounces at mailman.qth.net] On Behalf Of Bruce Freeman
Sent: Thursday, February 16, 2012 1:51 AM
To: Blacksmithing List Sponsored by ABANA
Subject: [TheForge] OT - fuses in a 230V (US) circuit

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.

(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.)

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?

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.

-- 
Bruce
NJ
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