I have a subpanel whose feed protected by two 30 amp old stlye (glass) fuses on each of the 120 volt lines.

In the subpanel is a 20 amp breaker for a pool pump.

The capacitor for the pool pump died and one of the 30 amp glass fuses blew.

My question is why didn't the 20 amp breaker trip before the 30 amp fuse blew?

Fuses in general especially if they are NOT time delay will open a circuit FASTER than a breaker will under a short circuit or severe overload condition, which I assume happened here..

A.D

[This message has been edited by Rewired (edited 06-28-2006).]

[This message has been edited by Rewired (edited 06-28-2006).]

Sometimes a 100A CB (or > ) will pop before say a 20A CB, depending on the trip curve of the CB, or just the general condition of the CB....

Yes, it happens all the time where a branch circuit doesnt trip but the main does - whether a fuse or breaker. This is one of the reasons why its bad practice try to short out a circuit to identify it at the panel.

Or NONE will trip at all....

Been working with Federal Pacific recently?

Oh, 1960s Schrack breakers (Austrian) were good at that too... few months ago I swapped out an entire panel at a friend's place because he complained dead shorts usually blew the 25A main fuse instead of the 10A breaker... the breaker was H characteristic, supposed to be much faster than a fuse.

Fuses tend to fatigue over time as well. They get weaker (particularly if they've been subjected to numerous faults) with age and will blow faster than their original published curves.

Could be one, or more, of the following:

<OL TYPE=A>

[*] Fuse was Non-Time Delay type (Fast Acting),


[*] Terminations at the Fuse were generating heat, which assisted in a more rapid melting of the Fuse's element,


[*] Circuit Breaker's contacts are welded closed, from continuous reclosing on a faulted circuit,


[*] Circuit Breaker is of the infamous "FPE Trip-Free", or "No-Blow Zinsco" designs,


[*] Motor Circuit is maybe not fed "Entirely" through the 20 Amp Breaker,


[*] Non-Selective coordination between the Feeder's OCPD and the Panelboard's OCPD.
</OL>

Sounds more likely that the Fuse was just a Fast Acting type.

At least the E base Fuse did not have a
"Capacity Extending Device"
or
"Trip Eliminator"
(AKA "Coin behind the Fuse")


Scott35

HEY NOW!
Easy on the FPE my place is full of it

( and believe it or not they DO work!!)

A.D