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

[Bob]

     I don't know the details of your circuit and I don't know what your 
SSR is controlling or how it is controlled.
The SSRs that I am familiar with are usually controlled by a lower 
voltage to control terminals and can be
either AC or DC. A 240V circuit does not require a neutral wire but does 
require an earth ground.
If the control circuit for the SSR is derived from a 120V circuit then a 
neutral is required for this circuit
and it could be fused separately from the 240V circuit. If voltage 
surges are an issue then a surge protector
could be used on the control circuit only.

Bob Willman
The Eagle's Anvil
Bowling Green, Ohio
WB8NQW

The natural progress of things is for liberty to yield and government to gain ground.

Thomas Jefferson
in a letter to E. Carrington,
May 27,1788


On 2/16/2012 1:50 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.
>
> (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.
>