Open circuit lamp

Or somebody put a in normal fuse that doesn't have the light in it.

TS21sso wrote:
I have a WFCO ULTRA III Distribution center (Fuse Box)
This unit has an open circuit lamp on each fuse. When fuse is blown or removed, the lamps light up. Problem: A fuse blew when I was extending the awning. This should have resulted in the lamp to be lit, it wasn't, nor did it light when I removed the blown fuse. Is this due to a fault in the box, or some other condition that would cause both the short and the unlit lamp? Like a loose connection some where in the circuit?

Probably nothing wrong with the sense circuit. It's a byproduct of how the circuit works. The lamp won't always light when fuse blows.

Why????? Well here is how that circuit works. When the fuse blows, IF the circuit is trying to draw current, a small amount of current will flow through the high resistance sense circuit, causing a transistor to turn on, which then lets current flow through the LED.

However, IF the circuit isn't trying to draw power, NO light. So it will work fine on circuits with a light on, radio, tv. etc. It will almost NEVER work on things like awnings, etc.

However, IF you were to try to extend or retract the awning, then I'll bet you will see the LED light with the fuse blown.

Hope this helps explain that circuit. It is handy, but not foolproof.