Old glass AGC fuses - what does voltage mean?

13 years ago

My heat pump (a Waterfurnace geothermal) uses AGC glass fuses on the circuit board to protect 14 gauge wire running form the circuit board (relays on the CB) to my ground loop water pumps (two pumps, two speed system). These fuses are the (old, like old automobile) glass AGC and are rated at 10 amp and 250 volts. The pumps run on 240 vac.

I have had some problems (not yet resolved) with these fuses blowing and when going out to purchase replacement fuses I found Bussmann fuses in 250 volts in amp ratings up to 10 amps. They also offer identical looking fuses (where I was shopping) in 15, 20, 25 and 30 amp, but they are all rated at (only) 32 volts. What is it about the fuse that limits the voltage to 32 volts? I assume when operating below the burn-out current the voltage drop across the fuse is only a low voltage regardless of what the line voltage is. And, even if it isn't something to do with heating of the fusible element, how can the same glass structure in one case be safe at 250 volts and at only 32 volts in another?

Comments (5)

brickeyee
13 years ago
The lower voltage fuses will not provide enough clearance from the melted fuse metal to open a high voltage circuit.
creek_side
13 years ago
The voltage is the rated maximum that can be applied to an open (blown) fuse without it possibly arcing over. Never replace a blown fuse with one of a lower voltage rating or a higher current rating.

You should also match the type exactly. I.E., a fast blow fuse should not be replaced with a slow blow fuse, even if they have the same current rating.
jerry_nj
Original Author
13 years ago
I guess I understand, the melted metal inside the fuse case is still there and the larger (higher amperage) fuses have more metal to melt, but no more space to store it. Still, I bet the jump form 250 volts to 32 volts in the size jump from 10 amp to 15 amp (I'm looking at the metal material, not a lot of difference) is not driven by that alone, some standardization going on - fuses at 30 amps may in fact pool enough metal to be a problem at voltages about 32 volts. The 30 amp fuse has a real "hunk" of metal inside. so the decision was made to make fuses in two voltage ratings, one is 32 volt and the 15 amp was included in that range.

Other information on the package is:
5A & 10A IR 10,000A, 125 Vac; IR 200A, 250 Vac
and
15A through 30A IR 1000A 32 Vac

The 5A and 10A also carry a UL listing
The higher amperage carry a SA listing.

I have no idea what 10,000A, 200A and IR mean in that information.
jerry_nj
Original Author
13 years ago
I was typing while other posts were being made.

Yes, I have fast acting fuses, as was used in the device.

As said, I think I understand the need for space inside the glass envelope to store the melted metal. As far as the external dimensions are concerned, they are the same and those factors should work at the same voltage limits, i.e., 250 volts for all on the exterior.
creek_side
13 years ago
IR means interruption rating. It is the instantaneous maximum current that can blow the fuse without the fuse essentially exploding from the metal in the link vaporizing instead of melting.